Presses and Equipment for SheetMetal Dies

Presses and Equipment for Sheet Metal dies POWER PRESS TYPES The types of power presses available for metal-cutting and forming operations are varied, the selection depending upon the type of operation. Not all types of presses will be described because of space limitations. The basic types of presses and press mechanisms will be described to give the beginner the necessary background for designing press tooling. Presses are classified by (1) type of frame, (2) source of power, (3) method of actuation of slides, (4) number of slides incorporated, and (5) intended use. Most presses are not classified by only category one but several.

For example,

a straight-side press may be mechanically

or

hydraulically driven and may be either single or double acting. Classification by frame type: The frame of a press is fabricated by casting or by welding heavy steel plates. Cast frames are quite stable and rigid but expensive. Cast frame construction also has the advantage of placing a mass of material where it is needed most.

Welded frames are generally

less expensive and are more resistant to shock loading because of the greater toughness of steel plate. The general classification by frame includes the gap frame and the straight side. The gap frame is cut back below the ram to form the shape of a letter C. This allows feeding a strip from the side. Some gap-frame presses have an open back to permit strip feeding from front to back or ejection of finished parts out the back. Gap-frame presses are manufactured with solid frames fixed in a vertical or inclined position. Others are manufactured with a separate frame mounted in a base, which allows the frame to be inclined at an angle in three different positions. The reason for inclining the press is to allow parts to fall through the open back by gravity. The three-position inclinable press is frequently referred to as an open-back inclinable (OBI) press (see Fig. 3-1). Solid gapframe presses are obtainable in higher tonnages than inclinable ones because of the rigid base and solid construction.

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The OBI press is the most common press in use today. It ranges from a small 1-ton bench press to floor presses rated up to 150 tons. Its main use is for blanking and piercing operations on relatively small work pieces, although bending, forming, and drawing operations can also be done. Fig, 3-2 shows the major components of an OBI press, as follows: 1) A rectangular bed, the stationary and usually horizontal part of the press, serving as a table to which a holster plate is mounted. 2) A bolster plate, consisting of a flat steel plate from 50 mm. to 125 mm. thick, secured to the press for locating and supporting the die assembly. 3) The ram, sometimes called the slide, which reciprocates within the press frame and to which the punch or upper-die assembly is fastened. 4) A knockout, consisting of a crossbar through a slot in the ram that contacts a pin in the die to eject the work piece.

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5) The flywheel, which absorbs energy from the motor continuously and delivers its stored energy to the work piece intermittently, making it possible to use a smaller motor. 6) The pitman, consisting of a connecting rod to convey motion and pressure from the main shaft or eccentric to the press slide.

Fig. 3-3 Single action straight side eccentric shaft mechanical press.

The straight slide press incorporates a slide or ram, which travels up and down between two straight sides or housing and commonly used for large and heavy work. The size of the press is limited to some extent because reduce the working area. However the frame construction does permit large bed areas and longer strokes. The drive mechanism is generally located above the bed, The straight slide press incorporates a slide or ram, which

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travels up and down between two straight sides or housing and commonly used for large and heavy work. The size of the press is limited to some extent because reduce the working area. However the frame construction does permit large bed areas and longer strokes. The drive mechanism is generally located above the bed, although under drive presses may be obtained with the drive mechanism located below the bed. Straight side presses are classified as single, two or four point suspension, depending upon the number of connection between the slide and the main drive shaft. Fig, 3-3 shows a typical straight slide press. Classification by source of power: The great majority of presses receive their power mechanically or hydraulically. A few manually operated presses are hand operated through levers or screws, but they are hardly suited for high production. Mechanical presses use a flywheel driven system to obtain ram movement. continuously

The and

heavy

flywheel

delivers

its

absorbs

stored

energy

energy

to

from

the

motor

the

work

piece

intermittently. The motor returns the flywheel to operating speed between strokes. The permission slowdown of the flywheel during the work period is about 7 to 10 percent in nongeared presses and 10 to 20 percent in geared presses. The flywheel is attached directly to the main shaft of the press (non geared), or, it is connected to the main shaft by a gear train. Nongear drives are used on presses of low tonnage and short strokes. The number of strokes per minute on nongear drives is usually quite high.

Gear driven presses transmit the energy of the flywheel through a single or double reduction gear. The single reduction gear drive is suited for heavier blanking operations or shallow drawing. The double-gear drive is used on large, heavy presses where it is necessary to move large amounts of mass at slower speeds. The double reduction greatly reduces the strokes per minute without reducing the flywheel speed.

Basic types of mechanical press drives a) Nongeared

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b) Single geared c) Single reduction gear d)Double

reduction

Fig. 3-5 Typical double action hydraulic press with a Die cushion

Hydraulic presses have a large cylinder and piston, coupled to a hydraulic pump. The piston and press ram are one unit. The tonnage capacity depends upon the cross-sectional area of the piston (or pistons) and the pressure developed by the pump. The cylinder is double acting in order to move the ram in either direction. The advantage of a hydraulic press is that it can exert its full tonnage at any position of the ram stroke. In addition, the stroke can be varied to any length within the limits of the hydraulic-cylinder travel. The speed and pressure are also

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constant throughout the entire stroke Fig. 3-5 shows a typical hydraulically driven press. Classification by method of slide actuation: The flywheel of a press drives the main shaft, which in turn changes the rotary motion of the flywheel into the linear motion of the slide or ram. This is generally accomplished by incorporating crankpins or eccentrics into the main drive shaft, as shown in Fig, 3-4. The number of points of suspension of the slide determines the number of throws or eccentrics on the main shaft. Points of suspension are places where pressure is transmitted by connection to the slide. Press connections, called connecting rods or pitman, are usually adjustable in length so that the shut height of the press can be varied. The most common driving device is the crankshaft, although many newer presses use the eccentric for ram movement. The main advantage of the eccentric is that it offers more surface area for bearing support for the pitman and main disadvantage is its limitations on the length of stroke. Therefore, presses having longer strokes generally utilize the crankshaft. In addition to eccentrics and crankpins, cams, toggles, rack and pinions, screws, and knuckles actuate slides. Space does not permit discussion of these mechanisms in this text. Information may be obtained by referring to the various die-design and press handbook. 3.1.4 Classification by the number of slides incorporated: The number of slides incorporated in a single press is called the action, i.e. the number of rams or slides on the press. Thus a single-action press has one slide. A double-action press has two slides, an inner and an outer slide (see Fig. 3-5). This type of press is generally used for drawing operations during which the outer slide carries the blank holder and the inner slide carries the punch. The outer, or blank-holder, slide, which usually has a shorter stroke than the inner, or punch-holder slide, dwells to hold the blank while the inner slide continues to descend, carrying the draw punch to perform the drawing operation. A triple-action press is the same as a double-action with the addition of a third ram, located in the press bed, which moves upward in the bed soon after the other two rams descend. All three-slide movements

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are properly synchronized for triple-action drawing, redrawing, and forming. GENERAL PRESS INFORMATION The tool designer must know certain fundamentals of press operation before he can successfully design press tooling. Press tonnage: The tonnage of a press is the force that the press ram is able to exert safely. Press slides exert forces greater than the rated tonnage because of the built-in safety factor, but this is not a license to overload. The tonnage of hydraulic presses is the piston area multiplied by the oil pressure in the cylinder.

Changing the oil pressure varies the

tonnage. The tonnage of mechanical presses is determined by the size of the bearings for the crankshaft or eccentric and is approximately equal to the shear strength of the crankshaft metal multiplied by the area of the crankshaft bearings. The tonnage of a mechanical press is always given when the slide is near the bottom of its stroke because it is greatest at this point. Stroke: The stroke of a press is the reciprocating motion of a press slide, usually specified as the number of inches between terminal points of the motion. The stroke is constant on a mechanical press but adjustable on a hydraulic press. Shutheight: The shut height of a press is the distance from the top of the bed to the bottom of the slide with the stroke down and the adjustment up. The thickness of the bolster plate must always be taken into consideration when determining the maximum die height. The shut height of the die must be equal to or less than the shut height of the press. The shut height of a press is always given with the adjustment up. Lowering the adjustment of the slide may decrease the opening of the press from the shut height down, but it does not increase the shut height. Thus the shut height of a die must not be greater than the shut height of the press. It may be less, because lowering the adjustment can reduce the die opening in the press. Die space:Die space is the area available for mounting dies in the press.

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Introduction to Die Cutting Structure THE FUNDAMENTALS OF DIE-CUTTING OPERATIONS While there are many die-cutting operations, some of which are very complex, they can all be reduced to the following simple fundamentals.

Fig. 4-1 Drop-through Blanking Die

Fig. 4-2 Piercing Die Assembly

Plain blanking: Fig. 4-1 shows a simple operation of this type. The material used is called the stock and is generally a ferrous or nonferrous strip. During the working stroke the punch goes through the material, and on the return stroke the material is lifted with the punch and is removed by the stripper plate. The stop pin is a gage for the operator. In practice, he feeds the stock by hand and locates the holes to be punched as shown. The part that is removed from the strip is always the work piece (blank) in a blanking operation. 4.1.2

Piercing: This operation consists of simple hole punching. It differs

from blanking in that the punching (or material cut from stock) is the scrap and the strip is the work piece. Piercing is nearly always accompanied by a blanking operation before, after, or at the same time. Fig. 4-2 shows a typical piercing die assembly. 4.1.3

Lancing: This is a combined bending and cutting operation along a

line in the work material. No metal is cut free during a lancing operation.

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The punch is designed to cut on two or three sides and bend along the fourth side. Fig. 4-3 and 4-4 show the principle of the lancing operation.

Fig. 4-3 lancing action

Fig. 4-4 Strip lanced for free metal for

forming

4.1.4

Cutting off and parting: A cutoff operation separates the work

material along a straight line in a single-line cut (Fig. 4-5). When the operation separates the work material along a straight line cut in a doubleline cut, it is known as parting (Fig. 4-6) . Cutting off to separate the work piece from the scrap strip. Cutting off and parting usually occur in the final stages of a progressive die. Cutting off is also used to chop up the scrap strip skeleton as it leaves the die. This makes the scrap much easier to handle. Fig. 4-7 shows the basic principles of cutting off and parting.

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Fig. 4-5 Cutoff action

Fig. 4-7 Layout for making blanks by

Cutoff

4.1.5

Notching: This operation removes metal from either or both edges

of the strip. Notching serves to shape the outer contours of the workspace in a progressive die or to remove excess metal before a drawing or forming operation in a progressive die. The removal of excess metal allows the metal to flow or from without interference from excess metal on the sides. Fig. 4-8 shows a typical example of notching

Fig. 4-8 Notching

4.1.6

Fig. 4-9 Shaving

Shaving: Shaving is a secondary operation, usually following

punching, in which the surface of the previously cut edge is finished smoothly to accurate dimensions. The excess metal is removed much as a chip is formed with a metal-cutting tool. There is very little clearance (close to zero) between the punch and die, and only a thin section of the edge is removed from the edge of the work piece. Fig. 4-9 described the shaving operation.

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Fig. 4-10 Trimming a horizontal flange 4.1.7 Trimming: This operation removes the distorted excess metal from drawn or formed parts and metal that has been needed in a previous operation. It also provides a smooth edge. Fig. 4-10 shows tooling for Trimming a horizontal flange on a drawn shell in a separate operation. After scrap from a sufficient number of trimmed shells has accumulated, the piece of scrap at the bottom is severed at each stroke of the press by scrap cutter shown in this figure and falls clear. CUTTING ACTION IN PUNCH AND DIE OPERATIONS The cutting action that occurs in blanking or piercing is quite similar to that of chip formation ahead of a cutting tool. The punch contacts the work material supported by the die and a pressure buildup occurs. When the elastic limit of the work material is exceeded, the material begins to flow plastically (plastic deformation). The punch penetrates the work material, and the blank, or slug, is displaced into the die opening a corresponding amount. A radius is formed on the top edge of the hole and the bottom edge of the slug, or blank, as shown in Fig. 4-11a. The radius is often referred to as rollover and its magnitude depends upon the ductility of the work material. Compression of the slug material against the walls of the die opening burnishes a portion of the edge of the blank, as shown in Fig. 4-11b. At the same time, the plastic flow pulls the material around the punch, causing a corresponding burnished area in the work material. Further continuation of the punching pressure then starts fractures at the cutting edge of the punch and die (see Fig. 4-11c). Under ideal cutting conditions, the fractures will meet and the remaining portion of the slug edge will be broken away. A slight tensile burr will be formed along the top edge of the slug edge will be broken away. A slight tensile burr will be formed along the top edge of the slug and the bottom edge of the work material (see Fig. 4-11d).

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Fig. 4-11 Cutting action progression when blanking and piercing metal

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Fig. 4-12 Characteristic appearance of edges of parts produced by piercing and blanking Fig. 4-12 Shows the characteristic appearance of the edges of parts produced by blanking and piercing operations in detail. The edge radius (or rollover) is produced during the initial stage of plastic deformation. The edge radius is more pronounced with soft materials. The highly burnished band is the result of the material’s being forced against the walls of the punch and die and rubbing during the final stages of plastic deformation. The sum of the edge radius depth and the burnished depth is referred to as penetration, i.e., the distance the punch penetrates into the work material before fracture occurs. Penetration is usually expressed as a percent of material thickness, and it depends upon the properties of the work material. As the work material becomes harder, the percent of penetration decreases. For this reason, harder materials have less deformation and burnished area. The remaining portion of the cut is the fractured area, or break. The angle of the fractured area is the breakout angle. The tensile burr is adjacent to the break. The burr side of blank or slug is toward the punch, and the burr side of the work material is toward the die opening.

Die Clearance: Clearance is defined as the intentional space between the punch cutting edge and die cutting edge. Clearance is always expressed as the

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amount of clearance per side. Theoretically, clearance is necessary to allow the fractures to meet when break occurs, as shown in Fig. 4-13. The amount of clearance depends upon the kind, thickness and hardness of the work material.

Excessive clearance allows a large edge radius (rollover) and excessive plastic deformation. The edges of the material tend to be drawn or pulled in the direction of the working force, and the break is not smooth. Large burrs are present at the break edge.

Fig. 4-13 The effect of clearance (a) too little clearance: fracture do not meet(b) Correct clearance: fracture do meet

Insufficient cutting clearance caused the fractures to miss and prevents a clean break, as shown in Fig. 4-13a. A partial break occurs, and a secondary break connects the original or main fractures. This is often referred to as secondary shear. The secondary break does not allow separation of the material without interference, and a second burnished ring is formed, as shown in Fig. 4-13b. The burnished ring may appear as a slight step around the outside edge of the blank or around the inside edge of the hole. Insufficient clearance increase pressure on the punch and die edge and has a marked effect on die life.

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Authorities disagree on the correct amount of clearance for a given work piece material. One reference may recommend 6 percent of stock material thickness per side, while another may recommend 12 percent for the same material. The difference is probably in the end result each is striving for. The designer should consider the application of the pierced or blanked work piece. When the purpose is only to make a hole, as in the case of structural steel, wide clearances may be used to increase die life. Blanked work pieces that assemble as an integral part of a mechanism require tighter clearances. stampings

with

a

description

Fig. 4-14 shows various edges for and

use

of

each.

Note

that

the

recommended clearance varies from 2 to 21 percent for mild steel. The diameter of the blank or pierced hole is determined by measurement of the burnished area. Since the burnished area on the blank is produced by the walls of the die, the diameter of the blank will be the same as the diameter of the die (disregarding a slight expansion after the blank is pushed from the die). The same principle applies to the diameter of the pierced hole. The burnished area in the hole is caused by the punch; thus the diameter of the pierced hole will be the same as the punch. Therefore, die clearance is either placed on the punch or the die, depending upon whether the pierced hole or the blank will be the desired work piece. If the blank is to become the work piece, the die diameter is made to the work piece size and the punch is reduced in size an amount equal to the die clearance. If the pierced hole is to become part of the work piece, the punch is made to the correct hole size and the die opening is made oversize an amount equal to the die clearance. In simple terms, the die controls blank s ize and the punch controls hole size

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Angular clearance is necessary to prevent backpressure caused by blank or slug buildup especially when the punches or die block are fragile. Recommended angular clearance varies from ¼ to 2 per side, depending upon the material and the shape of the work piece. Soft materials and heavy-gage materials require greater angular clearance. Larger angular clearance may be necessary for small and fragile punches.

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Fig. 4-15 The use of angular clearance Stripping: The forces that cause the blank to grip inside the die walls also cause the stock material to grip around the punch. The stock material will rise as the press ram is raised unless some means of stripping the stock material from the punch is provided. Fig. 4-15 shows the two basic types of stripping device.

Fig. 4-15 Basic types of stripping devices (a) Fixed type and (b) Spring loaded type The amount of pressure required to strip the stock material from the punch varies from 5 to 20 percent of the cutting-force requirements. This is only a rough estimate, as many variables affect stripping pressure. For example some materials cling more than others. Thicker

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materials require more stripping force because more material is in contact with the punch. Holes punched close to the strip edge do not require as much stripping force because there is less backing and the metal can give. Punches with polished sidewalls tend to strip easier than those with rough surfaces. More force is also required to strip punches that are close together. Cutting forces: The force required to penetrate the stock material with the punch is the cutting force. If the die contains more than one punch that penetrates t he stock material simultaneously, the cutting force for that die is the sum of the forces for each punch. Knowledge of cutting forces is important in order to prevent overloading the press or failure to use it to capacity. The formula for determining cutting forces takes into account the thickness of the stock material, the perimeter of the cut edge and the shear strength of the stock material. The shear strength of the stock material is the force necessary to sever 1 sq. in. of the material by direct shearing action. It sometimes becomes necessary to reduce cutting forces to prevent press overloading. One method of reducing cutting forces is to step punch lengths, as shown in Fig. 4-16. Punches or groups of punches progressively become shorter by about one stock-material thickness. A second method is to grind the face of the punch or die at a small shear angle with the horizontal. This has the effect of reducing the area in shear at any one time. Shear also reduces shock to the press and smoothes out the cutting operation. The shear angle chosen should provide a change in punch length of from 1 to 1 ½ times the stock thickness. Shear that is equal to or greater than the stock thickness is called full shear. Cutting forces are reduced by approximately 30 percent when full shear is applied

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Methods of reducing cutting forces: a) Stepping punches b) Single shear on punch c) Single shear on die d) Double shear on punches e) Double shear on punches f) Convex & concave shear

Scrap – Strip Trip Layout For Blanking In designing parts to be blanked from strip material, economical stock utilization is of high importance.

The goal should be at least 75 per cent

utilization. A very simple scrap-strip layout is shown in Fig. 6-1. SCRAP ALLOWANCE

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A scrap-strip layout having insufficient stock between the blank and the strip edge, and between blanks, will result in a weakened strip, subject to breakage and thereby causing misfeeds. Such troubles will cause unnecessary die maintenance owing to partial cuts, which defect the punches, resulting in nicked edges. The following formulas are used in calculating scrap-strip dimensions for all strips over 0.8 mm. thick. t = specified thickness of the material B = 1.25 t when C is less than 64 mm B = 1.5 t when C is 64 mm or longer C = L + B, or lead of the die Example:

A rectangular part, to be blanked from 1.5 mm thick steel

(Manufacturers Standard) is 10 X 27 mm. If the scrap strip is developed as in Fig. 6-2, the solution is t = 1.5 mm B = 1.25 X 1.5 = 1.875 mm C = 10 + 1.875 = 11.875 mm W = 27 + 3.75 Nearest commercial stock is 32 mm.

= 30.75 mm

Therefore, the distance B will equal

2.3mm. This is acceptable since it exceeds minimum requirements.

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Minimum Scrap-Strip Allowance: If the material to be blanked is 0.6 mm thick or less, the formulas above should not be used. Instead, dimension B is to be as follows: Strip width W

Dimension B

0 - 75 mm

1.3 mm

76 – 150 mm

2.4 mm

150 – 300 mm

3.2 mm

Other Scrap-Strip Allowance Applications: Figure 6-3, 6-4 and 6-5 illustrates special allowances for one-pass layouts:

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View D. For layouts with sharp corners of blanks adjacent, B = 1.25 t. Fig. 6-4 Allowances for one-pass layouts.

Percentage of Stock Used: If the area of the part is divided by the area of the scrap strip used, the result will be the percentage of stock used.

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If A = total area of strip used to produce a single blanked part, then A = CW (Fig. 3-35), and a = area of the part = LH. If C = 11.5 mm and W = 32 mm then A = 11.5 X 32 = 368 mm² If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6 mm² Percentage of stock used: a

256.5 =

A

=

70% approx.

368

EVOLUTION OF A BLANKING DIE In the planning of a die, the examination of the part print immediately determines the shape and size of both punch and die as well as the working area of the die set.

Die Set Selection

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A commercially available standardized two-post die set with 150 mm overall dimensions side-to-side and front-to-back allows the available 76 mm. wide stock to be fed through it. It is large enough for mounting the blanking punch on the upper shoe (with the die mounted on the lower shoe) for producing the blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of from 100 to 225 mm. Since the stock, in this case was available only in a width of 76 mm the length of the blanked portions extended across the stock left a distance between the edges of the stock and the ends of the blank of 6 mm or twice the stock thickness; this allowance is satisfactory for the 3.2 mm stock. Die Block Design By the usual „rule-of thumb‟ method previously described, die block thickness (of tool steel) should be a minimum of 20 mm for a blanking perimeter up to 75 mm and 25 mm for a perimeter between 75 and 100 mm. For longer perimeters, die block thickness should be 32 mm. Die blocks are seldom thinner than 22 mm finished thickness to allow for grinding and for blind screw holes. Since the perimeter of the blank is approximately 178 mm a die block thickness of 38 mm was specified, including a 6 mm grinding allowance. There should be a margin of 32 mm around the opening in the die block; its specified size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap screws and dia. 10 mm dowels are located at the corners 20 mm from the edges of the block. The wall of the die opening is straight for a distance of 3.2 mm (stock thickness); below this portion or the straight, an angular clearance of 1½° allows the blank to drop through the die block without jamming. The dimensions of the die opening are the same as that of the blank; those of the punch are smaller by the clearance (6 per cent of stock thickness, or 2 mm), which result in the production of blanks to print (and die) size. The top of the die was ground off a distance equal to stock thickness (Fig. 6-7) with the result that shearing of the stock starts at the ends of the die and progresses towards the center of the die, and less blanking pressure is required than if the top of the die where flat. Punch Design

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The shouldered punch (57 mm) long is held against a 6 mm thick hardened steel backup plate by a punch plate 20 mm thick) which is screwed and doweled to the upper shoe. The shut height of the die can be accommodated by a 32-ton (JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For the conditions of this case study, shear strength S = 42 kg/mm², blanked perimeter length L = 178 mm approx. and thickness T = 3.2 mm. From the equation P = SLT The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons. This value is well below the 32-ton capacity of the selected press. The shut height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the punch into the die cavity. print immediately determines the shape and size of both punch and die as well as the working area of the die set.

Die Sheet Selection

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A commercially available standardized two-post die set with 150 mm overall dimensions side-to-side and front-to-back allows the available 76 mm. wide stock to be fed through it.

It is large enough for mounting the blanking

punch on the upper shoe (with the die mounted on the lower shoe) for producing the blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of from 100 to 225 mm. Since the stock, in this case was available only in a width of 76 mm the length of the blanked portions extended across the stock left a distance between the edges of the stock and the ends of the blank of 6 mm or twice the stock thickness; this allowance is satisfactory for the 3.2 mm stock. Die Block Design By the usual „rule-of thumb‟ method previously described, die block thickness (of tool steel) should be a minimum of 20 mm for a blanking perimeter up to 75 mm and 25 mm for a perimeter between 75 and 100 mm. For longer perimeters, die block thickness should be 32 mm. Die blocks are seldom thinner than 22 mm finished thickness to allow for grinding and for blind screw holes. Since the perimeter of the blank is approximately 178 mm a die block thickness of 38 mm was specified, including a 6 mm grinding allowance. There should be a margin of 32 mm around the opening in the die block; its specified size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap screws and dia. 10 mm dowels are located at the corners 20 mm from the edges of the block. The wall of the die opening is straight for a distance of 3.2 mm (stock thickness); below this portion or the straight, an angular clearance of 1½° allows the blank to drop through the die block without jamming. The dimensions of the die opening are the same as that of the blank; those of the punch are smaller by the clearance (6 per cent of stock thickness, or 2 mm), which result in the production of blanks to print (and die) size. The top of the die was ground off a distance equal to stock thickness (Fig. 6-7) with the result that shearing of the stock starts at the ends of the die and progresses towards the center of the die, and less blanking pressure is required than if the top of the die where flat. Punch Design

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The shouldered punch (57 mm) long is held against a 6 mm thick hardened steel backup plate by a punch plate 20 mm thick) which is screwed and doweled to the upper shoe. The shut height of the die can be accommodated by a 32-ton (JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For the conditions of this case study, shear strength S = 42 kg/mm², blanked perimeter length L = 178 mm approx. and thickness T = 3.2 mm. From the equation P = SLT The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons. This value is well below the 32-ton capacity of the selected press. The shut height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the punch into the die cavity. Stripper Design The stripper that was designed is of the fixed type with a channel or slot having a height equal to 1.5 times stock thickness and a width of 80 mm to allow for variations in the stock width of 75 mm. The same screws that hold the die block to the lower shoe fasten the stripper to the top of the die block. If, instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a spring-loaded stripper would firmly hold the stock down on top of the die block and could, to some extent, flatten out wrinkles and waves in it.

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A spring-loaded stripper should clamp the stock until the punch is withdrawn from the stock. The pressure that strips the stock from the punch on the upstroke is difficult to evaluate exactly. A formula frequently used is Ps = 2.5 x L x t kgs.

Where Ps = stripping pressure, in kgs. L = perimeter of cut, in mm. t = stock thickness, in mm. Spring design is beyond the scope of this book; die spring data are available in the catalogues of spring manufacturers. Stock Stops The pin stop pressed in the die block is the simplest method for stopping the hand-fed strip. The right-hand edge of the blanked opening is pushed against the pin before descent of the ram and the blanking of the next blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked opening to ride over the pin and to engage the right-hand edge of every successive opening. The design of various types of stops adapted for manual and automatic feeding is covered in a preceding discussion.

A spring-loaded stripper should clamp the stock until the punch is withdrawn from the stock. The pressure that strips the stock from the punch on the upstroke is difficult to evaluate exactly. A formula frequently used is Ps = 2.5 x L x t kgs.

Where Ps = stripping pressure, in kgs. L = perimeter of cut, in mm.

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t = stock thickness, in mm. Spring design is beyond the scope of this book; die spring data are available in the catalogues of spring manufacturers. Stock Stops The pin stop pressed in the die block is the simplest method for stopping the hand-fed strip. The right-hand edge of the blanked opening is pushed against the pin before descent of the ram and the blanking of the next blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked opening to ride over the pin and to engage the right-hand edge of every successive opening. EVOLUTION OF A PROGRESSIVE BLANKING DIE Figure 6-8 gives the blanked dimensions of a linkage case cover of cold rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts made at one setup, with the possibility of three or four runs per year.

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Step 1, Part Specification 1. The production is of medium class; therefore a second-class die will be used. 2. Tolerances

required:

Except

for

location

of

the

slots,

all

dimensions are in fractions. The slot locations, though specified in decimals, are not very close. Thus a compound die is not necessary; a two or three-station progressive die will be adequate. 3. Type of press to be used: Available for this production are presses of 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or 200 mm. 4. Thickness of material: Specified as 32 mm standard cold rolled steel. Step 2, Scrap-Strip Development From the production requirements, a single-row strip will suffice. After several trials, the scrap strip shown in Fig. 6-9 was decided upon. Owing to the closeness of the holes it was decided to make a four-station die.

The scrap strip would be fed into the first finger stop, and the center hole would be pierced. The strip would then be moved in to the second finger stop, and the two holes would then be pierced. At the third stage and third finger stop, a pilot

30

would locate the strip and the four corner holes would then be pierced. At the fourth and final stage, a piloted blanking punch would blank out the finished part.

Step 3, Press Tonnage It is now in order to determine the amount of pressure needed. Only the actual blanking in the fourth stage need be calculated, since the work in the first three stages will be done by stepped punches. From Table, the shear strength S of cold rolled steel is 40 kgs/mm². The length L of the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut (stock thickness t) equals 3.2 mm. From the equation P = S L t P = 40 kgs./mm² x 240 mm x 3.2 mm = 30,720 kgs. Or 30.7 tons. This tonnage is greater than can be handled by the available presses. To lower the pressure, shear is ground on the blanking punch to reduce the needed pressure by on third. This, ? x 30.7 = 30.7 - 10.2 = 20.5 tons. A punch press of 25-ton capacity would do, but there is reported available only a 30-ton press with a 190 mm shut height and a 50 mm stroke. This press is selected. The bolster plate is found to be 300 mm deep, 140 mm from centerline of ram to back edge of bolster, and 600 mm wide. Shank diameter is 64 mm. Step 4, Calculation of the Die (a) The die.

The perimeter of the cut equals 240 mm and therefore the

thickness of the die must be 25 nm. The width of our scarp-strip opening is 60 mm with 32 mm extra material on each side of the opening, it will be 60 mm + 64 mm = 124 mm or 130 mm width. The distance from the left side of the opening in stage 4 to the edge of the opening in stage 1 equals 3 C + 30 + 6 = 192 + 30 + 6 = 228 mm and plus 62 mm = 290 mm or 296 mm long. Therefore the die should be 2.5 x 130 x 296 mm long. (b) The die plate. As a means of filling in between the die and the die shoe, a die plate of machinery steel is used. To secure the die plate to the die shoe M12 cap screws and dowels are used. A minimum of twice the size of the cap screw for the distance from the edge of the die to the edge of the die plate is needed, which will equal 25 mm. Twice this distance = 50 mm and 50 mm added to the size of the die will result in a die plate of 25 x 180 x 346 mm. Figure 6-10 shows the die

31

and die plate fitted together, and with the holes, which show the sharpening portion and the relief portion.

Step 5, Calculation of Punches Good practice requires 10 per cent of the metal thickness to be removed from the basic dimension of the blanking punch. This same value is used on the die opening, since holes are to be pierced in the blank. The clearance rule will be applied to the die opening in Stages 1, 2, and 3, and to the punch in Stage 4 (see fig6-11). For Stage 4: Blank to be 60 mm square, Stock thickness = 3.2 mm; 10% = 0.32 mm. Punch = 60 – 0.32 = 59.68 mm. Therefore the die opening will equal 60.01 to 60 mm and the punch will equal 59.68 to 59.67 mm. For Stage 2: Slot to be 8 mm wide by 34 mm long. Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm.

32

Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to 34.00 mm. long. The punch and the die opening will have straight sides for at least 311 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for the punches of Stage 2, and also the stepped arrangement of the punches for all stages

Step 6, springs A solid stripper plate can be used for this job. Step 7, Piloting Figures 6-9 and 6-11 illustrate the arrangement for piloting. In this case it is direct piloting. However, if the part did not have a center hole, and the slots and other holes were too small, indirect piloting would have to be provided. Step 8, Automatic Stops Finger stops will act as stops when a new scrap strip is being inserted but, after that, an automatic spring drop stop must be used to halt the scrap strip. Figures 6-12 illustrate details of the completed drawing of the die.

33

Die Design WIRE METHOD OF LOCATING THE CENTER OF PRESSURE: The center of pressure of a blank contour may be located mathematically, but it is a tedious computation. Location of the center of pressure within 12 mm of true mathematical location is normally sufficient.A simple procedure accurate within such limits is to bend a soft wire to the blank contour.

By

balancing this frame across a pencil, in two coordinates, the intersection of the two axes of balance will locate the desired point. As an example of the marked influence this factor may have on tool design, a rather unusual blank is shown in Fig. 5-1.

Here, the center of pressure is near one end of the

blank, and will require the indicated imbalance in the press tool design .

34

Die clearance is the space between the matting members of a die set. Proper clearance between cutting edges enables the fractures to meet and the fractured portion of the sheared edge has a clean appearance. For optimum finish of a cut edge, proper clearance is necessary and is a function of the kind, thickness and temper of the work material. At Fig. 5-2, which shows clearance C for blanks of a given size, make die to size and punch smaller by total clearance 2C. At B, which shows clearance for holes of given size, make punch to size and die larger by the amount of the total clearance 2C

35

The application of the clearance for holes of irregular shape is diagrammed in Fig. 5-13; at B the hole will be of punch size, while at A the blank will be of the same dimensions as the die.

DIE BLOCK GENERAL DESIGN: Overall dimensions of the die block will be determined by the minimum die wall thickness required for strength and by the space needed for mounting screws and dowels and for mounting the stripper plate. Wall thickness requirements for strength will depend on the thickness of the stock to be cut. Sharp corners in the contour may lead to cracking in heat treatment, and so require greater wall thickness at such points. Two, and only two, dowels should be provided in each block or element that requires accurate and permanent positioning. They should be located as far apart as possible for maximum locating effect, usually near diagonally opposite corners. Two or more screws will be used, depending on the size of the element mounted. Screws and dowels are preferably located about 1 ½ times their diameters from the outer edges or the blanking contour. Die block thickness (see Table 5-1) is governed by the strength necessary to resist the cutting forces, and will depend on the type and thickness of the material being cut.

On very thin materials 13 mm. thickness should be

36

sufficient but, except for temporary tools, finished thickness is seldom less than 22 mm., which allows for blind screw holes and also builds up the tool to a narrower range of shut height for press room convenience. DIE BLOCK CALCULATIONS: Method 1 (“Rule of Thumb”). Assuming a die block of tool steel its thickness should be 20 mm. minimums for a blanking perimeter of 75 mm. or less 25 mm. thick for perimeters between 75 mm. and 250 mm. and 32 mm. thick for larger perimeters. There should be a minimum of 32 mm. margins around the opening in the die block. The die opening should be straight for a maximum of 3 mm; the opening should then angle out at ¼ to 1 ½ to the side (draft). The straight sides provide for sharpenings of the die; the tapered portion enables the blanks to drop through without jamming. To secure the die to the die plate or die shoe, the following rules provide sound construction: 1) On die blocks up to 175 mm square, use two M10 cap screws and two dowels of dia. 10 mm. 2) On sections up to 200 mm. square, use three cap screws and two dowels. 3)

For blanking heavy stock, use cap screws and dowels of dia 12-mm.

diameter.

Counterbore the cap screws 3.2 mm. deeper than usual, to

compensate for die sharpening.

Method 2. This method of calculating the proper size of the die was derived from a series of tests, whereby die plates were made increasingly thinner until breakage became excessive. From these data the calculation of die thickness was divided into four steps:

1) Die thickness is provisionally selected from Table 5-1. This table takes into account the thickness of the stock and its ultimate shear strength (see Table 4-1). Table 5-1. DIE THICKNESS PER TON OF PRESSURE Stock

Die

Stock

37

Die

thickness thickness thickness thickness mm.

cm.

0.5 0 0.7 5 1.0 0 1.2 5

*

cm. *

*

0.2 5

mm.

0.5

0.

90

118

1.5

0.

1.8

236

2.0

0. 335

2.3

0.6 49 0.7 08 0.7

0.

2.5 48

0.

87

433

0.7

512

For each ton per sq. cm. of shear strength. Table 5-2. FACTORS FOR CUTTING EDGES EXCEEDING 50mm.

2)

Cutting perimeter mm.

Expansion Factor

50 to 75

1.25

75 to 150

1.5

150 to 300

1.75

300 to 500

2.0

The following corrections are then made: a) The die must never be thinner than 8 mm. to 10 mm. b)

Data in Table 5-1 apply to small dies, i.e. those with a cutting perimeter of 50-mm. or less. For larger dies, the thickness listed in Table 5-1 must be multiplied by the factors in Table 5-2.

c) Data in Tables 5-1 and 5-2 are for die members of toolsteel, properly machined and heat-treated. If a special alloy of steel is selected, die thickness can be decreased. d) Dies must be adequately supported on a flat die plate or die shoes. Thickness data above do not apply if the die is placed

38

over a large dopening or is not adequately supported. However, if the die is placed into a shoe, the thickness of the member can be Decreased up to 50 percent. e) A grinding allowance up to 0.25 to 0.5 mm must be added to the calculated die thickness. 3)

The critical distance A, Fig. 5-4, between the cutting edge and the die border must be determined. In small dies, A equals 1.5 to 2 times the die thickness; in larger dies it is 2 to 3 times the die thickness.

Table 5-3 MINIMUM CRITICAL AREA



Critical distance A must not less than 1.5 to 2 times die thickness.



The critical area between the die hole and the die border must be checked against minimum values in Table 3-4 and die thickness B corrected if necessary.

4) Finally, the die thickness must be checked against the empirical rule that the cross-sectional area A x B (Fig. 5-4) must bear a certain minimum relationship to the impact pressure for a die put on a flat base. In Table 5-3 impact pressure equals thickness times the perimeter of the cut times ultimate shearing strength. If the die height, as calculated by steps 1 and 2, does

39

not give sufficient area for the critical distance A (Fig. 5-4), the die thickness must be increased accordingly. With the die block size determined, the exact size of the die opening can now be determined. Assuming a clearance of approximately 10 percent of the metal thickness, and by the rule-ofthumb method. Metal thickness = 1.6 x 10% = 0.16 mm. If the finished die opening is 25 mm. dia., then add 0.16 mm., giving 26.16 ± 0.025 mm. If the blank were made according to size, the clearance would be applied to the punch. PUNCH DIMENSIONING: The determination of punch dimension has been generally based on practical experience. When the diameter of a pierced round hole equals stock thickness, the unit compressive stress on the punch is four times the unit shear stress on the cut area of the stock, from the formula. The diameter of most holes are greater than stock thickness; a value for the ratio

d / t of 1.1 is recommended. The maximum

allowable length of a punch can be calculated from the formula. This is not to say that holes having diameters less than stock thickness cannot be successfully punched. The punching of such holes can be facilitated by: 1.

Punch steels of high compressive strengths

2.

Greater than average clearances

3.

Optimum punch alignment, finish, and rigidity

4.

Shear on punches or dies or both

5.

Prevention of stock slippage

6.

Optimum stripper design

The determination of punch dimension has been generally based on practical experience. When the diameter of a pierced round hole equals stock thickness, the unit compressive stress on the punch is four times the unit shear stress on the cut area of the stock, from the METHODS OF PUNCH SUPPORT:

40

Following figure presents a number of methods to support punches to meet various production requirements:

41

42

43

STOCK STOPS: Finger Stops: In its simplest form, a stock stop may be a pin or small block, against which an edge of the previously blanked opening is pushed after each stroke of the press. With sufficient clearance in the stock channel, the stock is momentarily lifted by its clinging to the punch, and is thus released from the stop. Figure 6-23 shows an adjustable type of solid block stop which can be moved along a

44

support bar in increments up to 25 mm to allow various stock lengths to be cut off. A starting stop, used to position stock as it is initially fed to a die, is shown in Fig. 5-23, view A. Mounted on the stripper plate, it incorporates a latch, which is pushed inward by the operator until its shoulder (1) contacts the stripper plate. The latch is held in to engage the edge of the incoming stock; the first die operation is completed, and the latch is released.

Automatic Stops: The starting stops shown at view B, mounted between the die shoe and die block, upwardly actuates a stop plunger to initially position the incoming stock. Compression springs return the manually operated lever after the first die operation is completed. Trigger stops incorporated pivoted latches (1, Fig. 5-24, views A and B). At the ram‟s descent, these latches are moved out of the blanked-out stock area by actuating pins, 2. On the ascent of the ram, springs, 3, control the lateral movement of the latch (equal to the side relief) which rides on the surface of the advancing stock, and drops into the blanked area to rest against the cut edge of the cut-out area.

45

When feeding the stock strip from one stage to another, some method must be used to correctly locate and stop the strip. Automatic stops (trigger stops) register the strip at the final die station. They differ from finger stops in that they stop the strip automatically, the operator having only to keep the strip pushed against the stop in its travel through the die. A typical automatic stop designs shown in figure 5-24. In this lever end is raised by the trip screw as punch descends and cut the blank. On the return stroke end of lever drops and lever end would drop it former position if it were not for the endwise action o the lever, which causes the lever end to drop onto the top surface of the stock instead of into blank space. The mounting of the finger on the pivot is loose enough to allow for this endwise movement. When feeding the stock strip from one stage to another, some method must be used to correctly locate and stop the strip.

46

Automatic stops (trigger stops) register the strip at the final die station. They differ from finger stops in that they stop the strip automatically, the operator having only to keep the strip pushed against the stop in its travel through the die.

47

Cropping: An unusual stop that requires no moving mechanisms is known as the French stop or Cropping as shown in fig. 5-26. It operates on the principle of cutting a shoulder in the edge of the stock strip, which acts as stop. A strip wider than necessary is inserted into the strip channel until it contacts the shoulder built into back gage. The first hit of the press performs the first station operation and at the same time punches from the side of the strip a section of metal equal to the length of the pitch. This operation leaves a shoulder in the side of the strip. The strip is then advanced until the strip shoulder contacts the shoulder on the back gage on the return stroke of the ram.

48

The advantage of the cropping is its accuracy and speed of operation. It is especially well suited to the light-gage materials that are easily distorted when pushed or pulled against a stop pin. Its main disadvantages are the extra cost of tooling and extra stock scrap. PILOTS: Since pilot breakage can result in the production of inaccurate parts and the jamming or breaking of die elements, pilots should be made of good tool-steel, heat-treated for maximum toughness and to hardness of Rock-well C57 to 60. Press-fit Pilots:

Press-fit pilots (Figs. 5-28 and 5-29, view C), which may out of the punch holder, are not recommended for high-speed dies but are often used in lowspeed dies. For holes

20 mm in diameter or larger, the pilot may be held

49

by a socket-head screw shown at B.

A typical press-fit type is shown at C.

Pilots of less than 6 mm diameter may be headed and secured by a socket setscrew, as shown at D. Indirect Pilots: Designs of pilots that enter previously pierced holes in the strip as shown in Fig. 5-27. This practice provides more support under the strip. It helps to locate the pitch and prevent distortion. Spring-loaded pilots: Spring-loaded pilots should be used for stock exceeding 1.5 mm thickness sheet as shown in Fig. 5-30. This allows the pilot to retract in case of misfeed. Tapered slug-clearance holes through the die and lower shoe should be provided, since indirect pilots generally pierce the strip during a misfeed. Spring-loaded pilots are not necessary on thinner material because the pilot will pierce the strip rather than break in the event of misfeed. In this case tapered slug-clearance holes through the die and lower shoe should be provided. Misfeed detector: Fig. 5-31 shows a precision misfeed detector used in a manner similar to that of a pilot. The detector senses out-of-register position of stock and actuates a switch to cut off the electric power to the press.

50

51

STRIPPERS: Strippers are of two types, fixed or spring-operated. The primary function of either type is to strip the work piece from a cutting or non-cutting punch or die. A stripper that forces a part out of a die may also be called a knockout, an inside stripper, or an ejector.

Besides its primary function, a

stripper may also hold down or clamp, position, or guide the sheet, strip, or work piece. Fixed strippers: The stripper is usually of the same width and length as the die block. In the simpler dies, the stripper may be fastened with the same screws and dowels that fasten the die block, and the screw heads will be counter bored into the stripper.

In more complex tools and with

sectional die blocks the die block screws will usually be inverted, and the stripper fastener will be independent. Following fig. 5-32 is will make clear picture of fixed type stripper plate.

The stripper thickness must be sufficient to withstand the force required to strip the stock from the punch, plus whatever is required for the stock strip channel. Except for very heavy tools or large blank areas, the thickness required for screw head counter bores, in the range of 10 to 16 mm will be sufficient.

52

The height of the stock strip channel should be at least 1½ times the stock thickness. This height should be increased if the stock is to be lifted over a fixed pin stop. The channel width should be the width of the stock strip, plus adequate clearance to allow for variations in the width of the strip as cut, as follows: Stock thickness mm

Add to strip width in mm

Up to 1

2.0

1 to 2

2.4 2 to 3

Above 3

2.8 3.2

If the stripper length has been extended on the feed end for better stock guidance, a sheet metal plate should be fastened to the underside of the projecting stripper to support the stock. This plate should extend slightly in for convenience in inserting the strip. The entry edges of the channel should be beveled for the same reason.

5.9.2

Spring-operated strippers:

53

Where spring-operated strippers are used as shown in fig. 5-33, the force required for stripping is 35000 times cut perimeter times the stock thickness. It may be as high as 20 per cent of the blanking force, which will determine the number and type of springs required. The highest of these values should be used. Selection of stripper springs: Die springs are designed to resist fatigue failure under severe service conditions. They are available in medium, medium-heavy and heavy-duty grades, with corresponding permissible deflections ranging from 50 to 30 percent of free length. The number of springs for which space is available and the total required force will determine which grade is required. The required travel plus the preload deflection will be the total deflection, and will determine the length of spring required to stay within allowable percentage of deflection limits. As the punch is re-sharpened, deflections will increase, and should also be allowed for. To retain the stripper against the necessary preload of the springs, and to guide the stripper in its travel, a special type of shoulder screw known as a stripper bolt is used. Choice of the method of applying springs to stripper plates depends on the required pressure, space limitations, shape of the die and the nature of the work, and production requirements. The stripping pressure may be from 5 to 20 percent of the cutting pressure. The amount of pressure needed to hold thin firmly while it being cut must also be considered when selecting springs. With so many variable factors involved, exact results cannot be expected from formulas, although they may be useful as guides. The “Metal Handbook” gives the following formula for stripping force: L = KA

Where L = Stripping force

Approx. values for K, as determined by experiment: 105 for sheet metal thinner than 1.6 mm. when cut is near an edge or near a preceding cut; 150 for other cuts in sheet metal thinner than 1.6

K = Stripping mm; and 210 for sheet metal more than 1.6 mm. thick. constant, Kg / cm.2 A = Area of cut surface, cm.2

54

(stock thickness x length of cut)

Die springs are available in various service grades with corresponding permissible deflections ranging from 25 to 50 percent of free length. For example, DME Spring catalogue lists four load ratings. Based upon max. (See fig. 5-34) 50% deflection = medium pressure (MP) 37% deflection = medium high pressure (MHP) 30% deflection = high pressure (HP) 25% deflection = extra high pressure (XMP) An important feature of these springs is that they are similar dimensioned thus are interchangeable. The steps in selecting die springs are as follows: 1) Estimate the stripping force required according to formula. 2) Determine the amount of space available for spring mounting. 3) Select the max. allowable number of springs, which will fit into the available space and total required force, will determine which grade of spring is required. 4)

Determine the deflection. The required travel plus preload deflection will be the total deflection and will determine the length

of

spring

required

to

stop

within

the

allowable

percentage of deflection limits. Allowance should be made for punch sharpening, which will increase deflections over a period of time. Select a spring from the lowest (greatest deflection) load rating series from the table. It is important that the spring not be compressed beyond the specific limit for the highest-pressure spring of corresponding length and

55

diameter. Then if the springs prove too “light” for the stripping force required, each one is replaced with the next higher rated springs. This will provide more stripping pressure without the need changing spring pockets, support rods and the like.

KNOCKOUTS Since the cut blank will be retained, by friction, in the die block, some means of ejecting on the ram upstroke must be provided.

A knockout assembly

consists of a plate, a push rod and a retaining collar. The plate is a loose fit with the die opening contour, and moves upward as the blank is cut. Attached to the plate, usually by rivets, is a heavy pushrod, which slides in a hole in the shank of the die set. This rod projects above the shank, and a collar retains and limits the stroke of the assembly. Now the assembly of the ram stroke, a knockout bar in the press will contact the pushrod and eject the blank. It is essential that the means of retaining the knockout assembly be secure, since serious damage would otherwise occur. In the ejection of parts positive knockouts offer the following advantages over spring strippers where the part shape and the die selections allow their use:

56

1) Automatic part disposal; the blank, ejected near the top of the ram stroke, can be blown to the back of the press, or the press may be inclined and the same result obtained. 2) Lower die cost; knockouts are generally of lower cost than spring strippers. 3)

Positive action; knockouts do not stick as spring strippers occasionally do.

4) Lower pressure requirements, since there are no heavy springs to be compressed during the ram descent.

Fig. 5-35 shows a plain inverted compound die, is of the simplest type. It consists of an actuating plunger, knockout plate and a stop collar doweled to the plunger.

Shedder D consists of a shouldered pin

backed by a spring, which is confined by setscrew

57

Scrap – Strip Trip Layout For Blanking In designing parts to be blanked from strip material, economical stock utilization is of high importance.

The goal should be at least 75 per cent

utilization. A very simple scrap-strip layout is shown in Fig. 6-1.

SCRAP ALLOWANCE A scrap-strip layout having insufficient stock between the blank and the strip edge, and between blanks, will result in a weakened strip, subject to breakage and thereby causing misfeeds. Such troubles will cause unnecessary die maintenance owing to partial cuts, which defect the punches, resulting in nicked edges. The following formulas are used in calculating scrap-strip dimensions for all strips over 0.8 mm. thick. t = specified thickness of the material B = 1.25 t when C is less than 64 mm B = 1.5 t when C is 64 mm or longer C = L + B, or lead of the die

58

Example: A rectangular part, to be blanked from 1.5 mm thick steel (Manufacturers Standard) is 10 X 27 mm.

If the scrap

strip is developed as in Fig. 6-2, the solution is t = 1.5 mm B = 1.25 X 1.5 = 1.875 mm C = 10 + 1.875 = 11.875 mm W = 27 + 3.75

= 30.75 mm

Nearest commercial stock is 32 mm.

Therefore, the

distance B will equal 2.3mm. This is acceptable since it exceeds minimum requirements.

Minimum Scrap-Strip Allowance: If the material to be blanked is 0.6 mm thick or less, the formulas above should not be used.

Instead,

dimension B is to be as follows: Strip width W

Dimension B

0 - 75 mm

1.3 mm

76 – 150 mm

2.4 mm

59

150 – 300 mm Over 300 mm

3.2 mm 4.0 mm

Other Scrap-Strip Allowance Applications: Figure 6-3, 6-4 and 6-5 illustrates special allowances for onepass layouts:

View D. For layouts with sharp corners of blanks adjacent, B = 1.25 t. Fig. 6-4 Allowances for one-pass layouts.

60

Percentage of Stock Used: If the area of the part is divided by the area of the scrap strip used, the result will be the percentage of stock used. If A = total area of strip used to produce a single blanked part, then A = CW (Fig. 3-35), and a = area of the part = LH. If C = 11.5 mm and W = 32 mm then A = 11.5 X 32 = 368 mm² If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6 mm² Percentage of stock used: a

256.5 =

A

= 368

61

70% approx.

EVOLUTION OF A BLANKING DIE In the planning of a die, the examination of the part print immediately determines the shape and size of both punch and die as well as the working area of the die set.

Die Set Selection A commercially available standardized two-post die set with 150 mm overall dimensions side-to-side and front-to-back allows the available 76 mm. wide stock to be fed through it. It is large enough for mounting the blanking punch on the upper shoe (with the die mounted on the lower shoe) for producing the blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of from 100 to 225 mm. Since the stock, in this case was available only in a width of 76 mm the length of the blanked portions extended across the stock left a distance between the edges of the stock and the ends of the blank of 6 mm or twice the stock thickness; this allowance is satisfactory for the 3.2 mm stock. Die Block Design By the usual „rule-of thumb‟ method previously described, die block thickness (of tool steel) should be a minimum of 20 mm for a blanking

62

perimeter up to 75 mm and 25 mm for a perimeter between 75 and 100 mm. For longer perimeters, die block thickness should be 32 mm. Die blocks are seldom thinner than 22 mm finished thickness to allow for grinding and for blind screw holes. Since the perimeter of the blank is approximately 178 mm a die block thickness of 38 mm was specified, including a 6 mm grinding allowance. There should be a margin of 32 mm around the opening in the die block; its specified size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap screws and dia. 10 mm dowels are located at the corners 20 mm from the edges of the block. The wall of the die opening is straight for a distance of 3.2 mm (stock thickness); below this portion or the straight, an angular clearance of 1½° allows the blank to drop through the die block without jamming. The dimensions of the die opening are the same as that of the blank; those of the punch are smaller by the clearance (6 per cent of stock thickness, or 2 mm), which result in the production of blanks to print (and die) size. The top of the die was ground off a distance equal to stock thickness (Fig. 6-7) with the result that shearing of the stock starts at the ends of the die and progresses towards the center of the die, and less blanking pressure is required than if the top of the die where flat. Punch Design The shouldered punch (57 mm) long is held against a 6 mm thick hardened steel backup plate by a punch plate 20 mm thick) which is screwed and doweled to the upper shoe.

The shut height of the die can be

accommodated by a 32-ton (JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For the conditions of this case study, shear strength S = 42 kg/mm², blanked perimeter length L = 178 mm approx. and thickness T = 3.2 mm. From the equation P = SLT The pressure P = 42 kgs. X 178 mm X 3.2 tons.

63

= 23.92

This value is well below the 32-ton capacity of the selected press. The shut height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the punch into the die cavity. Stripper Design The stripper that was designed is of the fixed type with a channel or slot having a height equal to 1.5 times stock thickness and a width of 80 mm to allow for variations in the stock width of 75 mm. The same screws that hold the die block to the lower shoe fasten the stripper to the top of the die block. If, instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a spring-loaded stripper would firmly hold the stock down on top of the die block and could, to some extent, flatten out wrinkles and waves in it.

A spring-loaded stripper should clamp the stock until the punch is withdrawn from the stock. The pressure that strips the stock from the punch on the upstroke is difficult to evaluate exactly. A formula frequently used is Ps = 2.5 x L x t kgs.

64

Where Ps = stripping pressure, in kgs. L = perimeter of cut, in mm. t = stock thickness, in mm. Spring design is beyond the scope of this book; die spring

data

are

available

in

the

catalogues

of

spring

manufacturers. Stock Stops The pin stop pressed in the die block is the simplest method for stopping the hand-fed strip. The right-hand edge of the blanked opening is pushed against the pin before descent of the ram and the blanking of the next blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked opening to ride over the pin and to engage the right-hand edge of every successive opening. The design of various types of stops adapted for manual and automatic feeding is covered in a preceding discussion. EVOLUTION OF A PROGRESSIVE BLANKING DIE Figure 6-8 gives the blanked dimensions of a linkage case cover of cold rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts made at one setup, with the possibility of three or four runs per year.

65

Step 1, Part Specification 1. The production is of medium class; therefore a second-class die will be used. 2. Tolerances required:

Except for location of the slots, all

dimensions are in fractions. The slot locations, though specified in decimals, are not very close. Thus a compound die is not necessary; a two or three-station progressive die will be adequate. 3. Type of press to be used: Available for this production are presses of 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or 200 mm. 4. Thickness of material: Specified as 32 mm standard cold rolled steel.

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Step 2, Scrap-Strip Development From the production requirements, a single-row strip will suffice. After several trials, the scrap strip shown in Fig. 6-9 was decided upon. Owing to the closeness of the holes it was decided to make a four-station die.

The scrap strip would be fed into the first finger stop, and the center hole would be pierced.

The strip would then be moved in to the second finger

stop, and the two holes would then be pierced. At the third stage and third finger stop, a pilot would locate the strip and the four corner holes would then be pierced. At the fourth and final stage, a piloted blanking punch would blank out the finished part. Step 3, Press Tonnage It is now in order to determine the amount of pressure needed. Only the actual blanking in the fourth stage need be calculated, since the work in the first three stages will be done by stepped punches. From Table, the shear strength S of cold rolled steel is 40 kgs/mm². The length L of the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut (stock thickness t) equals 3.2 mm. From the equation P = S L t

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P = 40 kgs./mm² x 240 mm x 3.2 mm = 30,720 kgs. Or 30.7 tons. This tonnage is greater than can be handled by the available presses. To lower the pressure, shear is ground on the blanking punch to reduce the needed pressure by on third. This, ? x 30.7 = 30.7 - 10.2 = 20.5 tons. A punch press of 25-ton capacity would do, but there is reported available only a 30-ton press with a 190 mm shut height and a 50 mm stroke. This press is selected. The bolster plate is found to be 300 mm deep, 140 mm from centerline of ram to back edge of bolster, and 600 mm wide. Shank diameter is 64 mm. Step 4, Calculation of the Die (a) The die.

The perimeter of the cut equals 240 mm and therefore the

thickness of the die must be 25 nm. The width of our scarp-strip opening is 60 mm with 32 mm extra material on each side of the opening, it will be 60 mm + 64 mm = 124 mm or 130 mm width. The distance from the left side of the opening in stage 4 to the edge of the opening in stage 1 equals 3 C + 30 + 6 = 192 + 30 + 6 = 228 mm and plus 62 mm = 290 mm or 296 mm long. Therefore the die should be 2.5 x 130 x 296 mm long. (b) The die plate. As a means of filling in between the die and the die shoe, a die plate of machinery steel is used. To secure the die plate to the die shoe M12 cap screws and dowels are used. A minimum of twice the size of the cap screw for the distance from the edge of the die to the edge of the die plate is needed, which will equal 25 mm. Twice this distance = 50 mm and 50 mm added to the size of the die will result in a die plate of 25 x 180 x 346 mm. Figure 6-10 shows the die and die plate fitted together, and with the holes, which show the sharpening portion and the relief portion.

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Step 5, Calculation of Punches Good practice requires 10 per cent of the metal thickness to be removed from the basic dimension of the blanking punch. This same value is used on the die opening, since holes are to be pierced in the blank.

The

clearance rule will be applied to the die opening in Stages 1, 2, and 3, and to the punch in Stage 4 (see fig6-11). For Stage 4: Blank to be 60 mm square, Stock thickness = 3.2 mm; 10% = 0.32 mm. Punch = 60 – 0.32 = 59.68 mm. Therefore the die opening will equal 60.01 to 60 mm and the punch will equal 59.68 to 59.67 mm. For Stage 2: Slot to be 8 mm wide by 34 mm long. Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm. Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to 34.00 mm. long.

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The punch and the die opening will have straight sides for at least 3-11 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for the punches of Stage 2, and also the stepped arrangement of the punches for all stages

Step 6, springs A solid stripper plate can be used for this job. Step 7, Piloting Figures 6-9 and 6-11 illustrate the arrangement for piloting.

In this

case it is direct piloting. However, if the part did not have a center hole, and the slots and other holes were too small, indirect piloting would have to be provided. Step 8, Automatic Stops Finger stops will act as stops when a new scrap strip is being inserted but, after that, an automatic spring drop stop must be used to halt the scrap strip. Figures 6-12 illustrate details of the completed drawing of the die.

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Bending, Forming And Drawing Dies BENDING DIES Bending is the uniform straining of material, usually flat sheet or strip metal, around a straight axis, which lies in the neutral plane and normal to the lengthwise direction of the sheet or strip. Metal flow takes place within the plastic range of the metal, so that the bend retains a permanent set after removal of the applied stress. The inner surface of a bend is in compression; the outer surface is in tension. A pure bending action does not reproduce the exact shape of the punch and die in the metal; such a reproduction is one of forming. Terms used in bending are defined and illustrated in Fig. 7-1. The neutral axis is the plane area in bent metal where all strains are zero. Bend Radii: Minimum bend radii vary for the various metals; generally most annealed metals can be bent to a radius equal to the thickness of the metal without cracking or weakening.

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Bend Allowances: Since bent metal is longer after bending, its increased length, generally of concern to the product designer, may also have to be considered by the die designer if the length tolerance of the bent part is critical. The length of bent metal may be calculated from the equation A B =

2 ( IR + Kt )

(7-1)

360

Where B = bend allowance in mm. (along neutral axis) A = bend angle in deg. IR = inside radius of bend in mm. t = metal thickness in mm. K = 0.33 when IR is less than 2t and is 0.50 when IR is more than 2t.

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Bending Methods: Two bending methods are commonly made use of in press tools. Metal sheet or strip, supported by a V block (Fig. 7-2A), is forced by a wedge-shaped punch into the block. This method, termed V bending, produces a bend having an included angle which may be acute, obtuse, or of 90°. Friction between a spring-loaded knurled pin in the vee of a die and the part will prevent or reduce side creep of the part during its bending.

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Edge bending (Fig. 7-2B) is cantilever loading of a beam. The bending punch, step1, forces the metal against the supporting die, step 2 - The bend axis is parallel to the edge of the die. The work piece is clamped to the die block by a spring-loaded pad, step3, before the punch contacts the work piece to prevent its movement during downward travel of the punch. Bending Pressures: The pressure required for V bending is K L S t² P=

W

(7-2) Where P = bending force, tons K = die opening factor: 1.20 for a die opening of 16 times metal thickness, 1.33 for an opening of 8 times metal thickness L = length of part, cm. S = ultimate tensile strength, tons per sq cm. W = width of V or U die, cm.

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For U bending (channel bending) pressures will be approximately twice those required for V bending; edge bending requires about one-half those needed for V bending.

Spring back: After bending pressure on metal is released, the elastic stresses are also released, which causes metal movement resulting in a decrease in the bend angle (as well as an increase in the included angle between the bent portions). Such a metal movement, termed spring-back, varies in steel from ½ to 5°, depending upon its hardness; phosphor bronze may spring back from 10 to 15°. V-bending dies customarily compensate for spring-back with V blocks and wedge-shaped punches having included angles somewhat less than that required in the part. The part is bent through a greater angle than that required but it springs back to the desired angle. Parts produced in other types of bending dies are also over bent through an angle equal to the spring-back angle with an undercut or relieved punch. Evolution of a Bending Die The production of a work piece of Fig. 7-4 in the die of Fig. 7-5 required blank development before die design began.

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The straight length of the vertical leg is 25.0 – 1.5 or 23.4 mm. the straight length of the horizontal leg is 150.0 – 1.6 or 148.4 mm. The bend length (since IR is less than twice metal thickness) is, from Fig. (7-1) 90 B =

1.5 2  ( 1.6 +

360

) 3

= 3.3 mm.

The developed length is 23.4 + 148.4 + 3.3 = 175.1 mm.

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To hold the tolerance of ± ½ deg. allowed for the 90-deg bend, the designer decided that an edge-bending die, with a slight ironing action on the stock, be used. Based upon Fig. (7-2), the bending pressure needed without ironing is 1.33 X 25 X 4.65 X (0.15) ² P

=

= 0.483 ton 2 X 3.6

The total spring pressure required of six springs in the pressure pad is 480 kg.; each spring will supply a pressure of 80 kg. Commercial 25 mm. dia die springs, 50 mm. long, will easily supply this pressure. Almost any small OBI press will supply these pressures with an ample allowance for slight ironing of the blank and has a bed area large enough to accommodate a die set. There are no formulae for determining ironing pressure; it can be approximated by multiplying the yield strength of the metal by the thickness of the metal after reduction times its length. Since the size of the blank to be sent is 250 x 175 mm. the area of a die set 350 mm. (right-to-left) by 250 mm. (front-to-back) allows for mounting of the punch and pressure pad on the upper shoe and the die block and heel to the lower shoe. The blank is located on the die block against an end-stop pin and two rearstop pins. On the down stroke of the press, the pressure pad clamps the blank in this location. The descent of the punch forces the end of the blank against the end of the die block. Its wiping action results in some ironing of the blank, the amount of which is determined by the clearance between the heel block and the punch. To establish optimum clearance and to allow for wear on punch and heel block, shims can be inserted between the backup and heel blocks. The surface of the heel block against which the punch rubs can be hardened or can have a bronze wear strip as shown.

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FORMING DIES Forming dies, often considered in the same class with bending dies, are classified as tools that form or bend the blank along a curved axis instead of straight axis. There is very little stretching or compressing of the rail. The internal movement or the plastic flow of the material is localized and has little or no effect on the total area or thickness of the material. The operations classified as forming are embossing, curling, beading, twisting, and hole flanging. A large percentage of stampings used in the manufacturing of products require some forming operations. Some are simple forms that require tools of low cost and conventional design. Others may have complicated forms, which require dies that produce multiple forms in one stroke of the press. Some stampings may be of such nature that several dies must be used to produce the shapes and forms required. A first consideration in analyzing a stamping is to select the class of die to perform the work. Next to be considered is the number of stampings required, and this will govern the amount of money that should be spent in the design and building of the tools. Stampings of simple channels in limited production can be made on a die classed as a solid form die. It would be classified under channel forming dies. Others the block and pad type are also channel forming dies. Such operations as curling, flanging, and embossing as well as channeling employ pressure pads. A forming die may be designed in many ways and produce the same results; at this point the cost of the tool, safety of operation, and also the repairing and reworking must be considered. The tool that is cheapest and have the simplest design may not always be best because it may not produce the stamping to the drawing specifications. Where limited production is required, and a liberal tolerance is allowed in a stamping, a solid form die can be used.

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7.2.1

Solid Form Dies

The solid form die is usually of simple construction and design. Stampings produced in these dies are usually of the flanged clamp type, such as pipe straps, etc. They are made of metal, which is of a soft grade, mostly strip stock, and the grain runs parallel to the form. Some distortion is encountered; this can be compensated for in the designing of the templates. A male and a female template are usually made. The male template is made to the contour that will be shaped on the punch of the die – the same as the inside contour of the stamping. The female template is made to the outside contour of the stamping and will be shaped on the die halves. By so doing, allowance has been made for the thickness of the metal, when both die halves are set in place. Figure 7-6 illustrates both templates in place. A forming die of this kind need not be mounted on a die set. A die set should be considered because of the amount of time that can be saved in setting the die shoe production; also it eliminates the chances for misalignment in setting the die. A die that is not properly set could cause some pinching of the metal, thereby causing the stamping to break. Considering that die shoe and punch holders are required in each case, the cost of adding the leader pins to complete the die set is nominal, and should be saved in a short time. A great deal of side pressure is exerted on the die blocks, and must be considered in the designing. The die block should be made of more than one piece of tool-steel. This is necessary to eliminate the possibility of cracking the die block if the operator should feed a double blank or if metal of a thicker gage is used. The die blocks should be tied together by means of cross pieces. The die blocks should be sunk

79

into the die shoe, to obtain some support from the edge of the sinking. The blocks should be constructed wider than they are high, a proportion of at least 1:1½, with large sturdy dowel pins. The form edge of the blocks must be of proper radius to prevent digging on the side of the stamping. The radius should not be less than twice material thickness, and for best results the edge should have a high polish. A smaller radius could cause some fatigue in the material when formed. The punch is made of tool-steel, hardened according to the severe ness of the operation. It should be designed long enough to allow complete forming of the part without interference with the punch holder. The width and shape are governed by the part to be formed, and at all times it must be twice material thickness smaller than the width or span of the die blocks. The screws and dowel pins should be spaced properly and should be located so that they will not mar the stamping. Consideration must be given to stripping the formed part from the punch. This can be accomplished by means of a knockout, stripper hooks, or stripper-pin (spring) construction. It is important to consider and plan for the removing of the formed part. Figure 7-7 shows these details and also illustrates the gages necessary for locating the stamping. The shoe (1) should be thick enough to withstand the pressure required for the forming operation, and in selecting its thickness the size and shape of the hole in the bolster plate of the press must also be considered. The A and B dimensions of the shoe when placed over the hole of the bolster plate should also be long and wide enough to allow ample space for clamping it to the bolster plate. The depth of the sunk-in d section should be considered when selecting shoe thickness. The die shoes can be had in cast iron, semi steel, or steel. It is important that the proper material be selected for the die shoe, because the shoe must withstand a good share of the force applied during the forming operation. A die shoe made of steel will give good service, and costs only a little more than a cast or semi Cast shoe. The diameter of the leader pins (3) should be selected according to the working area of the shoe, or by consulting a die set manufacturers catalog. The length of the pins should be at least ¼ in. shorter than the shut height of the die, as listed on the drawing. The guide bushings (4) should be of the shoulder type, and for a die of this kind can be the regular-length type. (Guide bushings are made in three lengths – regular,

80

long, and extra long). Die sets are made in two types – precision and commercial. Precision die sets are considered for stamping work that requires great accuracy in alignment, such as between punches and die parts of cutting dies. For secondary operations, such as bending, forming, or other non-cutting operations, commercial die sets should be specified.

The punch holder (2) is the same as the shoe in the a and b dimensions. The thickness c should be 32 mm. and have a 50 mm. dia. shank. The shank is always placed on the centerline of the punch holder of a regular stock die set. If it is necessary to locate it off center, the set becomes a special, therefore costing more. The shanks of die sets come in various diameters – 38, 40, 50, 63.5 and 76 mm. – also can be had in special diameters when specified, at extra cost. The shank is used to align the centerlines of the die with the centerlines of the press. It also is clamped securely in the press ram, and must help lift the punch and stamping from the die. Punch shank diameters are selected according to the hole in the press ram. The 50 mm. dia. shank is the most popular one, because it provides a strong shank and eliminates the use of collars. The shanks 38 and 40 mm.diameter are 63 mm. long; the others are 73 mm. long. Die blocks (5) are of a two-piece construction. They should be made of tool-steel that will withstand excessive wear and galling. Toughness and shock resistance should be considered in selecting the kind of tool-steel. It should be an oil or air-hardening steel, and in a good many applications a double draw will produce some added toughness and abrasion (galling)

81

resistance in the steel. The die blocks take most of the wear, so care must be taken to design them properly. The length F of the die blocks must always be longer than the height G, at least one and one-half times as long. The width should be governed by the width of the stamping to be formed, taking into account also the space required to fasten gages, to locate the stamping. The screws and dowels should be spaced to help withstand some of the forces that will be encountered in the forming operation. The screw and dowel holes should be located so that they will not mar the stamping. The dowel pins should be large enough in diameter to help withstand some of the spreading force that will be encountered. The screws should be large enough to compensate for these forces and should be located to help control them. The screw and dowel hole locations should also be considered for locating the gage plate (6). Designing the gage plate shape when designing the die blocks, and then placing the screw sand dowel holes accordingly, accomplishes a dual purpose. It helps eliminate some of the holes required in the die blocks, as well as the work of drilling and tapping the holes and the cost of the extra screws.

The shape of the stamping to be formed should be studied, and if small radius corners are required it should be designed for a pad type rather than a solid form die (Fig. 7-8). The radius in the corners should be at least five times metal thickness in the solid form die design.

The gaging of the stamping should be studies, and whenever possible the blank should be fed the long way. The die should be located on the die set so as to permit the blank to feed the long way. When blanks are fed the short way, they have a chance to twist, and the operator loses control over them. This causes a loss of time in locating the blank in the die, and can cause misallocation of the blank.

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The gage plate should be made so that the blank can slide into the proper location for forming. A slide should be provided with sides (7) whose height is at least one and one-half times metal thickness. The pocket for locating the blank in the length should stop it on both ends and the clearance should be governed by the stamping drawing tolerances. The width of the slide pocket should be sufficient to include the mill tolerance of the strip. When the blanks are wide and flat it is necessary to provide some ribs or wires to help slide the blank into the die and reduce some of the friction caused by the oil on the blank surfaces. The slide should be long enough to prevent the operator from sliding his fingers under the punch. The safety factor most commonly used is to have it long enough for the operator to have the web of his thumb rest at the front edge of the gage, and with his hand spread forward, have his center finger clear the punch or die opening (approximately 152 mm. to 165 mm.) The punch (8) should be of tool-steel; it is usually made of the same kind as the die blocks. The width of the punch is usually governed by the width of the stamping being formed. When narrow-width stock, 13 mm. or less, is used, care must be taken to add flanges to the sides to make it wide and strong enough at the base. The screw and dowel holes should be placed to eliminate the possibility of marring the stamping. The screws and dowels should be large enough to take care of the forces encountered by the punch in the forming operation. The screws and dowels should not be located to cut into the punch shank. The only hole we should consider putting into the punch shank at any time is for a knockout rod, and this is most always in the center of the shank diameter. When possible, a knockout arrangement using a knockout Rod (9) should be employed for releasing the finished stamping from the punch. This usually is simple to design, and making it is most economical. Also, if the press bar for pushing the knockout rod is properly adjusted, the removing of the stamping is foolproof and should increase production. All the comments made in this section (on the solid form die) relating to die sets and their selection, screws and dowels, die blocks, punches, etc. apply to the design of other forming dies. They are necessary details to be considered for most die construction, and will not be referred to again. 7.2.2

Forming Dies with Pressure Pads

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When the forming of stampings requires accuracy, dies employing pressure pads are often designed. The pressure pad helps to hold the stock securely during the forming and eliminates shifting of the blank. The pressure can be applied to the pad by springs or by the use of an air cushion (Fig. 7-9 A and B). When springs are used, they can be located directly under the pad and confined in the die shoe (Fig.7-9 A). They may also be located in or under the press bolster plate; and by the use of pressure pins, which are located under the pad, and through the die shoe, pressure is applied to the pressure plate.

Pressure pins are also used with an air cushion. The construction of the pressure plate and pressure pins would be the same as shown in Fig. 7-9 B except that an air cushion is substituted for the springs. When springs are used to apply pressure to a pressure pad, spring pressure increases (in pounds) with the pad travel. Each fraction of an inch of travel increases the pressure on the pad. This could cause some trouble in stampings of light-gage material, because too much pressure may cause the metal to stretch. When springs are used, a certain amount of pressure is lost owing to the springs‟ setting (losing height after being worked). When an air cushion is used, the proper amount of pressure on the pressure pad is assured as long as air supply is set properly. It is important to have a set amount of pressure on the pressure pad to control the quality of the stampings.

84

The pressure pad, the moving member of the die, must always be controlled in its travel between the die blocks. This can be done by means of retaining shoulders or by shoulder screws. When using the retainer shoulder construction, a recess is machined into the form blocks, and a corresponding shoulder is machined on the pad. The retainer shoulder should always be made strong enough to withstand the pressure applied by either springs or air cushion. The size of the shoulder to be used varies according to the size and metal thickness of the stamping. A good rule to employ is to have the height of the shoulder one and one-half times the width (Fig. 7-10).

Always design the shoulders of the pad with a radius in the corner. When the pad is made of hardened tool-steel, heat treatment should specify a double draw of the shoulder section. When using shoulder screws to control the travel of the pad, the die shoe must be thick enough to permit sufficient travel. The pressure pad should always travel so that it extends slightly above the die blocks. This will insure uniform parts, because there will be pressure to lock the part between the punch and pad faces, before the actual forming takes place. The amount of travel the pad should have depends upon the height of the form die. It is not always necessary to travel the full height; in many cases half the die‟s form height is sufficient. When a blank is distorted, or has a tendency to curl, which may cause the completed blank to be out of square, it may be necessary for the pad to travel the full length. It is necessary for the pad to bottom on the die shoe, to allow the punch to give the part a definite set at the bottom of the stroke. When a stamping must have sides that are square with the bottom, after forming, the corner radius should be set. This is done by designing the die blocks with the correct radius A, Fig. 7-11. The pressure pad is made to match the height

85

of the die blocks‟ radius edge. The punch radius C is made slightly smaller, approximately 10 per cent less than the die block radius.

It may be necessary to machine a slight angle on the side of the punch to allow a slight over bending of the side being formed. This ensures that the sides of the formed part will be square with the base after forming.

Single and multiple pressure pads are used in die construction. The single pressure pad is used when the forming is done in one direction. It is most commonly used for forming short flanges, tabs, lugs, or ears at right angles to the base of the part. The pressure pad is used to support the base of the part accurately, either by pilot pins or other gages, and by it securing the part properly, the part is formed with great accuracy. The side or tab to be formed may be bent downward as well as upward. When the side or tab is being bent downward, the length may vary slightly, because the metal is stretched or drawn more. Some of this may be overcome by having an angle and radius on the punch as shown in Fig. 7-12. The greater the angle and radius, the less bending pressure is required. When a side is bent down, a heel block is required to help support the punch before it starts to do any forming. It should be at least two metal thicknesses higher than the die block. The pressure pad must travel at least 3 mm. beyond the edge of the form punch. This is done to assure holding pressure before any forming work is done. The punch should travel far enough beyond the corner radius to smooth out the formed side. Multiple pressure pads are used when a series of forms are necessary; they are used mostly in progressive dies, when several bends are required on small precision parts. A combination of stationary form blocks, supplied by pressure pads, helps lift the strip so it can be advanced from one station to the next. 7.3 EMBOSSING OPERATIONS

86

In an embossing operation of shallow surface detail is formed by displacement of the metal between two opposing mated tool surfaces. In one surface we have the depressed detail, on the other the relief detail. The metal is stretched into the detail rather than being compressed. Embossing is used for various purposes, the most common being the stiffening of the bottom of a pan or container; the embossing is designed to follow the outside profile of the part. A round can may have an embossed circle or raised grooves of various widths or panels. When the can or box is square or rectangular, such embossments follow their contours. Embossing often are ribs or crosses stamped in the metal to help make a section of a blank stronger by stiffening. An embossing die can be a male and female set of lettering dies, or profile of one of various shapes. The method of constructing the die blocks for an embossing operation depends on the size and shape of the form, also the accuracy and flatness required. When embossing simple shapes such as stiffening ribs, it is not necessary to fit up the die to strike the bottom. The metal stretches over the punch and across the two radius edges of the die hole (Fig. 7-13).

The die opening has the same width of the rib or embossing a, and a slight radius b is added to the edges of the opening to allow the metal to flow freely. The punch is made slightly smaller than the required metal thickness per side, so that it does not strike along this area. By constructing it in this manner the pressure required to stamp the embossing is reduced.When embossing are of the lettering type, such as depressed or raised letters for name plates, care must be taken to see that both dies are properly located and doweled in place (Fig7-14).

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The male die is located in a pocket or recess, and keyed in place. By lining up the female die profile to correspond with the male die profile, and keying it in place, a good stamping or embossing can be made. Stamping operations of this kind require precision work by the toolmaker; the dies are easily damaged by misalignment. As small embossing is often used as a weld projection nib. These nibs are used to weld piece-parts together. There are two kinds: a button type, which we use for light-gage metal, 3/32 in, or less thick, and a cone type for heavier-gage metal (Fig. 7-15). Care must be taken in their design, because if the projection is too weak the nib will collapse before a good weld can be made. A nib that is too thick or heavy through the section will require too much pressure to produce good welds. The piece-parts are heated electrically, and this causes the projections to melt, so if the projections are too heavy or too light, the heat and pressure required can cause trouble. Table 7-15 lists the correct design for both kinds of projections.

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Fig. 7-15 Die for embossing weld-projection nibs.

Table 7-1 DESIGN GUIDES FOR PROJECTIONS (24 TO 5 GAUGE STEEL) Type ButtonType projecti on

ConeType

U.S.S. Ga. 24 23 22 21 20 19 18 17 16 15 14 13

12 11

A

B

D C

0.63 5 0.71 374 0.79 248 0.87 376 0.95 25 1.10 998 1.27 1.42 748 1.58 75 1.78 562 1.98 374 2.37 998

1.27 1.27 1.27 1.27 1.27 1.27 1.27 1.39 7 1.52 4 1.90 5 1.90 5 1.90 5

2.77 622

2.03 2

0.6 2.7 35 686 0.6 2.7 35 686 0.7 3.1 62 75 0.7 3.1 62 75 0.8 3.1 89 75 1.0 3.1 16 75 1.0 3.9 16 624 1.0 3.9 16 624 1.1 4.3 43 688 1.1 4.3 43 688 1.2 4.5 7 72 1.2 4.5 7 72 (For projections in 1010 SAE steel)

89

1.3 97

F

G

2.2 86

3.3 274

E

4.3 688

0.6 35 0.6 35 0.7 62 0.7 62 0.8 89 0.8 89 1.0 16 1.1 43 1.2 7 1.3 97 1.3 97 1.6 51

2.0 32

projec tion

10 9 8 7 6 5

3.17 2.03 1.3 5 2 97 3.57 2.03 1.5 124 2 24 3.96 2.03 1.5 748 2 24 4.36 2.03 1.5 372 2 24 4.76 2.03 1.7 25 2 78 5.15 2.03 1.7 874 2 78 5.53 2.03 1.9 72 2 05 All dimensions are given in mm.

4.3 688 4.3 688 4.3 688 4.8 26 5.1 562 5.1 562 5.3 34

2.0 32 2.0 32 2.0 32 2.0 32 2.3 876 2.3 876 0.2 54

0.2 54 2.7 94 3.0 988 3.5 052 4.2 164 4.6 228 5.0 8

One or more projections can be used on a part for welding purposes. The design of the part and its use govern the number and size of the projections. When embossing ribs or shapes in blanks, it is best to have the die blocks large enough to cover the whole blank. A die block that is too small holds the metal between the die and pressure pad. The portion of the blank not held will distort and cause the metal to twist, wrinkle, and pucker. When designing a die with embossing, regardless of whether they are formed up or down, a way must be provided to lift the form out of the die pocket. This is usually done by pressure pads or ejector pins. BEADING AND CURLING DIES In beading and curling operations, the edges of the metal are formed into a roll or curl. This is done to strengthen the part or to produce a better-looking product with a protective edge. Curls are used in the manufacturing of hinges, pots, pans, and other items. The size of the curl should be governed by the thickness of the metal; it should not have a radius less than twice metal thickness. To make good curls and beads, the material must be ductile; otherwise it will not roll and will cause flaws in the metal. If the metal is too hard the curls will become flat instead of round. If possible, the burr edge of the blank should be the inside edge of the curl.

90

3.5 052 3.6 83 3.9 37 4.2 164 4.6 99 5.2 324 5.5 88

This location facilitates metal flow and also helps keep the die radius from wearing or galling. In making curls and beads a starting radius is always helpful and should be provided if possible (Fig. 7-16).

The curling radius of the die must always be smoothly polished and free of tool marks. Any groove or roughness will tend to back up the metal while it is rolling and cause defective curls. The inside surface of the blank must be held positively in line with the inside curling radius of the punch (Fig. 7-17). When curling or beading pots, pans, cans, or pails, wires are often rolled inside the curls to make them stronger. The wire is made to the contour of the pan and placed on a spring pad. When the curling die descends, the edge of the pan is forced to curl around the wire as shown in Fig. 7-18. BULGING OPERATIONS In a bulging operation a die forms or stretches the metal into the desired contour. By using rubber, heavy grease, water, or oil the metal is forced, under pressure, to take the shape of the die. To facilitate the removal of the part from the die after bulging, the die is of a split construction. Dies that are split in

91

halves or sections must have strong hinges and latches. The sections when operated must work freely and rapidly and, when under pressure, be tight enough so that the sections will not mark the stamping. The pressures required for these operations are usually found by experimenting during the first setup of the die. This is necessary because other variables besides metal thickness and the annealed condition of the stamping must be considered. Most stampings that are to be bulged have been work-hardened by previous drawing operations and therefore need annealing. Most metal that is not soft enough will rupture in the bulging operation.Rubber is the cleanest and easiest material to use for bulging operations (Fig. 7-19). Once it is designed and tried out, the shape and hardness can be duplicated for replacement. Neoprene rubber of medium hardness is the most suitable.

The rubber should be confined between two punch sections. The upper section slides on the arbor on which the rubber pad is assembled. The lower section is fastened to the arbor. It acts as a stop to locate the rubber in the correct place for the bulging operation. As the punch descends, the rubber forces the metal into the exact shape of the die. Once the press is set to the required height, all the stampings should be of equal quality. The use of grease or tallow results in a handling problem. First of all, the same amount must be used for each stamping. Secondly, these substances are messy to handle both before and after the bulging operation. They also require extra work in the cleaning of the finished stamping because all traces of grease or oil must be removed. The die and punch must be designed to confine the grease securely in the desired area. A loss of grease owing to escaping or squirting out of the die are results in inferior stampings.

92

Water or oil requires a pump to produce the required pressure. This also causes other undesirable conditions such as operators‟ clothing getting wet, rusting of the tools and press, and also the necessity for pump maintenance. The die for bulging using water pressure must be designed with care. Its open end or the ends of the stamping should be sealed securely. 7.6

COINING DIES

When designing dies for a coining operation, extra caution must be taken to build the dies strong enough to withstand severe pressures. The pressures of the metal being coined, regardless of the kind, are severe, and heavy die sections are necessary. Polish and the finish of the die surfaces must be the best. The slightest mark could cause the die to split.

The matching sections of the dies must be

made to very close tolerances, especially if the die has moving parts. The slightest opening will allow the metal to squirt, or be forced into the opening, and could cause the die to burst. The die shoe and punch holder are designed thicker, or heavier, than for most dies because of the heavy tonnage required.

Tool-steel

backup plates hardened to a good tough hardness of RC 50-52 are used to back up the dies on the shoe and punch holder to prevent the dies from sagging, or bending, under pressure (Fig. 7-20). The plates should be wider and longer than the die blocks, so that they will support the full face of the die blocks. The bolster plate of the press should be solid, or have as small a hole as possible, to help support the die. The screws and dowels must be located away from the coining detail, so that they will not mar the part, nor weaken the dies. The screws and dowels should be large enough to hold the dies firmly and securely. Dies for coining receive rough wear, and they wash out owing to the pressure of the metal as it flows into the detail. This makes for short die life, adding to the cost of the operation.

93

SWAGING DIES A swaging operation is similar to a coining operation, except that the part contours are not as precise. The metal is force to flow into depressions in the tool faces, but the remaining metal is unconfined, and it flows generally at an angle to the direction of the applied force. The flow of the metal is restricted somewhat by the tool faces, but an overflow flash is usually encountered, and must be removed in a subsequent operation. The upsetting of heads of bolts, rivets, pins and many cold and hot-forging operations are classified as swaging operations. The sizing of faces or areas on castings, which is often referred to as planishing, is also classified as swaging. The planishing of faces, especially around hubs or bosses of casting or forging, is considered desirable, because it increases the wear resistance of the area as much as 80 percent, as compared to a similar machined surface. The faces of connecting rods and piston rings are cold sized, in order to increase the hardness of the wearing surfaces and to make them smooth. A surface of this kind cannot be accomplished by milling or machining; moreover; the squeezing operation can be done with a die about ten times as fast as by milling. Copper electrical terminals are also made by swaging dies. The presses used for swaging operations must be selected according to the size of the work and the interval of time necessary to complete the operation. Knuckle joint presses and hydraulic presses, of extra-heavy tonnage capacities, are usually used.

Hydraulic presses have a decided advantage over knuckle-joint presses

because of the extra dwell at the bottom of the stroke, which puts a definite “set” in the work. Knuckle-joint presses have short powerful strokes and can compress the metal but, lacking the extra-dwell feature at the bottom of the stroke, slight variations of the swaged parts result. Pressures up to 15.5 tons per square cm. are applied to metals in swaging operations. Presses ranging from 25 to 2500 tons are used. The most practical way of determining the size of the press for swaging a part is to squeeze the first parts with the dies placed in a hydraulic press provided with tonnage gages, which will eliminate any guesswork.

If a mechanical or knuckle

joint press is used, one should be selected with a safety factor of three or more times the maximum work pressure. Swaged and cold-sized parts are highly compressed; the metal becomes harder (work-hardened) and denser. The more metal to be moved, the greater its resistance. All these factors should be considered when selecting a press for swaging to avoid the possibility of broken

94

machinery. When computing the pressures required for swaging or cold sizing, the correct ultimate compression strength of the metal must be used. The formula used to compute the pressure is AxS P = (7-3) 2000 where P = pressure required, tons A = area to be sized, sq. cm. S = ultimate compressive strength of the metal, kgs. per sq. cm. Coining dies are used to emboss on the part the detail engraved on the faces of the punch and die. By compressing the metal between the punch and die, metal flows into the detail or embossing. Coining dies produce coins, medals, medallions, jewelry parts, ornamental hardware, plates, and escutcheons. Hydraulic or knuckle joint presses are usually used to perform coining operations. To force the metal to flow into the detail requires very high tonnage. One of the reasons for the high tonnage is that when metal is compressed it hardens and toughens. This is called work hardening of the metal, and the more complex a part, the greater its resistance and the tonnage required. During the coining operation the part must be properly confined. The thickness of the part is controlled by the die block surfaces. The closed height of the die is checked by the surfaces of the blocks. The die block surfaces, on which the detail is inscribed, must be highly polished and free of scratches or mars. That is necessary because the slightest scratch or mark will be embossed into the part. The sides of the die that control the outside contour of the part must be slightly tapered to allow removal of the part from the dies.

95

Tool-steel used for swaging and sizing operations must be of high strength. Chrome-tungsten

oil-hardening

steel,

which

combines

high

hardness

with

maximum toughness, is used for swaging dies. High-carbon high-chrome steels are also used, but they are more difficult to machine and their resistance to shock is not as high as that of chrome-tungsten oil-hardening steels. The die body sizes must be extra heavy. The shut height of the die is checked by the surfaces of the blocks. The area of the stop and the top and bottom surfaces of the block should be large enough to allow the block to withstand three times the yield strength of the work piece metal. The contour or profile of the part to be sized is usually machined into the surface of the die blocks. These surfaces are in a plane coinciding with the longitudinal center plane of the works piece. To avoid using excessive pressures, the dies are relieved where no pressing is done. A draft is provided along all edges of the work that are not squeezed. There is also a 45deg draft around the bosses to be sized to facilitate removal of the finished work. In some cases the parts are machined about 0.8 mm. oversize before being placed in the sizing dies, which eliminates the chances of overtaxing the dies and also improves the dimensional quality of the parts. Die shoes and punch holders are similar to the ones used for coining dies. They must be heavy, with wide and long base surfaces. Figure 7-21 illustrates general principles for designing a swaging or sizing die.

96

The die blocks have identical thickness, A and B, and lengths, C. The center of the part is located in exactly the same position, D, in each block. The screw and dowel holes must not be located near or in the area of the part to avoid marking it. Hole Flanging or Extruding Dies The forming or stretching of a flange around a hole in sheet metal is termed hole flanging or extruding. The shape of the flange can vary according to the part requirements. Flanges are made as countersunk, burred, or dimpled holes. When countersunk shaped extruded holes are made in steel, it is necessary to coin the metal around the upper face and beveled sides to set the material. The holes are also made about 0.125 mm. deeper than the required height of the rivet or screw head, which allows bunching that, occurs when squeezing the rivet in place. A section of a die for this purpose is shown in Fig. 4-26. The hole can be pierced before it is placed in the countersinking die, or it can be formed and pierced in a single stroke of the press. As shown in Fig. 4-26, the sheet is placed over the pilot dia. A locates it centrally in the die.

The die body, 1, descends and forces the metal down around the

flange surface of the punch. Spring pressure strips the part from the punch and releases the formed part from the die. Figure 7-22 shows a two-step punch, 1, which first punches the hole in the part and then forces the metal around to the countersunk shape of the die block, 2. The hole punched by this method is always somewhat smaller than the size of the hole in the finished part. Spring pressure is used to strip the finished part from the punch. A shedder pin should be provided in the piercing point of the punch to remove the slug.

97

The size of the pierced hole for a 90 hole flange can be calculated, but should never be used until it has been proved correct by using the same tools that will be employed in the die. To calculate the hole size the same principles are employed when finding a 90 bend. Dimensional details of Fig. 7-23 are identified as follows:

T = thickness of metal to be flanged A = dia. of calculated hole B = dia. of hole inside of flange (punch body size) G = dia. of hole in die (outside of flange) R

=

radius on edge of die (usually one-third metal

thickness) H = height of flanged hub T

98

T

R

can be specified as from

to 3

4

When the flanges are stretched more than 2 ½ times metal thickness in height, the wall can split.

This can be prevented to some extent by burring the edge

around the hole before the extruding operation. 90 Hole Flanging: Forming a flange around a previously pierced hole at a bend angle of 90 (the most common operation) is nothing more than the formation of a stretch flange at that angle. One manufacturer has standardized flange widths (Fig. 7-24, dimension H) for holes to be tapped in low-carbon-steel stamping stock, as follows: 5T B = A +

when T is less than 1.14 mm.

(7-4) 4 B = A + T

when T is more than 1.14 mm. H = T

(7-5)

when T is less than 0.9 mm. (7-6)

H =

4T when T is 0.9 to 1.3 mm. (7-7)

5

3T H =

when T is more than 1.3 mm. (7-8)

(7-9)

99

The radius on he nose of the punch should be blended into the body diameter, eliminating any sharpness, which could cause the metal to score as it passes over it. The radius on the body B or hole-sizing portion of the punch must be as large as possible, and smooth. The portion between the A and B diameters of the punch should have a radius C that should be as large as possible (See Fig. 7-25). DRAWING DIES

Drawing is a process of changing a flat, precut metal blank into a hollow vessel without excessive wrinkling, thinning, or fracturing. The various forms produced may be cylindrical or box-shaped with straight or tapered sides or a combination of straight, tapered, or curved sides. The size of the parts may vary from 6 mm. diameter or smaller, to aircraft or automotive parts large enough to require the use of mechanical handling equipment. Metal Flow When a metal blank is drawn into a die, a change in its shape is brought about by forcing the metal to flow on a plane parallel to the die face, with the result that its thickness and surface area remain about the same as the blank. Figure 7-25 shows schematically the flow of metal in circular shells. The units within one pair of radial boundaries have been numbered and each unit moved progressively toward the center in three steps. If the shell were drawn in this manner, and a certain unit are examined after each depth shown, it would show (1) a size change only as the metal moves toward the die radius; (2) a shape change only as the metal moves over the die radius. Observe that no change takes place in area 1, and the maximum change is noted in area 5. The relative amount of movement in one unit or in groups of units is shown in Fig. 7-26 A and B, in which two methods of marking the blanks are used to illustrate size, shape, and position of the units of area, before and after drawing. The blank in view A is marked with radial lines and concentric circles, and are used to illustrate size, shape, and position of the units of area, before and after drawing. The blank in view A is marked with radial lines and concentric circles, and in view B with squares. If, after these blanks are marked and drawn, sections are cut out of the shell, flattened, and compared with the original triangular portions, a change in shape of the triangular pieces will be found. The illustration shows that the inner portion of the triangle, which becomes the base of the shell, remains

100

unchanged throughout the operation. The portion which becomes the side wall of the shell is changed from an angular figure to a longer parallel-sided one as it is drawn over the die radius, from which point no further change takes place. The particular areas observed have been enlarged and superimposed upon each other, respectively, to show more clearly their size, shape, and position before and after drawing.

Fig. 7-26 Two methods of marking blanks to illustrate size, shape and position of the units of area, before and after drawing. The general change in circular draws, due to flow, may be summarized as follows: 1) Little or no change in the bottom area because no cold work was done in this area. 2) All radial boundaries of the units of area remain radial in the bottom area. The units in the top flange area remain radial until they move over the die radius; then they become parallel and assume dimensions equal to their dimensions at the point where they move over the die radius. 3)

There is a slight decrease in surface area and increase of thickness in the units involving maximum flow. The increase in thickness is limited to the space between the punch and die.

4)

The flow lines on a circular shell indicate that the metal movement is uniform.

101

7.9.2 Flow in Rectangular Shells:

Fig. 7-27 Metal flow in rectangle draws: (A) Blank marked before drawing (B) Corner areas after drawing. The variation in flow in different parts of the rectangular shell divides the blank into two areas. The corners are the drawing area, which includes all the metal in the corners of the blank necessary to make a full corner on the drawn shell. The sides and ends are the forming area, which includes all the metal necessary to make the sides and ends full depth. To illustrate the flow of metal in a rectangular draw, the developed blank in Fig. 7-27B has been divided into unit areas by two different methods. In Fig. 7-27A the corners of the shell drawn from the blank in view B are shown. The upper view is the corner area, which was marked with squares and the lower view is the corner area, which was marked with radial lines and concentric circles. The severe flow in the corner areas is clearly shown in the lower view by the radial lines of the blank being moved parallel and close together, and the lines of the concentric circles becoming farther apart the nearer they are to the center of the corner and the edge of the blank. The relatively parallel lines of the sides and ends show that little or no flow occurred in these areas. The upward bending of these lines indicates the flow from the corner area to the sides and ends to equalize the height where these areas on the blank were blended to eliminate sharp corners.

102

Single-action Dies The drawing of a rectangular shell involves varying degrees of flow severity. Some parts of the shell may require severe cold working and others, simple bending. In contrast to circular shells, in which pressure is uniform on all diameters, some areas of rectangular and irregular shells may require more pressure than others. True drawing occurs at the corners only; at the sides and ends metal movement is more closely allied to bending. The stresses at the corner of the shell are compressive on the metal moving toward the die radius and are tensile on the metal that has already moved over the radius. The metal between the corners is in tension only on both the side wall and flange areas.

The simplest type of draw die is one with only a punch and die.

Each

component may be designed in one piece without a shoe by incorporating features for attaching them to the ram and bolster plate of the press. Figure 7-28A shows a simple type of draw die in which the precut blank is placed in the recess on top of the die, and the punch descends, pushing the cup through the die. As the punch ascends, the cup is stripped from the punch by the counterbore in the bottom of the die. The top edge of the shell expands slightly to make this possible. The punch has an air vent to eliminate suction, which would hold the cup on the punch and damage the cup when it is

103

stripped from the punch. The method by which the blank is held in position is important, because successful drawing is somewhat dependent upon the proper control of blank holder pressure. A simple form of drawing die with a rigid flat blank holder for use with 13-gage and heavier stock is shown in Fig. 7-28B. When the punch comes in contact with the stock, it will be drawn into the die without allowing wrinkles to form.

Another type of drawing die for use in single-action press is shown in Fig. 729. This die is a plain single-action type where the punch pushes the metal blank into the die, using a spring-loaded pressure pad to control the metal flow. The cup either drops through the die or is stripped off the punch by the pressure pad. The sketch shows the pressure pad extending over the nest, which acts as a spacer and is ground to such a thickness that an even and proper pressure is exerted on the blank at all times. If the spring pressure pad is used without the spacer, the more the springs are depressed the greater the pressure exerted on the blank, thereby limiting the depth of draw. Because of limited pressures obtainable, this type of die should be used with light-gage stock and shallow depths. A single-action die for drawing flanged parts, having a spring-loaded pressure pad and stripper is shown in Fig. 7-30. The stripper may also be used to form slight indentations or re-entrant curves in the bottom of a cup, with or without a flange. Draw tools in which the pressure pad is attached to the punch are suitable only for shallow draws. The pressure cannot be easily adjusted, and the short springs tend to build up pressure too quickly for deep draws. This type of die is often constructed in an inverted position with the punch fastened to the lower portion of the die.

104

Fig. 7-31Cross section of an inverted draw die for a single-action press; die is attached to the ram; punch and pressure pad are on the lower shoe. Fig. 7-32 A typical double-action cylindrical draw die. Double-action Dies In dies designed for use in a double-action press, the blank holder is fastened to the outer ram which descends first and grips the blank; then the punch, which is fastened to the inner ram, descends, forming the part. These dies may be a push-through type or the parts may be ejected from the die with a knockout attached to the die cushion or by means of a delayed action kicker. Figure 7-32 shows a cross section of a typical double-action draw die. DEVELOPMENT OF BLANKS The development of the approximate blank size should be done first (1) to determine the size of a blank to produce the shell to the required depth and (2) to determine how many draws will be necessary to produce the shell. This is determined by the ratio of the blank size to the shell size. Various methods have been developed to determine the size of blanks for drawn shells. These methods are based on (1) mathematics alone; (2) the use of graphical layouts; (3) a combination of graphical layouts and mathematics. The majority of these methods are for use on symmetrical shells. It is rarely possible to compute any bank size to close accuracy or to maintain perfectly uniform height of shells in production, because the thickening and thinning of the wall vary with the completeness of annealing. The height of ironed shells varies with commercial variations in sheet thickness and the top edge varies from square to irregular, usually with four more or less pronounced high spots

105

resulting from the effect of the direction of the crystalline structure of the metal. Thorough annealing should largely remove the directional effect. For all these reasons it is ordinarily necessary to figure the blank sufficiently large to permit a trimming operation. The drawing tools should be made first; then the blank size should be determined by trial before the blanking die is made. There are times, however, when the metal required to produce the product is not immediately available from stock and must be ordered at the same time as the tools are ordered. This situation makes it necessary to estimate the blank size as closely as possible by formula or graphically in order to know what sizes to order. Blank Diameters The following equations may be used to calculate the blank size for cylindrical shells of relatively thin metal. The ratio of the shell diameter to the corner radius (d / r) can affect the blank diameter and should be taken into consideration. When d / r is 20 or more.

D =

d2 + 4dh

(7 – 12) When d / r is between 15 and 20,

106

The relationship of the punch-nose and draw-die radii to minimize stock thinning is shown in Fig. 7-33. The center point of the radius should be approximately 3 mm. outside the previous cup, as illustrated in A. The center point of the punch-nose radius should be slightly inside the following shell as in B. The center point of the punch-nose radii on the last two operations is about on the same line, thereby maintaining the flat on the bottom of the cup as in C.

D =

d2 + 4dh – 0.5r

(7 – 13)

When d / r is between 10 and 15,

107

d2 + 4dh – r

D =

(7 –

14)

When d / r is below 10,

(d – 2r) 2 + 4d (h – r) + 2 ? r (d – 0.7r)

D =

where

D = blank diameter;

(7 – 15)

d = shell diameter

h = shell height

r

=

corner radius

of punch The above equations are based on the assumption that the surface area of the blank is equal to the surface area of the finished shell. In cases where the shell wall is to be ironed thinner than the shell bottom, the volume of metal in the blank must equal the volume of the metal in the finished shell. Where the wall-thickness reduction is considerable, as in brass

108

shell cases, the final blank size is developed by trial. A tentative blank size for an ironed shell can be obtained from the equation t d2 + 4dh

D =

( 7 – 16) T where t = wall thickness;

7.10.2

T = bottom thickness

Reduction Factors

After the approximate blank size has been determined, the next step is to estimate the number of draws that will be required to produce the shell and the best reduction rate per draw. As regards diameter reduction, the area of metal held between the blank holding faces must be reasonably proportional to the area on which the punch is pressing, since there is a limit to the amount of metal which can be made to flow in one operation. The greater the difference between blank and shell diameters, the greater the area that must be made to flow, and therefore the higher the stress required to make it flow. General practice has established that, for the first draw, the area of the blank should not be more than three and one-half to four times the cross-sectional area of the punch.

d Percentage reduction is calculated from

P = 100

1

–

( 7 – 17)

D where P = Percentage reduction; Blank dia.

109

d = Shell ID;

D

=

The theoretical maximum percentage reduction for one draw is approximately 50%, although the figure is hard to obtain in production. For practical purpose, it is better to figure a maximum of 40% reduction for a single draw. 7.10.3

Determine Draw Ratio: The depth of draw expressed as the ratio

h / d. When the value exceeds 0.75, more than one reduction is necessary, referring to following table, two reduction are needed, with 40% reduction for the first draw and 25% reduction for second draw. It is now necessary to determine the cup size from the blank size for a 40% reduction and cup size from the previous draw for a 25% reduction. Table 7-2 Possible number of reduction for a given ratio of shell height to diameter.

Number of reduction

Reduction %

Up to 0.75

1

40

0.75 to 1.5

2

40

25

1.5 to 3.0

3

40

25

15

3.0 to 4.5

4

40

25

15

Ratio h/d

7.10.4

First draw

Second draw

Third draw

Fourth draw

10

Determine radius on Die: theoretically, the radius on the draw die

(draw ring) should be as large as possible to permit full freedom of metal flow as it passes over the radius. The draw ring causes the metal to begin flowing plastically and aids in compressing and thickening the outer portion of the blank. However, if the draw radius is too large, the blank holder will release the metal too soon and wrinkling will result. Too sharp a radius will hinder the normal flow of the metal and cause uneven thinning of the cup wall, with resultant tearing. The general rule is to make the draw radius 4 times the material thickness. The draw radius may be increased to 6 to 8 times the metal thickness when drawing shallow cups of heavy gauge metal without a blank holder. The nomograph in Fig. 10.30 gives a more exact method of determining draw-die radius, based on the relationship of the blank diameter to the cup diameter.

110

Deep draws may be made on single-action dies, where the pressure on the blank holder is more evenly controlled by a die cushion or pad attached to the bed of the press. The typical construction of such a die is shown in Fig. 7-31. This is an inverted die with the punch on the die‟s lower portion.

Progressive Dies A progressive die performs a series of fundamentalsheet-metal operations at two or more stations during each press stroke in orderto develop a workpiece as the strip stock moves through the die. This type of die is sometimes calledcut-and-carry, follow, or gang die. Each working station performs one or moredistinct die operations, but the strip must move from the first through eachsucceeding station to produce a complete part. One or more idle stations may be incorporated in the die, not to performwork on the metal but to locate the strip, to facilitate interstation striptravel, to provide maximum-size die sections, or to simplify their construction. The linear travel of the strip stock at each press stroke iscalled the

progression,

advance,

or

interstationdistance.

111

pitch

and

is

equal

to

the

The unwanted parts of the strip are cut out as it advances throughthe die, and one or more ribbons or tabs are left connected to each partiallycompleted part to carry it though the stations of the die. Sometimes parts aremade from individual blanks, neither a part of, nor connected to a strip; insuch cases, mechanical fingers or other devices are employed for thestation-to-station movement of the workpiece. The operations performed in a progressive die could be done inindividual dies as separate operations but would require individual feeding andpositioning. In a progressive die, the part remains connected to the stockstrip, which is fed through the die with automatic feeds and positioned bypilots with speed and accuracy. SELECTION OF PROGRESSIVEDIES The selection of any multi operation tool, such as a progressivedie, is justified by the principle that the number of operations achieved withone handling of the stock and the produced part is more economical thanproduction by a series of single-operation dies and a number of handlings foreach single die. Where totalproduction requirements are high, particularly if production rates are large,total handling costs (man-hours) saved by progressive fabrication compared witha series of single operations are frequently greater than the costs of theprogressive die. The fabrication ofparts with a progressive die under the abovementioned production conditions isfurther indicated when, 1.Stock material is not sothin that it cannot be piloted or so thick that there are stock-straightening problems. 2.Overall size of die(functions of part size and strip length) is not too large for availablepresses. 3.Total press capacityrequired is available.

112

STRIP DEVELOPMENT FOR PROGRESSIVE DIES Individualoperations performed in a progressive die are often relatively simple, but whenthey are combined in several stations, the most practical and economical stripdesign for optimum operation of the die often becomes difficult to devise. The sequence ofoperations on a strip and the details of each operation must be carefullydeveloped to assist in the design of a die to produce good parts. A tentative sequence of operations should beestablished and the following items considered as the final sequence ofoperations is developed: 1.Pierce piloting holesand piloting notches in the first station. Other holes may be pierced that willnot be affected by subsequent non-cutting operations. 2.Develop

blank

fordrawing

or

forming

operations

for

free

movement of metal. 3.Distribute pierced areasover several stations if they are close together or are close to the edge of dieopening. 4.Analyze the shape ofblanked areas in the strip for division into simple Shapes so that punches ofsimple contours may partially cut an area at one station and cut out remainingareas in later stations.

This

may

suggest

the

use

of

commercially

availablepunch shapes. 5.Use idle stations tostrengthen die blocks, stripper plates, and punch retainers and to facilitatestrip movement. 6.Determine whether stripgrain direction will adversely affect or facilitate an operation. 7.Plan the forming ordrawing operations either in an upward or a downward direction, whichever willassure the best die design and strip movement.

113

8.The shape of thefinished part may dictate that the cutoff operation should precede the lastnon-cutting operation. 9.Design adequate carrierstrips or tabs. 10.Check strip layout forminimum scrap; use a multiple layout if feasible. 11.Locate cutting andforming areas to provide uniform loading of the pres slides. 12.Design the strip so thatscrap and part can be ejected without interference.

Figure 8-1 illustrates the use of a three-station die toavoid weak die blocks. At A the pierced hole is near the edge of the part whereit is cut off, thereby weakening the die block at this point.

114

If an idle stationis added so that the piercing operation is moved ahead one station, the dieblock is stronger and there is less chance of cracking in operation orfabrication. At B, the pierced holes are centered on the strip but closetogether. In this case the holes should be pierced in two stations to avoid thinsections in the die block between the holes. The adding of stations alsoprovides better support for the piercing punches.

Figure 8-2 shows the use of one die station insteadof two stations to maintain a close-toleranced dimension. If two stations were used, the variation inthe location of the stock guides and cutting punches could make it difficult tohold the ± 0.02 mm. tolerance. The strip development for shallow and deep drawing inprogressive dies must allow for movement of the metal without affecting thepositioning of the part in each successive station. Figure 8-3 shows various types of cutouts andtypical distortions to the carrier strips as the cup-shaped parts are formed andthen blanked out of the strip. Piercingand lancing of the strip around the periphery of the part as shown at A, leavingone or two tabs connected to the carrier strip, is a commonly used method. The semicircular lancing as shown at B isused for shallow draws. The use of this type of relief for deeper draws placesan extra strain on the metal in the tab and causes it to tear. The carrier strip is distorted to providestock for the draw. A popular cutout for fairly deep draws is shown at C. Thisdoublelanced relief suspends the blank on narrow ribbons, and no distortiontakes place in the carrier strips. Two sets of single rounded lanced reliefs

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ofslightly different diameters are placed diametrically opposite each other toproduce the ribbon suspension. The hourglass cutout in D is an economical methodof making the blank for shallow draws. The connection to the carrier strips is wide, and a deep draw would causeconsiderable distortion. An hourglass cutout for deep draws is shown in E, whichprovides a narrow tab connecting the carrier strip to the blank. The cupping operations narrow the width ofthe strip as the metal is drawn into the cup shape. The hourglass cutout may be made in two stations bypiercing two separated triangular-shaped cutouts in one station, and lancing ornotching the material between them in a second station. The cutouts shown at F and G provide anexpansion-type carrier ribbon that tends to straighten out when the draw isperformed.

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Fig. 8-3 Cutout reliefs forprogressive draws: (A) Lanced outline;(B) Circular lance; (C) Double lance suspension; (D) HourglasscutoutConventionaldrafting techniques are followed in tool design with the exception of a fewpractices that vary somewhat. The following section explains the differences andhow they are used. No attempt is made to teach the basics of drafting. It isassumed that the student has a sound working knowledge of orthographicprojection and is familiar with conventional drafting techniques.

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Often tooldrawings are used only once, when the tool is constructed. They are brought backinto use only when changes become necessary, such as those caused by productredesign or changes made to improve tooling performance. They are used only byhighly skilled toolmakers, tool room personnel, and tooling buyers. For thisreason, many shortcuts can be used in tool drawings that would cause problems onproduct drawings. Product drawings have a greater circulation and are used morefrequently and usually over a greater period of time; therefore, the shortcutsused in tool drawings are not permitted on product drawings. 2.1DRAFTING PRACTICE The following listof drafting rules generally applies to tool drawings and is intended as a guideto help maintain uniformity. All lines must bedark enough to produce a clear and sharp print All drawingsshould be on standard size that will allow the resulting prints to fold tostandard A4 size. All drawingsshould have a border line drawn 5 or 10 mm from each side of the paper,depending upon the size of the drawing. The material andtitle block should be located in the lower right-hand corner of the drawing. All dimensionsshould be expressed in mm, with the mm sign omitted. Full-scaledrawings should be used whenever possible. Otherwise, use half or as per IS: 696standard. Drawing anddimensioning must help the person who will use the drawing to make the item inthe tool room. The toolmaker should not have to make calculations before he canbegin producing the tool. Only as many viewsas necessary to show all required detail should be given. Use uppercaseengineering lettering (3 mm high) throughout the drawing. A name is alwaysassigned to each tool and placed in the title block. The name usually is thetool name plus the name of the part as noted on the part drawing. For example,if the name in the title block of a part drawing is „Horizontal actuating rod‟the correct title of the drill jig is „Drill jig-horizontal actuating rod‟. Only criticaldimensions, overall dimensions, and location dimensions should be shown on tooldrawings. Dimensions of individual pieces can be indicated in the bill ofmaterials and need not appear on the drawing. Standard purchasedtool components need not be dimensioned. These include die

sets,

screws,

dowels,springs,

knobs,

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and

tooling

specialty

items.

Dimensions are not necessarybecause the components come ready-made and are identified in the material listby number. Standard purchasedtool components that are to be altered by the toolmaker should have the alteredportion dimensioned. Special toolingcomponents that have been standardized by a particular company do not needdimensions. Dimensions thatcan be determined by or calculated from, dimensions on the part print need notbe shown on the tool drawing. Examples would be the center of the nest, cuttingedges on a punch, die clearance etc. DRAWINGLAYOUT There aretwo different methods of preparing tool-design drawings. One is to show allinformation, including the details, on one sheet. The tool is shown assembledwith only the necessary views to give pertinent information. Detail drawings areincluded when necessary. The method is generally adopted by companies whosetool-making department is such that one toolmaker builds the entire tool or dieand does most of the work on it. Thismethod will be explained in detail in the following section. The other method ofpreparing tool-design drawings is similar to the method of preparingproduct-design drawings. The assembly is drawn on one sheet, and each componentis detailed completely on a separate sheet. In this case the tool is generally built by several people, each doingone operation on each component. Another person may complete the assembly. Thisallows the company to utilize different skill levels in the tool room. Thismethod of drawing also ensures interchangeable components, which may be a realasset when repairing tools used on continuous production.

These cutouts are made in two stationsto allow for stronger die construction. Satisfactory multiple layouts may be designed using most of the reliefsby using a longitudinal lance or slitting station to divide the wide strip intonarrower strips as the stock advances. The I-shaped relief cutout in H is amodified hourglass cutout used for relatively wide strips from which rectangularor oblong shapes are produced. Straight slots or lances crosswise of the stock are sometimes used onvery shallow draws or where the forming is in the central portion of the blank.On the deeper draws, this type of relief tends to tear out the carrier strips orcause excessive distortion in the blank and is not too satisfactory touse.

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Fig. 8-3(Contd.) Cutoutreliefs for progressive draws: (E)Cutout providing expansion-type carrier ribbon for circular draws (G)Cutout

providing

expansion-type

rectangulardraws (H) Ishaped relief for rectangular draws

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carrier

ribbon

for

STOCK POSITIONING Of prime importance in the strip developmentis the positioning of the stock in each station. The stock must be positionedaccurately in each station so that the operation can be done in the properlocation. A commonly used method of stock positioning is the incorporation ofpilots in the die. There are two methods of piloting in dies: direct andindirect. Direct piloting consists of piloting in holes punched in the part at aprevious station. Indirect piloting consists of piercing holes in thescrap-strip and locating these holes with pilots at later operations. Directpiloting is the ideal method for locating the part in subsequent die operations.Unfortunately, ideal conditions may not exist, and in such cases indirectpiloting must be used to achieve the desired results of part accuracy and highproduction speeds. Theadvantagesof locating pilots inthe scrap material area are: 1)Not readily affects byworkpiece change. 2)Size and location not aslimited. Disadvantagesof locating pilots in the scrap section of stripare: 1)Materialwidth and lead may increase. 2)Scrap-strip carriersdistort on certain types of operations and make subsequent station useimpossible.

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How to pilot is an arbitrary decision that the tooldesigner must make. It is impossible to give definite rules and formulas,because the material and the hardness of the stock influence the decision.However, in the indirect piloting method, it is possible to use pilots ofgreater diameter than if holes in the part are used for piloting such as in Fig.8-4 A. The greater the diameter of the pilot, the less chance there is ofdistortion of either the strip or the pilot. Also, small-diameter pilots introduce the possibility of broken pilots. When holes in thepart are held to close tolerances (Fig 8-4 B) it is possible for the pilots toaffect the hole size in their effort to move the strip to proper location. When holes in thepart are too close to the edges (Fig. 8-4 C) the weak outer portions of the partare likely to distort upon contact with the pilots, instead of the strip‟smoving to the correct location. These possibilities is often overlooked inplanning a progressive die, and gives to subsequent runs of scrap parts andexpensive die alterations. Just what constitutes a condition where the edge ofthe hole is too close to the edge of the part is, like many aspects of design, amatter of personal judgment. Manydesigners use the rule-if-thumb: Thedistance between the two must be at least twice the stock thickness.

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A similar problemexists when the part holes are located in a weak portion of the inside area ofthe part (Fig. 8-4 D). Here, there is a possibility of the part‟s bucklingbefore the pilots can position the stock strip. In this case it is advisable topilot in the scrap strip. To achieveaccurate part location, the pilots must be placed as far apart as possible. When the holes in the work piece are closetogether, as in Fig. 8-4 E, holes in the scrap strip should be used forpiloting. A second method would be toplace a pilot in one hole in one station and in the same hole in a succeedingstation. The feasibility of the secondmethod depends upon the availability of an additional die station. When slots are punched in the blank parallel to the stock movement (Fig.8-4 F) the slots are not suitable piloting holes. Therefore, indirect pilots must beused.

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8.4DISPOSITION OF SCRAP STRIP

A strip development is illustrated in Fig. 8-5 B utilizing pierce,trim, form, and blank-through operations and carriers on both sides of thestrip. The work piece is dropped throughthe die, while the carrier bars continue to the scrap cutters to be cut intoshort lengths. The dropping of thework-piece through the die is the most desirable method of part ejection, butcannot always be obtained. Cutting thescrap into small sections simplifies the material handling problems and producesa greater dollar return when sold as scrap metal. Figure 8-5 C shows an alternate stripdevelopment with one side carrier. Theworkpiece is pierced, trimmed, cut off, and formed on a pad with air or gravityejection, and the carrier bar is cut into short pieces by the scrap cutter. It is well to remember that if a part is tobe ejected as this one is, the double carrier bar design in Fig. 8-5 B should beavoided, because the part may become trapped in these bars and cause die damage. The design of the part in Fig. 8-5 A requires that the carrier be outsidethe part configuration. Thisnecessitates the use of stock wider than the part width plus

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the normal trimmingallowance. The part shown in Fig. 8-5Acan be made of stock the same width as the part.

The strip development of Fig. 8-6 D illustrates how the strip ispierced, trimmed, and the part cut off and formed. A slug-type cutoff punch is used and theflange is formed downward. The part isthen ejected by an air jet or by gravity. This arrangement is often referred to as a scrap less development sinceno carrier strips remain after the part is cut from the strip.

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Figure 8-6 E shows a strip development for the same part using ashear-type cutoff. The flange is formedupward as the combination cutoff and form punch descends. A spring-loaded pad supports the workpieceduring forming and assists in ejecting the part from the die. The progression of this type of developmentis shortened by the width of the cutoff slug.

Figure 8-7 shows another development in which the stock is thesame as the developed width of the workpiece. The strip is pierced in station 1; piloted and notched in station 2;piloted, pierced, and formed in station 3. Progressive Die Element The die elements used in progressive dies such as punches, stops,pilots, strippers, die buttons, punch guide bushings, die sets guide posts, andguide post bushings are of similar design to those used in other types of dies. General Die Design Aprogressive die should be heavily constructedto withstand the repeated shock

and

continuous

runs

to

which

it

issubjected.

Precision

or

antifrictionguideposts and bushings should be used to maintain accuracy. The stripper plates (if spring-loaded andmovable), when also serving as guides for the punches, should engage guide pinsbefore contacting the strip stock. Lifters should be provided in die cavities to lift up or eject the formedparts, and carrier rails or pins should be provided to support and guide thestrip when it is being moved to the next station. A positive ejector should be provided at thelast station. Where practical, punchesshould contain shedder or oil-seal-breaker pins to aid in the disposal of theslug. Adequate piloting should

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beprovided to ensure proper location of the strip as it advances through thedie.

Fine Blanking FINE-EDGE BLANKINGAND PIERCING Fine-edge Blanking(also known as fine blanking, smooth-edge blanking, or fine-flow blanking)produces precise blanks in a single operation without the fractured edgescharacteristically produced in conventional blanking and piercing. In fine-edgeblanking, a V-shape impingement ring (Fig. 9-1) is forced into the stock to lockit tightly against the die and to force the work metal to flow toward the punch,so that the part can be extruded out of the strip without fracture or die break.Die clearance is extremely small, and punch speed much slower than inconventional blanking. Fine-edge piercing canbe done either separately or at the same time as fineedge blanking. In piercingshall holes, an impingement ring may not be needed.

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No further finishing or machining operations are necessaryto obtain blank or hole edges comparable to machined edges, or to those that areconventionally blanked orblanking a simple shape.pierced and then shaved. A quick touchup on an abrasivebelt or a short treatment in a vibratory finisher may be used to remove thesmall burr on the blank. Specially designedsingle-operation or compound blanking and piecing dies are generally used forthe process. PROCESSCAPABILITIES Holes can bepierced in low-carbon steel with a diameter as small as 50% of stock thickness.In high-carbon steel, the smallest hole diameter is about 75% of stockthickness. Holes can be spaced as close to each other, or to the edge of theblank, as 50 to 75% of stock thickness. Total tolerances obtainable are: 0.0125mm. on hole diameter and for accuracy of blank outline; 0.025 mm. on holelocation with respect to a datum surface and 0.025 mm. on flatness. No die break showson the sheared surface of the hole. Blank edges may be rough for a fewthousandths of an inch of thickness on the burr side of the part when the widthof the part is about twice the stock the stock thickness or less. Finish on thesheared edge is governed by the condition of the die edge and the land withinthe die. Parts fine-edge blanked from stainless steel will have a surface finishof 1.3 micro-mm. or better. Smooth edges also are produced onspheroidize-annealed steel parts. Burr formation increases rapidly during a run,necessitating frequent grinding of the cutting elements. Chamfers can becoined around holes and on edges. Forming near the cut edge, or forming offsetparts with a bend angle up to 30°, is possible under restricted conditions. Metals up to 3.2mm. thick having a tensile strength of 6,000 to 8,000 Kg. / sq. cm. are easilyblanked. Parts up to 13 mm. thick can be blanked if press capacity is available.Material thicker than 3.2 mm., especially steel having a carbon content of 0.25%or more, requires an impingement ring on the die so that the corners on the partwill not break down. The edges of parts made of 1018 steel work harden as muchas 7 to 12 points Rockwell C during blanking. In tests on 0.60%carbon spring steel with a hardness of Rockwell C 37 to 40, the surface finishon the sheared edges was 32 micro-in. or better, but punch

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life was only 6000pieces. The cutting speed for fine-edge blanking is 7.6 mm. to 15.2 mm. per sec. WORKMETALS Low-carbon and medium-carbon steels (1008 to1035), annealed or half-hard, give good blanked edges and normal tool wear.High-carbon steels in the spheroidize-annealed condition can be blanked easily;blanking of steel with 0.35% carbon or higher is recommended only when it isspheroidize-annealed. Steels quenched and tempered to about Rockwell C 30 arewell suited to fineedge blanking, because they do not require subsequent heattreatment, which could result in deformation. High-carbon steelsand alloy steels such as 4130, 4140, 8620 and 8630 cause considerably highertool wears than low-carbon plain carbon steels, but surface finish is smoother.Leaded steels are not suitable for fine-edge blanking because of their lowdeformability. Parts made ofstainless steels of types 301, 302, 303, 304, 316, 416 and 430 in the form ofbright rolled fully annealed strip, have good blanked edges, but cause highertool wear than steels of low and medium carbon content. Good results havebeen experienced with aluminum alloys 1100 (all tempers), 5052-O to 5052-H38,6061-O to 6061-T6 and others having similar yield strength and elongation.Blanked edges on parts made of aluminum alloy 2024 generally

are

rougher

thanedges

on

other

aluminum

alloys.

Brasses

containing more than 64% copper areespecially suitable. Nickel alloys, nickel silver, beryllium copper and god andsilver also are easily fine-edge blanked. BLANK DESIGN Limitations onblank size depend on stock thickness, tensile strength and hardness of the workmetal, and available press capacity. For example, perimeters of approximately63.5 cm. can be blanked in 3.2 mm. thick lowcarbon steel (1008 or 1010). It ispossible to blank smaller parts from lowcarbon or medium-carbon steel about12.7 mm. thick. Sharp corner andfillet radii should be avoided when possible. A radius of 10 to 20% of stockthickness is preferred, particularly on parts over 3.2 mm. thick or

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those madeof alloy steel. External angles should be at least 90°. The radius should beincreased on sharper corners or on hard materials. Parts with tinyholes or narrow slots to be pierced or with narrow teeth or projections to beblanked may be unsuited to fine-edge blanking. The ratio of hole diameter, slotwidth, or projection width to metal thickness should be at least 0.7 forreasonably efficient blanking, although a ratio as small as 0.5 has beensuccessful with some parts. The spacing, between holes or between a hole and theedge of the blank should not be less than 0.5 to 0.7 times metal thickness. Inorder to maintain the quality of hole-wall and blank-edge surfaces, and to avoiddistortion. These limitationshave been exceeded. For instance, a 16-mm. dia. hole was pierced in each end ofa 1018 steel link 25 mm. wide and 8 mm. thick. Since the part had a 12 mm.radius on each end the wall thickness was 5mm. The part was offset 2.5 mm. inthe same die. In a part made of 4 mm. thick aluminum alloy 5052-H34, 3.2 mm.dia. holes were pierced leaving a wall thickness of 0.1 mm. A 15.8 mm. dia. holewas pierced in the same part. The sheared facesof holes pierced during fine-edge blanking are usually break, provided themaximum hole dimensions are not more than a few times the stock thickness. As inconventional piercing, there is a slight radius around the punch side of thehole, but there are no torn edges on the die side of the blank. A rough shearedsurface on the blank may be caused by too great a punch-to-die clearance orimproper location and height of the impingement ring for the material beingblanked. On parts blanked to a small width-tothickness ratio, a small roughsurface may be noticeable, but may not be detrimental (see Example 1). PRESSES A triple-actionhydraulic press or a combination hydraulic and mechanical press is used forfine-edge blanking. The action is similar to that of a double-action pressworking against a die cushion. An outer slide holds the stock firmly against thedie ring and forces a V-shape impingement ring into the metal surrounding theoutline of the part. The stock isstripped from the punch during the upstroke of the inner and outer slides. An inner slide carries the blanking punch. Alower slide furnishes the counteraction to hold the blank flat and securelyagainst the punch. This slide also ejects the blank.

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The stripping andejection actions are delayed until after the die has opened at least to twicethe stock thickness, to prevent the blank from being forced into the strip, orslugs from being forced into the blank. Because loads are high and clearancebetween punch and die is extremely small, the clearance between the gibs andpress slides must be so close that they are separated by only an oil film. Force requirementsfor fine-edge blanking presses are influenced not only by the work metal and thepart dimensions, but also by the special design of the dies and pressure padsused for fine-edge blanking. Depending on part size and shape, a 100-ton presscan blank stock up to 8 mm. thick; a 250-ton press, up 12 mm. thick; and a400-ton press, up to 13mm. thick. The total load onthe press in fine-edge blanking is the sum of three components: the cuttingforce (Lc); the lower blank holder force (L LB) or counterforce andthe clamping force on the impingement ring (L LR) on the pressure pad.The first two components comprise the total force on the inner slide, and thethird component is the force on the outer slide. The cutting force,in Kgs., is calculated from the equation: Lc = 0.8 S l Bt Where 0.8 is anexperimentally determined constant; S is the tensile strength of the work metal,kg/cm.²; l Bis the total length of cut (sum of perimeters of blankand holes pierced in blank), cm. and t is the thickness of the work metal, cm. The counterforce,or lower blank-holder force, in pounds, is calculated from the equation:LLB = Pc A Where Pc is thecounter pressure on the lower side of the blank, psi; and A is the area of theblank, sq. in. The counter pressure usually is about 10% of the tensile strengthof the work metal. The clamping force on the impingement ring on the pressurepad, in pounds, can be obtained from: LIR= Lr lIR

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Where LIis the force to embed a 1-in. Length of theimpingement ring into the work metal, lb; and lIRis the totallength of the impingement ring, in. The force LIfor different workmetals, as determined by experience in fine-edge blanking, is given in Fig. 9-2.When impingement rings are used on both the pressure pad and the die, thecalculation of force is still based only on the pressure-pad impingementring. The reduced heightof impingement rings when used in pairs allows the use of a lower clampingforce, and thereby reduces the over-all load on the press. This is because the lower impingement ring isimpressed into the work piece by the reaction force. If coining,embossing or other forming is done during the blanking, the additional forcerequired for those operations must be added to the force requirements ascalculated above.

TOOLS The design oftools for fine-edge blanking is based on the shape of the part, the method ofmaking the die, the required load and the extremely small punch-to-dieclearance. The considerable loading and intended accuracy require that the presstools be sturdy and well supported to prevent deflection. The small clearancepresupposes precise alignment of the punch and die .

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9.6.1Design: A basic tool comprises three functionalcomponents: the die, the punch, andbackpressure components. To produce good-quality blanks, the punch-to-dieclearance must be uniform along the entire profile and must be suitable for thethickness and strength of the work metal. The clearance varies between 0.0050and 0.01 mm. The components ofa typical tooling setup for fine-edge blanking of a part of simple shape areshown in Fig. 9-3. The pressure pad guides the profile part of the blankingpunch. A round punch is prevented from rotating by a key fastened to the upperdie shoe. The hardened pressure pad is centered by a slightly conical seat inthe upper die shoe; this pad contains the V-shape impingement ring. Some die makersput a small radius on the cutting edge of the die. This causes slight bell-mouthconditions, which produces a burnishing action as the blank is pushed into thedie, improving the edge finish. If holes are to bepierced in the part, the blanking punch contains the piercing die. The slug is ejected by ejector pins, orthrough holes in the punch. The die is centered in the lower die shoe by a slightlyconical seat, as is the upper pressure pad. Both the die and the upper pressurepad are preloaded to minimize movement caused by compression. The pressure andejector pad is guided by the die

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profile, and issupported by pressure pins and the lower slide. The backup block for thepiercing punch also guides the pressure pins. The die componentsare mounted in a precision die set with precision guide pins and bushings. Somedesigners prefer pressing the guide pins into the upper shoe. 9.6.2 Materialsand Life: Because of the high loads, close tolerances, and small clearancesinvolved in fine-edge blanking, the die elements are made of highcarbonhigh-chromium tool steels, such as D2 or D3, or of A2 tool steel, heat treatedto about Rockwell C 62. Punch and the dielife vary with tool material and hardness, punch-to-die clearance, type of workmetal, and work piece dimensional and surface-finish tolerances. For most workmetals under the usual operating conditions, punch life for fineedge blankingof 13.2 mm. thick stock is 10,000 to 15,000 blanks between regrinds – assumingthat the blanks are of simple shape and that punch wear

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is such that only 0.05to 0.125 mm. of metal need be removed to restore the punch to its originalcondition. The effect of workmaterial on punch and die life can be illustrated by the following data. In oneapplication, after blanking 33,000 pieces made of 1010 cold rolled steel (No. 2temper), 0.225 mm. was ground from the punch and 0.15 mm. from the die. Production rate was 35 pieces per minute.When blanking 8617 and 8620 steel, it was necessary to grind 0.225 mm. from thepunch after 12,000 pieces and 0.175 mm. from the die after 23,000 pieces. Theproduction rate was 27 to 30 pieces per minute. In anotherinstance, 15,000 to 30,000 pieces per punch grind were produced when blankingannealed 1040 and 1050 steel; and 25,000 to 50,000 pieces for 1010 steel (No. 3and 4 temper). Punch life for blanking fine-tooth gears made of annealedhigh-carbon steel was 10,000 to 15,000 pieces, and for steel with a hardness ofRockwell C 32 to 34 was 5,000 to 15,000 pieces. The reason for grinding thepunch was to remove the small radius on the edge of the punch, which must bekept sharp and flat to obtain a good edge on the part. Total die life maybe 200,000 to 300,000 blanks per tool. The die is usually sharpened once foreach two or three punch sharpening. It may be necessary to remove from the dieas amount of metal up to half the work-metal thickness to restore the die to itsoriginal condition. In some productionapplications of blanking simple shapes from 2.5 mm. thick 1010 steel, lifebetween regrinds was about 40,000 blanks for punches and about 80,000 blanks fordies, when punch and die wear of 0.1255 to 0.175 mm. was allowable and thesurface finish of the cut edge was 2.5 micro-mm. or better. PRESSURE-PADIMPINGEMENT RINGS The most importantconsideration in the design of a pressure pad for fine-edge blanking is thespecial construction required to lock the work piece tightly against the die andforce the metal to flow against the punch. A V-shape impingement ring isprovided on the pressure pad surrounding the outline of the part (see Fig. 9-1).The lip of the pressure pad between the ring and the shear line has a differencein elevation of 0.05 to 0.125 mm. from the outer

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surface of the pressure pad(see inset in Fig. 9-4). The ring (which penetrates to its full depth into thescrap metal outside the shear line) and the lip of the pressure pad hinder metalflow at the shear line during blanking. The outline of animpingement ring is a closed shape conforming to that of the blank to beproduced. Figure 9-4 shows the minimum and maximum distance of the right fromthe edge of the die opening, for rings of various heights. An impingement ringwith a 60° angle, instead of 45° angle shown in Fig. 3, has been used, but moreforce was required to imbed it into the work metal.

Height of impingement ring depends on thethickness and ductility of the work metal. The height (penetration) of the V-shape is 20% of stock thickness formaterials of low ductility. The more ductile materials require a penetration of32 to 35% of stock thickness. If rings are usedon both sides of the stock, the height of each ring should be half the totalpenetration required for the metal. Thus, if the penetration required were 1.07mm., the height of each ring would be 0.52 mm. The

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distance to the edge of thedie opening (s, in Fig. 9-4) would be reduced, and so would the length of theimpingement ring. The selection ofthe ring height and of the exact location within the range defined in Fig. 9-4is based on experience. A shallow ring near the shear line has about the sameeffect as a deeper one farther from the shear line. If the ring is too near thedie opening, a large portion of the metal in the zone will flow into the shearor edge radius and impair the efficiency of the ring. When a larger ring islocated a greater distance from the die opening, a larger amount of stock isused, and more force is required to impress the ring into the work metal.Improper location and size of the impingement ring can cause rough sheared edgeson the blank. 9.7.2Effect of Stock Thickness: Stock up to 4 mm.thick usually requires a ring on the pressure pad only. Stock up to 4.8 mm. thick may need a partialring on the die in addition to a full ring on the pressure pad. Full rings onthe pressure pad and the die may be necessary for stock over 4.8 mm. thick. Although animpingement ring on the die reduces the edge radius on the blank more than doesa similar ring on the pressure pad, its use is avoided when possible, because itmakes re-sharpening of the die difficult. The need for afull or partial ring on the die, to supplement the ring on the pressure pad, canbe reduced by properly orienting the blank design on the strip. More precise andintricate cutting can be done on the side of the blank adjacent to the incomingstrip, where ample stock is available to restrain metal flow, than alongsurfaces near the edges of the strip or along the narrow portion of the webwhere blanks have already been removed. When a blank cannot be oriented on thestrip so that an ample width of stock is adjacent to all critical sections ofthe shear line, it is usually more economical to use partial rings or straightknife-edge projections on the die than to provide large edge and web widths. 9.7.3Effect of Part Shape: The impingement ringordinarily follows the contour of the part at a distance depending on ringheight (see Fig. 9-4), but it cannot follow narrow slots in the part. Impingement ringsare not necessary around holes pierced in blanked parts, particularly holes withdimensions that are only a few times the metal

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thickness. However, the blankmust be securely clamped between the punch and the pressure pad. LUBRICATION The work metalmust have a film of oil on both sides to lubricate the punch and die duringfine-edge blanking. The lack of a lubricant on either side can reduce punch ordie life between sharpening as much as 50%. Oils used for conventional blankingusually are satisfactory. In severe applications, a wax lubricant may beused. In Examples 2, 3 and 4,sulfur-free oil was used to lubricate the strip. Extreme-pressure chlorinatedoil was used for the part in Example 1. EXAMPLES OFAPPLICATION The lock lever inthe following example was made of low-carbon steel and had a lowwidth-to-thickness ratio. Although a conforming impingement ring was used, arough surface appeared on the cut surface adjacent to the upper (punch) side ofthe blank. Example 1. Blanking of a Long Slender Lock Lever toClose Tolerances (Fig. 95) A lever for a pushbuttonlock (Fig. 4) was fine-edge blanked to a minimum total tolerance of 0.075 mm.The maximum total dimensional tolerance was 0.25 mm. except on fractionaldimensions, which had a tolerance of 0.8 mm.

138

The lever wasblanked from cold rolled, commercial quality 1010 steel, 3.2 mm. thick and 70mm. wide. The coil stock had a No. 4 temper (soft), No. 3 edge (slit), and a No.2 finish (bright). The blank design was positioned at an angle on the strip,with a progression of 22.2 mm. An impingement ring 1 mm. high was used on thepressure pad. The edge of theblank was smooth and perpendicular to the top and bottom surfaces. On the dieside of the blank, edge radius (rollover) was noticeable, particularly at theoutside corners. (This effect, typical of low-carbon steel in fine-edgeblanking, is less pronounced on high-carbon and alloy steels.) The die was madeof D2 tool steel, hardened to Rockwell C 57 to 60, and ground to a fine finish.During blanking, the die was lubricated with EP chlorinated oil. Die life was 30,000 pieces per grind. The diewas mounted in a special 40-ton triple-action hydraulic press operating at 40 to50 strokes per minute. The two examplesthat follow describe applications in which fine-edge blanking replacedconventional blanking, thereby eliminating the need for subsequent drilling,reaming and shaving. In the first example, the smaller of the two holes piercedduring fine-edge blanking had a diameter only 62% of stock thickness. In the second example, the distance betweenthe edge of a large hole and the edge of the part was only 50% of stockthickness. Example 2 and 3:Fine-Edge Blanking and Piercing to Final Size and Finish, Which could not bedone by conventional blanking.

139

Example 2 - The clutch dog shown in Fig. 9-6 was fine-edgeblanked from annealed cold rolled. Commercial quality 8617 steel, 3.2 mm. thickand 38 mm. wide. The two holes were pierced at the same time the outline wasblanked. The periphery of the part and the holes had a 100% land. There was noedge radius on the die side, and the burr on the punch side was small andeasy to remove. The sequence ofoperations used to make the part by conventional methods was: blank outline andpierce a 4.8 mm. dia. hole; drill the 2.032 / 1.9558 mm. dia. hole; ream the 4.8mm. hole to 6.3881 / 0.0625 mm. and shave periphery of part to printrequirements. In fine-edgeblanking, impingement rings 0.889to 1.016 mm. high was used on both sides of theblank. On the punch side, the metal between the punch and the ring was deformed,indicating draw-in. The same area on the die side of the stock had a sharpoutline, but it was about 0.25 mm. below the surface. The blanking dieswere made of D2 tool steel, hardened to Rockwell C 60 to 61, and ground to afinish of 0.63 micro-mm. The life between grinds was 12,000 parts. The die was mounted in a special 110-tontriple-action hydraulic press operating at 20 to 30 strokes per minute. Die set up time was 30 min.

140

The clutch dog was madein lots of 10,000 pieces; yearly production was 50,000 to70,000.

Example 3 - Thepositive clutch detent shown in Fig. 9-7 was fine-edge blanked from annealed,cold rolled, commercial quality 8617 steel, 3.2258 / 3.1242 mm. thick. The twoholes were pierced and the blank was severed from the stock in one press stroke.There was no distortion where the edge distance was less than work-metalthickness. The conventionalmethod of making this part was: blank, drill and ream the two holes, shave twosurfaces, and deburr. The part had asmall edge radius along the die-side surface. This radius was more pronounced atthe outside corners. The dimensional tolerances were 0.25 mm. total. Surfacefinish in two areas along the cut edge was 1.26 micromm. The die was madeof D2 tool steel, hardened to Rockwell C 60 to 61, and ground to a finish of0.63 micro-mm. Die life was 12,000 pieces per grind. The impingement ring was79.35468 to 1.0 mm. high and in the pressure pad only. The die was set up in aspecial 110-ton triple-action hydraulic press operating at 20 to 30 strokes perminute.

141

A sulfur-free oilwas used as lubricant for both methods. Production rate for fine-edge blanking was 26 pieces per minute; lot sizewas 10,000 pieces, for a total production of 50,000 pieces per year. The blanking of a16-pitch gear is described in the following example. The impingement ring wascircular instead of scalloped because of the small teeth. Example 4. Blankinga Spur Gear With a Smooth Edge - The teeth, center hole, and key of the16-pitch, 48-tooth spur gear shown in Fig. 9-8 were fine-edge blanked in onepress stroke from annealed, cold rolled, commercial quality 8620 steel strip,3.20 / 3.15 mm. thick and 357 mm. wide. The edges of the hole and teeth weresmooth, having no fractured area. On the die side of the gear, the corners atthe tip of each tooth had a noticeable edge radius, which decreased along withtooth flank until it was zero in the root. On the punch side, the corners weresharp but had a small burr, which was easily removed by vibratory finishing orbelt lapping.

142

The blanking die was made of D2 tool steel, hardened toRockwell C 60 to 61, and ground to a finish of 0.63 micro-mm. Impingement rings,0.889 to 1.016 mm. high and 81.7372 mm. in diameter, were used on both thepressure pad and the die. The feed length (strip progression per stroke) was83.312 mm. The die was set up in a special 110-ton hydraulic press with threeslides. The press operated at 10 to 15 strokes per minute. Die life was 12,000pieces per grind. Lubricant was sulfur-free oil, applied to both sides of thestock. The gears weremade in 15,000-piece lots at a yearly production of 105,000 pieces. Die setup time was 30 min. The conventionalmethod for making the gear would have required three or four operations: blank and pierce in a compound die, shavehole and outside diameter, and hob the teeth. Two shave operations might havebeen required, because the tolerance on the hole diameter and key width was0.0125 mm. In the followingexample, it was more economical to form the part by fineedge blanking than byconventional blanking and machining.

edge blanking and piercing than by conventional blanking andmachining. Example 5. Fine-Edge Blanking vs Blanking andMachining-The latch part shown in Fig. 9-9 wasmade of 3.2 mm thick low-carbon steel having a No. 1 temper and a No. 2 finish.

143

Originally, thepart was made by blanking and shaving, and then was deburred by vibratoryfinishing. The three holes were drilled and reamed and the part was disk groundto the required flatness. The method waschanged to fine-edge blanking. The complete part was made in one press stroke,except that a small burr was removed by fine-belt sanding. Fine-edge blankingresulted in savings of $5000 for annual production of 35,000 pieces.

Materials UsedFor Die Parts KIND OFTOOLING In general, short-run orsingle-operation dies are used for smallvolume production where the cost oftooling must be kept low because it is the major cost factor, and the cheapesttool materials are used. Compound dies are used for medium to large-volumeproduction where intricacy is not the dominant problem and where accuracy oftenis. Progressive dies are used for medium-volume, and particularly forlarge-volume, production where the die is preferably not made as intricate asthe part. In adjacent and simultaneousblanking operations where the die sections are thin or intricate, type A2 toolsteel is preferred for runs up to 100,000 parts of most materials. M2, D2, D3 or carbide is preferred for longerruns. DIECOMPONENTS Piercing punches. The usuallimiting slenderness ratio of punch diameter to sheet thickness for aluminum,brass and steel is 2.5 to – 1 for unguided punches and 1 - to - 1 for guidedpunches.

144

The limiting slenderness ratio of punch diameter to sheet thickness forpiercing spring steel and stainless steel is from 3-to1 to 1.5-to-1 forunguided, punches, and from 1 – to - 1 to 0.5 – to - 1 for accurately guidedpunches. Where these usual limits areexceeded and breakage cannot be eliminated by stepping the punches, tool steelssuch as O1, A2 and M2 are used. W1 is used if the diameter is greater than10 mm.but less than 20 mm. or 25 mm. Piercing-Punch

Bushings.

Therecommended

materials

for

piercing-punch bushings of all three types (quillretainer, guide or stripper, and die button), particularly for bushings of theprecision type, for instance, where the outside diameter is ground to – 0, +0.0075 mm., concentric with the inside diameter within 0.005 mm. The hardness ofthe W1 bushing should be Rockwell C62 to 64; that of the D2 bushing, Rockwell C61 to 63. Die plates and die parts thathold inserts are made of class 50 gray iron, alloy steel, or (for heavy work)tool steel, and of cast iron or low-carbon wrought steel for blanking andpiercing soft and thin materials. For blanking or piercing thicksheets or hard materials, either gray iron of 2,800 to 4,200 Kgs. / cm.² tensilestrength or 4140 treated to Rockwell C 30 to 40 should be used. Particularly on heavy-gage or hard materialand on long runs for which inserts are pressed in, steels like 4340 or H11 areused; when inserts are screwed into the die plate, 4340 is nearly always used. Die plates for blanking orpiercing thin or soft sheets may be made of gray iron of 2,100 to 4,200 Kgs. /cm.² tensile strength, or mild steel.

145

Punch holders and die shoes forcarbide dies are of high-strength gray iron or mild steel plate. Yokes retainingcarbide sections are usually made of O1, hardened to Rockwell C 55 to 60. Backupplates for carbide tools are preferably made of O1, hardened to Rockwell C 48 to52. Stripper plates can ordinarilybe made of some low-carbon or medium-carbon steel like 1020 or 1035. Where ahardened plate, is used for medium-production work, the preferred steels areflame-hardened 4140, conventionally hardened W1 or for intricate shapes,cyanided and oil-quenched W1. For carbide dies and high-production D2 or D4dies, hardened strippers are of O1 or A2 Rockwell C 50 to 54. Guides and locator pins can bemade from W1 or W2 for most dies, or from alloy steels such as 4140 forshort-run low-cost dies.

Many

commercial

guide

pins

are

made

from

1117,carburized, hardened, and finished to 0.6 micro-mm. Combined operations likeblank-and-draw or pierce-and-extrude give rise to selection problems best solvedby determining which of the operations is the more severe, and selecting forthat operation. Selection of material for pierce-and-extrude sections of diesshould follow the recommendations of this article. Wear of extrusion or embossingdies can be offset by nitriding A2 and D2 materials. However, nitriding mayshorten the life of blanking tools because edges are likely to chip, unless thedies are used for thin or soft sheet. 10.3TOOLMATERIALS Table

10

shows

nominalcompositions

of

the

tool

steels

recommended in the selection tables. All ofthese steels serve

146

best when used at maximum tempered hardness, particularly inblanking thin material and when shock will be absent. For conditions of shock,the hardness is lowered to produce a tolerable level of breakage. Whether longerdie life can be achieved by tempering to a lower hardness or by using toughersteel at full hardness cannot be readily predicted. W1 and W2 are readilyavailable, readily machinable, wearresistant and highly versatilewater-hardening grades, furnished with various carbon contents in 10%ranges. W1 and W2 are interchangeable inperformance, but W2 is of little advantage except that coarse grain is lesslikely to develop in the steel as a result of overheating. The depth of hardness of thewater-hardening grades is shallow and for this reason such steels should not beused where grinding of the hard case will be needed to correct for distortiondue to heat-treating, except for short-run dies. W1 may make a brittle, easilybroken punch if less than 1o mm. in diameter, but a tough one if the diameter isabout 20 mm. Hardness should be thehighest obtainable at a temperature of 325 to 375 F – usually Rockwell C 62 to66. Shock-resisting tool steels S1and S5 are used for punches only where the probability of breakage is high. With normal heat treatment they haveunacceptable levels of wear resistance, and they are economical only if they arecarburized to obtain 0.25 to 0.50 mm. case containing 0.70 to 0.75% C. S1 shouldbe used at Rockwell C 57 to 60, and S5 at Rockwell C 59 to 62. Oil-hardening steel O1 is saferto harden and distorts less than W1 steel. O2 is preferred to O1 for dies thatare to be made by broaching. It distorts less in hardening. Steel O6 is easier to

147

weld, has consistentlybetter life in blanking and piercing dies than O1 and has reduced regrinding andmaintenance by about one-half in blanking 1040 and other steels up to 10 mm.thick. Although less widely available than O1, the usage of O6 steel hasincreased greatly during recent years. Advantages derived from the use of O6 indie applications relate to its greater resistance

to

sliding

compared with

other

wear O

and

its

bettermachinability,

grades; however, it may

as

distort

moreduring heat treatment. A2 air-hardening medium-alloy(5% Cr) tool steel has wear resistance about halfway between that ofoil-hardening steels and that of D2. A2 presents the least hazard of size changeand cracking in heat treatment of the entire tool steels, followed closely byD2, air-hardened D4, and then by oil-hardened O and oil-hardened S types.LikeD2, the A2 steel can be nitrided for dies for thin or soft materials orreinforced plastics, to resist wear and heat. D2

high-carbon

probably

the

high-chromiumair-hardening

most

commonly

used

and

tool

steel

may

be

is the

mostsatisfactory and most widely available tool steel for largevolume production ofblanks. It is about the second-best steel for high accuracy and for safety inheat treatment and it throughhardness

in

3-in.

sections.

Its

highest

usablehardness

of

Rockwell C 62 to 63 is recommended for punches and dies wherebreakage is not a problem, as in dies blanking steel less than 0.062 in. thickand softer than Rockwell B 90. Maximum resistance to breakage may be developedby tempering back to Rockwell C 58 to 60, but only at a sacrifice in wearresistance. For lamination dies, the hardness should not be less than Rockwell C61 or 62.

148

D4

high-carbon

high-chromiumair

hardening

tool

steel

is

somewhat more wear resistant than D2 and D3,particularly in blanking and piercing electrical sheet, where, at Rockwell C 63to 65 it often wears about 20% less than D and D3 and about the same as M2 highspeed steel. All of the high-carbonhighchromium steels should be nitrided to extend die life only for blankingreinforced plastics or for soft or thin materials. D5 high-carbon high-chromiumtool steel has replaced D2, D3, M2 and M3 in some plants for the piercing,trimming and blanking of austenitic stainless steel. Metal pickup and scoringhave been minimized in such applications by the use of D5, with an increase of100 to 200% in die life for some stainless steel parts. M2 high speed steel is theleast costly, most used, and most readily available high-speed steel forblanking dies and punches. It is equal to or better than, D4 in wearresistance. For blanking and piercingelectrical sheet, the conventionally hardened M2 is surpassed only by carbide,cast alloys, and carburized M2. When carburized, M2 is about30% more resistant to punch wear in making laminations than it is with standardheat treatment. It is equaled only by D4 and three less widely used highspeedsteels: M4 carburized, T1 carburized, and T15 carburized. It is recommended inTable 5 with standard heat treatment because it is less likely to break thanother steels of equal wear resistance than the shock-resisting steels S1 or S5in blanking dies. M3 high speed steel, with its1% carbon and high vanadium content, is more wear-resistant than M2 and the Dgrades. Its wear resistance can be improved by liquid nitriding. Selection of M3depends on whether the dies can be ground economically; to

149

reduce the amount ofgrinding, M3 is generally used only for inserts. M3 is more difficult to grindthan M2; caution must be used to avoid “burning” and the formation of surfacecracks. Hot rolled mild steel platewith carbon content from 0.10 to 0.20% may beused for short runs of small partsafter it has been surface hardened, either by carburizing to a depth of 0.25 to0.50 mm. or by cyaniding to 0.1 to 0.2 mm. Because it distorts in heattreatment, its use is limited to small, symmetrical shapes. 4140 alloy steel is generallyavailable in various sizes of plate of aircraft quality. It is flame hardened toabout Rockwell C 50 for long blanking runs on soft materials. However, flamehardened tools that have either inside or outside corners are likely to havesoft spots that will wear rapidly. For large dies, flame hardening the workingedge only, instead of hardening the entire die, has the advantage of minimizingthe changes in size and the warpage that occur as a result of heat treatment. Carbide tooling is usuallyconsidered where production is four or more times the life of a D4 tool steeldie, especially where close tolerances and minimum burr are required and aheavier press is available. Partial or complete inserts of carbide in tool steelmay be considered for lower quantities or where the tool life between grindsneeds to be extended. However, brazed sections are hazardous, and dovetailed ormechanically held sections will approach the cost of a complete carbide die. The first material should beused where shock is appreciable. The second of the above combines toughness andwear resistance and is preferred for heavy-duty service, such as piercingsilicon steel. Where

close

tolerances

must

be

held

in

piercing

silicon

steellaminations, the third material is useful. The last of the

150

carbides listed willbe best for guides and guide rolls, or for applications involving very highshock. Selection of die material forpress tool depends mainly on the type of metal being cut, bend or formed and onproduction quantities. Following table

will

gives recommendations for

materialsfor die, punches, housing, punch holder, stripper plate, shank, strip guides,stopper and die base parts are listed here.

Positio n 1

Part Description

Material

Die shoes

Cast iron or M. S. / En 8

151

HRc

2

Die housing, Punch holderplate, M. S. or En 8 Stripper

plate,

spacer

plates,

Shank and Strip support. 3

All guide pillar and bush.

Case steel

Spring loaded stripper guidepin

hardening Case 54 –

36 58

En

orEquivalent.

and bushes.

Core 38 – 42

O1

Floating die guide pin andbushes.

54 – 58 4 5

Stripper

insert,

Guide OHNS (O1) / W1 / 54 – 58

plates,stopper pin, pilot punches

W2

Cutting Die and Punches.

D2 / D3 / D4 / M2 58–62

Coining,

Bending,

forming

Dieand Punches.

/ Carbide

60–64

O1 / A2 / D2

86 – 90

/

56 – 60 Table 10. NOMINAL COMPOSITION OF TOOL STEELS FOR DIE AND PUNCH Recommended in Selection Table. Steel

Description

C

W1 W2

0.6 Water-hardening tool steels

Mn

Cr

Mo

T0

1.4 0.6

T0

0.25

1.4 S1 S5

V

0.50 Shock resisting tool steels

0.55

1.5

2.5

0

W

0.8

0.4

0 O1

Oil-hardened cold-work steels

152

Other

0.90

2.00 Si

1.0

0.5

0.50

0

0

W

A2

Air-hardened

medium-alloycold- 1.00

work steels D2 D3 D4

1.50 High-carbon high-chromiumcoldwork steels

2.25

M2

high-speed

toolsteels

1.0

0

0

12.

1.0

00

0

120

1.0

0

0

4.0

5.0

6.25

0

0

W

2.25

0.85 Molybdenum

5.0

2.00 V

153