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Thread: Alarm 421 on Fanuc 6M

  1. #1
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    Default Alarm 421 on Fanuc 6M

    I replaced the Y axis Velocity Control unit with a used known good (?) unit from a different machine.
    After setting the jumpers and Trim POTs RV1, RV2, RV3, RV4, RV5 to the same position as the removed VCU, the machine is up and running.

    But I get the occasional Alarm 421 while rapid traveling at 100%, it does not occur at 50%.

    The Fanuc 6M repair (?? the cover is missing so I don't know the real title) manual lists the jumper and Trim pot settings for the Velocity Control Unit. For trim Pot RV4, the Tacho-generator compensation, it says:
    1) normally 5 scales
    2)Use for fine adjustment of loop gain. Refer to maintanance manual of NC

    My question is "Which manual should I refer to?, The machine manufacturers?, Some other Fanuc manual?

    Or "How do I adjust this issue out of my machine?"

    The 421 does sound like an issue with the Tacho gain and over shoot.

    I have an Oscilloscope and the ability to use it, just not the VCU knowledge.

    Any help, comments, questions, condolences or snide remarks would be apperciated.

    vfsi

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  2. #2

    Default 421 Alarm

    Hello
    Sounds like you may have a bad motor.
    Check the brushes and Megger the motor.



  3. #3
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    Default

    cncdiag,

    Thanks for the suggestion, but I just had a good motor shop replace the brushes and test the motor. That all checks fine.



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