I can't decide on suitable homing switches for the X and Y axes yet. These need to be reasonably repeatable and precise, since they ensure you can pick up the machine coordinates where you left off, in the absence of a more sophisticated absolute coordinate system. Failure to achieve that can lead to toolpaths becoming offset from each other if you have to power off or force a reset. This in turn will bugger up your work.
It will be some time before I have the Z axis system in place, so for now, any inaccuracy / inconsistency won't be critical. So I have plenty of time to decide what to do about "proper" switches.
So here's the (temporary) Y axis homing switch in place. I previously ran a 4 core screened cable to the limit switch, so I just need to run the switch connections into that switch body. The homing switches will be NO, unlike the limit switches which are NC.
I like drilling and tapping holes in this machine.
And here's the X axis homing switch. I had to extend the roller spindle so it would contact the movable stop without said stop interfering with the actual limit switches. It's pretty ropey but for now it just provides a basic level of functionality.
All done now - and connected up inside the cabinet. The homing functions work nicely. This is it at the X and Y home position:
Check axis scaling and backlash:
The ballscrews are both 5mm pitch and I have a 2.5:1 reduction in the toothed belt drive. That results in a very simple ratio for the number of pulses per mm travel. However, having programmed in the parameters in the "Axis Parameter" area, it's sensible to check the result. Rather conveniently, I previously fitted a 3 axis DRO to this machine and it's really not worth taking the scales off, so I have a simple means of directly comparing desired movement against actual movement.
Time to remount the display head. I took a look inside this recently and although it's a bit Chinesium, it didn't look hazardous as such. Besides, I have a good strong protective earth and the workshop circuits are protected with RCDs.
Here we go:
The way to check for backlash on each axis is to creep up on a nominal position (eg X100), note the measurement on a "real" DRO, then overshoot the position (eg to X110) and creep back to the same nominal position (X100 again). Any backlash will show up as a difference between the "real" DRO and the CNC's DRO display.
Homing repeatability:
Finally, having checked the scaling is correct, it's a good time to see how repeatable the homing function actually is.
The X axis seems to be repeatable to within 20-30um which is rather better than I'd expected. However, the poor tolerances and resulting slop in the switch mechanism suggests I'd be foolish to count on that.
The Y axis seems a bit worse, most likely due to the different actuation method and some differences between the 2 switches themselves.
Still, it seems to be a reasonable result.
Some buggering about with max feedrates, accel rates etc and I seem to have a reasonable result for now.
What next?
Before I start on the Z axis, there are a few things to finish off:
- Refit the X-axis DRO scale cover.
- Do some tidying up of the cabinet wiring after the recent limit / home switch work.
- Possibly wire up the spindle VFD.
- Connect up the servo error line back to the controller. Currently, if either of the servos bomb out, the controller has no means of knowing. Not ideal.
- Fit comms / diagnostic cables to both servo drives. I have 2 Dsub cutouts in the cabinet that are ideal for the job. Just requires making up a couple of simple tails.
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