Workholding on the Lathe: Three Jaws, Four Jaws, and Which to Reach For

Before a lathe can cut anything accurately it has to hold the work in a known place, and every accuracy problem that follows is really a holding problem wearing a disguise. Work that runs out by a tenth of a millimetre cannot be turned to a hundredth, no matter how carefully the cut is taken. There are only a few ways to hold round work, and they trade speed against truth. A three-jaw chuck grips in one movement and is never quite concentric. A four-jaw is adjusted one jaw at a time, takes minutes, and can be dialled truer than most people can measure. Collets are truer still and only fit one size each. Knowing which to reach for is most of the skill, and it is decided by the tolerance on the drawing rather than by preference.