Machinery's Handbook, 31st Edition
Blanking and Punching 1413 where k = a coefficient that depends on the shear strength of the material and the thickness of the material
F = deformation force (lb) T = material thickness (in.)
Table 7. Values for Coefficient k for Some Materials
Material Thickness, inch (mm)
Shear Strength lb/in 2 (MPa) 35,000–50,000 (240–345) 50,000–70,000 (345–483) 70,000–95,000 (483–655)
< 0.040 in. < (1.0 mm) 0.70–0.65 0.60–0.55 0.45–0.42
> 0.157 in. > 4.0 mm 0.45–0.35 0.40–0.30 0.30–0.20
0.078–0.157 in. (2.0 − 4.0 mm)
0.040 − 0.078 in. (1.0 − 2.0 mm)
Material
Low carbon steel
0.58–0.50 0.49–0.42 0.36–0.32 0.64–0.55
0.64 − 0.60 0.54–0.50 0.41 − 0.38 0.70–0.65
Medium carbon steel 0.20 to 0.25% carbon Hard steel 0.40 to 0.60% carbon Copper, annealed
21,000 (145) 0.50–0.40 Stripper Force.— Elastic Stripper: When spring strippers are used, it is necessary to calculate the amount of force required to effect stripping. This force may be calculated by the following equation: (23a) where F s = stripping force (lb) P = sum of the perimeters of all the punching or blanking faces (in.) T = thickness of material (in.) (23b) where F s = stripping force (N) P = sum of the perimeters of all the punching or blanking faces (mm) T = thickness of material (mm) This formula has been used for many years by a number of manufacturers and has been found to be satisfactory for most punching and blanking operations. 0.75–0.69 After the total stripping force has been determined, the stripping force per spring must be found in order to establish the number and dimensions of springs required. Maximum force per spring is usually listed in the manufacturer’s catalog. The correct determined force per spring must satisfy the following relationship: (24) where F max = maximum force per spring, lb (newton) F SO = stripping force per spring, lb (newton) F s = total stripping force, lb (newton) n = number of springs Blanking Pressure.— When designing parts that are to be blanked in a press, it is often necessary to work out the pressure required. A press of sufficient tonnage, providing the necessary blanking force, must be used. Otherwise, the press is likely to stall. To calculate the pressure required in tons for blanking various materials, use the following formula: P = A × T × S where . PT PT 000117 1 855 = F s = . F PT 59 s = F F n F max so s 2 2
P = pressure required (tons) A = perimeter of blank (in.) T = thickness of material (in.) S = shear strength of material (tons per in 2 )
Example: Calculate the pressure required in tons for the blank illustrated by Fig. 11. Soft sheet brass is to be used with a thickness of 0.125 inch and a shear strength of 17 tons per square inch.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online