Spindle thread breakage

Hey all,

I experienced something new tonight and was curious if anyone else has had a similar experience? I went to remove my collet nut and found that it had locked onto the threads pretty good. I used some WD-40 to get it loose, but it appears that part of my thread broke off and obstructed the nut from coming off, which ended up welding the broken off portion into the threads during the removal process. The nut was NOT overtorqued or cross-threaded. It appears that there is a second set of threads going from the collet nut attachment location into the actual spindle. I’m hoping that I can just get another set of threads and change it out to get back up and running. Any help would be very appreciated. I’ve already sent a ticket in to get some support from the staff.

Thanks in advance!

Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.

We’ve seen this more times than I’d like. It’s usually due to material being stuck in the threads of the spindle or collet nut when tightening it down. Can also be caused by over torque, corrosion, metal flakes from component wear, and a number of other things. As such it’s recommend to at a bare minimum blow out the spindle, collet, and collet nut between every bit change. It’s better to actually clean them.

Worst case you can clean the threads of the spindle up with some needle files. As long as you have at least 2 full threads left it shouldn’t effect the system too much. If you go that route make sure that you taper off the lead in thread so it doesn’t catch.

I know you said that you didn’t over torque the nut. However, most people aren’t aware that there are torque maxes listed for the setup (ER size and nut type) in addition to a recommend torque for the bore size of the collet. As an example an ER16 with a UM/A/Hex nut has a max torque of 41 ft-lb. However, here’s the recommend torque base on bore diameter:

  • 1.0mm-1.4mm - 6 ft-lb
  • 1.5mm-3.9mm - 15 ft-lb
  • 4.0mm-4.9mm - 30 ft-lb
  • 5.0mm-10mm - 41 ft-lb

The above numbers also come from pricey industrial spindles (HSD, Columbo, Mechatron) and matching grade nuts. If either are hardened wrong, made from the wrong steel, lack proper lubrication, and/or aren’t clean they may not hold up to the maxes.

One last thing. Collets and collet nuts are consumable parts. Each cycle wears them and can effect runout, clamping force, and the material of the components. In your pictures your nut is past it life. I say this from the groove that has been ground at the end of the chamfer. In general once you get a groove or spalling material in the chamfer the nut needs to be replaced. At this level I would also check the collets. If they also have grooves or transferred material from the nut they should be replaced regardless of any other factors. This is due to the fact that they will damage or accelerate the wear of the new nut.

Hope that helps. Let me know if there’s something I can help with or expand on.

Thank you very much for your response! Ironically, I bought a new collet nut and 1/4” collet online and they arrived in the mail the same day that this breakage occurred because I’d noticed there was unusual wear on the collet. Are you aware of any torque wrench options that will work on a horizontal orientation? I have inch/lbs and ft/lbs torque wrenches, but they are for typical 3/8” sockets. I am also still waiting to hear back from support about being able to get new set of threads and taper. It looks like there are holes for a special wrench to hold the spindle still while unscrewing the threads from the spindle motor.


No problem.

Wrenches depend on the nut type and I don’t know what you are using. In general though you can usually find spigot end wrenches that will let you rotate the head to different orientations. Norbar is the one manufacturer I know of off the top of my head for those.

I don’t know what you mean by getting a new set of threads and taper. The part that holds the collet is the armature and it goes through almost the entire length of the spindle. It sits inside of multiple bearing sets that would need specialized tooling to remove and remount without damage. In rare cases I’ve seen things called spindles that have a removable chuck. No real spindle I’ve seen has that though. Closest you get is an ATC spindle and those are real pricey.

Let me know if there’s something I can help with.

This is an extreamly helpful response. Thank you for taking the time to help us all here on the forum.

@AndrewSwann There are “Crows Foot” wrenches which work well in a torque wrench. I bought mine at Harbor Afraid… I mean Freight (lol)…

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No problem. Glad it was useful.

If there’s something I can help with here or in another topic let me know (@ or email me). I usually try to check the forum a couple times a week but there’s not been much that I can address over the last year or so. I have a pretty broad knowledge of cutters their use and associated parts. I have to be careful about comparisons and brand specifics though as I don’t want to be advertising or marketing.

You might try a product called Musclechuck. Should solve thread problem and provide quicker bit changes.