Mandrel
1. A tapered or cylindrical axle or spindle which can be inserted into a hole in a piece of work to support it during machining.
2. A metal bar that serves as a core around which material (usually metal) may be cast, molded, forged, bent, or otherwise shaped.
3. The shaft and bearings on which a tool (as a circular saw) is mounted.
4. A miner’s pick.
According to the OED the word is usually believed to be an alteration of the French mandrin, which has the senses 1 & 2 above. However the French word has not been traced earlier than 1690 and is of obscure origin.

Unless one is a tool-maker or similar craftsman the most likely place most of us will encounter a mandrel is as the stick which a jeweller uses to size rings.