A precision sleeve with lengthwise slits and a tapered outside, drawn or pushed into a matching taper so it closes evenly on a tool or part. Collets grip with better concentricity and less marking than jaw chucks over a narrow size range. Common systems include R8 for knee mill spindles, ER for tool holders and 5C for lathe and fixture work.
Run the half-inch endmill in an ER collet, not the drill chuck, it'll pull out and wreck the part.
Related terms
Endmill
A rotating cutter with teeth on its end and sides, the main tool for milling slots, pockets and profiles.
Four-jaw
Shop name for an independent four-jaw lathe chuck, each jaw adjusted separately to center or offset work.
Chuck key
The T-handle wrench that opens and closes a geared drill chuck or lathe chuck, never left in the chuck.
Bridgeport
Shop name for any vertical turret knee mill, after the brand that made the design standard.