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Author Topic: Questions re "PWM bumped to minimum" on lathe  (Read 685 times)

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Questions re "PWM bumped to minimum" on lathe
« on: September 18, 2017, 03:57:10 AM »
I spent the last couple of days playing with spindle control on my CNC lathe with some interesting results. I am wondering whether anyone else has explored this area.

System:
W7, Mach3 .062, ESS, BoBs, KBWT-26 motor DC power supply, Baldor DC motor 180 VDC max, 1/2 kW. PWM control used. (NOT a VFD.)
There is a home-brew optical isolater between the BoB and the KBWT because the input is floating at ~-100 VDC. Artifact of the KBWT design apparently.

First of all, I checked out the max pulley rpm when the motor was getting 'full scale volts'. It was 3860 rpm, but 4000 rpm was a better match over the range. That was set in.
Then I set pulley min rpm to 1.
Question: has anyone used 0 for the pulley min? Does it make any difference?

Then I went to ports&pins, spindle control, to look at PWM Minimum. Now for many VFDs you have to set this to something like 30% to avoid burning out the VFD, but with a DC motor you can go down to 0% in theory. However, while 1% worked fine, 0% seemed to freeze the whole spindle output. It was as though the M3 command was being ignored.
Question: has anyone managed to get 0% to work?

Then I tried 0.5%, but Mach seemed to round this off to 0% and the system didn't work.
Question: has anyone managed to get a decimal place working here?

I then ran a lathe program to step through the speed range (eg 5% steps) with M1 used to pause the loop at each setting so I could record the results: m98 p10 L20 for 20 steps. However, a few times Mach seemed to ignore the M1 and it whipped through the program 'real fast' and finished. I checked the M1 setting was correct, and tried again - whereupon it worked.
Question: has anyone found that the M1 setting and command was being ignored, even though the Config said it should be obeyed?

None of this would be relevant to a mill of course, but it does matter on a lathe when you want to do some threading.
Yes, the system is running just fine. I was fine-tuning.

Cheers
Roger