Sheet Metal Working Questions and Answers

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.

Answer: A

Earing is associated with deep drawing.

When a circular blank of sheet metal is deep drawn into a cup, the rim often develops wavy peaks and troughs called ears. They arise from planar anisotropy of the sheet: its plastic strain ratio differs with direction relative to the rolling direction, so the metal flows unevenly and the cup wall ends up taller in some directions. The ears must then be trimmed off, wasting material.

Rolling creates the anisotropy (through crystallographic texture) but does not itself produce ears. Extrusion and forging do not involve drawing a flat blank into a cup.

Answer: C

Leak tightness of a riveted joint is improved by fullering.

Fullering is done with a tool whose face is as wide as the plate edge: hammered along the edge of the plate, it presses the whole edge down against the plate below and closes the seam. With caulking, which bevels and drives down just the edge, it makes boiler and tank joints steam- and water-tight.

Shot peening induces compressive stress in a surface for fatigue strength. Bucking is holding a bucking bar against the rivet tail while the head is driven. Spot facing machines a flat seat around a hole for a bolt head or nut. None seals the joint.