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Miltronics Servo Mystery, help wanted


Nov 2, 2022
Hi All, long shot here but I wanted to see if anyone had seen anything as weird as what I'm experiencing. My local rep and I are both stumped.

We have an older Miltronics partner 5 and for the most part it gets new parts when it needs them, tech comes in on an annual to check and calibrate the machine, all the regular stuff.

A few months ago my guys reported that the machine was shuddering in the X axis but by the time that I got there to look at it everything seemed fine. Ran for months with no issue.

Then it happened again but didn't go away. The bed moved perfectly in -X but was absolutely violent in a +X move. Tech came out and announced that the ballscrew was trash, needed to be replaced.

Skip forward 2 weeks, new ballscrew and all bearings installed and, absolutely no change, except for the hole in my pocket.

I've taken the servo out and put it on the bed, still wired in, and with the handwheel I get perfect motion going 1 way but the opposite is also jerky and in most tests it kept spinning long after I had taken my hand off the handwheel, sometimes cogging a little, sometimes not. I've inspected the motor internals, tach looks clean, encoder is attached firmly. I've also swapped drives between axes, no change.

At this point it's either send the servo in for factory inspection, replace the cables or light the entire thing on fire.

Has anyone seen anything remotely like this? I'm beyond stumped and we're getting further and further behind on part backlog.

Thanks in advance!
Same thing happened to me on my ML-17 lathe.
Ballscrew reground etc.....
Finally I was able to talk with one of the "old" guys and he told me to take the servo motor out and remove the endcap/encoder etc... and using compressed air, blow out the carbon residue from the brushes that has accumulated on the armature. Worked like a champ!
As Machfab suggested, make sure all the carbon dust is out of the brush area, that your brushes are in serviceable condition. If that doesn't improve things, take a look at you following error and the low voltage power supply. On the DC servo machines you should have plus and minus 15V I think it is, to drive the servos forwards and backwards. There are voltage regulators that tend to go and my machine was +15V and -24V and I couldn't get it to run worth a darn. The regulators are cheap, couple bucks each and you have two different styles, one for positive, one for negative.

Hope that helps...
I recall reading a paper about Servo Tuning, and the adjustments required to make them run smoothly. It was written specifically about the GlenTek drivers. I'll see if I can find it, if so, will post.
I have most of the tuning and trouble shooting files, but they are too large to upload here. email me direct if you want them. blamb11 at cox dot net
I second the cable- Not sure what mill it's on but if the servo cable is moving it could be a bad encoder cable. Had that happen to the A axis on my VM16
Update.. Well after replacing the ballscrew, motor&cable, all bearings and having my service guy in here for 2 days the machine is all back up and running. all axes are holding 0.0001.

I think that machfab had it on the nose. There was a bunch of carbon dust in the motor but I didn't find that until we'd replaced the screw and by the time I blew out the motor I think I had buggered the encoder trying to get it apart so it wasn't really reusable. I'm sure the ball screw needed to be replaced eventually but it's definitely a much more costly repair than if I had just blown out the motor to start with. Word to the wise, if anyone has any jittering on their machine, pull the motor and check for carbon dust, I'm pretty sure that I flushed a bunch of cash needlessly.

thanks everyone for the insights!