ER Collet Basics — Everything You Should Know to Get Started

Since their introduction in 1973, ER collets have become a staple in machine shops everywhere. Whether you need to grip tooling or hold round stock, ER collets are a go-to, industry-standard solution for general machining work.

Standards and Series of ER Collets

ER collets are produced in a common series that typically includes ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The model number corresponds to the diameter (in millimeters) of the tapered receptacle that receives the collet. As the nut is tightened on the holder, the collet contracts and clamps round tools or stock down to a diameter that is between 0.5 mm and 2 mm smaller than the collet’s nominal internal bore — the exact collapse range depends on the collet size. Although the ER system originated using metric dimensions, imperial-sized ER collets and sets are also available.

How to Choose ER Collet Sizes

A simple answer to “which ER collet size do I need?” is: a set covering the whole series. The real versatility of ER collets comes from their ability to contract to fit a range of tool diameters, but any single collet cannot cover the entire range of tools you might use. Because each collet only collapses by a limited amount (roughly 0.5–2 mm depending on size), keeping a set with many intermediate sizes lets you clamp almost any tool within that series’ span.

For drilling and other operations where runout matters, it’s particularly important to have a collet whose nominal size closely matches the tool diameter — the closer the fit, the lower the runout. ER collets are highly flexible holders, but if your priority is ultimate rigidity and repeatability, other holder types (setscrew holders, heat-shrink, milling chucks) can offer higher stiffness and accuracy — they simply don’t match the ER’s wide clamping range.

One more frequently overlooked factor is how tightly you torque the collet nut. There’s a running joke among machinists about some mythical “German torque spec,” but in practice the applied torque is critical: under-torque risks slippage, overtightening risks deforming or damaging the collet and can actually worsen runout.

Proper ER Collet Setup

Although ER collets look straightforward, there are a few details that often confuse newcomers. The collet nut is intentionally machined with an internal offset or retaining lip (sometimes implemented as a separate collar) so the collet can be inserted and held. You must snap or clip the collet into the nut before threading the nut onto the holder. When correctly seated, the front face of the collet will sit flush with the nut face — almost a press fit. Only after the collet is fully engaged in the nut should you insert your tool and tighten the assembly into the holder.

There are two common styles of collet nuts that retain the collet and compress it: a standard one-piece nut and a multi-part ball-bearing clamping nut. The standard nut is simpler and less expensive, while the ball-bearing style typically increases clamping capacity and may be chosen for specific high-force or special applications.

Holding End Mills and Other Tools or Stock

To maximize holding strength with an ER collet, engage as much of the tool shank as possible—ideally the tool should be supported by the full length of the collet. This practice has two closely related benefits:

Holding power — The more surface area of the tool gripped by the collet, the less likely the tool will pull out during machining. Greater clamping engagement also tends to reduce runout.

Rigidity — Increased engagement reduces deflection while cutting. Better rigidity allows higher feed rates, longer tool life, and improved surface finish.

If you’re building out accessories for a machine, investing in a comprehensive set of ER collets is one of the most useful purchases. Begin with a full range so you can clamp nearly any tool, then add extra copies of the most frequently used sizes. Having several collets in the common diameters that you use most often keeps changeover fast and minimizes downtime.