Diameter range is the most notable difference between collets and chucks. Generally collets are used on smaller work pieces than typical three-jaw chucks. The general range for collets are 1/16in to 2.5in whereas chucks can be used on work pieces that are larger.

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. While the initial ER system is based in metric, there are ER collets available with imperial sizing as well.

A multi chuck is used to easily and quickly change accessories on your Dremel Multi Tool without using collets. This will save you valuable time in the workshop and save you from buying a collet for each differently sized accessory.