. Made of premium carbon steel, straight shank with clamping nut to be tightened. Extension rod diameter and the other parts are precision ground to eliminate unbalance, increase safety when cutting. Widely used in engraving, milling, drilling, grinding and other processing. Installation: shaft diameter is 0.02mm smaller. Through the 300 high-frequency heating, thermal expansion and contraction principle, the hole will be heated to open, and then put the spindle into the lock top wire. After cooling, the extension rod is completely fixed . Specification: Material: Carbon Steel Size: ER20A 8mm Package Includes: 1 Piece Collet Chuck Motor Shaft Extension Rod Holder Note: 1.Please allow a slight difference between the size from the above and the actual item due to manual measurement. 2.Due to the difference between different monitors,there could be a little color difference acceptable, please kindly note.