ER40 series collets have a capacity of .094-1.024 inches. Each ER40 collet has a range of .039 inches(1mm). The size indicated on the collet is the largest size it can hold and can be collapsed smaller within its collapse range mentioned above. For example, ER40-1/4 can grab a round shank from .250 diameter to .211 diameter. Please refer to our ER40 Collet print for general ER40 dimensions.