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Troubleshooting · Toolingintermediate~3 min read

Bit slipped in the collet

Short answer

A bit pulling down through the collet during a cut makes the carve get deeper and deeper. Causes: a dirty collet, a worn collet, the wrong collet for the shank, not enough shank gripped, or an upcut bit pulling itself down in a heavy cut.

Symptom

The cut starts at the right depth and gets progressively deeper as the job goes on, often ending with the bit cutting into the spoilboard. The bit is usually visibly further out of the collet than when you started.

Check
first
Take the bit out and look inside the collet. Dust, resin or a single chip in there is enough to stop it gripping. Clean it, the nut and the spindle taper, and inspect the collet for wear, cracks or a bell-mouthed end.

Most likely causes

  1. Dirty colletMost common
    Dust and resin prevent the collet closing evenly on the shank. Clean at every tool change.
  2. Worn or damaged colletCommon
    Collets are consumables. They lose grip over time and they are cheap to replace. See collets.
  3. Not enough shank in the colletCommon
    The grip is proportional to how much shank is engaged. Short insertion means poor grip and more deflection at the same time.
  4. Wrong collet for the shankCommon
    A 6 mm shank in a 1/4 inch collet is undersize, so the collet closes past its range and grips poorly. It feels tight and it is not. See collets.
  5. Nut not tightened enoughCommon
    Firmly, with the proper wrenches. Hand tight is not enough.
  6. Heavy cut with an upcut bitOccasional
    The spiral generates a real downward pull on the tool. A marginal grip fails under it.
  7. Oil or lubricant inside the colletOccasional
    Anything slippery in there defeats the whole mechanism.
  8. Bit bottomed out in the colletOccasional
    Tightening then pushes the bit back out rather than clamping it. See installing a bit.

Diagnostic steps

  1. Measure how far the bit moved. Compare the stickout now with what you set, or with the shank mark if you made one. That distance is also how much deeper the cut went.
  2. Read the shank and the inside of the collet. Bright rub marks along the shank mean it has been sliding. Dust packed in the collet slots is the usual cause.
  3. Check the collet size against the shank with calipers, not by eye. 6 mm and 1/4 inch look identical.
  4. Note the bit type and the cut. An upcut bit in a deep, fast pass is the one that pulls itself down. If it only happens on that combination, the grip was marginal and the cut found it.

Fixes

  1. Clean everything: collet, nut, spindle taper, bit shank.
  2. Replace the collet if there is any wear, cracking or belling. They are inexpensive.
  3. Use the correct collet size for the actual shank, remembering 1/4 inch is 6.35 mm and 1/8 inch is 3.175 mm.
  4. Insert more shank, leaving a small gap at the bottom and keeping the flutes clear.
  5. Tighten properly with the right wrenches.
  6. Reduce the cut if an upcut bit under heavy load is pulling itself down.

Why it happens

A collet holds a bit by friction alone. It is a tapered, slotted sleeve, and the nut drives it into a matching taper so it squeezes the shank evenly all the way round. That only works if the collet can close evenly, which a chip in one slot prevents, and if the shank is within the small range the collet was made for. An upcut spiral is a screw thread, and under load it tries to screw the bit down out of the collet; a good grip resists that easily, a marginal one does not, and a marginal grip feels exactly like a good one when you tighten the nut.

Prevention

Mark the shank
Put a line on the bit shank with a marker, right at the collet nut, after you tighten it. If the line moves, the bit moved. It takes five seconds and it turns an invisible failure into an obvious one, which is particularly worth doing before a long job.
  • Blow out the collet and nut at every tool change, and wipe the shank.
  • Own the right collet for every shank size you use, and label the metric ones.
  • Replace collets on a schedule. They are the cheapest wear part on the machine.
  • On a long carve, pause once and check the mark. A bit that has crept a little is a warning; a bit that has crept a lot has already ruined the piece.
Read next
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