Machinery's Handbook, 31st Edition
1412 Blanking and Punching bulging area disappears and the roll-over surface is stretched and will retract after the slug breaks free. Less heat is generated with increases in the blanking and punching clearance, and the edge breakdown rate is reduced. The deformation force is greatest when the punch diameter is small compared to the thickness of the work material. In one example, a punching force of about 142 kN was required to punch 19 mm holes into 8 mm mild steel when the clearance was about 10 percent. With a clearance of about 4.5 percent, the punching force increased to 147 kN, and a clearance of 2.75 percent resulted in a force of 153.5 kN. Die Opening Profile.— Die opening profiles depend on the purpose and required toler ance of the workpiece. Two opening profiles are shown in Fig. 10a and Fig. 10b.
Fig. 10a. Opening Profile for High Quality Part Fig. 10b. Opening for Low Accuracy Part The profile in Fig. 10a gives the highest quality workpiece. To allow a die block to be sharpened more times, the height h of the block needs to be greater than the thickness of the workpiece. The value of h is given in Table 6 . The die opening profile in Fig. 10b is used for making a small part with low accuracy from very soft material, such as soft thin brass. The angle of the cone α = 15 ′ to 45 ′ . Table 6. Value of Dimension h Based on Material Thickness Work material thickness, T
< 0.04 in. ( < 1 mm)
> 0.04 to 0.2 in. ( >1 to 5 mm ) 0.26 in (6.5 mm)
> 0.2 to 0.4 in. ( > 5 to 10 mm) 0.45 in (11.5 mm)
Height h
0.14 in (3.5 mm)
Angle α = 3 ° to 5 ° Deformation Force, Deformation Work, and Force of Press.— Deformation force F for punching and blanking with flat face of punch is defined by the following equation: (20) where F = deformation force, lb (N) . F LT LT UTS 08 m τ = = ^ h
L = the total length of cutting, in. (mm) T = thickness of the material, in. (mm) τ m = shear stress, lb/in 2 (MPa) UTS = the ultimate tensile strength of the work material, lb/in 2 (MPa)
Force of Press: Such variables as unequal thickness of the material, friction between the punch and workpiece, or dull cutting edges, can increase the necessary force by up to 30 percent, so these variables must be considered in selecting the power requirements of the press. That is, the force requirement of the press, F p is (21) The blanking and punching force can be reduced if the punch or die has bevel-cut edges. In blanking operations, bevel shear angles should be used on the die to ensure workpiece remains flat. In punching operations, bevel shear angles should be used on the punch. Deformation Work W for punching and blanking with flat face of punch is defined by the following equation: (22) W kFT = . F F 13 p =
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