How far should the bit go into the collet, how much should stick out, and when should I replace a collet?

Why it matters

The software assumes the bit runs perfectly true at the diameter in the Tool Database. A worn or dirty collet lets the bit run off center (runout), so it cuts wider than programmed, leaves a poor finish and breaks sooner. A bit hanging out too far bends under load. Harvey Performance states that deflection grows as overhang grows and that the tool should be held as deep as possible.

How to seat the bit

1. Clean the collet, nut and spindle taper. C.R. Onsrud says to do this whenever a bit is changed. Edge of Arlington Saw & Tool says to use only a nylon or brass brush.
2. Insert the shank so it fills about 80% of the collet's depth (C.R. Onsrud and Edge of Arlington both give this rule).
3. Do not let the flute fade out, the point where the flutes end on the shank, sit inside the collet. C.R. Onsrud says this causes runout.
4. Amana Tool words it for routers: insert the shank completely, then back it off about 1/16 inch, and never partially insert a bit.
5. Sienci Labs gives a bare minimum of 3/8 inch of tool inside the router for light cuts, and recommends the shortest tool that will do the job.
6. Tighten firmly but not excessively. Edge of Arlington warns over tightening distorts the collet and causes runout, while under tightening lets the bit slip.

When to replace a collet

Makers disagree on the interval:

• C.R. Onsrud: about 3 months when used 8 hours a day.
• Edge of Arlington: at least every six months of daily use (400 to 600 hours), or at any sign of wear.

For hobby hours, go by condition. Amana says dark marks or grooves on a bit's shank usually mean slippage and a worn collet that should be replaced immediately, and that even new collets should not be assumed to be true.

Pro tips

• Amana advises against reducing sleeves. Use a collet of the correct size for the shank.
• If a job needs long reach, lower Pass Depth and Feed Rate for that tool. See the entry on chatter and deflection.
• A bit that slips changes your cut depth mid job. See the entry on wrong depth after a tool change for re-zeroing.