Re: [CAD_CAM_EDM_DRO] Strange lost step problem, grounding related

Jon Anderson wrote:

> Noticed today a real strange "lost step" problem on my mill. Machining
> some SS shafts, I discovered to my dismay, that I was losing .025 in the
> Z every time I ran through a fixture of 11 parts. Now, there are but 4 Z
> moves in the entire program, so I was rather perplexed. I've always had
> a niggling little creep in the Z, but within a range I could deal with.
> This was too much. To make the long story short, I first ran a ground
> wire from the inverter ground outside to a waterpipe that runs several
> hundred feet underground. This knocked it down to about .003-.005 per
> run, better but still a problem.
> I have run many parts and multiparts, with peck drill cycles and minimal
> or no Z creep, so I was looking for some variable that might account for
> this. Discovered that if I run the inverter at 55hz, it really moves the
> Z around. 60hz, no problem, or rather, back to the level of creep I can
> deal with. Placing my hand on the graduated dial atop the quill ball
> screw, I can feel it pulse now and then.
>
> So, the inverter is bolted to the side of the motor on the mill and
> presently grounded to a good earth ground, but the wiring is micky
> (wanted to know if I was on the right track before doing it proper).
> I have the cabinet with stepper drivers pretty well covered and
> shielded, but wonder if I ought to attach a ground buss somewhere on the
> mill and run all grounds to this, then run a proper ground wire outside.
> Also, should I insulate the inverter from the mill itself?

Yes, definitely. This is most likely your problem. I have my inverter
mounted on the wall behind the mill. I had a stout ground wire to the
power panel, which is only a foot away from the inverter. My CNC
system was not at all disturbed by the inverter, but my computer-controlled
air compressor would go crazy whenever the inverter was running the
motor. As the only connection was through the power lines, the inverter
must have been radiating noise back into the power lines. So, I put a
large (15 A) power line filter module at the input to the inverter, and
it cured 90% of the trouble.

>
> All power and logic lines to the mill are several feet from any 220
> lines, around the mill, but the extension cord that powers the computer
> and power supply for the drivers runs alongside the 220/3ph line that
> also runs down along the center of my shop (follows a structural beam).

Keep ALL cables away from the line running from the inverter to the
motor. You probably want to separate those wires as well, but a line
filter unit is the best approach. If you are running your inverter from
a 3-phase source, the filter will be more expensive, unless you can find
a suitable unit surplus.

Jon