Precise ER 20 - 5 mm ER Collet - Precise ER 20 is a standard ER collet most widely used clamping system in the world. It is made up of alloy spring steel and fully hardened this ER collet. It is ideal for many machining applications such as boring, milling, reaming, tapping, grinding. 7mm ER Collet can also be used for holding a work-piece or a tool. Also, it has good flexibility and abrasion resistance.