Standards of ER Collets The numbers represent the opening diameter of the tapered receptacle in millimeters. As you tighten the nut on your holder, the ER collet collapses together to hold round tools or stock from 0.5-2 mm (or 0.0197-0.0787 in., depending on collet size) smaller than the collet internal diameter.

3C collets have a size range of 1/16" to 1/2". We have 3C collet closers with adapters for most lathes with a 3MT spindle bore. If you have a production job using small diameter material, they may be a good solution. You can pass material up to 7/16" through the collet and drawtube so you can use long material.

The collet should be selected to match the tool's shank diameter precisely. Using the correct collet size ensures a secure grip and prevents tool slippage during machining. Consider the diameter of the workpiece you'll be machining. The collet size should be suitable for securely holding the workpiece in place.