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SHEET METAL EMPIRICAL FORMULAE & DESIGN GUIDELINES
CONSULTANCY SERVICES
Experience certainty. For internal use only
CONTENTS:
1. BLANKING 2. PIERCING (PUNCHING) 3. EXTRUDED HOLE 4. FORMING AND BENDING 5. EMBOSSING 6. DRAWING 7. GAUGING 8. NESTING 9. TOLERANCE AND FLATNESS VALUE 10. KFACTOR 11. DESIGN FOR MANUFACTURABILITY AND GUIDELINES 12. REFERENCES
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COST EFFECTIVE DESIGN PRINCIPLES IN SHEETMETAL PROCESS BLANKING Minimum Practical Section should never be less than material thickness or 0.060in. A minimum section must be one and one half times material thickness for high shear strength material for the most practical stamping. Corners can be sharp if material thickness is 1/16in or less. For over 1/16in thick sheet allow corner radii should be a minimum radius of 0.5 x material thickness or 0.4 mm (0.016in) whichever is greater. Sharper corners can be produced but at a greater die maintenance costs and more burrs. Slots or tabs widths should be greater than 1.5 X stock thickness. The length can be a maximum of 5 times slot/tab width. These rules can be violated at an increased tooling cost width as low as 1 X thickness and length as high as 7 X thickness can be achieved. Avoid full radii across the width of stock (Tab). A square cut is best. If radius is necessary, then an angle –blended radius is best to avoid feather edges.
NOTCH AND TAB GUIDELINES
PRACTICAL DESIGN FOR ECONOMY MANUFACTURE OF TAB W = .060 MINIMUM FOR MATERIALS THINNER THAN .060" WIDER IF POSSIBLE. W1 = NEVER LESS THAN MATERIAL THICKNESS, WIDER IF POSSIBLE. L = 5 X W IS MAXIMUM DEPTH, SHOULD BE LESS IF POSSIBLE. L1 = 5 X W IS MAXIMUM LENGTH, SHOULD BE LESS IF POSSIBLE
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PIERCING Do piercing together to ensure good hole to hole tolerance. To avoid distortion, produce holes by staggered pattern shown in Fig A. To pierce holes with economical tools and operations, the hole diameter must not be less than the stock thickness. If the hole diameter is less than the material thickness (or less than .060") it usually must be drilled and deburred and each of these operations is slower than punching. Minimum hole (and short slot) to bend distance should be 2.5 X the stock thickness + bend radius. For long slots, the distance should be 4 X the stock thickness + bend radius. Minimum hole diameter should be at least 20 % greater than stock thickness. In the case of stainless steels, it should be 2 times the material thickness. Minimum wall thickness (distance from hole to edge or hole to hole) should be at least 2 times stock thickness. For nonround slots, the minimum wall thickness should be 2 times thickness for short slots < 10 thickness long; and 4 times thickness for long slots > 10 thickness long. For long slots, the distance should be 4 × the stock thickness + bend radius. The addition of the word "thru" to any hole diameter, regardless of tolerances, indicates the requirement of the hole to be reamed. Reaming and the additional chamfer to remove burr add two extra operations to the cost of the part. Piercing also can combined with Lance – Form and Extrude operation If Diameter of hole less than 5 times of sheet thickness, the hole edge to edge should 1.5 times of sheet thick; and 2 times thickness between edge to edge if diameter of hole less than 10 x sheet thickness.
Fig A
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Fig.B indicates a hole diameter with a tolerance of plus or minus .002". We can pierce a hole within these limits on the punch side for approximately 25% to 30% of the material thickness as indicated in Fig C. The percent of thickness varies with the shear strength of the materials. On holes where a machine finish is required, they can be punched undersized, redrilled and reamed to size as shown.(See Fig "E".) If the web (distance between the hole and edge of material) is a minimum of the stock thickness, the hole can be punched which is less expensive than drilling and deburring.(See Fig D.) A web that is less than the stock thickness will result in a bulge on the blank. Bulge conditions would increase progressively as the web decreases, until there would be a complete break through. (See Fig F.) As a suggestion, if the web is too narrow, the profile of the blank could be changed by adding an ear of sufficient dimensions and shape to eliminate the problem.(See Fig G.) Another alternate suggestion would be to change the contour of the blank to include the hole as a notch. (See Fig H.)
PIERCING – ADJACENT TO BENDS:
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The minimum inside distance required from the edge of a hole to a bend is 11/2 times the material thickness plus the bend radius. Fig.A Otherwise distortion will occur as indicated in fig "B"  or piercing after form must be considered. In Fig C indicates a similar condition to “A” except for openings with an edge parallel to bend. In this case the following requirements apply for economical tooling and production When "L" = up to 1"  2T + R (minimum). When "L" = 1" to 2"  21/2T + R (minimum). When "L" = 2" or more  3T to 31/2T + R (minimum). Minimum recommended tolerances are shown below: (on multiple bends each bend is made separately).
DEFINITION AND LIMIT OF EXTRUDED HOLE An extruded hole is formed by punching a smaller hole and then flanging the sides. R = Outer radius H = Flange height D = Inner diameter T = Material thickness
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FORMING AND BENDING: FORMING DESIGN CONSIDERATION On bends, the short leg (inside length) should be a minimum of 2.5 x T + bend radius. Minimum hole (Slot) to bend distance should be 2.5 x T + bend radius for short slots. For long slots , the distance should be 4 x T + bend radius. Bending using tight radiuses or in hard materials often results in burrs and fractures on the outside of bends. This can be eliminated by using larger bend radiuses and by providing relief notches. Bend relief notches should be provided by the following ratios, Notch width should be 2 x T or At least minimum 1.5mm / 0.060 in Length should be Bend radius + T
AIR BENDING: The inner radius of the bend is the same as the radius on the punch.
BOTTOMING OR COINING: The inner radius of the work piece should be a minimum of 1 material thickness in the case of bottoming; and up to 0.75 material thickness, in the case of coining.
BEND RELIEF:
In the Fig. A the left side design is not desirable for quality or economy. When the form is inside the blank profile, as shown, the material must be torn through the stock thickness and the bend radius. if
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the part is under stress, this tear will likely cause fatigue failures. In addition, stock tooling cannot be adopted because the flat area adjacent to the form must be held in position during forming, which means extra tooling expense.
Fig.B is a similar condition, but with the form just outside the blank profile. In this case, the tear extends to the center of the required bend radius. Fig C and D shows the possible solution by changing the blank profile to provide relief for bends. Besides eliminating the chance of fatigue under stress, there is a possibility of using stock 90degree “Vee” punches and dies. The results are better quality and less expensive engineering charges. If the relief notches shown in fig D are wide enough compared to the material thickness and shear strength, or are designed like the shown in Fig E, they can be included in the blanking operation for little engineering cost and no extra operation.
Use the above empirical relation to solve the bend relief.
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BEND ALLOWANCE FORMULA:
MINIMUM BEND RADII: The bend radii listed are the standard minimum for manufacturing in Aerospace and Space applications. Commercial sheet metal radii are created with less concern for stresses created during forming and radii can approach zero (sharp internal corner) for many thin sheet metal gauges. When bending to an angle of 90 °, the minimum bend radius = 2 to 5t for CSteel = 1t for Stainless Steel = 2 to 3.5 for Titanium Alloy = .3 to .5 for Brass = .35t for Aluminum However to prevent any damage to punch and die, the bend radius should not be less than 0.8mm. ALUMINUM Note: Bend radius of zero is achievable for 0.120.050 material thickness. MATERIAL
SHEET THICKNESS

.012 .016 .020 .025 .032 .040 .050 .063 .071 .080 .090 .100 .125 .160 .190
20240 & W .06
.06
.06
.06
.06
.06
.09
.09
.12
.12
.16
.19
.22
.31
.36
.75 1.00
2024T3
.06
.06
.06
.09
.09
.12
.16
.22
.25
.31
.38
.44
.62
2024T36
.06
.09
.09
.09
.12
.16
.19
.25
.31
.38
.44
.50
.75 1.00 1.25
30030
.06
.06
.06
.06
.06
.06
.06
.06
.09
.09
.09
.12
.12
.16
.19
3003H14
.06
.06
.06
.06
.06
.09
.09
.12
.12
.16
.19
.22
.31
.38
.44
50520
.06
.06
.06
.06
.06
.06
.06
.09
.09
.09
.12
.12
.16
.19
.22
60610 & W .06
.06
.06
.06
.06
.06
.06
.09
.09
.09
.12
.12
.16
.19
.22
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6061T4 & T6
.06
.06
.06
.06
.06
.06
.09
.09
.12
.12
.16
.19
.22
.31
.38
70750 & W .06
.06
.06
.06
.09
.09
.12
.16
.19
.22
.25
.31
.38
.50
.62
.06
.09
.12
.12
.16
.22
.25
.31
.41
.44
.50
.69
.87 1.00 1.25
71780 & W .06
.06
.06
.06
.09
.09
.12
.19
.22
.25
.31
.38
.50
.75

.09
.16
.19
.22
.31
.38
.50
.56
.62
.62
.75 1.00 1.25

7075T6
7178T6
.06
STAINLESS STEEL MATERIAL
SHEET THICKNESS

.012 .016 .020 .025 .032 .036 .040 .045 .050 .063 .080 .090 .112 .125 .160 .190
302 Annealed
.06
.06
.06
.06
.06
.06
.09
.09
.09
.09
.12
.12
.16
.19
.22
.25
3471A
.06
.06
.06
.09
.09
.06
.06
.09
.09
.09
.12
.12
.16
.19
.22
.25
1/4 Hard Cres
.06
.06
.06
.06
.06
.09
.09
.09
.12
.12
.16
.19
.22
.25
.31
.38
1/2 Hard Cres
.06
.06
.06
.09
.09
.12
.12
.12
.16
.16
.25
.25
.31
.38
.50
.62
Full Hard Cres
.06
.06
.09
.12
.12
.16
.16
.19
.22
.25
.31
.38
.44
.50
.62
.87
EMBOSSING VBEAD
L = 3T Reduce to 2T for commercial grades of steel, Onequarter hard tempers and alloys of aluminum
OFFSET
L = R1 + R2 Reduce to .5(R1 + R2) for commercial grades of steel, Onequarter hard tempers and alloys of aluminum.
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STYLE OF EMBOSSING
EDGE CONDITIONS OF FORMED PARTS “Hemmed Edge” – This is formed for smoothness and stiffness.
“Curled Edge” – This is formed for maximum stiffness and smoothness
“Lance – Formed”
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DRAWING Round shapes are easiest to draw. Square shapes can also be drawn if the inside and outside radiuses are at least 6 x T Draft should added to deep drawing
EDGE CONDITION OF DRAWN PARTS
GAUGING Standard Steel Thickness (inches)
3
0.2391
0.2294
4
0.2242
0.2043
5
0.2092
0.1819
6
0.1943
0.1620
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Galvanized Steel Thickness (inches)
Aluminum Thickness (inches)
Gauge (ga)
11
7
0.1793
0.1443
8
0.1644
0.1285
9
0.1495
0.1532
0.1144
10
0.1345
0.1382
0.1019
11
0.1196
0.1233
0.0907
12
0.1046
0.1084
0.0808
13
0.0897
0.0934
0.0720
14
0.0747
0.0785
0.0641
15
0.0673
0.0710
0.0571
16
0.0598
0.0635
0.0508
17
0.0538
0.0575
0.0453
18
0.0478
0.0516
0.0403
19
0.0418
0.0456
0.0359
20
0.0359
0.0396
0.0320
21
0.0329
0.0366
0.0285
22
0.0299
0.0336
0.0253
23
0.0269
0.0306
0.0226
24
0.0239
0.0276
0.0201
25
0.0209
0.0247
0.0179
26
0.0179
0.0217
0.0159
27
0.0164
0.0202
0.0142
28
0.0149
0.0187
0.0126
29
0.0135
0.0172
0.0113
30
0.0120
0.0157
0.0100
31
0.0105
0.0142
0.0089
32
0.0097
0.0134
0.0080
33
0.0090
0.0071
34
0.0082
0.0063
35
0.0075
0.0056
NESTING In the design of a blanking die set, the first step is to prepare blanking layout. While doing so, the major consideration is the economy of the material. The different ways of arranging to blank the given work piece as shown. The arrangement at Figure a can be worked at single row, single pass with a single punch. For arrangement in Figure b, the strip either has to be fed twice, once for
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each row or double blanking will have to be employed. Figure c shows a single row, double pass strip. Here the strip will have to be passed through the dies once, turned over and passed through dies a second time.
Fig a
Fig b
Fig c
SCRAP BRIDGE (B)
W
BACK SCRAP (A)
H
T
W
B
FEED (S)
FRONT SCRAP (a)
B Y
L
Fig d
With reference to Fig d the distance between the blank and the edge of the strip, known as Back Scrap (A) is determined by the equation, A = T + 0.015 H The distance between successive blanks and also the scrap bridge (B) is given in table below:
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Material Thickness (mm)
B (mm)
0.8
0.8
0.8 to 3.2
T
Over 3.2
3.2
13
The feed or advance or the length of once piece of stock needed to produce one blank is: S = W + B The number of blanks which can be produced from one length of stock can be found out as, N = (L – B)/ S The scrap remaining at the end of one length of strip may be calculated from, Y = L – (N x S+B)
TOLERANCE AND FLATNESS VALUE OF STAMPED PARTS For lowest tooling price we can give plus or minus .005 tolerances between holes centers. For slightly higher tooling price we can give plus or minus .002 tolerances between holes centers. From 0” – 1” Flatness 0.005” T.I.R From 1”  4” Flatness 0.005”/inch T.I.R Over 4"  .020" plus or minus .004/inch of additional length, T.I.R.
T.I.R – Total Indicator Reading
The typical tolerance and size graph is shown below.
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KFACTOR KFactor is where the neutral axis is situated in the bend. It is signified as “k” in the development formulas. Since the inside compression cannot exceed the outside tension, the Kfactor can never exceed .50 in practical use. This means that the neutral axis cannot migrate paste the midpoint of the material (i.e. towards the outside). A reasonable assumption is that the Kfactor cannot be less than 0.25.
DIFFERENT BEND TYPES AND KFACTORS WRAPPED HEM (.29 KFACTOR)
MACHINE BEND WITH SET (.33 K FACTOR)
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MACHINE BEND WITH NO SET (.38 KFACTOR)
VBEND WITH BRAKE TOOL (.42KFACTOR)
ROTARY BENDERS (.43KFACTOR)
GRADUAL BENDS /LARGE RADII (.50 KFACTOR)
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DESIGN FOR MANUFACTURABILITY GUIDELINES BENDS Bends should be tolerance plus or minus onehalf degree at a location adjacent to the bends. For the ease of manufacturing, multiple bends on the same plane should occur in the same direction. Avoid large sheet metal parts with small bent flanges. In low carbon steel sheet metal, the minimum radius of a bend should be onehalf the material thickness or 0.80 mm (0.03 inch), whichever is larger. COUNTERBORES The minimum distance between two counter bores is eight times the material thickness. The minimum distance from a counter bore to an edge is four times the material thickness. The minimum distance from a counter bore to a bend is four times the material thickness plus the bend radius. COUNTERSINKS The maximum depth is 3.5 times the material thickness at an angle of the hardware. A minimum of 50% contact between the hardware and the countersink is required. The minimum distance between two countersinks is eight times the material thickness. The minimum distance from one countersink and an edge is four times the material thickness. The minimum distance from a countersink and a bend is four times the material thickness plus the bend radius. CURLS The minimum radius is two times the material thickness with an opening to a minimum of one material thickness. The minimum distance between a curl and the edge of a hole is the radius of the curl plus the material thickness. The minimum distance a curl should be from an internal bend is six times the material thickness plus the radius of the curl. The minimum distance a curl should be from an external bend is nine times the material thickness plus the radius of the curl. DIMPLES The maximum diameter should be six times the material thickness, and a maximum depth of onehalf the inside diameter.
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The minimum distance that a dimple should be from a hole is three times the material thickness plus the radius of the dimple. The minimum distance that a dimple should be from the edge is four times the material thickness plus the inside radius of the dimple. The minimum distance that a dimple should be from a bend is two times the material thickness plus the inside radius of the dimple plus the radius of the bend. The minimum distance between one dimple and another is four times the material thickness plus the inside radius of each dimple. EMBOSSMENTS The maximum depth is proportional to the internal radius or material thickness. The maximum depth for a flat embossment is equal to the internal radius plus the external radius. The maximum depth for a V embossment is equal to three times the material thickness. EXTRUDED HOLES The minimum distance between two extruded holes is six times the material thickness. The minimum distance from an extruded hole to an edge is three times the material thickness. The minimum distance from an extruded hole to a bend is three times the material thickness plus the bend radius. FLANGES The minimum height of a bent flange is directly related to the material thickness, bend radius, and length of bend. The minimum width of a bend relief is one material thickness or 1.50 mm (0.06 inch), whichever is greater. GUSSETS The width and depth, recommended at an angle of 45 degrees, is directly proportional to the radius and material thickness. The minimum distance that a gusset should be from the edge of a hole in a parallel plane is eight times the material thickness plus the radius of the gusset. HEMS The minimum diameter of a teardrop hem is equal to the material thickness, with a return flange height equal to or greater than four times the material thickness, and a minimum opening of 1/4 of the material thickness.
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The minimum diameter of an open hem is equal to the material thickness with a return flange height equal to or greater than four times the material thickness. The minimum return flange height of a closed hem is equal to or greater than four times the material thickness (the diameter is zero). NOTE: Closed hems tend to fracture at the bend and cause entrapment of solutions during the finishing process. The minimum distance from a hole to a hem is two times the material thickness plus the radius of the hem. The minimum distance a hem should be from an internal bend is five times the material thickness. The minimum distance a hem should be from an external bend is eight times the material thickness. HOLES The minimum diameter of a hole should be equal to the materials thickness or 1.00 mm (0.04 inch), whichever is greater. The minimum distance between holes is directly proportional to the size and shape for the hole feature and the material thickness The minimum distance the edge of a hole should be from a form is three times the material thickness plus the form radius. The minimum distance the edge of a hole should be from a bend is two times the material thickness plus the bend radius. The minimum distance between a hole and the edge of the material is directly proportional to the size and shape of the hole and the material thickness. The minimum distance between the leading edge of a hole through a bend should be equal to the thickness of material plus the bend radius or two times the material thickness, whichever is greater. LANCES The minimum width of an open lance is two times the material thickness or 3.00 mm (0.125 inch), whichever is greater, with a maximum length of five times the width. The minimum width of a closed lance is two times the material thickness or 1.60 mm (0.06 inch), whichever is greater, and a maximum height of five times the material thickness at a 45degree angle. The minimum distance from a lance to a bend in a parallel plane is eight times the material thickness plus the radius of the bend. The minimum distance from a lance to a bend in a perpendicular plane is ten times the material thickness plus the radius of the bend. The minimum distance from a lance to a hole is three times the material thickness.
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NOTCHES The minimum width is equal to the material thickness or 1.00 mm (0.04 inch), whichever is greater. The maximum length for a straight/radius end notch is equal to five times the width. The maximum length for a V notch is equal to two times the width. The minimum distance between a hole and the edge of a notch is directly proportional to the size/shape of the hole and the material thickness. The minimum distance from a notch to a bend in a parallel plane is eight times the material thickness plus the radius of the bend. The minimum distance from a notch to a bend in a perpendicular plane is three times the material thickness plus the radius of the bend. The minimum distance beyond the bend on the side edge is equal to the thickness of the material plus the bend radius, or two times the material thickness, whichever is greater. The minimum distance between two notches is two times the material thickness or 3.200 mm (0.125 inch), whichever is greater. RIBS The maximum inside radius is equal to three times the material thickness, with a maximum depth of the inside radius. The minimum distance from a center line of a rib to the edge of a hole is three times the material thickness plus the radius of the rib. The minimum distance a rib should be from an edge in a perpendicular plane is four times the material thickness plus the radius of the rib. The minimum distance a rib should be from an edge in a parallel plane is eight times the material thickness plus the radius of the rib. The minimum distance a rib should be from a bend perpendicular to the rib is two times the material thickness, plus the radius of the rib, plus the radius of the bend. The minimum distance between two parallel ribs is ten times the material thickness plus the radii of the ribs. SEMIPIERCED HOLE The minimum distance from a semipierced hole and a form is three times the material thickness plus the form radius. The minimum distance from a semipierced hole and a bend is two times the material thickness plus the bend radius. The minimum distance between semipierced holes is eight times the material thickness. SLOTS The minimum width of a slot is equal to the material thickness or 1.00 mm (0.04 inch), whichever is greater. The minimum distance from the inside surface of a bend to the edge of a slot is directly proportional to the length of the slot, material thickness, and radius of the bend.
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When using slots and tabs the maximum width of the slot must be greater than the thickness of the tab and the tab length should equal the material thickness. TABS The minimum width is equal to two times the material thickness or 3.200 mm, whichever is greater, while the maximum length is five times the width. The minimum distance between tabs is equal to the material thickness or 1.00 mm (0.04 inch), whichever is greater.
WELDING Spot welding should be restricted to joining coplanar surfaces. The minimum distance between welds is 10 times the material thickness. Using 20 times the material thickness is ideal. The minimum distance between a weld and the edge is two times the diameter of the spot weld. The minimum distance from a weld to a form is the spot diameter plus the bend radius. Use PEMs instead of threaded inserts. PLATING Outside sharp corners receive twice as much plating as flat surfaces. Allow for pitch diameters for screw threads, which can increase four times the plating thickness. Tapped holes may need to be retapped after plating to ensure accuracy. Projections accumulate more plating than other areas. Recessed areas may be difficult to plate, resulting in little or no coverage. Lapwelded joints trap plating solutions. One solution is to raise welds on embossed areas by 0.015 in. (0.3 mm) to allow for flushing and blow drying between the surfaces. Masking of stampings and fabrications to anodize certain areas is not recommended. Design drain holes/vent holes for plating solutions and rinsing. Design tabs/holes for attachment to part racks.
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REFERENCES: www.sheetmetaldesign.com www.engineersedge.com www.efunda.com www.sheetmetalworld.com www.daytonrogers.com BOOKS:
Sheet metal handbook, by Ron Fourier and Sue Fourier. Working sheet metal, by David j. Gingery. Advanced sheet metal fabrication, by Tim Remus. Sheet metal fabrication, by Jack Rudman. Sheet metal fabrication basics, by Timothy Remus. Mechanics of sheet metal forming, by Jack Hu. Sheet metal forming, by Roger Pearce. Sheet metal forming process and die design, by Vukota Boljanovic. Drawing Requirements Manual, by Jerome H. Lieblich
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