A mandrel is a tapered, cylindrical rod – typically made from hardened steel or aluminum – used by jewelers to shape, resize, and measure rings. Graduated markings along the taper correspond to standard ring sizes, allowing a jeweler to determine the size of any ring by slipping it onto the rod and reading where it stops. It is one of the most essential tools at any jewelry bench, present in virtually every repair shop that handles ring work.
The mandrel serves several functions during repair and fabrication. When resizing a ring up, the jeweler stretches the band over the mandrel and works the metal down the taper using a rawhide or plastic mallet, gradually expanding the ring to the target size. For reshaping a bent or distorted ring, the mandrel acts as a form: the ring is placed over it and gently tapped back into a true round. The steel surface of the mandrel provides the resistance needed to move the metal without marring the finish.
Any ring repair that involves changing the shape or size of the band will pass through a mandrel at some point in the process. Ring sizing, reshaping after bending, and rounding out a ring that has gone oval from wear all depend on it. The quality of the final result is directly tied to how carefully the jeweler works the metal against the mandrel – rushing the process can introduce stress or leave the ring slightly out of round.