When using a large lathe with 5C, ER or other large collets it is sometimes helpful to have a way to change to a small chuck quickly. These adapters have either a 1/2" shank (1066) or a 5/8" shank (1065). On the other end is a 3/4-16 thread that accepts any Sherline 2.5" or 3.1" chuck. This allows you, without changing your spindle or going to another machine, to quickly install a Sherline 3- or 4-jaw chuck. Just insert the shank of the adapter attached into a 1/2" or 5/8" collet. Now you can chuck up parts that could not be easily held in a collet. T.I.R. of the adapter is specified to be 0.0005" (0.013mm) or less. The unhardened steel adapters have a black oxide finish to prevent rust.