Spindle RPM calibration

[aethersis]

Hello,
I have a Chinese GDZ-80 spindle (it has no sensor) and a pretty decent Delta MS300 VFD. I'd like to be able to calibrate the spindle in such the way that the RPM under no load is exactly as what was set. Currently, I'm using analog voltage output of AXBB-E and the voltage corresponds to the setpoint, so the issue is definitely on the VFD/spindle side.

In Mach 3 there was a way you could do measurements e.g. with 1000 rpm step and save a table that had set rpm against measured (actual) rpm in linearity.dat file that was used to proportionally compensate for the error. Is there a similar functionality or plugin in UCCNC? I couldn't find anything relevant in the forum and again - I am using a handheld laser tachometer. The spindle does not have a sensor I could use in the VFD to have a PID loop.

[cncdrive]

No, this is not possible unfortunately.
Honestly, noone asked about this feature yet, but I will make a new entry about it now in our "to be implemented features" list.

[aethersis]

Thank you! I think it might be a fairly niche feature because those who care about accurate rpm use encoders and those who use cheap Chinese VFD usually don't care as long as it works. I'm in the middle - I upgraded the Chinese garbage with Chinese premium (Delta ms300 that's actually really good) but still no encoder and thanks to your motion controller, I can now really push my machine hard with pretty crazy feeds with aluminum and reasonably accurate rpm is important. It can live with it being off by 5% (hence no real need for an encoder) but currently it can be off by as much as nearly 20%! The voltage to rpm relationship is pretty non linear from what I've seen and it's like half a parabola...

[aethersis]

I just realized a huge error from my side! The minimum RPM of my spindle was set to 5000 in UCCNC and I completely misunderstood this setting! I thought it simply acts as a threshold, but apparently it does some RPM scaling. I though it's simply a protection function to limit the lower and upper end of the RPM. After I set it to 0 and 24k max, the spindle RPM is super precise with <1% error!