CNC Collet Maintenance and Repair

Collets should not wear out or fail for many years unless something is wrong. Here's advice on keeping the collets in good shape. April 19, 2011

I am using a Biesse Rover and for some reason my 3/8" compression keeps coming loose. The HSK is new (less than 6 months) and I changed my collet about 3 weeks ago because the old one was passing 4 months old. My chip load is at .023 (846" @ 18500) which is in range for melamine (MDF/particleboard .020-.023) and I didn't have this problem with the old collect. Could I have a bad collet? When I look inside the collet, there are wear marks that are shiny as if the bit were vibrating. Could it be the end of my router bit? I have had it sharpened 3 times. Is there a chance that the shaft diameter has been varied? I do clean the inside of the HSK during every bit change with some soft abrasive (green pads) to get some buildup off the walls. Could I have altered the inside walls that the collet can't be tightened?

Forum Responses
(CNC Forum)
From contributor M:
Are you using range collets? By range, I mean are they sized for 9-10mm for 0.375" tools? If so, toss them out and buy collets for 3/8" bits. I haven't used range collets since the early 90's when I had similar problems.

As for cleaning, compressed air and a soft cloth should be sufficient. I wouldn't ever use abrasives. Just wipe them free of dust every tool change. My other suggestion would be to guarantee proper and consistent tightening by using a torque wrench.

From the original questioner:
I am using collets specifically for 3/8" bits. As for the torque wrench, I crank down rather hard on the HSK to lock it into place, so I am probably going above recommended torque. I am wondering if I am over torqueing the HSK and causing the collet to strain there by wearing it out earlier? But I did the same on my old collet and never had any issues.

From contributor J:
What brand is the collet? If it is a Chinese made part, they do allow tools to loosen, as sometimes they are not machined/ground to proper clearances. Big surprise there, I know. US or European sourced collets are typically dependable quality and not that much more expensive.

From the original questioner:
When we got the machine, our rep sent us some HSKs, collets and drill bits and if I remember correctly, he got them all from Vortex. I wouldn't order Chinese parts for my CNC. I don't buy their knock-off slides, hinges, plates, particleboard, etc. I'm not about to start ordering parts for one of the most important machines in my shop (next to the coffee maker) from someone whose tolerances and quality control may be questionable. And before anybody can crack a joke about it, look at who makes my coffeemaker - it's a Miele.

From contributor D:
"I changed my collet about 3 weeks ago because the old one was passing 4 months old."

I missed something there. Why change it because it's 4 months old? I have 3 Biesse Rovers and have never changed a collet. I'm still using the same collets that came with the machines. They are all over 11 years old. I too had problems with multi-size collets. Not really coming loose, but over a few days, the tool would move up inside the collet. Fixed size collets, no problems.

From contributor H:
There are many reasons for this to happen. I also didn't replace a collet unless it was severely worn or scratched. You could be having multiple issues here that are all likely. First, the collet may not be seated right with the locking nut and when you tighten it, it could be damaging the collet and not clamping effectively. Over tightening will damage the collet and cause issues. Collets can have different internal material makeup due to metal compositions. By over tightening the new collet, it may actually be weaker than your old collet. I would switch back to your old collet and continue to use it.

Another issue could be your router bit shank may be warped or damaged and may appear fine. Have you tried a different router bit in the same new collet? Replacing the items one at a time would be the best answer to your cause. Without seeing it in person, we will all have ideas.

From contributor M:
First let me say, the coffee maker is the most important machine in any shop. From your original post, the shiny areas on the inside of the collet could be indicative of the tool spinning in the collet. Once the inner diameter of the collet is compromised, you no longer have the precision cylindrical engagement needed to secure the tool shank. By itself, this can create a condition of run out (everything not spinning on the same axis) and unbalance. While they cost more, ask your tooling rep about UP collets. The UP stands for Ultra Precision.

From contributor S:
Very unlikely, but make sure the shank is in the collet to the gullets, ensure the length is correct in the dati mac, and that the depth of cut doesn't exceed the cutting length (edge) of the tool.