I can regularly get within .005", and often .003" or better. To do that, everything has to be dialed in just right and maintained, and you also have to calibrate and maintain calibration.
It sounds like you've hit most of the possible causes of lost steps, but here's a few things to consider: 1. Belt tension-Too tight and the motors have to work hard, you lose steps. Too loose, and there's slop. How are you measuring belt tension? 2. Potentiometers-Have you adjusted your pots? Too low and motors don't get the power they need, too high and they thermally overload. 3. V-wheels-Yeah, these are hard to quantify. I typically set mine so I can put 2 fingers on opposite sides of the wheel (across the diameter), and if they're fairly easy to spin without a lot of force they're good. Easily spinning them with 1 finger is too loose. 4. Pulleys-One of the screws needs to be seated snugly down in the center of the flat. Loctite (blue) is recommended. 5. Modifications-If you stiffened the X and Y axes, your pockets will be consistently deeper and your axes stiffer. If not, you may have some twist/flex issues. This is especially probable if you have a heavy router and a non-stiffened X axis.
There's quite a few more things, but those are the most common issues. Here's a couple threads I've started that you may be interested in:
It takes some work, and some things seem counterintuitive, but once you get it dialed in you'll be getting .005" accuracy.
I agree. I have my mill dialed in rather well and have done a number of mods to keep it locked in. I mill 95% aluminum on my 500x500 and I keep tolerances within 0.05mm-0.10mm with ease one run to the next in 1/4" aluminum. It really is just a matter of working to dial the mill in and then keep up on maintenance and calibrations to keep it dialed in. But also keep in mind I do not run a stock controller, and I use leadshine stepper drivers (same drivers used on Tormach mills) so.. With mods and work the mill can hold a tight tolerance just fine.
After a bit of tuning, not much actually, on my standard 500mm machine, happily using a laser to around 0.05mm accuracy (no real load). Closer to 0.1mm with the spindle (standard 300w 24v one) but that is with load on plus potential errors in measuring bit diameters, runout etc.
So it's possible and I've really only spent a couple of hours tuning.
I think my v-wheels were a bit too tight, so that's resolved for now.
As for the potentiometers, I adjusted them when I first got the machine, and the motors are definitely getting the power they need.
Yeah I sealed the pulleys with thinset (OEM, goes by a million retailer names.. bob's industries for this one) as soon as I had them up. I haven't noticed any slipping.
The help you provided did make me take a closer look at the mechanics so thank you.
I think I know exactly what the problem is. I'm using the dewalt p611 or whatever it is, and the z wobbles like nonsense when stepping. The bracket fitting the acme screw is too far from the z rail, making it flex slightly. It shouldn't be a problem, but when plunging into a cut, especially a radial one, those small errors are compounded.
I do not currently own the Tormach. We are buying a 770 in a month or so. I was not aware the Tormach used the Leadshine drivers until after I had already made the upgrade on my XC and other mills in the shop. The Xc does do a very good job for all the projects I ask of it in aluminum so far. Not sure how much aluminum work the XC will see once we install the 770 though. I think the Xc will be down to carbon fiber for FPV drones and engraving work once the 770 is here. But until then it will work hard to help pay for the new 770.
I made all of my products on small manual lathe until a couple months ago (to fund the CNC). Now that I use the CNC, I still use the small lathe for small dedicated tasks (mostly scotch briting, and quick sanding jobs).
I hope to have one of their mills someday as well. I bet you're going to love it. Their customer service has been nothing less than awesome.