Troubleshooting a Noisy Spindle

CNC router spindles are rebuildable, but the noise problem could have other sources. April 6, 2007

I believe the spindle on our Gerber CNC router is thinking about failing. Recently it's developed a pronounced objectionable whine when cutting and not during above-surface travel. The noise seems to be coming from the lower portion of the assembly. Can anyone offer some insight into the pitfalls of rebuild or replacement of a spindle motor?

Forum Responses
(CNC Forum)
From contributor T:
We put a rebuilt spindle in one of our machines about 6 or 7 years ago, still running. The bearings are definitely on the way out on that one now, but it ran 2 shifts for most of those years. Can't complain about that. I have no complaints against a rebuilt spindle. We were able to order a rebuilt for our machine, and then basically ran the old spindle until pretty much complete failure, and then swap it out and send the old one in as a core. They didn't care whether the bearings were noisy or totally trashed.

From contributor C:
If you are very tech savvy, strip the unit and rebuild it yourself, now, before the bearings let go and cause damage to the housing. (If this has occurred, it can be re-sleeved by any good machine shop). Spindles are not difficult to repair, although some would have you believe otherwise. Some of them require special tools, which you can make or buy.

Doing it this way will save thousands. A full set of bearings for my spindle ran to less than $1K, but the "experts" wanted $9K to repair it. If you are not too technical, ask your local electrical motor rebuild shop to see if they can do it. After all, that is all it is, and the collet holding devices are not too difficult to deal with either. Don't let the experts scare you with "bearing preload settings," as this info and also the how-to is in the SKF bearing books and web site.

If your spindle requires balancing, not necessary if it is only bearing replacement, as the bearings are all marked for positional installation for this purpose. You can get one of the dynamic balance companies to do that, i.e. Pratt and Whitney jet engine re-builders, and it is only a few hundred dollars at worst.

From contributor D:

I agree with contributor T that there is nothing wrong with a properly rebuilt spindle. Unless you have a cartridge type spindle, I would not generally recommend in-house rebuild. I do not know how your particular spindle is built, but most require expertise and facilities beyond the typical shop maintenance group. If bearing failure is the only thing wrong with your spindle, then you may get lucky, but you also could end up with a mess. High speed spindles, especially the ones with larger diameter bearings, press the limits of ball bearing performance. Preload, squareness requirements, and fitting conditions must be tightly controlled, often to tolerances of +/-0.00005". Most motor shops, let alone end users, cannot begin to do this kind of work. Dings, scratches, distortions, or tiny bits of dirt on the bearing mount surfaces can also ruin your rebuild.

As for prices, a major rebuild of a 10 hp tool changer spindle averages about $4500. Inspection and simple bearing replacement should be much less. A reputable spindle house will also give you a warranty.

From contributor S:
Before you go through the trouble of spindle repair or replacement, be sure to eliminate any other possible source of noise. It could be dust in the tool holder, crud in the cone assembly/drawbar…? Do the scheduled maintenance before you take it apart.

From contributor G:
Contributor S is right here. Replace your cutter and clean out the collet, as the problem could be in one of those two areas. In other words, it could be a balance issue which leads to another possible problem. Your spindle could be slightly bent. One other quick thing to check is if your spindle has a fan for cooling, one of the blades could be chipped or broken off, causing the noise. This may not be the case, as you stated that the noise doesn't happen when not cutting.