cnc way shudders
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    Default cnc way shudders

    I have a kiwa excel center from 84. The x axis shudders when it moves faster then about 30 IPM. I had a few suggestions mentioned. One was that the box ways are not getting enough oil. I'm not sure what style the oiler is on the thing. It does have a manual pumping handle on the top. I tried pumping it a few times today not sure yet if it helped. Anyone run into this before? Any suggestions on what to do? Thanks Dan

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    On a machine that age, you can just about guarantee that some of the oil metering valves are plugged/restricted. Should pull the covers back and see if there is any oil on the ways.

    Shudder can also be a sign of poor servo tuning

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    This was happening on a Mori Seiki lathe I just bought and it turned out to be a bent ballscrew. In my case I bought the machine like that and I assume someone at the last shop that owned it crashed it hard enough to bend the screw.

    In my case I knew it wasn't a tuning or stiction issue because the servo and ballscrew pulley rotated smoothly while the cross slide still shuddered. You could pull some covers and watch the motor and screw as you jog around - a sticking slide would cause the screw to bind at the motor, but a bent screw might rotate fine at the motor end.

    You can also pull the motor and see how it runs with no load, to check for an electrical problem.

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    My Fadal did that; the oil gallies were plugged, the thrust bearings wasted, and the motor bearings shot, the ballscrew was wasted, and gibs were flaking apart due to galvanic corrosion.

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    Quote Originally Posted by Perry Harrington View Post
    My Fadal did that; the oil gallies were plugged, the thrust bearings wasted, and the motor bearings shot, the ballscrew was wasted, and gibs were flaking apart due to galvanic corrosion.
    That's all?

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    Believe it or not, someone thought that was okay once upon a time. There was evidence the machine was used to cut MDF and some sort of scummy rubber or stuff that left a black greasy residue.

    Kinda reminds me of those Lincoln Navigators with blown air-ride and trashed interiors for dirt cheap on Craigslist. Once upon a time someone maintained the car and cared for it, but when lowlife Joe owns it, you're bound to find ash ground into the carpet and the vehicle sitting on the bumps.

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    Thanks for all the replies, looks like I have a few things to check, The machine has box ways and I always heard that box ways need more oil then the other types. I tried the manual puming handle on the oil tank and I did notice a small difference in the way it felt and sounded. From what I'm gathering it seems to me to either be lack of oil or servo tuning. If its servo tuning I dont even know where to start, I have an osciliscope available but the fanuc manuals I got with the machine are confusing.

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    Servo tuning rarely goes awry without parts failing or someone tweaking something.

    Pull the table and check the oil galleys and the Bijur metering units. The older oil has paraffin in it and would clog the metering units over time.

    I suspect you are thinking of going to the "easiest" thing first, but if you have mechanical problems and start messing with servo tuning, you can end up with an even worse situation.

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    If that is a machining center most of the time with power down you can move the table by hand, if so and it moves smoothly in both directions to the travel limits means it is an electrical issue, not mechanical. If it is a lathe turn the ball screw by hand to test for mechanical issues. Plenty of time an electrical issue will mimic a mechanical one under power in this instance. You could have a bad electrical component in the X-axis system causing the problem.

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    Quote Originally Posted by Perry Harrington View Post
    Servo tuning rarely goes awry without parts failing or someone tweaking something.

    Pull the table and check the oil galleys and the Bijur metering units. The older oil has paraffin in it and would clog the metering units over time.

    I suspect you are thinking of going to the "easiest" thing first, but if you have mechanical problems and start messing with servo tuning, you can end up with an even worse situation.
    This is all pretty solid advice. A machine this old will have Fanuc DC analog servos. Tuning this servo and control setup is a combination of control parameters in the control and pots on the drives. Oxidation on the pot's wiper can cause tuning to drift. Servo tuning would not be my first thought in trying to diagnose this type problem.

    First step should be to verify all mechanical aspects of the axis. Oiling failure, bad thrust bearings, bad ballscrew, unsecured gib, trashed Turcite (as a result of bad wipers letting chips embed or lube failure), are all places to inspect. Another occasional problem on the old DC servos is wear on the Oldham coupler between the servo motor and encoder. That more often manifests it self as loss in accuracy and "hunting" while stopped than shuddering while moving.

    Trying to tune out a mechanical/lube problem is poor practice and not a long term solution. Poor motor commutation (worn brushes) can cause instability in servo performance, but again, not as likely as a mechanical issue.

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    I would replace all the metering units, 32 years is a long time. Can you switch servo amps and motors from the x and y axis?

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    Or it could just be worn out brushes in the motor.
    Bob

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    And without doing a complete teardown, you won't know if it is A, B, C, D, E, AND F.

    BTDT, have the pictures to prove it.

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    I still have this problem I added a bunch of oil to the ways with an oil can and no difference. I took the brushes out and cleaned the carbon powder off them.The commutator didnt look to bad still shiny copper. There is tape around the end cap where the encoder is and it looks like the end might be bad on the cable. How do I figure out what cable I need if the ends bad?

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    Quote Originally Posted by camaro_dan67 View Post
    I still have this problem I added a bunch of oil to the ways with an oil can and no difference. I took the brushes out and cleaned the carbon powder off them.The commutator didnt look to bad still shiny copper. There is tape around the end cap where the encoder is and it looks like the end might be bad on the cable. How do I figure out what cable I need if the ends bad?
    What I would do is determine if we have a mechanical problem with the ways/ball screw/ball nut/thrust bearings...
    OR a problem with the motor/servo/wiring/tuning...

    Right now, it seems you're going at it kind of willy nilly. Have you eliminated a clunky ball nut yet? Thrust
    bearings??

    Pull the motor off and see what it does by itself.. Then put a drill on the coupling and see what you
    can feel.. Now we have easily halved where our problems can be.. If its on the motor side, swap the
    X and the Y motors.. If it follows the motor, we know where the problem is, if it doesn't, we've
    eliminated another thing.. Now swap the driver boards/control cards.. If all the X axis pieces end
    up on the Y, and the problem is still on the X, now we got a wiring problem..

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    Ok how do I check the ball nut and thrust bearings? I dont have experience checking the mechanical workings of the machine.

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    Are the ways getting oil???? Squirting some on them doesn't do much if the wipers are any good. You need to know that the auto oiler is doing it's job. If not your machine will be ready for the scrapper in little time.

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    Quote Originally Posted by camaro_dan67 View Post
    I still have this problem I added a bunch of oil to the ways with an oil can and no difference. I took the brushes out and cleaned the carbon powder off them.The commutator didnt look to bad still shiny copper. There is tape around the end cap where the encoder is and it looks like the end might be bad on the cable. How do I figure out what cable I need if the ends bad?
    Been a year and a half, and you squirted some oil on the ways, cleaned the brushes up and now you are scratching your head??? Read through the thread again, follow some of the advice given that long ago. Obviously squirting oil on stuff isn't going to fix the problem.

    R


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