Lesson learned on the small CNC - tightening the collet enough

Today was my first solo attempt on the CNC, and I got to learn what happens if you don’t tighten the bit enough. The bit clearly started to drop lower and lower. It cut too far into the spoilboard and then broke the bit when it encountered one of the screws holding it into place.

James was nearby to help sort out what happened, and it’s been discussed thoroughly. It also caused the first scratch on the aluminum.

It is functioning normally. I finished the remainder of my piece after a bit change.

And the broken bit

You’ll find the Tormach has the same issue. This was from not checking the spring collet to be free of chips and debris before tightening the endmill in it. Lesson learned - if it sounds wrong it is wrong. You can see where the bit started to pull out and how long it took me to acknowledge that it was wrong.

I deburred the aluminum T tracks. Next time if someone hits those please deburr them using a file. Slight damage to the T tracks don’t affect their use much but burs do and can be pretty sharp


Will do. Thanks Hanna

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I have had bits slip. I have noticed it happens more with inexpensive bits or inexpensive collets also. Might not be your fault.

Thought I’d add my 2 cents on collet care. Years ago at another makerspace, I had a similar issue with bits slipping on 3d carvings. Thankfully a guy there gave me great advice and have not had the issue again.

  • Use compressed air to blow any saw dust off the inside of the collet and nut before you assemble them and make sure the shank of the bit is clean too. Even just a little saw dust trapped between the bit and the collet might cause slippage.

  • Collets are consumables, they can wear out over time, get damaged, or rust, which can reduce their ability to hold the bit. (So I ended up buying a couple Technik collets for myself, and kept them in good condition)

  • If you need to change to another bit disassemble the collet and nut and air them off again, instead of just loosening the nut taking the old bit out and sliding the new bit in.

It does take a little extra time, but for me its worth it because it was extremely frustrating having to redo hours of 3d carving due to bit slippage and also wasted some good wood too.


Really good point! I often find the collet is full of dust and needs to be cleaned out when I go to use it.

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