ER collets and router collets: sizes, care and mistakes
A collet is a split sleeve that squeezes the bit shank when the nut tightens it. Insert the bit so most of the shank is gripped but it does not touch the bottom, keep the collet spotlessly clean, and never grip the fluted part of the bit.
How a collet works
It is a tapered, slotted sleeve. As the nut tightens, the taper is forced into a matching taper in the spindle nose, and the slots let the sleeve squeeze down evenly on the bit shank. The grip is around the whole circumference, which is what makes it concentric and strong. This is fundamentally different from a drill chuck, which grips at three points and is nowhere near as accurate.
ER sizes
| Type | Typical shank range | Found on |
|---|---|---|
| ER11 | Up to about 7 mm | Small spindles, often 0.8 to 1.5 kW |
| ER16 | Up to about 10 mm | Mid size spindles |
| ER20 | Up to about 13 mm | 2.2 kW spindles, the common hobby size |
| Router proprietary | Usually 1/4 inch and 1/8 inch | Trim routers, with brand specific parts |
Each individual ER collet has a narrow clamping range, typically about 1 mm. An ER20 collet marked for 6 mm will hold 6 mm and a little under, and it will not hold a 6.35 mm (1/4 inch) shank properly. Use the right collet for the shank, not the nearest one.
Installing a bit, correctly
- Clean everything. The collet, the nut, the inside of the spindle taper, and the bit shank. A single chip creates runout.
- Snap the collet into the nut first. ER collets click into an eccentric groove in the nut. Do not put the collet into the spindle and then the nut on top: the collet will not extract properly and can jam.
- Thread the nut and collet onto the spindle together, by hand, a few turns.
- Insert the bit. Get as much shank inside as you can while leaving a small gap at the bottom, and keep the flutes clear of the collet.
- Tighten with the correct wrenches, firmly. Not with all your strength, and not with a cheater bar.
Do not bottom the bit out
If the bit touches the bottom of the collet or the spindle bore, tightening the nut pushes the bit back out as the collet closes, or it locks the assembly so the collet cannot release properly. Leave a small gap, a couple of millimetres is plenty.
Do not grip the flutes
The collet must grip the smooth shank. Clamping on the fluted section damages the collet, damages the bit, and gives a grip that will not hold. If you cannot get enough shank into the collet without gripping flutes, you have the wrong bit for that stickout.
And do not leave too much stickout
Every extra millimetre outside the collet multiplies deflection. See stickout. The rule is simple: as deep in the collet as safely possible, as little exposed as the cut requires.
Keeping collets working
- Clean at every tool change. A quick wipe and a look inside. Chips and resin are the enemy.
- Do not tighten an empty collet. It deforms the slots and it is the fastest way to ruin one.
- Replace them. Collets are consumables. They lose grip and gain runout over time, and they are cheap next to what they affect.
- Do not use rust penetrants or oils inside a collet. A slippery grip is not a grip.
- Inspect for cracks and bell-mouthing. Any damage means replacement, not persistence.
When a bit slips
A bit that pulls down in the collet during a cut is a specific and recognisable failure: the carve gets progressively deeper, often ruining the piece and sometimes cutting into the spoilboard. Causes and fixes are in bit slipped in the collet.