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Help with C-beam losing position (missing steps?)

Discussion in 'CNC Mills/Routers' started by Richmond Maker Labs, May 4, 2020.

  1. Richmond Maker Labs

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    Hi all,

    I'm currently trying to come up with a list of things to check as to why the Y-axis is losing it's position/ slipping....

    We're a small makers club and lucky enough to have a CNC, but the guy that built it has left, and I need some help with learning/ fixing it please.
    I'm reasonably confident that it was probably working at some point as people had made things on it in the past but I'm also willing to bet that some fundamentals may have been missed/ I could be doing things that haven't shown up before as I'm learning it.

    First thing is that the y-axis seems to be losing it's position. Here's a photo of where it's returned to for the second pass - probably a good inch from the zero point. I'm pretty confident the fusion/ gcode is ok but happy to share them if someone is kind enough to check for me
    File:CNC slipping.jpg - Richmond Maker Labs

    • I don't *think* it's the motor coupling. Hard to get to at the back but the grub screws seem tight but happy - trying not to strip it all down until I've exhausted less dramatic routes.
    • It's only happening in Y-axis (so far), I've seen it happen when I've done a test above the piece so I don't think it's to do with load (it also went throgh the wood in the above pic like hot knife through butter and, I think, the y travels were off piece anyway)
    • It's intermittent
    • Not ruling out the problem being me!
    Things I'm unsure of/ future tasks:
    • I'm not 100% that the motor current has ever been 'tuned'
    • Similarly, looking at the grbl settings below I suspect the steps/mm may need a little tuning
    • Need to look at updating grble on the board at some point but trying to change one thing at a time at the moment.
    Loud
    The motors make quite a bit of noise and I'm not sure whether this is a sign of something wrong, partly the housing acting as a speaker, or just normal. I'll get a video to show this when next there.

    Huge thanks in advance for any suggestions you can help with,
    Andy
    Richmond Maker Labs, UK

    Details

    Pretty sure it's a C-Beam, plate maker, with a Spark Concepts CNC xPRO Controller V3. I haven't tackled upgrading GRBL yet. Pic here
    File:CNCRouter.jpg - Richmond Maker Labs

    GRBL settings -

    Code:
    >>> $$
    $0 = 10    (Step pulse time, microseconds)
    $1 = 255    (Step idle delay, milliseconds)
    $2 = 0    (Step pulse invert, mask)
    $3 = 6    (Step direction invert, mask)
    $4 = 0    (Invert step enable pin, boolean)
    $5 = 1    (Invert limit pins, boolean)
    $6 = 0    (Invert probe pin, boolean)
    $10 = 1    (Status report options, mask)
    $11 = 0.010    (Junction deviation, millimeters)
    $12 = 0.002    (Arc tolerance, millimeters)
    $13 = 0    (Report in inches, boolean)
    $20 = 1    (Soft limits enable, boolean)
    $21 = 0    (Hard limits enable, boolean)
    $22 = 1    (Homing cycle enable, boolean)
    $23 = 0    (Homing direction invert, mask)
    $24 = 20.000    (Homing locate feed rate, mm/min)
    $25 = 2000.000    (Homing search seek rate, mm/min)
    $26 = 15    (Homing switch debounce delay, milliseconds)
    $27 = 1.000    (Homing switch pull-off distance, millimeters)
    $30 = 1000    (Maximum spindle speed, RPM)
    $31 = 0    (Minimum spindle speed, RPM)
    $32 = 0    (Laser-mode enable, boolean)
    $100 = 200.000    (X-axis travel resolution, step/mm)
    $101 = 200.000    (Y-axis travel resolution, step/mm)
    $102 = 200.000    (Z-axis travel resolution, step/mm)
    $110 = 2000.000    (X-axis maximum rate, mm/min)
    $111 = 2000.000    (Y-axis maximum rate, mm/min)
    $112 = 2000.000    (Z-axis maximum rate, mm/min)
    $120 = 100.000    (X-axis acceleration, mm/sec^2)
    $121 = 100.000    (Y-axis acceleration, mm/sec^2)
    $122 = 100.000    (Z-axis acceleration, mm/sec^2)
    $130 = 350.000    (X-axis maximum travel, millimeters)
    $131 = 248.000    (Y-axis maximum travel, millimeters)
    $132 = 200.000    (Z-axis maximum travel, millimeters)
    ok
    
     
  2. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
    Staff Member Moderator Project Maker Builder Resident Builder

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    You do have a Fan blowing onto the xPro? If not, it could be thermal shutdown (briefly cuts the driver when it overheats, then re-enables it, looks like skipped steps). a Fan blowing onto the board helps keep the drivers cool.

    Try turning the current setting down just a little too to help with overheating, the xPro was kinda known for that

    Or consider upgrading to an OpenBuilds BlackBox (see OpenBuilds BlackBox 4X Documentation )
     
  3. Richmond Maker Labs

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    Thanks, that's a great thing to test.

    Just been told new information -
    The Z-axis also sometimes doesn't hold, though haven't seen that myself.
    This may have all started when someone connected the spindle and stop buttons to the Xpro. Will revert that and test
     
  4. Richmond Maker Labs

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    PS Love a black box but out of our price range at the moment I'm afraid
     
  5. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Could also be thermal - it cuts out, start sagging under gravity. Its getting warmer your side of the world right? :) spring time?

    (; $10/month/member donation gets a BlackBox in no time (;
     
  6. Richmond Maker Labs

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    I think it may be warmer outside - but we're not allowed in the mystical outside land ;)

    This was happening in the cold months but I've just got round to looking at it. You know how it is.

    As you're there, and I will read more, is the current per motor, or one setting.

    Reason I'm asking is whether connecting the buttons/ spindle could have, in some way, altered that current? That's purely from a 'what has changed' perspective rather than my understnading how it happened?
    Was going to disconnect the spindle again to see if that made a difference?
    Is that a worthwhile test? (I'll try the fan/ current first though)

    Someone also suggested a possible wire break - not ruling anything out at the moment

    I'm off to get the video of the noise of the motors as well. I've always suspected they don't sound right (current/ steps again?) and would be very greatful for a second opinion please.

    PS Nice try, we charge £1, if people want to, when they turn up ;)
    (I know but we're about being accessible)
     
  7. Gary Caruso

    Gary Caruso OpenBuilds Team
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    Hi Richmond, halfway down this page you will see instructions on setting motor current.
    For sure get a cooling fan blowing on it, and check those wire connections.
    Gary
     
  8. Richmond Maker Labs

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    Thanks!

    Is there much to be gained by updating it from grbl .9 to 1,1?
    I'd rather change one thing at a time but for future
     
  9. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Nothing that will address this problem

    Also be very careful when upgrading on an xPro - it doesnt use a standard atmega328p. it uses the 328pb, the extra B being a slighly different chip. So using tools like xLoader bricks the board. (;
     
  10. Richmond Maker Labs

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    Notes to self/ the next guy of some other possible things to check (though I suspect the overheating is our issue)

    1. Is the duration of pulses being sent to the stepper driver too short, causing it to miss some pulses. If yes you'll need to extend the high-time on the pulse and maybe reduce the maximum pulse frequency
    2. Drive current too low/high. Either can cause missed steps, check the motor serial number and de-code it to find the recommended current but you can just tweak it up and down and see if either way helps
    3. Maximum speed set too high, causing it to miss steps when at top speed
    4. Acceleration rates in the firmware set too high, if it tries to accelerate the axis faster than the motor is capable of then steps will be missed
    5. Step/direction timing error when changing direction, does the direction line change too slowly, resulting on 1 or more steps being backwards? Scope could tell you.
    6. Loose/high-resistance motor connections
    7. Current scales with target acceleration, faster acceleration needs higher drive currents. So long as the motor isn't hot to the touch after a half an hour's running you're fine, warm is OK and expected from normal use, hot isn't
     
  11. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Yes, adding a fan to an xPro is almost a must. The drivers need to run cool. Turning current down a bit also helps as the drivers work a little less hard then making less heat
     

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