there are many types of collet used in the metalworking industry. common industry-standard designs are r8 (internally threaded for mills) and 5c (usually externally threaded for lathes). there are also proprietary designs which only fit one manufacturer's equipment. collets can range in holding capacity from zero to several inches in diameter. the most common type of collet grips a round bar or tool, but there are collets for square, hexagonal, and other shapes. in addition to the outside-holding collets, there are collets used for holding a part on its inside surface so that it can be machined on the outside surface (similar to an expanding mandrel). furthermore, it is not uncommon for machinists to make a custom collet to hold any unusual size or shape of part. these are often called emergency collets (e-collets) or soft collets (from the fact that they are bought in a soft (unhardened) state and machined as needed). yet another type of collet is a step collet which steps up to a larger diameter from the spindle and allows holding of larger workpieces.