The insert’s external thread fits into the back of the chuck. The internal thread matches that of your lathe’s headstock spindle. NOVA produces a wide range of inserts to fit many different, popular lathe models. The technical data PDF will help you choose the correct insert if you do not know for certain which one your lathe requires. The big advantage of this system is the flexibility it brings. You may be lucky enough to have more than one lathe in your workshop. If the lathes have different spindle sizes then the purchase of a second insert, means you can use the same chuck on both lathes. You won’t need to own two separate chucks. Similarly, if one day you change or upgrade your lathe, you only require a suitable insert to adapt your existing NOVA chuck.NOVA also produces directly threaded chuck bodies. You cannot alter them to an insert type body or another thread size once threaded, so it pays to be sure that your lathe thread size is not covered by the insert/adaptor first.