Bits keep slipping

[jkaras]

Hello. I have a Shopbot PRS Alpha with a spindle router. I laminated together a 2" stack of masonite (hard board) with wood glue. Whenever I try to mill into this material with any of the 1/2" bits displayed in my attachment, they tend to slip in the collet (no matter how tight I wrench it) and pull themselves into the material digging themselves a nice unplanned trench. I have tried slowing my move speeds and it helps but it still can happen. I noticed the two bits on the right have a flat spot on their shank, and I thought that this was allowing poor gripping in the collet, however I tried the bit on the left with the smooth shank and it still did the same thing. I have been using a straight fluted 1/2" bit that works good, however it is not long enough to cut through 2 inches of material. I have been using a single fluted upcut 1/4" bit to do the cutting, but its barely long enough and I am constantly worried it will break. Any suggestions?

22094

[knight_toolworks]

the flat spot is an issue it may have damaged the collet. it is best to replace it.

[bleeth]

Those bits are made for a tool holder. Also make sure you are chucking up tight enough. I position my wrenches so I can squeeze them together to tighten with one hand and really bear down on it for larger bits.
I agree with Steve that you may have damaged your collet at this point.

[bobmoore]

Your bits are not made for cnc routers. There are plenty of router bits with 2 inch depth of cut. Go to a cnc router bit manufacturer ( I.E. vortex, onsrud, whiteside and order what you need from them. Your collet/spindle manufacturer can supply you with new collets and a torque spec to follow. Too tight is as bad as too loose and do not bottom out the bit in the collet.
Bob

[Burkhardt]

Are you trying to cut the 2" full depth at one time? What is the feed and speed you are using?

I machined 2.5" thick piece of softwood with a 1/2" ball end at full depth a while ago and (although the tool path overlap was only 20/1000"), the same problem happened. The well tightened bit without flats shook loose after maybe 2 hours, probably due to the machining forces and vibration. I cleaned the bit with air and the special collet care fluid, tightened again and finished the job without problem.

Recently I bought a few collets with built-in ball bearings that somebody had suggested here. Have not tried such a deep curring job with those yet but just from the feel it appears the ball bearings help reduce the friction substantially and help applying the tightening torque as axial clamping force.

As for the flats, I bought a few bits in a blowout sale from Onsrud that had such a flat and the instructions said clearly the the flat must not be clamped but stick out of the collet to prevent bit or collet damage. But these bits had ample shank left above the flat.

[Brady Watson]

If your tool slips in the collet even once - It's trashed. Get a new one & buy 2-flute solid carbide end mills without the flat on them. 4-flute doesn't have sufficient chip clearance & unless you are cutting soft foam, should not be used. 4 FL ball end mills are OK as long as you do a 3D roughing pass first.

-B

[shilala]

Here's a fun thing...
I got a call from my girlfriend's father years ago, the shop he worked for needed some refrigeration work done, fixing a new chiller that was woefully screwed up.
It chilled the RO water that cooled the machine that put the gold coat on those Niagra bits.
That job ended up getting me in the pipefitters local. I just got a 25 year pin the other day.
/completely unrelated story.