Troubleshooting Slipping Router Bits

A thread from the CNC forum discusses the factors that could be causing router bits to slip off the collet on a CNC router. March 21, 2006

We are relatively new to CNC routing. Up to now, we have used the same down cut spiral bits that we used in our overhead router in our CNC. We just purchased a three wing up cut spiral and found it to run cooler and faster, but while running it through solid wood at 150 inches per minute and 18000 rpm, we have had a great deal of trouble with it pulling itself further out of the collet very quickly. I don't think I can get it any tighter than it already is. Any ideas what we might be doing wrong?

Forum Responses
(CNC Forum)
From contributor C:
I had the same problem with a Chinese bit (and it bit into my expensive aluminum table) and I think the bit was not round to start with. Threw it away and will never use their bits again. Only good USA or European bits, and have not had a problem since.

From contributor M:
As for collets, I don't know what type spindle or tool holder you are using, but here are the rules I live by. Don't use metric collets for inch sizes. I know they say that the collet will hold up to a 1mm difference in diameter, but I have had them slip in the past. Use good quality tools and change them as they dull. Keep an eye on worn tools - they can give you lots of grief if you don't change them when the edge begins to degrade. Use a torque wrench set to the recommendations of the manufacturer of the collets so that tightening is the same every time. If a bit does slip, toss the collet. Finally, collets are springs. They don't last forever. I throw them out after approximately 700 hours of cutting. They aren't that expensive and you won't have slipping problems.

From contributor A:
It is correct that collets are not a forever item and as your tool dulls, it will pull harder than when it is sharp, obviously.

From contributor M:
One other thing I forgot to mention. It is so simple, but I have seen guys not do it. When changing tools, be sure to thoroughly clean and check collet and holder for damage. Unless you are using a collet dust shield, you will find that quite a bit of dust can find its way into the assembly.

From contributor B:
Contributors M and A are giving good info. When you over tighten a collet, you actually loosen the collet. Check with your toolholder manufacturer for specs, get a torque wrench and tightening stand, and keep everything clean. Avoid petroleum based cleaners, as oil attracts dust, which is not good. I also question the use of a three edge cutter at such a slow feed speed. Consult Onsrud Cutter or Courmatt about your application, as there is much more to tooling than you think. You need to make chips, not dust, and keep the tool cool.

From contributor U:
I recommend checking your tooling catalog for correct speeds and feeds. 18000 rpm sounds high. You might be heating up the bit, which could cause slippage.

From contributor G:
All the above responds provide you with great info. Usually 100 pounds of torque will do. I believe 3 wings is too much for what you are going. A bearing collet nut will provide 50% more holding power vs. a conventional nut. If you insist on using a 3 fluted tool, put in chip breakers, as this will provide less stress on the tool and assist it from slipping out.

From contributor J:
I agree, 3 flutes, or wings as you call them, is too much. You should probably be using a 2 flute compression tool. And 18000 rpms is only high if your machine doesn't cut fast enough, and 150 isn't fast enough. Cutting that slow means you are re-cutting your chip load, causing your tool and machine to work much harder than they need to, and in conjunction with 3 flutes, you're generating a crap load of heat, which kills your tool and your collet. Then your tool will slip. Anybody at Active Machine will be able to help you with anything. I've dealt with them for years and they are the best in the business.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor N:
One thing you should check is the shank size. I have seen this happen when the shank is undersized. It is also best to use a torque wrench when tightening collets. Worn collets and dust will affect gripping power also. Always wipe out the taper, clean the treads with a brass wire brush and air blow dust out.

Comment from contributor K:
This problem is often caused by static nuts. Ball bearing nuts can prevent tool slippage. Tool holders made in Taiwan generally do not have ball hearing nuts, German ones do, some Italian ones do as well.