mandrel

Short definition from Wiktionary

Noun

1. 1
   A round object used as an aid for shaping a material, e.g. shaping or enlarging a ring, or bending or enlarging a pipe without creasing or kinking it.
2. 2
   A tool or component of a tool that guides, grips or clamps something, such as a workpiece to be machined, a machining tool or a part while it is moved.

Verb

1. 1
   To shape with a mandrel.