an external collet is a sleeve with a (normally) cylindrical inner surface and a conical outer surface. the collet can be squeezed against a matching taper such that its inner surface contracts to a slightly smaller diameter, squeezing the tool or workpiece to hold it securely. most often this is achieved with a spring collet, made of spring steel, with one or more kerf cuts along its length to allow it to expand and contract. an alternative collet design is one that has several tapered steel blocks (essentially tapered gauge blocks) held in circular position (like the points of a star, or indeed the jaws of a jawed chuck) by a flexible binding medium (typically synthetic or natural rubber). regardless of the collet design, the operating principle is the same: squeeze the collet against the tool or workpiece to be held, resulting in high static friction and accurate alignment.