The easiest way to determine what type of collet your machine has is to measure the length and diameter of the collet. All collets have a distinctive length and diameter.

They are an essential part of toolholding systems, ensuring that cutting tools are held firmly during machining operations. ER nuts are typically made from high-quality materials like steel or stainless steel to provide durability and reliability in machining environments.

As the collet is pushed into the pocket by the collet nut, the slots in the collet allow the I.D. bore to collapse and apply clamping pressure to the cutting tool shank. The result is a very strong and rigid clamping force on the cutting tool.

The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The "ER" name came from an existing "E" collet (which were a letter series of names) which Rego-Fix modified and appended "R" for "Rego-Fix". The series number is the opening diameter of the tapered receptacle, in millimetres.