formulas for designing press tools

THEORY OF SHEARING • Shearing is the method of cutting sheets or strips without forming chips. • The material is stressed in a section which lies parallel to the forces applied. • The forces are applied by means of shearing blades or punch and die. Critical stages in shearing 1. Plastic deformation. 2. Penetration. 3. Fracture. 1. Plastic deformation: The pressure applied by the punch on the stock material tends to deform it into the die opening when the elastic limit is exceeded by further loading, a portion of the material will be forced into the die opening in the form of an embossed on the lower face of the material and will result in a corresponding depression on its upper face. This stage imparts a radius on the lower edge of the punched out material. This is called the stage of “plastic deformation”.

2. Penetration stage: As the load is further increased, the punch will penetrate the material to a certain depth and force an equally thick portion of metal into the die. This stage imparts a bright polished finish on both the strip and the blank or slug. This is “penetration stage”.

3. Fracture stage: In this stage, fracture will starts from both upper and lower cutting edges. As the punch travels further, these fractures will extend towards each other and eventually meet, causing complete separation. This stage imparts a dull fractured edge. This is the “fracture stage”.

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1. Press Force calculation: The essential considerations are: • Cutting force • Stripping force • Ejection force 1. Cutting force “Cutting force is the force applied on the stock material in order to cut out the blank or slug”. This determines the capacity of the press to be used for particular tool. The area to be cut is found by multiplying the length of cut by stock thickness. Cutting force (F) = L x S x T max L = Length of periphery to be cut in ‘mm’. S = Sheet thickness in ‘mm’ T max = Shear strength in N/mm2 Shear and tensile strengths for most materials are not the same. Shear strength for: Aluminum is approximately 50’% of its tensile strength Cold roll steel is approximately 80% of its tensile strength Stainless steel is approximately 90% of its tensile strength 2. Stripping force The main purpose of a stripper is to the part material from the ends of the punches. This function occurs at the Withdrawal phase of the cutting process. Stripping force varies based on part material type and thickness as well as punch to die clearance. Most applications do not exceed 10% of the cutting force. If the die has more than one punch the stripping force for that die is the sum of stripping forces required for each punch. Striping force = 10% - 20% of cutting force (F) Movement of stripper Ystr = t + 2 Where Y sIr = Movement of stripper t= Thickness of stock Spring deflection (Y) Y = (3 to 4) Y str = (3 to 4) (t+2) Where Y = Total spring deflection at F max load 3. Ejection force The force required to eject the component from the punch. Ejection force = 10% cutting force (F) Press force = Cutting force + stripping force The following table gives the shear strength (T max = 0.2 for tensile strength σ max ) of several materials.

Material

T max in N/mm2

Steel with 0.1% carbon Steel with 0.2% carbon content (deep draw steel) Steel with 0.3% carbon

240 - 300 320 - 400 360 - 420

2

Steel with 0.4% carbon Steel with 0.6% carbon Steel with 0.9% carbon Silicon steel Stainless steel Copper Brass Bronze German silver (2 - 20% Ni, 45 - 75% Cu) Tin Zinc Lead Alluminium 99% pure Alluminium manganese alloy Alluminium silicon alloy Paper & card board Hard board Laminated paper or rosin impregnated paper Laminated fabrics Mica Plywood Leather Soft rubber Hard rubber Celluloid

450 - 560 550 - 700 700 - 900 450 - 550 350 - 450 200 - 400 350 - 400 360 - 450 300 - 20 30 - 40 100 - 120 20 - 30 20 - 120 150 - 320 120 - 250 20 - 50 70 - 90 100 - 140 90 - 120 50 - 20 20 - 40 7 7 20 - 60 40 - 60

2. Cutting clearance: It is the small amount of gap maintained between the side of the punch and the corresponding die opening on one side of the edge, when punch is entered in to the die opening. So the cutting clearance should expressed as the amount of clearance per side Clearance for sheet thickness up to 3 mm

√ T 10max

cxsx Clearance for sheet thickness above 3 mm

√T10max

(1.5 x s) x (s-0.015) x

'C' constant = 0.005 or 0.01 as the case may be. T max, Shear strength 80% UTS. It is expressed in N/ mm2 If 'c' is 0.005 we get a clearance, which yields a better and cleanest work piece, but requires a higher cutting force and considerably more energy. If 'c' is 0.01, the cutting force energy as its minimum, but finish wil;l not b good. The usual practice 'c' will be conceded as 0.01 Ii is also expressed in terms of % of stock thickness (s) per side C=c x t Since the edge characteristics, dimensional accuracy and die life depends upon Clearance its value should be taken according to the requirements.

Material

I

II

Type of edge III IV

V 3

Steel (SAE 102O) Steel (High carbon) Stainless steel Copper (1/2 hard) Copper (annealed) Brass (1/2 hard) Phosphors bronze Lead Aluminum (Hard) Aluminum (250) Magnesium

21% 26 1/2% 22 1/2% 25% 26% 21% 25% 22% 20% 17% 16%

12% 18% 12.5% 11% 8.1/2% 9% 13% 9% 14 1/2% 9% 6%

9% 14 1/2% 10 1/2% 8% 6% 6% 11.5% 7% 10% 7% 4%

6 1/2% 12% 4% 3 1/2% 3% 3% 4 1/2% 5% 6% 3% 2 .5%

2% 4.5% 2% 2% 1% 1% 3 1/2% 2 '1/2% 1% 1% 1%

Type I - large dished radius large burr - only for structural rough work.Type II - large radius / Die roll. Normal burr Max, tool life for average sheet metal work. Type III - Normal Radius/die roll Burr free For Components to be formed later Type IV - Less radius Normal burr, Signs of secondary shear, for good quality components to be shaved, reamed, polished and also for close tolerances Type V - Negligible radius Normal burr, complete secondary shear Recommended for accuracy for soft materials And for hard materials the die life reduces considerably.

Recommended total shearing clearance, for precision stamping

Sheet thickness S in mm 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.0 1.2 1.5 1.8 2.0 2.2 2.5 2.8 3.0 3.5 4.0 4.5 5.0 6.0 7.0

High carbon Card board, steel, High Laminated leather, alloy steel plastics paper, hard, Brass, rubber and Bronze Clearance on both sides in microns 7 4 2 14 5 3 21 +10 6 +5 4 +3 28 8 5 35 10 6 42 12 8 49 14 9 56 +20 16 +10 10 +8 63 18 12 70 20 15 100 +30 24 +15 10 +12 120 30 22 140 36 27 160 40 30 200 +50 44 +25 40 +20 230 50 45 250 56 48 270 60 53 350 70 60 400 +100 200 +50 60 +20 540 90 60 600 100 60 700 1000 +200

Mild steel, Copper, Brass, Aluminum

med. carbon steel Duraluminium, Bronze

5 10 15 +10 20 25 30 35 40 +20 45 50 70 +30 80 110 120 160 +50 180 200 210 280 320 +100 360 400 500 700 +200

6 12 18 +10 24 30 36 42 48 +20 54 60 80 +30 110 130 140 180 +50 200 220 240 320 360 +100 450 500 600 900 +200

magnesium alloys

---- +10 -17 20 25 30 +10 34 35 42 +15 52 62 70 77 +25 90 98 105 122 77 +25 157 175 210 +50

4

8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0

800 1100 1200 1600 1700 +300 2100 2300 2700 2900 3400 +500 3600 4200 4400

1000 1300 1400 1800 1900 +300 2500 3000 3000 3200 3800 +500 4000 4600 4800

1100 1400 1600 2000 2200 +300 2800 3300 3300 3500 4100 +500 4300 5000 5200

Applying Clearance Given diagram illustrates how to apply clearance to obtain correct size of hole and blank. When the metal is punched out from the functional part and the metal around the opening is scrap, the die is made to desired part size and the clearance is subtracted from (applied to) the punch size as shown in figure A. when the slug is discarded and the punched opening is functional, the required clearance is applied (added to) the die opening as shown in figure B.

Determination of punch and die size Piercing Piercing punch = Pierced hole size Die = Hole size + total clearance.

Blanking Blanking punch = Blank size-total clearance Die = Blank size

Land and angular clearance. To avoid brakeage of cutting edge of the die plate die walls are kept straight only to a certain amount from the cutting edge. The straight wall is called “The Land.” An amount of 3mm land for stock thickness up to 3mm and the thicker materials equal to their thickness has proved to be good practice. The die wall below the land is relieved at an angle for the purpose of enabling the blanks or slugs to clear the die. Generally, soft materials require greater angular clearance than hard materials. Soft thicker materials above 3mm require more angular clearance. An angular clearance of 1.50 per side will meet the usual requirements Pitch punches (side cutters) Allowance for stock cut off at the side of the strip Stock thickness t (mm)

4t,

k = 0.33t k = 0.4t k = 0.5t

Calculation for R max and R min. In order to obtain a permanent set the stress which occurs on bending must be higher than yield point of the material. The above formula therefore gives the condition for R max. Radius which produces a permanent. R max could be calculated by following formula. Rmax R max = SE 2σy R min could be calculated by following formula. Rmin

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R min = C 3 S S = sheet thickness C = Constant referred to the following table. If ri is greater than the R max, no permanent deformation takes place. 01. Mild steel 1.5 02. Deep drawing tool 0.5 03. Construction steel 2.0 04. Copper 0.27 05. German Silver 0.45 06. Brass 0.4 07. Aluminum hard 0.4 08. Aluminum pure 0.7 09. Aluminum half hard 1.4 10. Gun Metal 1.2 11. Stainless Steel 0.5 12. Brass 0.3 Bending Force Bending force for Edge bending or Wiping die.

2

Bending force Fb: 0.33 x Su x W x t L W = width of stock 2

Su= ultimate tensile strength. (Kg/mm ) L = Span = rd + rp + c C = Die clearance. rd= die radius rp=punch radius. Pad force: Fp = 0.5 x Fb Total force: Fn = Fb + Fp

U - Bending or channel bending:

Force required Fs = 2 times force required for edge bending. 12

Bending force: 0.667 x Su x W x T2 L Pad force: FB = 0.4 x W x t x Su Total load: FN = 0.8 x W x t x Su V - Bending Force:

FB = C x Su x W x t2 L C=1+4t L L = Width of opening. The following formula is also frequently used for V- bending. FB = 1.2 x Su x W x t2

L Curling: Force required for curling Fc = 0.8 x Tmax x w x s2 4 x Rc x (1 - µ) Rc = radius of curling µ = co efficient of friction (0.05-0.1)

Off-setting or joggling: Force required for off setting is 3 times the bending force (90º bend) if off setting is 6t and more, 8-10 times the V – bending force (90º bend) if offset is less then 6t

Spring Back Degrees of spring back Degrees of bend

Material Aluminium 3003-0 CRCA (SAE 1008) BRASS (dead soft)70/30 Dead soft stain less steel

5 2.2 3.0 3.5 4.0

10 2.7 3.5 4.0 5.0

20 3.2 4.0 5.0 5.8

30 3.6 4.2 5.4 6.2

40 3.8 4.6 6.0 6.8

50 4.0 4.7 6.3 7.1

60 4.3 4.8 7.0 7.5

70 4.5 5.0 7.3 8.0

80 4.7 5.1 7.8 8.4

90 4.9 5.3 8.2 8.8

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9. Forming: Flanging

B = A + 5t for t < 1.2 mm 4 =A+t H = t where t > 1.2 mm H = 4t where t is 0.9 to 1.25 mm 5 H = 3t for t > 1.25 mm 5 R = t/4 for t < 1.20 mm = t/3 for t > 1.2 mm Pre pierced hole size. d=

Force required for direct piercing and flanging (with single stepped punch) Ff = (2-2, 5) π d t Ssh Force required for hole flanging after pre-punching the hole: Ff = (1,5-2) π d t Ssh Embossing/Beading Force for embossing Fe = Su t L L = height of embossing or bead Su = uts Bottoming force Fb=Sy A A= plan area of bottoming zone Sy=yield of strength

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Coining: Force of coining Fc=A Pc A = total area of deformed surface (mm²) Pc = surface pressure (Kg/mm²) Coining pressure p in kg/mm2 Kind of coining

Ultimate tensile stress Kg/mm²

Letter and pattern

Both sides

Light coining

99% of Al

8-10

5-8

8-12

5-8

Al alloy

18-32

15

35

14

Brass 63% Cu

29-41

20-30

150-150

20-30

Soft copper

21-24

20-30

80-100

10-25

Hard copper

--

30-50

100-150

--

Pure nickel

40-45

30-60

160-180

25-35

German silver

35-45

30-40

120-150

35-40

Steel

28-42

30-40

120-150

35-40

Stainless steel

--

60-80

250-320

60-90

Silver

--

--

150-180

Gold

--

--

120-150

Material

Heavy coining For depth mm Up to 0.4 0.4-0.7

1-12 6-10

--

20

Up to 0.4 0.4-0.7 Upto 0.4 0.4-0.7 Over0.7

100-120 70-100 100-120 70-100 60-80

--

--

Upto 0.4 0.4-0.7 Over0.7 Upto 0.4 0.4-0.7 Over0.7 Upto 0.4 0.4-0.7 Over0.7 Upto 0.4 0.4-0.7 Over0.7

100-150 70-90 60-80 120-150 100-120 70-100 180-250 125-160 100-120 220-300 160-200 120-150

--

--

--

--

--

--

Pc

Note: Pressures for 0.4 over 0.7 are the excess pressures given up to 0.4 Flattening (planishing) Force required for flattening Ff = A × P A = surface area of flattening portion P = surface pressure

Calculations for tool elements

Movement of stripper Ystr = t + 2 Where Y sIr = Movement of stripper t= Thickness of stock

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Spring deflection (Y) Y = (3 to 4) Y str = (3 to 4) (t+2) Where Y = Total spring deflection at F max load

Sharpening allowance 'S' is provided on the tools Y max = [(3 to 4) (t +- 2)] + S Rubber blocks: Shore hardness recommended 65-68 Possible Max. Deflection =40% of it’s original height. Force developed under this deflection 25-35 kg/cm2 Expected life of rubber blocks =b 3 lakh cycles where rubber blocks are used. Space between blocks should be more than 1.6D to allow for bulging.

Press Tool design check list

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Preliminary planning 1. Has the blank been developed with due regard to best grain direction, to stresses and Strains, involved and to the press working equipment to be used? 2. Are idle s stations needed in a planned-progressive die? 3. Check for required dimensional accuracy be realized from the planned stock strip layout? 4. Can the burr be so placed as to require no removal? 5. Is the correct side of the blank up with respect to any shaved portions? 6. Will any forming be done across the grain (optimum), or not to exceed 450 7. Is material utilization maximum? 8. Have proper provisions been made for clamping the die set to press? 9. Have the design feature been checked against the shut height of the closed die? 10. Have unavoidable delicate projections been designed as inserts, for easy replacement? 11. Has the centerline of pressure been properly established? 12. Have any pilot-hole punches been suitably located? 13. Has the final sequence of operations been thoroughly checked and established. Punch planning 14. Have any notching punches been located and, if needed, provide with heel blocks or other backup support? 15. Has it been determined whether shedder provision is needed on any forming punches? 16. Where small pierce or blank punches are to be grouped closely together, are they stepped to reduce total shearing pressure? 17. If punches must be used having more than about 4-in. unguided length, have spacers or filler plates been considered? 18. Have heel punch fillets been made as large as possible? 19. Have spanking punches, if any, been located at next-to-last station and, preferably, combined with bending or forming? Die plate and punch plate 20. Provided the intended service requires it, has the die block been Specified to be finished square on all sides 21 Have edges of die openings been designed a minimum distance of 1 to 11/2 times block thickness from outside edge of block 22 Have the punches and dies been designed sectional, where feasible, for easy construction, hardening, sharpening, and replacement? 23. Are any finger stops so located as to avoid cutting on only one edge of the die? 24. Will inserts and bushings be planned wherever needed to-facilitate die making, heat treatment, or easy Replacement of worn or broken sections? 25. Has a selected die set been checked for parallelism of mounting surfaces? For of guideposts in their bushings? 26. Have needed scrap cutters been suitably located? 27. Have adequate provisions been made for scrap disposal? 28. Has doweling been checked for sufficient size to withstand shearing action; for spacing far enough apart; for means of removal from blind holes; for advisable staggering to prevent miss assembly? 29. Is the punch plate sufficiently thick to support all punches adequately? 30. Have any necessary clearance holes in die block or stripper been checked for transport of blanks or slugs? 31. Have any hardened punches been designed to be mounted in a soft plug, rather than pressed directly into a hardened punch plate? General design details 32. If die setup are to be used, are they large enough for needed rigidity, and far enough apart? 33. Have any needed release or vacuum pins been checked as to location and action? 34. Have blank hole, scrap hole clearances been checked? 35. Have the sizes of all springs been calculated? 36. Have bushing decisions been checked as to need, location, and optimum length? 37 Has the planned piloting practice been checked as to removability to facilitate punch grinding, for adjustability, And to avoid miss feeds? 38 Have bolt heads in die plates been set sufficiently below the top surface to permit maximum die sharpening? 39 Have any necessary air vent holes been located? 40 Are stop or bumper blocks needed anywhere? 17

41 Has a thorough check been made to ensure safety to the operator, the die, and the press? Heat treatment.

• • •

Design is the sum total of many variables among which are geometry, mass, surface area, surface finish, material used, method of fabrication, and heat treatment. Heat treatment is the most severe operation any tool or die must go through and it is necessary that ease or safety in heat treatment be given every possible consideration when designing tools and dies. Some of the basic rules for design, directly related to heat treatment, are given in the following slides

1. Use sufficiently oversize stock to insure freedom from surface defects and decarburization after grade selection is made. 2. Generous fillets should be used whenever possible to minimize stress Concentration during heat treatment. 3. Avoid sharp re-entrant angles; also square inside corners. 4. Avoid thin-walled areas. Increase cross section in such areas if possible. 5. Avoid drastic changes in cross section. Use steps of taper whenever possible. 6. Use sectional dies if the design is considered to be hazardous 7. Avoid the use of blind holes when possible, because they tend to alter uniformity of cooling. 8. Use fillets at base of keyways to minimize stress concentration. 9. Avoid use of large masses. If design permits, incorporate a hole to facilitate

Spring Selection

Steps

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I n determining the length of a spring, it should be remembered that maximum delivered spring load is obtained by selecting longer springs. For best economy and saving of space, choose Light and Medium Load springs or the Heavy Load spring having a free length equal to six times the travel, or an Extra Heavy Load spring having a free length equal to eight times the travel. If ratios lower than these are used because of height limitations, the number of springs required will be substantially increased. Step 1 Estimate the level of production Required of the die - short run, constant production, etc. Step 2 Determine compressed spring length “H” and operating travel “T” from the die layout. Step 3 Determine free length “C” as follows: Decide which load classification the spring should be selected from -Light, Medium, Heavy, or ExtraHeavy Load. Then choose the figure nearest the compressed length “H” required by the die design from the appropriate charts of spring supplier. Read corresponding free length. Step 4 Estimate total initial spring load “L” required for all springs when springs are compressed “X” inches or millimeters. Step 5 Determine “X” (initial compression) by using the following formula: X = C-H-T Step 6 Determine “R” (total rate for all springs in N/mm) by using the following formula: R= L. X Step 7 Select springs as follows: 1. The free length “C” must comply with the length determined in Step 3. 2. Divide “R” in Step 6 by the number of springs to be used (if known) in order to get the rate per spring. Then refer to the spring supplier catalogs for springs having the desired rate. If the number of springs is not known, divide “R” from Step 6 by the rate of the spring you select for the correct number of springs.

Hardness conversion

19

Vickers ,HV

Rockwell, c

Brinell

Vickers ,HV

Rockwell, c

Brinell

20

940 920 900 880 860 840 820 800 780 760 740 720 700 690 680 670 660 650 640 630 620 610 600 590 580 570 560 550 540 530 520 51 0 500 490 480 470 460 450 440 430 420

68 67 67 66 65 65 64 64 63 62 61 61 60 59 59 58 58 57 57 56 56 55 55 54 54 53 53 52 51 51 50 49 49 48 47 46 46 45 44 43 42

767 757 745 733 722 710 698 684 680 656 647 638 630 620 611 601 591 582 573 564 554 545 535 525 51 7 507 497 488 479 471 460 452 442 433 425 41 5 405 397

410 400 390 380 370 360 350 340 330 320 310 300 295 290 285 280 275 270 265 260 255 250 245 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 95 90 85

41 40 39 38 37 36 35 34 33 32 31 29 29 28 27 27 26 25 24 24 23 22 21 20 18 15 13 11 9 6 3 0 -

388 379 369 360 350 341 331 322 313 303 294 284 280 275 270 265 261 256 252 252 243 238 233 228 219 209 200 190 181 171 162 152 143 133 124 114 105 95 90 86 81

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