Design and Calculations of Piercing & Blanking Die

Module2 Design and Calculations of Piercing & Blanking Die

Contents • • • • • •

Economics for strip layout Calculations of cutting force and stripping force Minimization of press tonnage Centre of pressure Design aspects of press elements Die set selection

STRIP LAYOUT •

A strip layout represents the sequence of the logical, workable operations, which is to say a sequence of ideas. If this sequence of operations has error, the error will be surely emerge in a try out press. Factors to be considered while designing the layout are. – – – – –

Shape of the blank. Production requirement. Grain direction. Burr side. Stock material.

• STRIP LAYOUT FOR BLANKING TOOLS. Choice of strip layout method. • Blanking tools produce blanks out of the strip or unit stock. • None of the edges of the strip or unit stock forms an edge of the blank. • Blanking is the most efficient and popular way of producing intricate and closely tolerated blanks. • The profile and accuracy built into the tool will be reproduced on the blank. • In the strip layout, blanks can be positioned in different ways in the strip.

Shape of the blank: • The contour of the blank, decides the position of the strip. • Some of the blanks are laid at an angle. Production Requirement: • If production requirement is less, then material conservation is necessary. • This must not increase the tool cost. • Gang die may be suitable for the mass production. Grain Direction: • The grains are found in the sheets when they are rolled. • Bending the strip along the grain direction results in crack and fracture. Burr Side: • It is a decisive factor in laying the strip. • In blanking, burr is found on the punch side. • In piercing ,burr is found on the die side. Stock Material: • Every means is to be seen to conserve the stock material. • A double pass layout would justify the cost of stock material conserved.

ECONOMY FACTOR: The designer should try out every possible means to attain a min 60% usage of any strip, without sacrificing the accuracy of the piece part. Economy Factor = Area of the blank x No of rows x 100 Width of the strip x Pitch

The position of the blank in the strip decides the economy factor

TERMS USED IN STRIP LAYOUT Pitch :Distance between two consecutive operations on a strip. Scrap bridge: This is the portion of the material remaining after blanking operation between one edge of the strip and the cutout portion. The portion of material remaining between the two adjacent openings after blanking is also called as the scrap bridge. Front Scrap: This is the scrap bridge on that edge of the strip which is towards the operator. Back Scrap: This is the scrap bridge on that edge of the strip which is away from the operator

EXAMPLE:1 Calculate the economy factor to punch the mild steel washer in single row feeding. Outside diameter is 30mm, Inside diameter is 18mm and Thickness is 2mm

Scrap bridge width is = 1.2 s. Scrap bridge width = 1.2 x 2 =2.4 mm Pitch = 30 + 2.4 = 32 mm Strip width = 30 + 4.8 = 34.8 mm Number of rows = one Area of blank = Л D² 4 = Л 30² 4 = 706.65 mm² Area of blank x No. of rows x 100 pitch x strip width

Problem1: Calculate the economy factor to punch the same washer in double row feeding?

• Narrow run.

• Wide run

STRIP LAYOUT FOR CUT OFF AND PARTING • Cut off punch cuts with only one edge. • No scrap is produced. • A parting punch cuts with two opposite edges thereby producing a scrap.

STRIP LAYOUT FOR CUT OFF • Cut off punch cuts with only one edge. • No scrap is produced. • A parting punch cuts with two opposite edges thereby producing a scrap. NOTCHING • Notching is a cutting operation for cutting off small portions from the edge of a strip or a pre blanked component.

• Trimming is an operation of cutting off material to alter the shape of the strip or blank. • In notching only a small area of the blank is cut off. • In trimming a larger area of material is removed. • Blank can be produced by combining notching, trimming and piercing operations with cut off or parting operations.

Die block

Strippers • The main function of the stripper is to strip or discard the work piece from the punch or the die after the end of the cutting or forming operations. – Fig. illustrates a stripper attached to the punch holder. The stripper plate 3 is connected to the holder by means of two helical springs 2. – The punch 1 passes through a hole in the stripper 3. – When the punch descends to cut or form a material placed on the die block, the stripper plate 3 bears against the blank and holds it down by the spring pressure. In the upward stroke of the punch, the blank is stripped off from the punch cutting edge and prevents it from being lifted along with the punch by the stripper plate.

Pilots

Pilots: The pilots in progressive dies are used in order to bring the stock into the correct position for the succeeding blanking or piercing operations. The stock when fed manually is slightly over fed against a stop. The pilot when moves further down brings the sheet properly in position. The fit between the pilot and the pierced hole determines the accuracy of the component produced. Too tight a fit between the pilot and the pierced hole results in friction which would cause excessive wear of pilot and may spoil the component.

• The main purpose of a pilot is to position the stock strip accurately. Pilots also bring the stock strip into proper position for blanking and piercing operation simultaneously. If strip is fed by hand, it may go beyond proper position due to strip stop. In such a situation, pilot takes it back to proper position in a direction away from the strip stop. Pilot also prevents buckling of strip. When the strip is fed by hand, under feeding of strip occurs and pilot pulls the strip forward.

• Classification of Pilot Pilots are : direct pilot, indirect pilot and spring loaded pilot. Direct Pilot Direct pilots are mounted on punch face. • Direct pilot:-It acts as a misfeed detector, which detects overfeeding or underfeeding of strip. If it finds misfeed, it actuates a switch to cut off the electric power to press.

• The pilot size may deviate from the pierced hole by the magnitude given here.

Parts of standard die set

Die Sets The die set consists of a bottom plate and top plate together with guide pillars and bushes. The guide pillars and bushes align the top and bottom plates. The advantages of die sets are - Accuracy of set up. - improved piece part quality. - Increased die life. - Minimum set up time. - Easy maintenance. - Alignment of punch and die. - Easiness of storing. Die sets can be classified as  - Precision - Commercial The difference between them are the accuracy of the fits between the bushes and the pillars. Precision die sets are used for cutting operation tools. Commercial die sets are used for operations like bending, forming and other non cutting operations. Die set materials Die sets are manufactured by using the following materials: - Cast iron containing 10 to25% steel. - Hot rolled steel. - Semi hardened or hardened tool steels.

• Die set components: Top plate, Guide bushes, Guide pillars, Bottom plate. Top plate The upper working member of the die set is called the top plate. The punch holder is clamped to the top plate. Bottom plate The bottom plate is the lower working member of the die set. It's shape corresponds with that of the top plate except that it is provided with clamping flanges. The flanges have provision for fastening the die holder to the bolster plate of the press. Usually the bottom plate is made thicker than the top plate. This is to compensate for the weakening effect of slug and blank holes. Guide pillar Guide pillars are precision ground pins which are press fitted into accurately bored holes in the bottom plate. Guide pillars are assembled into corresponding guide bushes to align punch and die components with a high degree of accuracy. • Guide Bushes Guide bushes are precision ground bushes which are press fitted into accurately bored holes in the top plate.

• Types of die set 1. Standard die set. 2. Non standard die set 1.Standard die sets - used for bending tools. - secondary operation tools. Center pillar die set - used for round working area. Diagonal pillar die set - used for progressive tools with rectangular working area. Four pillar die sets - used for heavier press working operation. Non standard die sets These die sets are made for a particular design when standard die sets are not suitable or not available. These are usually made of mild steel with case hardened pillars and bushes.

• CENTER-POST DIE SETS • These die sets are ordinarily employed for secondary-operation work such as piercing, coining, and the like. Parts are loaded from the front. The die sets are available in semi-steel in the style illustrated at A, and in steel in the style shown at B. Components may be supplied in combination, with a steel die holder used in conjunction with a semi-steel punch holder as at C. Another important application for center-post die sets is the performing of secondary operations on work pieces having a right hand and a left hand. Parts of one hand may be conveniently loaded from one side. When the other hand is to be run, the die set is turned around 180 degrees in the press for ease in loading.