Collet sizes: 1/16", 1/8" 3/16" 1/4" 5/16", 3/8". The collet set is used for chucking small diameter rods of the diameters listed. The collet set allows speed and accuracy in those ranges. Stock can be fed through the headstock into the collet for production work. They are also used for holding end mills burrs and router bits in milling operations.