X-axis Servo 70 Power Feed Running Sluggishly

[Inflight]

My Bridgeport's Servo brand type 70 power feed is running very slowly lately. Rapids are about 1/2 of what they've previously been and dial speeds are slow too. I know the type 70 is obsolete, but it's been working great for the last 10 years and I'm not about to replace it.

I can move the table by hand with no problem; handles spin nice and freely.
I removed the power feed and totally cleaned the leadscrew with solvent and re-lubed with fresh oil. I also pumped a ton of fresh oil into the table ways. Like I said, the table moves fine manually.

So I took apart the servo drive unit and cleaned everything inside and out. I used electrical contact cleaner to totally clean the armature and coil. I also cleaned and lubed the upper and lower gear assemblies. With the servo still removed from the table, I ran the motor through its speed range and rapid too. That worked well, sounded great and was moving nice and fast.

I then put everything back together on the mill table and it still seems slow. A little better than it was before the cleaning, but nothing like it should be.

I should mention that I replaced the motor brushes last year and the drive was working fine after that. During my cleaning, I inspected the brushes and they looked fine. The servo unit does not run hot or anything obvious and the machine does not get used very much; maybe 4 hrs per month.

So, where should I be looking for the culprit?

 

[David VanNorman]

If you can get a look see at where the brushes run I think you will see it needs cleaned. Take the motor apart get an armature stone and clean it up.

 

[Black13]

How did the commutator look when you removed the armature. The commutator can develop a glaze on the bars from airborne contaminants (coolant mist, cutting oils, even cigar smoke). As "DavidVanNorman" said a commutator stone and some careful cleaning between the bars may solve the problem. Contact cleaner will get rid of carbon dust, but will not remove the glaze on the bars. Most hobby shops with an R/C car section carry spray commutator cleaner that works good.

If the comm has any deep groves or pitting is should be turned on a lathe. Most motor shops will do this for a small fee.

 

[Inflight]

Today I decided to inspect the commutator. It looked good but since I had it disassembled again I went ahead and turned it between centers, taking a very light skim cut with a sharp tool. I then cleaned out all the grooves and gave it a quick polish. Reassembled and it runs the same. I think I'm going to just live with it and see how long it lasts. Thanks for all the help.

 

[John Hasler]

So, where should I be looking for the culprit?

At the controller. How is the supply voltage under load? How does it perform at lower speeds?

 

[Inflight]

At the controller. How is the supply voltage under load? How does it perform at lower speeds?

All the speeds seem 50% weak, like there is excess drag. I didn't check the voltage at all. Where should I connect the DMM leads to properly test voltage.

 

[Riaan]

Have you tested for short circuits between commutator segments?

 

[Inflight]

Have you tested for short circuits between commutator segments?

Yes, I checked between each bar and all was good.

 

[John Hasler]

Is this a true servo system with a tachometer on the motor and a closed-loop speed control? If so I would expect a weak motor or power supply to cause poor performance at high speed and high load but normal performance at low speed and low load. I'd have see a schematic or the unit itself to know where to check the supply voltage.

That motor looks pretty good.

[Edit] I see that you found that it works normally off the machine. That makes me suspect the power supply.

 

[Riaan]

Could be a red herring, because the torque load when off the machine is obviously pretty much zero by comparison. But it does sound like supply problems. I was going to suggest taking a look at the bearings too, assuming it has bearings instead of bushings.