1.Made of special elastic stell material 2.power clamping ability,precise accuracy 3.used for boring,milling ,drilling,etc. OZ257PCS: 1/4,5/16,3/8,1/2,5/8,,3/4,1”15PCS:1/8,3/16,1/4,5/16,3/8,7/16,1/2,9/16,5/8,11/16,3/4,13/16,7/8,15/16,1”SET OZ257PCS: 6,8,10,12,16,20,2515PCS: 3,4,5,6,7,8,10,11,12,13,16,18,20,25SET