First CNC Mill VMC40 Z-axis shudder

And my first post here! Hello Everyone! I've been doing machining and fabrication (bridgeport, manual lathe, waterjet, and tig welding) for about 15 years now (started when i was 15) have owned my own shop for the last 5 or so years and just bought my first CNC machine, a late 80's VMC40 with a CNC88 controller.

I just got it powered up after cleaning it, oiling the ways, etc, have only jogged the servos so far, haven't turned on the spindle yet. My issue is with the z-axis, it has a shudder at regular inch intervals, i should say the shudder goes away at regular intervals (every 0.200") and gets worse as you approach halfway between those intervals, it will just sit there and vibrate shaking the whole machine, I did not leave it there very long, maybe only 15-20 seconds it didn't go away so i moved it back to where it stops, it does it the whole way up the travel in 0.200" increments. Now this is my first foray into a CNC mill, but i am somewhat familiar servos and controls. It seems to me this is maybe an encoder/tach adjustment issue? like the board doesn't think the servo has moved so it tries again, maybe overshoots, or is in hysteresis on the feedback, and then gets into an oscillation?

A little background on the machine, it has been sitting in mothballs for 10 years probably, so this is it's first powerup in a long time, i have not done any backlash adjustment in the controller, just turned it on and lifted the Z off the wood parking block, and cycled the table x/y.

What should be the first thing i start with here?

So after a little bit of research i found that these DC brushed servos don't use encoders, they use an external resolver, and an integrated tach. After testing the Resolver's AC voltage at the bullet connector in the cabinet (tested fin at ~1.7 VAC) I swapped the resolver from the Z with the Y and vice versa per an instruction sheet from Fadal, still had the problem in the Z. So I'm thinking that leaves the tach armature or brushes possibly?

Any guidance is greatly appreciated!

It could also need to have the servo tuned at the drive.

Ben

What exactly does this entail? I am pretty mechanically and electronically savy, I just don't want to start dicking with stuff without some reasoning and guidance behind it lol!

His point is if the drive control loop has too high a gain, overshoot and oscillation are possible. This would demonstrate itself in a shudder or bounce, control loop instability. Lowering the gain should help, if this is the cause.

I would suggest making sure their is no mechanical cause first, i.e. verify lube oil is making it's way to both z ways as well as the ballscrew, and make sure the gibs are adjusted. You may even try loosening/tightening the gibs to see if the shudder changes any.

If all mechanical means are checked and still a shudder problem, try tuning the drive per Fadal maintenance manual March 2003, Section 06: Axis Drive Systems, Page 191 a copy on the web here http://www.flintmachine.com/pdfs/fad...ve_Systems.pdf

If that doesn't do it, sounds like you are on the right track with either a suspect Resolver or suspect drive in 'reading' the resolver SIN and COS input.

Good Luck.

Sorry, missed that you tried the resolver swap. Are the z and y drives the same size? You might try swapping all of the drive's input's and outputs from z to y to see if it is a drive issue.

Quote:

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Originally Posted by 123CNC

His point is if the drive control loop has too high a gain, overshoot and oscillation are possible. This would demonstrate itself in a shudder or bounce, control loop instability. Lowering the gain should help, if this is the cause.

I would suggest making sure their is no mechanical cause first, i.e. verify lube oil is making it's way to both z ways as well as the ballscrew, and make sure the gibs are adjusted. You may even try loosening/tightening the gibs to see if the shudder changes any.

If all mechanical means are checked and still a shudder problem, try tuning the drive per Fadal maintenance manual March 2003, Section 06: Axis Drive Systems, Page 191 a copy on the web here http://www.flintmachine.com/pdfs/fad...ve_Systems.pdf

If that doesn't do it, sounds like you are on the right track with either a suspect Resolver or suspect drive in 'reading' the resolver SIN and COS input.

Good Luck.

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I did notice when swapping the resolvers that the Z axis turns with incredible ease, almost no force to get the head to move up or down, as i had to manually rotate it by grabbing the spider coupling to get to the resolver's shaft pinch screw. If it is 1 inch pound id be surprised, it seemed way too effortless up or down (down is slightly easier). The ways and screw are getting oil as i manually pumped the oiler probably 10 times, could visibly see oil residue coming down the ways, and i pulled a line at the top and got drops out of it after pulling the oiler. As for the ball screw i think it is fine also, seems to be getting good oil and turning with the greatest of ease, in the lash and oiling department. In a half ass way to check it for lash I put a dial indicator on it, and lowered the head by hand onto a 4x4 block of wood centered on the table to take the weight off the screw looking for lash. Dial indicator reads the same at the way end, and the spindle end, no measurable lash on a 0.001" indicator, and it springs the same way when you load on the wood at either end, i loaded it about 0.006" by applying pressure to the spider coupling by hand, absolute minimal force. Does this sound right for it to turn so easily? I mean i understand it's not like the x and y that have all the weight on the way, not on the screw, but it just seems very easy to turn.


I am going to look into the gib adjustment spec now. Just trying to go about this as methodically as possible.

If i turn down the control loop gain on the Z the shudder does go away, but the axis seems laggy while i'm jogging it with the knob? is this just my imagination?



On a side note does anyone know what the DC voltage should be at the Amplifier's inputs or the large capacitor? I can't find anything in the maintenance manual or online to specify that voltage (It's a Glentek Card, late 80's VMC40 once again). I'm showing around 98-99 VDC. Is this correct?

I am running 208 3phase, and have already rewired the main transformer for that voltage, as well as the proper 208 procedure for the Toshiba VFD spindle drive (though i have not powered the spindle up yet).

The ring in this picture (green arrow) under the retaining cap and between the screw rotates about 0.015" in lash when you twist the screw back and fourth, is this normal? or should it be tight? Is that the visible end of the actual ball nut?

http://i.imgur.com/wzDubz7.jpg

The ring (green arrow) should just be a seal, preventing swarf in and trying to keep the ball nut well oiled. It shouldn't really rotate, but the small amount you've witnessed really shouldn't be a great cause of concern either.

Your counterweight really should help make the z screw feel nearly effortless,

'Laggy' when using the jog wheel is difficult to interpret, as opinions and perceptions will vary. What resolution you have the jog settings on and your own smoothness or sense of moving the jog encoder will play a big part in anticipated verses actual movement. At a high resolution trying to make minute moves, I feel it is laggy or notchy. But I think that's just me.

Ok so i turned the control gain down far enough that it doesn't shudder badly, but not far enough that i got no response from the servo, did a cold start procedure on the machine. I then, per the instruction manual, tried to set the gain by the steps below. But i get an error when trying to run it.

http://i.imgur.com/2x4Yj5n.jpg
http://i.imgur.com/zYIwGRY.jpg
http://i.imgur.com/WNsUUbT.jpg

Not sure if this helps, but I have a 1996 fadal 3016 that the Z kept not repeating and when you jogged it, it shudder and make weird noises. I switched the resolver with the X axis and it fixed Z and continued the problem into the X axis. So I bought a new resolver and haven't had a problem since then. Also be careful when you remove the resolver because there is a tiny allen set screw that you have to loosen on the shaft. I made the mistake of trying to pry it up and luckily it was on the bad resolver. I own 5 of these machines from 1989 to 2005 fadals and have done everything from spindle replacement to table replacement. Let me know if you have any other questions.

The picture of the pages in the manual look like the procedure for the later software and AC drives. I'm not sure what year of machine you have or if they are AC drives. Don't know when they changed, but would guess all the 80's machines were DC brushed servos. If so, you may want to try

DC Amplifier
Adjustment
1) Enter the following program:
a. X-4. Y-4. Z-4. G1 F150. M49,M91.
b. X4.Y4.Z4.
c. M99P1
2) Press Auto, to start the program running.
3) At the amplifier to be adjusted, use a small screwdriver and turn the SIG2
potentiometer until the monitor’s display of the appropriate axis reads the
following error: 595 - inch ballscrew / 302 - metric ballscrew.
4) Press the SLIDE HOLD button on the keypad and adjust the balance of the
amplifier to read between 0 - 1.
5) Press the START button on the keypad and the program will start again.
Verify that the following error is still 595 or 302.
6) Press the SLIDE HOLD key to stop the machine. Press the MANUAL key and
then type SETCS and HO, to send the machine home. Adjustment of the
amplifier is complete. Repeat the procedure for all axes

Maybe?