I have a problem that first came to light when making some 150mm diameter gear wheels for a wooden clock on my CNC router. Essentially the gears weren't quite round - one axis seemed undersize and the other oversize - the difference being about 0.5mm, which was enough for the clock to bind after a couple of minutes. I've been using the router for about a year for lots of jobs, but this was the first task I’d done that relied on a decent level of accuracy.

Having analysed the problem, it looks like my X-axis works oversize by about 0.22%, and my Y-axis works undersize by about 0.13%. Doesn't sound like a lot for a wood router, but on a 150mm circle, it does work out at about a 0.5mm discrepancy. Surely we can do better than that? My machine is home-made of the gantry style, and uses C7 grade ballscrews directly driven by 4Nm steppers from Mach3 in the usual way. The screws appear to have an accuracy spec of at least 10x better than these errors suggest, especially over a short 150mm run. I have therefore been looking at all aspects of the machine I can think of, assuming there are other issues to be corrected.

First check was backlash, and it measures at no more than 0.04mm on either axis. The error I see seems to scale with cutting size anyway, so it really does appear to be an accuracy issue in the axis drive. Slowing the motors via Mach3 (speed or acceleration) has no effect at all. Swapping the axis drive electronics between the two motors has no effect - the issue remains with the axis, so my conclusion is that the issue is not in the electronics.

I did discover that the X and Y axes were slightly out of square, which meant that the circles were out of round, but the errors were "off axis" - since corrected. I also discovered about 0.1mm of runout on my Kress spindle - which isn't ideal, but surely wouldn't cause this issue. Just oversize width cuts.

I did feel that the Z-axis assembly is not as rigid as I might have liked. I can flex the router side to side along either axis by maybe 0.5mm without excessive finger pressure, so for a while I wondered if this was my issue. However to try and prove this, I cut simple 150mm diam circular slot in MDF with a clockwise cut and a second one with a counter clockwise cut, and the two circles were essentially identical in their distortion. I should explain further - the X and Y diameters are in error as described, but the "45 degree" diameters are slightly different to one another. This strongly suggested to me a rigidity issue, such that cut direction mattered. With the two cut directions I had therefore expected the errors to be mirror images if you like, but they weren't! I think this suggests it isn't a rigidity issue, but I'm ready to be shot down on that, as this feels like a strong contender to me.

However, I am wondering if the absolute error of a C7 ballscrew is such that you would expect to have to enter a measured rather than calculated "steps per mm" setting in Mach3? I suspect I could null out these errors by entering something other than the current 320 "steps per mm" entry I am using, as it seems you can use any number - doesn't have to be an integer. And you can use the inbuilt calibration button to help you do this along with precision measuring kit. But that seems like a cheat and potentially just covering the real issue, which I can't believe is simply a ballscrew error.

The forum's thoughts and comments would therefore be much appreciated.....

Cheers
Graeme