Made of special elastic steel material:65Mn USed for boring,milling,drilling,etc... ER Collets, full sizes from ER8 till ER50, high precision, made from 65Mn.ER Collets, detailed sizes are as below: ER89PCS1-5mm (1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5mm)ER115PCS3-7mm(3;4;5;6;7mm) if 7PCS,includes 1&2mmER168PCS3-10mm(3;4;5;6;7;8;9;10mm)ER206PCS3-13mm(3;4;6;8;10;13mm)11PCS3-13mm(3;4;5;6;7;8;9;10;11;12; ...