What is "triblet"

Triblet \Trib"let\, Tribolet \Trib"o*let\, n.] [F. triboulet.]

1. A goldsmith's tool used in making rings.
   --Ainsworth.

2. A steel cylinder round which metal is drawn in the process of forming tubes.
   --Tomlinson.

3. (Blacksmithing) A tapering mandrel.

n. A tapering cylinder used in metal crafting, usually made of steel, which is used to expand or measure a ring or hole in another piece of metal up to a desired size.