Discussion
Calculate the punch size in mm, for a circular blanking operation for which details are given below.
| Size of the blank | 25 mm |
| Thickness of the sheet | 2 mm |
| Radial clearance between punch and die | 0.06 mm |
| Die allowance | 0.05 mm |
Answer: A
The punch size is 24.83 mm.
In blanking, the blank takes the size of the die opening, so the die is sized from the blank and the clearance is taken off the punch. The die allowance is taken off first:
\[ \begin{aligned} D_{die} &= 25 - 0.05 = 24.95\ \text{mm} \\ D_{punch} &= 24.95 - 2(0.06) = 24.83\ \text{mm} \end{aligned} \]
The radial clearance is subtracted twice because it applies on both sides of the diameter. Subtracting it once gives 24.89 mm. Adding the allowance and the clearance instead, which gives 25.17 mm, is the rule for piercing, where the punch, not the die, is sized from the hole. 25.01 mm follows from neither rule.
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