Blanking and Punching Machinery's Handbook, 31st Edition
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d p
c
D d
Fig. 9. Punch and Die Clearance Where Clearance is Applied: Whether clearance is deducted from the dimensions of the punch or added to the dimensions of the die opening depends upon the nature of the work piece. In the blanking process (a blank of given size is required), the die opening is made to that size and the punch is made smaller. Conversely, in the punching process (when holes of a given size are required), the punch is made to the dimensions and the die opening is made larger. Therefore, for blanking, the clearance is deducted from the size of the punch, and for piercing the clearance is added to the size of the die opening. Value for Clearance: Clearance is generally expressed as a percentage of the material thickness, although an absolute value is sometimes specified. Table 4 shows the value of the shear clearance in percentages, depending on the type and thickness of the material. Table 4. Values for Clearance as a Percentage of the Thickness of the Material
Material Thickness, T
Material
< 0.040 in. (< 1.0 mm)
0.040-0.080 in. (1.0-2.0 mm)
0.082-0.118 in. (2.1- 3.0 mm)
0.122-0.197 in. (3.1-5.0 mm)
0.200-0.275 in. (5.1-7.0 mm)
Low carbon steel Copper and soft brass Medium carbon steel 0.20% to 0.25% carbon Hard steel, 0.40% to 0.60% carbon Hard brass
5.0 5.0 6.0 6.0 7.0
6.0 6.0 7.0 7.0 8.0
7.0 7.0 8.0 8.0 9.0
8.0 8.0 9.0 9.0
9.0 9.0
10.0 10.0
10.0 12.0 Table 5 shows absolute values for the blanking and punching clearance for high-carbon steel (0.60% to 1.0% carbon) depending on the thickness of the work material. Table 5. Absolute Values of Clearance for Blanking and Punching High-Carbon Steel Material Thickness, T Clearance, c Material Thickness, T Clearance, c (in.) (mm) (in.) (mm) (in.) (mm) (in.) (mm) 0.012 0.3048 0.00006 0.001524 0.157 3.9878 0.0095 0.24130 0.020 0.5004 0.0009 0.02286 0.177 4.4958 0.0116 0.29464 0.032 0.8001 0.0013 0.03302 0.197 5.0038 0.0138 0.35052 0.040 1.0160 0.0016 0.04064 0.236 5.9944 0.0177 0.44958 0.047 1.1938 0.0020 0.05080 0.275 6.9850 0.0226 0.57404 0.060 1.5240 0.0026 0.06604 0.315 8.0010 0.0285 0.72390 0.078 1.9812 0.0035 0.08890 0.394 10.0076 0.0394 1.00076 0.098 2.4892 0.0047 0.11938 0.472 11.9888 0.0502 1.27508 0.118 2.9972 0.0059 0.14986 0.590 14.9860 0.0689 1.75006 0.138 3.5052 0.0077 0.19558 0.748 18.9992 0.0935 2.37490 Effect of Clearance: Manufacturers have performed many studies on the effect of clear ance on punching and blanking. Clearance affects not only the smoothness of the fracture, but also the deformation force and work. A tighter blanking and punching clearance generates more heat on the cutting edge and the bulging area tightens around the punch. These effects produce a faster breakdown of the cutting edge. If the clearance increases, the
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