Bit Slippage in Tough Material

A CNC owner who's making stair stringers from laminated veneer lumber (LVL) gets advice on good practice for inserting and tightening the bits. September 8, 2007

We have been cutting Micro=lam (aka laminated veneer lumber) to build a circular stairway. The problem we are having is holding the bit into the collet. We are using a 30 degree up shear, .5" diameter bit with chip breakers. The cutter is from BAM tools out of Oregon. We have changed up the depth of cut and feed speed. In general we are cutting from .75" to .5" deep and 60 ipm to 150 ipm feed speed. We are using a R32 collet and we just cannot get the bit to stay in place. It gets pulled out with bad consequences. We have replaced the collet a number of times. We've changed depth of cut and feed speeds and are still having problems. Ideas?

Forum Responses
(CNC Forum)
From contributor B:
A few things to check... Is the collet, nut, and cone clean, no debris or dust to keep the collet from compressing consistently all the way around the tool? The tool should be ~80% of the way into the collet. Any less and problems like you mention can occur, any more and only the very end is secured, with the same problem. A good visual check of the tool should let you know if slipping is occurring and where on the tool shaft.

If you have a locking spring or clip in the locking nut, is the collet being locked in place under the spring? Obvious but sometimes missed. If this is the case, the tool will not stay locked in placed.

Is the collet size correct for the tool shaft? Measure the tool shaft and be sure it is what it is supposed to be, and not significantly smaller. Also make sure the tool shaft is parallel, the same diameter throughout the area where the collet makes contact with it.

Last but not least, make sure the nut is being torqued down properly.

From contributor M:
Everything contributor B said is good info to follow. Also, you said you have replaced the collet a couple times already. Have you tried replacing the bit? It is possible you have a bad one. It could be slightly out of round or bent, either one of which can cause the problem you mention. If you have had no previous problem with other tools, that is the first place I would look.

From contributor A:
One thing to keep in mind is that if the bit is bottomed out in the collet when you tighten it down, it will prevent the collet from tightening correctly, as it tries to pull down on the sides of the bit. Leave a little space below the bit.

From contributor G:
You did not tell us the RPMs you run. Tools try to move because they are dull, incorrect torque (ER32 105-115 pounds), or an incorrect relief angle. A bearing collet nut will also give you 50% more holding power, ensuring tool doesn't move.

From contributor L:
Purchase of a torque wrench to tighten to the proper torque for the toolholder is critical. If you are (like many) tightening by feel, you will eventually experience failure. A proper torque wrench, in my view, is a requirement for proper tool holding.

From the original questioner:
Good suggestions from everyone. Two things... Where do I get a torque wrench that will work on my collet? And where do I get a collet with bearings? All of the other things we do on a regular basis (clean out collet, make sure of proper insertion of bit, use new or sharp bits, etc.). Thanks for the input.

From contributor L:
I suspect some of your tooling companies might have them. We purchased ours through NEMI. They have a large selection of CNC accessories. You will need to tell them what size collets you are using. We have one torque wrench and 3 wrench ends for different sized collets.

From contributor G:
Courmatt has all of these in stock.