CNC Grinding machine not repeating size
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  1. #1
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    Default CNC Grinding machine not repeating size

    Hello,

    I have a CNC grinding machine with a Fagor 8025 GP control, Indramat Drives and Power supply, Heidenhein linear scales and Inramat servo motors with encoders. (It is a dual loop system).
    The machine is 20 years old and saw heavy usage grinding parts to .00005" (half a tenth)

    A couple of months ago we noticed the machine was not repeating on the x axis. The machine said it was at location but the diameter varied +/-.0001" and then it gradually increased to +/- .0004. Also at position the servo kept hunting for position moving a tenth or two back and forth.

    This is what we have done so far:

    Sent linear scale out for cleaning and testing. Had it professionally installed back into machine.
    Sent CNC control to Fagor for testing...no problems found
    Sent Ball Screw out for rework
    Replaced Indramat drive
    Sent Servo Motor out for testing it's ok (zero backlash coupling)

    Every component on x axis has been tested or reworked except the thrust bearing. Mechanic said it was OK?

    At the end of the line here and many sleepless nights.....any ideas?

    GP

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    Maybe clean or replace the encoder, if it's 20 years old, it's old enough to have a tired component.

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    Machine might have stick slip problems. You don't say what kind of machine but if it is iron on iron X axis ways the scraping may be about gone and the oil pockets are worn away and it builds up friction and it jumps a bit when moving. If it has Turcite on it, Turcite can get stick slip too once the scraping is gone. Have you check the spindle bearings? They could be worn too. Slick slip is like ringing together gage blocks. Also have you cleaned the lubrication system? Are the ways wet with oil? Did you change brands of the way oil by chance?
    Last edited by Richard King; 01-24-2018 at 04:51 PM.

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    I think that Richard's reply is on the mark. I'm no fan of Indramat stuff and would have said to look there except for the fact that the system is using the Heidenhain scale for positioning.

    Back in my days of having to service Maho machines with Indramat servos and drives, everyone in the service dept referred to them as Laundramat drives as we all had the opinion that they were better suited to run a washing machine than a machine tool.

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    I know you rebuilt the ball screw, but did you check the actual backlash when it was assembled? Dual feedback is a real head scratcher from a control perspective. If there is too much mechanical backlash, the servo loop tuning can often cause the oscillation you are seeing.

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    Thanks for the comments. I checked the ways and there is a light coat of oil on them. Used oil from the ways is collected in a container that can be used elsewhere. New oil is added to replace old.

    The oscillation is coming from the motor. I can actually see it moving left to right.

    I called Indramat about the encoder. They want to test the servo motor on a drive and a test station for feedback and other tests.

    They want $500.00 for the test and over $7000.00 to totally rebuild the servo motor. Yikes!! On the other hand machine is no good to me now.

    GP

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    Quote Originally Posted by Gsharp10 View Post
    Thanks for the comments. I checked the ways and there is a light coat of oil on them. Used oil from the ways is collected in a container that can be used elsewhere. New oil is added to replace old.

    The oscillation is coming from the motor. I can actually see it moving left to right.

    I called Indramat about the encoder. They want to test the servo motor on a drive and a test station for feedback and other tests.

    They want $500.00 for the test and over $7000.00 to totally rebuild the servo motor. Yikes!! On the other hand machine is no good to me now.

    GP
    I wouldn't do that. Not sure about the older indramat stuff, but if they are using resolvers like the newer indradrive it won't be the issue. Resolvers and servos, if there is a problem with either you'll know 'cause the drive will know there's a problem and won't turn more than a poofteenth. It'll go straight to error.

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    Quote Originally Posted by Gsharp10 View Post
    .....The oscillation is coming from the motor. I can actually see it moving left to right.
    This type of servo behavior is sometimes a result of having mechanical backlash in the drive mechanism.

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    Honestly you need to get a decent tech in there, one that actually diagnoses whats wrong with stuff or simply fixes it. You have a parts swapper and there the mouth breathers of the tech world! Equally you need a tech not a mechanic, mechanic's fixes plant not tooling, big difference in results on stuff like this!!

    The servo dither is a clear indication the problem lies in the positioning loop, mearly rebuilding random bits of it won't get you there, the rebuilt parts need to be near perfect to hit these resolutions of movement and need to be installed properly. Equally drives need to be tuned to the mass there shifting other wise you get the oscillations your seeing.

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    Thanks adama,

    I think I'll hold off on the Servo motor repair (again) and see if I can get a technician in here to tune the drive.

    GP

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    FWIW I'm currently working on a grinder that would not hold size. I found that someone tightened the gibs up so tight that the axis would bind up and overshoot. Similar dual feedback system.

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    I tried getting a technician to come in and check the machine, but for some reason none of them want to work on grinding machines. So today I will switch servo motors. I know the z axis repeats so I will switch it with the x servo motor and see what happens.

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    I switched motors and the z axis repeats. The x axis still does not repeat. Using an indicator now the x axis goes past position and then springs back with various readings on the indicator.

    Every component except the thrust bearing on x axis has been repaired or replaced. Thinking thrust bearing is worn, or the ways need to be flaked.

    Any ideas?

    Thanks

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    replace those thrusts whether you think they are bad or not. some things are hard to requalify in grinders. your tech needs to really baby the adjusments- too tight is way worse than 0.0001 free play.

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    I'd say is pretty safe to say it's a mechanical issue, i'd start by checking to see how it moves by turning the screw by hand, compare the z and x and see what you think. It should give you a pretty good idea of what is going on. Another thing to think about is your oil. You say you are using the same oil, but sometimes, albeit rare, they forget to put additives in oils.... I had a coolant issue once that haunted me for months until I figured out it was a bad batch of coolant, it was missing the sulphur additive I think. All it took was for me to add a quart of another type of coolant to the 300 gallon sump and I could form tap cast aluminum again. Later after the next tank cleanout there wasn't a problem and we didn't have to add the other type of coolant. If you are still working off the same drum of way lube, it could be the culprit. Otherwise, as mentioned I'd start looking at the screw thrust bearings.

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    Have you checked for lost motion in the screw assembly? That would be a good way to check out your thrust bearings.

    Set up a mag base and indicator on the table and place the indicator's tip on the face of the ball screw. If you have lost motion, the screw will move independently from the table for just a bit.

    This is an easy method to see if your thrust bearings are still good.

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    I tend to agree that it is a mechanical issue. We have two identical machines that use the same oil, but that's a good thought. We just had the lead screw replaced an by hand it turns nice and smooth.

    What happens now is when I indicate the x axis the machine sometimes comes to position but most times the indicator needle stops after position and then springs back out of position. It seems worse the faster the x axis travels.
    My machine mechanic says the thrust bearing is fine but I am beginning to wonder about that.

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    When the head springs back, what does the X position display on the control show?

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    Hi...It shows that it is in position and does not show that springing effect. Thanks

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    Then it seems like you have an issue with the scale. The position feedback to the machine should come from the scale, not the encoder. Typically in a dual feedback system, the rotary encoder does the velocity loop and the scale does the proportional and integral.

    To me that seems like an issue with the scale or the P or I portion of the servo tuning.


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