Controlling three phase motors

I think I know the answer to this question, but I'd be interested in the communities thoughts.

I am in the process of updating a 1980 Fehlmann mill-drill my plan is to use LinuxCNC with a Mesa control card. This video from a couple of years ago more or less shows where I am with the machine:

I've been busy with other projects and am now returning to the Fehlmann.

I now have the stops set up on the control of the milling head and find that repeatability to the micro-switches is not great at 0.1mm.

The vertical travel is effected by a special 0.21kW 660rpm three phase induction motor built-in to the head.

Later machines such as the Picomax 54 and 55 seem to have precision cnc control over the vertical position of the head as can be clearly seen here:

I suspect this is achieved by replacing the induction motor with a servo and a glass or magnetic scale to give the absolute vertical postion.

Finally, my question: I am just seeking confirmation that even with the addition of a dro scale it would not be possible to achieve control over the absolute postion of the vertical head using the current three phase induction motor.

Given a negative answer, then I guess that if I replace the induction motor with a suitable servo motor - maybe even a stepper given the low speed - along with a dro scale that it should be possible to have control over the absolute vertical position of the head.

Whether it is possible to integrate this into LCNC would be my next quetion!!

Thanks for your patience!

David

Thanks Tommy, I have to admit that I was ignorant about using a servo drive on a conventional squirel-cage induction motor.

Can you point me to somewhere where I can read about controlling two axes in the same plane?

That's OK, I think I probably wasn't clear enough in my initial description. It looks like using a stepper would be the simplest thing.

If you know of any threads on controlling and integrating two motors on the same axis let me know.

Just for my curiosity:
Would this be an application in witch i would use W as second vertical axis to adjust both 'Z'-axis independent by gcode? (could be a nice idea for my old FM4)
Mike

Hi Mike, Fehlmann certainly refer to the "coarse" vertical positioning of the milling head as the W axis.

Out of curiosity what is an FM4?

David

FM4 is an old Stadelmann mill. Similar to todays BM4
Nothing special but good enough for an old "electronics" engineer.
Mike

Ah right, I found your website Mike. Looks like you're in Switzerland.

Yes, your little mill drill has the same issue as I have with the Fehlmann. How do you control the quill Z axis - I'm guessing with a ball screw? Mine uses the quill rack which has an astonishingly small amount of backlash. The return springs also act on the quill itself rather than the pinion which means the tiny amount of backlash shouln't be a problem except with very large cuts.

If you decide to add a W axis to your coarse positioning I'd be interest to see how you go about it and how it integrates into the CNC control.

David