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First time CNC having some stepper issues

Discussion in 'CNC Mills/Routers' started by FUGO, Aug 17, 2017.

  1. FUGO

    FUGO New
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    Hi all, this is my first time setting up a CNC router. I have the OX CNC with Nema23 steppers and the CNC shield paired to GRBL running V1.1. After having it all setup and messing around with it getting it to jog using Universal G-code I first found the steppers didn't move but vibrated like it's stalling and can't make it to the next pole. The steppers ran fine with the belts off and so I loosened up the wheels and it moved but with the slightest touch of my hand it stalled and struggled to move. I bumped up the pots bit by bit until they gave some resistance but at this point the pots were maxed out. Im not familiar with tuning CNC machines could anyone shed some light?
     
  2. SugarJ

    SugarJ Journeyman
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    Which drivers do you have on your CNC Shield?
     
  3. FUGO

    FUGO New
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    A4988's
     
  4. David the swarfer

    David the swarfer OpenBuilds Team
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    set GRBL to low max rate and low accelerations then give G0 commands to move (not jogging!).
    Do one axis at a time. Don't go so low that the step rate is below 40Hz, GRBL cannot do that.

    Double(1) the max rate until it stalls during a move. Now try 75% of that and reduce until it works fine.
    Note that you will need quite a long move to get up to max speed with the acceleration set low.

    Now you can double the acceleration rate until it fails, and back that off till it works reliably.

    The above gives you a high max rate with a max acceleration that is lower than it could be.
    for a Z axis you may want to exchange some max rate for higher acceleration, ie lower the max rate and increase the acceleration.
    This is because Z moves shorter distances more often, so by accelerating faster you get a higher average speed.

    note 1: why double? this gives you a quick progression to failure. say 500 and 1000 works but 2000 does not, now we know that somewhere between 1000 and 2000 will still work. we can try 1500, if that works, try 1750, if 1500 fails, try 1250.

    Note 2 : the commands you can use are quite easy.
    first put it into relative mode by giving a 'G91' command
    now you can move any axis from the current position and back by giving
    G0 X100
    G0 X-100
    change X to the axis you want to move. I am assuming millimeter mode, adjust your numbers accordingly.
    (GRBL calibration numbers are ALWAYS in mm)

    Note 3 : steppers run better at higher voltages. use about 80% of the max your drivers can handle. so if they are rated to 30 volts, use a 24 volt power supply. set the current currectly for the motors you are using. too low is useless.
     
    Rick 2.0 likes this.
  5. FUGO

    FUGO New
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    Thanks for the info I put that all through and tested it out and had numbers such as 300 for my max acc and up to 65,000 for max speed and played around a bit with 2000 max acc and 5000 max speed and in all cases if put a tiny bit of force against it moving it will seem to stall but make a vibrating noise instead of the higher pitch stalling sound when testing the max speed. I don't see it coping with cutting
     
  6. David the swarfer

    David the swarfer OpenBuilds Team
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    an OX would never cut at 5000mm/min, but if it reliably rapids at that speed that is good.
    for cutting you are looking at more like 1000 to 2000mm/min, depends on cutters, RPM and material.
    you should use one of the many online feeds and speeds calculators, then use a shallow depth of cut to suite the machine.

    what current are the drivers set at? are you using forced air cooling? the spec says it needs heatsinks and fan if set for more than 1 amp.
    what voltage are you using?
    have you set the roller preload according to the video?

     

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