Jacobs Die Grinder Chuck The Jacobs Die Grinder Chuck for 3/8-24 Mount Model 100-61 is a compact tap chuck that is designed for use with Rubber-Flex® collets holds mounted grinding burrs and rotary files on pneumatic and electric die grinders. The Jacobs Die grinder chuck features a compact design that facilitates use for a wide range of applications, and an integral seal that protects collet components from abrasives and swarf.