3.1″ 4-Jaw Self-Centering Chuck Body- ER-16 Thread
This chuck will automatically center square or round stock. (The stock must be accurately shaped for all four jaws to grip.) It will also grip thin-wall tubing in four places rather than three, which spreads out the load and allows more grip without crushing the tubing.
Centering accuracy on self-centering chucks is to within a few thousandths of an inch. If perfect centering accuracy is required using an independent chuck is recommended.