ER8 series collets have a capacity of .020-.197 inches. Each ER8 collet has a range of .020 inches(0.5mm). The size indicated on the collet is the largest size it can hold and can be collapsed smaller within its collapse range of .020inches(0.5mm). For example, ER8-3mm can grab a round shank from 3mm diameter to 2.5mm diameter. Please refer to our ER8 Collet print for general ER8 dimensions.