Boss8 Series II ** low speed servo chattering, sounds horrible!

The machine is a R2E3 Boss8 Series II, with SEM servo motors, and Heidenhain ER-120 encoders stock.

This only concerns the Y axis. I realize now this problem started a couple years of years when I was
engraving at probably around 1 or 1-1/2 ipm, I noticed a "rude sound" only when the Y axis was moving.
It was kind of a buzzing sound. I thought it was odd, but I did not see any problem with the engraved
pattern so I let it go.

Then 2 months ago I was using the jog wheel to move the Y axis while indicating a diameter. When
I moved the jog wheel fast there seemed to be no problem, but when moving it slowly I would hear a crunching sound that was absolutely horrible sounding, kind of like you might imagine a destroyed bearing to sound like. When watching my indicator, and slowly moving the Y axis using the jog wheel I could see
the Indicator needle "buzzing" in the range of about plus and minus .002 to .003 (.004 to .006 TIR.) When I stopped moving the jog wheel the buzzing would completely stop.

Then just last week I was cutting a 4 cavity "o ring" mold, with approx 7/16 OD and 5/16 ID using a 1/16 cutter. During setup the crunching showed up again. Then I ran the roughing PGM at about 1-1/2 to 2-1/2 ipm with no problem. Then "on the finish pass!" the crunching started, and by manually speeding up the feed with manual override I was able to get axis motion to smooth out, but not before leaving unacceptable gouge marks on 2 cavities.

Has anyone out there experienced anything like this on their Boss8 or Boss9 Series II (or Series I) machine? And maybe fixed this problem? I know I could swap x and y motors to help isolate the problem, but I'm hoping this sounds familiar to someone, before I have to take that action.

What drives are you using? older Westamp or similar? if so the drive sounds out of tune probably caused by component drift as these drives were prone to with age.
You could try swapping the drives and see of the problem follows.
Al.

The drives are in a rack and can easily be swapped. From left to right, they are x, y and Z. So swap the X and Y and see if the issue follows the drive.
Are you using flood coolant? It might have found its way into the motor. Remove a brush with the machine off. Bridgeport recommended that these motors be overhauled every 4000 hours of operation. Most motors can go 7 years before they need a going over. Look for moisture or oil on the brush. Check the length of the brush. look at the commutator with a flashlight for burnt spots.

George

Quote:

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Originally Posted by MrBoss8

The machine is a R2E3 Boss8 Series II, with SEM servo motors, and Heidenhain ER-120 encoders stock.

when moving it slowly I would hear a crunching sound that was absolutely horrible sounding, kind of like you might imagine a destroyed bearing to sound like..

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You may be exactly right! It may have chips in the ballscrews, or possibly a damaged
bearing on the ballscrew. I'd take the belt off and turn it by hand and see if you can
feel the roughness. One trick I have used to clean ballscrews is to wipe axle grease
on the screw and run it back and forth a few times, then wipe the grease off and examine
the paper towel for sparkles. If you see sparkling dust in the grease, you definitely
need to clean it out. Just keep repeating this procedure until the grease comes off clean.

In the case of the Y axis, you pretty much need to remove it from the machine, it is just
too hard to get inside the knee to do anything.

Jon

I'm using Contraves NC-400 drives all stock, and I probably will try the amp swap rather than switching servo motors. I believe the servo motors to be good, as I want thru them 10 years ago and replaced bearings, turned the commutators, cleaned everything completely, and installed new brushes on all motors. I only run the machine about 10 hrs per month. I only use spray mist, no flood at all. all motors look clean and run super cool compared to the Boss9's I've encountered which ran way hot, comparably speaking.

In response to John B from Ottawa:

I always run my FIST monitor whenever my machine is running. My backlash is set at .0008. I do not have the NC-400 manual so I could use a copy of that if it is available to access pages 6 to 10 (or did you mean the Bridgeport manual where pages are designated in chapter-page format, i.e. 5-6 etc) If someone has the complete NC-400 manual I would love to obtain it.

The only amp alignment I've done (years ago) is to null the drives (all 3) by disconnecting the encoder cable, then adjust the "offset pot" until drive movement stopped completely.

In response to jmelson:

The Y ball screw was rebuilt about 6 years ago, and is clean, no chip contamination at all. When I drive the ball screw manually using the nut at the end of the screw with the machine off, and it feels totally smooth everywhere. I get the feeling this could be in the encoder or servo amp, but I'm open to all suggestions

Your responses are appreciated, thanks everyone.

Here is a thought. Check the encoder coupling to see if it is loose. It might set up a oscillation that sounds like a growl.
When a drive is sent out for repair (age dependent) standard operating procedure is to replace the electrolytic capacitors as they suffer from age and heat to a greater degree than other components. It may be that one of the drives is giving up.
Good to know that you are familiar with the fist event monitor and have read other posts or were astute enough to figure out how to set the drift/balance. i have to find my cheat sheet on drive tuning.

George

Quote:

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Originally Posted by machintek

Here is a thought. Check the encoder coupling to see if it is loose. It might set up a oscillation that sounds like a growl.
When a drive is sent out for repair (age dependent) standard operating procedure is to replace the electrolytic capacitors as they suffer from age and heat to a greater degree than other components. It may be that one of the drives is giving up.
Good to know that you are familiar with the fist event monitor and have read other posts or were astute enough to figure out how to set the drift/balance. i have to find my cheat sheet on drive tuning.

George

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Yes, George is right, this is the next place to check if the ballscrew has no issues.
One other thing I have seen is that as the toothed belts wear in, they get loose,
and it can cause vibrations which are clearly audible and can be felt, too.
I saw this on an EzTrak lathe, but it is all the same servo system. So, check
the belt tension, it needs to be pretty tight.

Jon

RE: i have to find my cheat sheet on drive tuning.

George, if you find your drive tuning notes, I would sure like to see them. Thanks for your thoughts.

Although I would still like to see a "cheat sheet on drive tuning." I have come across the Contraves NC400 series controllers, PDF covering the mdl 407, 414, and 421. I believe this has been recently made available, and is available here:

www.contraves-drives.com/pdf/M502001_d.pdf

I'm starting to think my problem has something to do with the analog speed command signal. Since lowering the speed over ride potentiometer to reduce axis feed to under 1 IPM results in an analog input signal to the NC414 of very close to zero VDC, which is were I'm having my problem.

I'm thinking of looking for ripple voltage (possible filter cap failure) on the DC input that dictates y axis drive amp output, and polarity. At a value so close to 0 VDC, a total AC ripple voltage on this line could be greater that 2 times the DC voltage command glitching the NC414 into direction reversal at the frequency of the ripple!? It sounds like the buzzing axis noise could be in the range of 60 HZ!

What do you think?

First I will swap amps, and see if the problem follows the amp. If it does I'll check the DC speed command input for any ripple via scope, and I'll post it here.

PS Does anyone conveniently know how to locate the correct + - 10 VDC "analog input command velocity" terminal? before I try to dig it out of my manuals? Maybe point to a spot in the maintenance manual? Also, are the NC414 drives placed in the cabinet from left to right as follows: Z axis, X axis, Y axis? I used to know 8 years ago, or so, and did not mark them as I should have.

The latest is: I swapped the X and Y amps, and all seems fine!? At this point I'll guess it may have been reseating the NC400's that did it? I have not done this for 7 or 8 years. Thanks for your input everyone. And I hope this is the end of this series of posts.

Yes, I have experienced this as well at very slow feeds. I have the same machine. It was a couple years ago and have not seen it since. And yes,it did it on the FINAL PASS of course. Did you figure this one out?