Precise ER 20 - 3 mm ER Collet - Precise ER collet is made from alloy spring steel and fully hardened covering. It is made up of many machining applications that include boring, milling, reaming, tapping and grinding. It is used for holding cylindrical shank tools without flats. The ER collet has high durability and can clamp for a specific standard size and its next size. 3mm er collet has a special angled slot that improves the rigidity.