mandrel

kara 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.
A tool or component of a tool that guides, grip grips or clamp clamps something, such as a workpiece to be machined, a machining tool or a part while it is moved.
1920, Lester Gray French, Machinery, Volume 26, http://books.google.com.au/books?id=5olNAAAAYAAJ&q=%22mandrel%22&dq=%22mandrel%22&hl=en&ei=-Jq1TcK6Fo6IvgPL6YmgBw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CGMQ6AEwBQ page 491,

This socket forms the starting point of the piercing operation, enabling the mandrel to center itself on the work.

When inserted into the staple, the outside of the mandrel should fit the inside of the staple exactly.