Precise ER 20 - 6 mm ER Collet - ER 20 Collet is made from alloy spring steel and is fully hardened. The surface of the ER collet is of precision ground for an accurate fit. It works in all ER20 Collet chucks. It has a full .039 range of collapse while maintaining full accuracy. The tool has a range of .236-.197 inches. It is ideal for many machining applications like boring, milling, reaming, tapping and grinding.