Quarter bending press

A fixture designed for cleanly bending US quarters on a benchtop arbor press. Built for a Stanford fixture-design exercise, a constrained problem with all the texture of real machine-shop work.

The challenge was force and geometry. A quarter is a cupronickel-clad copper sandwich, roughly 1.75 mm thick and 24.26 mm in diameter. Bending it cleanly requires localized force in the kilonewton range without cracking the cladding or galling the die. I worked the force budget from the yield strength and section modulus of the coin, designed a die geometry that produced a controlled fold without strain concentrations at the corners, and machined the fixture from tool steel to fit a standard one-ton arbor press.

The surprise was how much the fixture geometry mattered relative to the press itself. The bench press has more than enough tonnage; the limit was how cleanly the coin could be constrained during the bend. Get the support geometry right and the fold is repeatable across many trials. Get it slightly off and the coin cracks at the cladding interface, slips out of the die, or curls unpredictably. Full report linked below.