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Z-Axis Troubles

Discussion in 'CNC Mills/Routers' started by hasemar, Jul 9, 2016.

  1. hasemar

    hasemar Well-Known
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    I've been running my C-beam machine for a little while. I have gotten a few good runs in. There has been some serious learning along the way!!

    I am having a problem with my Z-axis that I cannot figure out.

    I have been trying to machine an aluminum fixture that is basically a .5" dia hole .75" depth with an 1/8" slot cut in the side ( making a clamping fixture to hold a .5" piece of round bar.) Anyway, when I run the job it never cuts the whole .75" depth. (I am taking .010" cuts). Then when the tool goes back to w-pos zero it is higher than where I zeroed it.

    I ran the job again and slightly grabbed the z-axis shaft coupling to feel what it was doing during the run and I can feel it "let go" every once in awhile and I can physically turn it any direction I want like the motor isn't being driven. The same thing occurs when I am routing wood too.
    I thought maybe I burned out the z-axis driver, so I changed it out and ran the job again with the same result.

    Any ideas?

    I am driving Nema23 2.8A 1.79in-oz motor with DRV8825 drivers VREF set at 1 v. I have 1/8 microstepping enabled. I am using BCNC the code is running from the RPi SD card instead of the usb. No other programs running.


    Thanks in advance for any help.
     
  2. David the swarfer

    David the swarfer OpenBuilds Team
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    You need one or more of:
    more voltage,
    more current into that motor,
    less microstepping,
    lower speed,
    lower acceleration,
    more cooling.​

    Do you have cooling on that driver? it may be overheating and shutting down, they need heatsinks and forced air to keep them cool at 2amps.
     
  3. JustinTime

    JustinTime Master
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    I don't know the software you are using but you should never be able to turn the stepper while the machine is turned on. There should be an electrical brake that prevents the stepper from moving. It's done by shorting the field connections. I don't know if in your software this feature should be manually turned on or if it is automatically turned on. Maybe this is why you can turn the stepper.
     
  4. hasemar

    hasemar Well-Known
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    I will do more investigation with those parameters in mind. Thank you.

    JT, never? I turn all the axiis manually if my mpos isn't zeroed correctly. ( The machine is in idle mode). I don't have homing switches installed yet. The only time I cannot turn the steppers is when they are on motion or suppose to be holding a position.
     
  5. JustinTime

    JustinTime Master
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    Theoretically, when you turn on your machine and the driver boards are powered, the motors should hold position. My router does. If it doesn't, the weight of the spindle may cause the z axis to move down. It's a problem with 3d printers where they don't have that feature and there the weight of the print head is not as high as a router spindle, by far.
     
  6. David the swarfer

    David the swarfer OpenBuilds Team
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    Many CNC controllers have a timeout after which they will disable the stepper drivers.
    This is great if your motors get too hot because you are running too much power through them, shutting them down when they are not needed is useful to keep them cool.

    however....

    with open loop microstepping drivers the motors will instantly shift position to the nearest magnetic pole (the click you feel when you turn them by hand) and next time you enable the drivers the motor will not be where you left it.
    a Z axis with a ball screw might turn under the weight of the router, losing position quite throroughly.

    Closed loop systems (they have a sensor to tell the drivers where the motor shaft is) should not have this problem.

    all that aside, being able to turn your motors by hand when powered means there is just too little torque i the system.
    My NEMA23 motors (the ~170oz/in ones) are impossible to turn by hand with 1amp at 24 volts. I will have to try and do it with a spanner (-:
     
  7. Barry Danks

    Barry Danks Veteran
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    I would swap out the Z Axis stepper. The armature is pressed onto the stepper shaft it might be possible that armature is slipping on the shaft. At least it would eliminate one possibility :)
     

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