Hello CD,
Our team is looking into purchasing a CNC router for next year and we would like to know what kind of tolerances we would need to hold for things like belts / pulley C-C distances, gearboxes, press fit bearings, etc. We greatly appreciate any information.
Thanks,
18sampson
You want to be within a few thous (thousandths of an inch) for gearboxes and belt spacing. IIRC 1678 said their CNC router holds +/-0.003" and they’re happy with those results. For press fit bearings, you probably want to be within 0.0005" or less if you want precision press fits.
There’s a lot of great information on CNC routers in this thread.
I’d agree with Ari about those tolerances.
Another note about bearing hole press fits. You need to be incredibly accurate as Ari said but as long as your CNC is incredibly precise you can manipulate the CAD to make sure the bearing hole is the right size.
We did this with our CNC. If given a bearing hole size it can cut 10 instances of the hole with +/- .001 of each other but the holes were consistently .007 larger than they were supposed to be. We fixed this by under-sizing all our bearing holes for press fits by .007 and then we could cut 10 bearing press fits no problem.
Not sure if this is a problem that lots of people have, or if it’s just us but I figured it might help to know that.
We specifically post ream all our bearing holes to absolutely ensure the desired fit. But we could easily get away with not if we wanted.
Thank you for posting that link, it makes me very happy seeing others using it as a shared resource!
Thank you 1678 for creating that phenomenal resource. Our team just recently purchased our CNC Router and we spent the majority of the year learning to use it well. We don’t have a machinist as a mentor or a machine shop to work with making the discussion even more useful to us. I’d highly suggest reading the thread for anyone even remotely interested.
With my experience if all your holes are .007 larger then you model them the problem is not your Mill/Router its with your end mills. I would bet your end mill is .0035 larger then is says. For high precision through holes reaming is the best option. For pocket bearing seats where reaming is not an option I prefer to use high quality end mills and program the CAM with there exact dimensions. It is effectively the same as your doing by under sizing your model but if you make the parts on a different machine or try to share with another team everything will have to be adjusted again.
It’s almost inconceivable that an end mill would be .0035" oversize (also your math is wrong. .0035" radial increase in size makes the diameter .007" bigger. The end mill would be .007" oversize, which is absolutely not possible). They are specifically ground such that any deviation from nominal occurs on the minus side.
Likely culprits are spindle/toolholder runout and just the fact that it’s an open loop control system and probably rack and pinion drive. I have no experience with CNC routers and don’t know how much runout you can expect in your average router spindle. I wouldn’t be surprised if it was .0015-.002". But .007" or more would be shocking.
On a poor router 0.007" is easily achievable. 
299’s router when we got it had about that much runout in it (maybe lightly more), although with a new collet it’s gone down to 0.002" TIR typically. I want to replace it with a proper spindle in a few months for sure.