The 3-jaw chuck is the easiest chuck to use. It can hold round and hex shaped materials. Unlike the 4-jaw chuck, the jaws on the 3-jaw chuck all move at the same time allowing the user to load and unload parts very quickly. Soft jaws are easy to bore out, thereby making the chuck run very concentric to the spindle.
When using 3 jaw chucks the operator should bore out the jaws to enable the chuck to run concentric to the spindle. To bore out the jaws you put a round washer in the back of the jaws (washer touching chuck body) and tighten down on it. You then take light cuts on the jaws with a boring bar to clean up the jaw tips. When done there will be a small protrusion on the jaws where the washer was. This can be removed with a file. Once bored out, the chuck should run concentric to the lathe.
Note: Does not fit the CNC lathes with ER16 headstock.