Collet

A collet is a split, tapered sleeve that grips a router bit's or spindle's shank when tightened, distributing clamping pressure evenly around the shank's circumference rather than pinching from just one or two points the way a simple set-screw chuck would. Router collets are the most commonly encountered example, sized to standard shank diameters (1/4" and 1/2" being the two most common), and a worn, damaged, or debris-fouled collet is one of the most common real causes of blade runout and bits working loose mid-cut.

Keeping a collet clean and free of resin buildup, and not over-tightening it (which can deform the sleeve over time), is basic router maintenance that directly affects both cut quality and bit safety.