Router Bit coming loose in CNC Spindle

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Forum topic by TechTeacher posted 11-14-2013 03:00 PM 3958 views 0 times favorited 5 replies Add to Favorites Watch
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53 posts in 3733 days

11-14-2013 03:00 PM

I Have a CNC router with a columbo (Spelling?) spindle. I am attempting to mill out some deep pockets in some maple. I am using a 1/2” x 2.5” upcut spiral bit. I am having trouble keeping the bit in the collet. As the cut progresses the bit tends to creep down in the collet and will come out if I don’t stop the machine and reset/tighten. I have cleaned out the collet and tighten it as tight as I can. Any suggestions to keeping the bit from coming out?

5 replies so far

View bigblockyeti's profile


8007 posts in 3058 days

#1 posted 11-14-2013 03:19 PM

Take the cleaning one step further and use some lacquer thinner on the tapered cone as well as the collect itself. Once everything is total spotless, you can also use a very, very slight amount of grease on the contact surfaces between the collect and cone as well as the threads. DO NOT get any on the inside of the collect where it contacts the bit. This slight amount of lubrication will allow you to generate greater retaining forces with the same amount of applied torque. It will also attract dust, so the cleaning/regreasing regiment will become the norm. Also, these are very sensitive to surface finish, so any burrs, scratches or metal galling with make it hard to tighten while providing little clamping force on the bit shank.

-- "Lack of effort will result in failure with amazing predictability" - Me

View Underdog's profile


1804 posts in 3372 days

#2 posted 11-14-2013 03:53 PM

Ditto on what Yeti said. Everything must be well cleaned, free of burrs and nicks, and oiled properly. Unlike Yeti, I oil everything that makes contact, all tapers, threads and shank. I use Boeshield T-9. (Follow the directions!)

What type of toolholder are you using?

What torque specs do you use when tightening the nut? I use 120 ft/lbs on my HSK ER40 nuts for our KOMO router. If I use less, then the bits start backing out.

Is the collet old? Most CNC manufacturers will tell you to change your collets out at least once a year, if not sooner.

Also ensure that the collet contacts the full length of the shank.

-- Jim, Georgia, USA

View DLCW's profile


530 posts in 3991 days

#3 posted 11-15-2013 05:25 PM

Techteacher – how old is the collet in your spindle? I replace collets every 6 months on my CNC (production shop). Cheap insurance against this type of dangerous problem.

Spindles on a CNC get much harder use then a handheld or table mounted router. There are significant side loads that hand held routing doesn’t achieve. By your “name” it looks like you are a woodshop teacher? I consult and volunteer teach at my local high school woodshop and I have convinced them that for liability and safety sake they should change their collets at 6 to 9 month intervals. So far (after 3 years of running) they have never had a bit slip. I taught the teacher and the kids how much to tighten the collet nut. Not to tight and not to loose.

-- Don, Diamond Lake Custom Woodworks - - "If you make something idiot proof, all they do is make a better idiot"

View TechTeacher's profile


53 posts in 3733 days

#4 posted 11-15-2013 06:47 PM

Thanks everyone for the responses. Sorry for not including all possible technical info in original post. The collet in question is an ER-25 with the upcut spiral bit. I am using a .75” stepdown (pass depth). Based on the responses received I am going to order a couple new collets as I am sure mine is now damaged. My existing collet probably only has 3-4 Hrs of cut time on it but after this I am sure it is now junk. I will try and find the torque specs on the collet/spindle. I doubt that I am achieving 120 ft lbs with the specialized spindle wrench based on wrench length. Current plan is to modify my profile tool paths to remove additional material next to my part to aid in chip removal and reduce step-over and plunge rate. Once I get the new collet I will clean up everything and give it another try. Thanks Again

View Underdog's profile


1804 posts in 3372 days

#5 posted 11-15-2013 08:36 PM

With the smaller nut, you may not need that much torque. It’s best to ask the manufacturer of the collet/nut/toolholder what the recommended torque is.

-- Jim, Georgia, USA

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