ER Collet Size Guide: How to Choose the Right CNC Collet

The collet is the cheapest part of the cutting system and the one most often to blame. A worn, dirty or wrong-sized collet causes runout. Runout means one flute does all the cutting, which leads to chatter, poor finish and broken small bits. This guide covers which collet series your spindle takes, which size to order, and when to replace it.

Step 1: identify your spindle's collet

• Trim router (Makita RT0700 / 3620 / RP0900 and similar): these use their own collet, not ER. See collets for Makita trim routers, from 3.175mm to 9.53mm.
• Water- or air-cooled CNC spindle: almost always ER (ER11, ER16, ER20, ER25 or ER32). Measure the collet using the table below.
• Perske RDO spindle: uses DIN 6388A collets (OZ12A / EOC12A), not ER. See RDO20 collets. Check the code stamped on your existing collet.
• ATC spindle (ISO30, BT30, HSK63F): the tool holder carries an ER collet. See ISO30 ER32, BT30 ER and HSK63F ER32 holders.

Step 2: measure an ER collet

Measure the overall length and the largest outside diameter of the collet you have:

The ER series also tells you the largest shank you can run. An ER11 tops out at about 7mm, so it can't take a 12.7mm (1/2") shank bit. An ER20 or larger can.

Step 3: order the size that matches your shank

Each ER collet clamps from its nominal size down to 1mm under it. For example, a 6mm collet grips 5–6mm. That's for holding, not a reason to share collets: order the collet that matches your shank exactly. Common shanks are 3.175mm (1/8"), 4mm, 6mm, 6.35mm (1/4"), 8mm, 10mm, 12mm and 12.7mm (1/2"). Our router bit listings give the shank as the middle number in each size (cutting diameter × shank × cutting length).

A metric 6mm and an imperial 6.35mm shank look almost the same but aren't. A 6.35mm shank won't go into a 6mm collet, and a 6mm shank in a 6.35mm collet is held at the limit. Buy both if you run both.

For an occasional small-shank bit, a collet reducer sleeve works in an existing collet. It adds one more interface, so expect a little more runout than a dedicated collet. For regular use, buy the right collet.

Why precision matters

Our ER collets are rated to 0.005mm (5 micron) runout. Cheap unrated collets don't state a precision at all. Lower runout means every flute does its share of the cutting, which gives quieter machining, longer tool life and better accuracy. It matters most on small cutters, where a few hundredths of a millimetre of wobble is a large share of the bit's size.

Adam's Tip

Clean the collet, the nut and the spindle taper every bit change. Dust in the taper is a common cause of runout. Never tighten the nut on an empty collet, use two spanners, and tighten firmly but don't overdo it. Replace a collet that has been dropped or has a crash behind it, or when bits start breaking for no reason.

Frequently asked questions

How do I know what ER collet size my spindle uses?

Measure the length and outside diameter of your existing collet and match them to the table: ER11 is 18mm long and 11.5mm across, ER16 is 27.5 × 17mm, ER20 is 31.5 × 21mm, ER25 is 34 × 26mm and ER32 is 40 × 33mm.

Can I use a 6mm collet for a 6.35mm (1/4") bit?

No. A 6.35mm shank won't fit a 6mm collet. Use a 6.35mm collet for imperial 1/4" shanks and a 6mm collet for metric shanks.

How often should I replace CNC collets?

Replace a collet when it's damaged, after a crash or drop, or when you see chatter, poor finish or unexplained bit breakage. Collets wear, so the sizes you use most will need replacing first.

Do Makita trim routers use ER collets?

No. The Makita RT0700, 3620 and RP0900 use their own collet. We stock replacements from 3.175mm to 9.53mm.