The jaws have to under pressure when you grind them. There are several methods that are used. The simplest is to clamp a washer at the very inside end of the jaws and then grind up to the washer. After completing the jaw grind, remove the washer and with a grinding disc grind off the raised edge where the washer was and cut a little deeper to be sure you don't have a high spot. This will shorten the effective length of the jaws by the thickness of the washer.