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Z axis losing steps - diagnosis tips?

Discussion in 'CNC Mills/Routers' started by Techvette, Sep 14, 2018.

  1. Techvette

    Techvette Journeyman
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    So, after going through about 10 different versions of this machine (1250 x 1000 c-beam router, custom plates yada yada), it's finally working correctly.

    Most of the time.

    For 2D cuts, where the Z axis doesn't move very often relative to the others, everything is great. But, in a 2.5D cut, the Z occasionally loses steps - like, a LOT, amounting to about 1/4" - resulting in a trashed workpiece.

    I'm using the CNC xPro - which I absolutely love - with four motors wired as bipolar parallel so as not to exceed the current capacity of the onboard drivers.

    I have the Z configured for 1/4 microstepping, and have the current down about as low as it'll go (roughly 25%) that will keep the motor moving.

    I suspect thermal cycling on the motor or the driver, but I have no real way to verify that.

    Any suggestions on how to approach this issue?

    It may be worth noting that the Z motor is a lot smaller than my X, Y and A motors. Still a NEMA 23, but one of those that's about 2" long. I can't find any specs on it, and I bought the thing ~10 years ago. :/
     
  2. Andreas Bockert

    Andreas Bockert Veteran
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    I’ve had similar issues twice.

    The first time it was a problem that the pinion was slipping on the lead screw and it wasn’t retracting enough.

    The second time was due to thermal issues. The fan used to cool my DRV8825s broke and this caused them to lose steps. If you don’t have active cooling then this might be a good idea. With cooling they performed quite impressively. I think I had them set to about 2A and used 48v PSU.
     
  3. GrayUK

    GrayUK Openbuilds Team Elder
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    Sorry this is a cross post.
    Wow! That's an old stepper! Treat yourself to a new one for starters. :)
    So everything is good in 2D but trouble comes with 2.5D. Due to the extra work being demanded from the Z-axis, I personally, and if you haven't done so yet, would check all the mechanical side first. Does it always happen after a set amount of time, or random? Is it always by the same amount? :rolleyes:
    Swop the Z=axis motor with the X-axis and see what happens then. :)
    If you start the same job again, does it need the Z-axis to be zeroed again? If you didn't zero it, would it then be 1/2" lower on the next run? i.e. Cumulative. :rolleyes:
    Just shots in the dark really, but something might ring a bell.
    :thumbsup:
     
  4. Techvette

    Techvette Journeyman
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    Thanks for the input, y'all.

    I ordered a new stepper today, with a bit more oomf.

    The Z is driven by an ACME lead screw, with one of those plastic anti-backlash blocks. So, no slippage there.

    I have two 40mm fans on top of the xPro, and three more in the enclosure. The temp gauge indicates that it doesn't get over about 90* F within the case (which is roughly 13" x 11" x 4"). Given the amount of work X/Y/A are doing, at higher currents, I'd expect them to shutdown due to heat before the Z did. For grins, I took the top off of the (VERY well-ventilated) box and tried the job again. The same thing happened.

    The error happens at *roughly* (not exactly) the same time into the run. If I didn't re-home the machine, it would continue to think that it was ~1/4" (visual estimate) higher than it really was. It's not off by exactly the same amount, but close.

    Swapping with the X motor is a good plan. I'll give it a shot. Or wait for the Amazon same-day delivery guy to show up and use the new one. ¯\_(ツ)_/¯

    Thanks again!
     
    GrayUK likes this.
  5. Andreas Bockert

    Andreas Bockert Veteran
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    My slipping was between the lead screw and the clamping part of the pinion (using the belt reduction coupler). Not between the AB-nut and the screw.
     
  6. Techvette

    Techvette Journeyman
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    Ah.

    I don't have a gear reduction system. Motor -> shaft coupler -> lead screw -> AB block.
     
  7. CNCMD

    CNCMD Veteran
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    So did you check the shaft coupler, those can work themselves loose. I too second going for the mechanical side.
     
  8. Techvette

    Techvette Journeyman
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    Yes. It's rock solid. I've been over the mechanical bits a dozen times already; nothing new. :/ (That doesn't mean there's not a problem, just that, if there is, I'm not seeing it.)
     
  9. GrayUK

    GrayUK Openbuilds Team Elder
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    Another thought. You haven't mentioned what Spindle/router you are using. Are you using the usual 8mm screw? I ask because if you have a screw which is fast, i.e. the angle of the thread is quite acute, on a Z-axis it can cause a spindle/router to be preferential to sliding down the thread at any given opportunity. :rolleyes:
     
  10. Techvette

    Techvette Journeyman
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    Well, considering I can park this thing at the limit switch, walk away and come back a week later, and it hasn't moved, I'm don't think it's just drifting.

    I can switch it over to half-step (really should've designed the enclosure to allow easier access!) and see what that does. I'm not sure how changing the microstep setting will impact this; it works for quite a while before suddenly stopping. If that was the problem, shouldn't it be evident immediately?

    BUT, given that it runs great for a period of time, then suddenly has a *huge* change, it has to be (correct me here, please):

    1. Motor doesn't get a good signal. Could be an intermittent break in the cable or, possibly, interference - though the Z cable isn't running near anything else, and it doesn't have an issue on 2D runs.
    2. Motor decides to take a break every 5 minutes. **** millennials.
    3. Driver cycles. Again, current is nearly all the way down.
    4. Driver cycles, because current is too low for what the motor needs. Hmm. Weird since this is a finishing pass, and, other than moving a lot, there's not a lot of force on the cutter.
    5. Motor sucks. It was cheap circa 2008.
    6. Cosmic rays.
    7. Infinite improbability drive.

    ... argh.
     
    #10 Techvette, Sep 14, 2018
    Last edited: Sep 14, 2018
  11. Andreas Bockert

    Andreas Bockert Veteran
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    Could it be that the current is too low and it’s losing steps simply because it doesn’t have enough torque?

    You could try generating some g-code that simply jog the spindle up and down for 5 minutes and see if it drifts when cutting air.

    Possibly combine it with fast movement along x and to push the drivers.
     
    GrayUK likes this.
  12. Kevon Ritter

    Kevon Ritter Master
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    What is being used for the spindle/router? Or did I miss that? Weight could be a simple factor that shouldn't be overlooked.
     
  13. Techvette

    Techvette Journeyman
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    It's a Bosch Colt. I have a little 400W spindle around here somewhere I could try if replacing the motor doesn't solve the problem.

    I'd need to make an adapter plate to let the spindle mount connect to my Z - I haven't used it in awhile. The previous iteration had a completely different linear motion system, with mounts for everything.

    IMG_4141.JPG

    That router *is* pretty heavy, especially relative to the size of the motor. It's weird that it runs for quite awhile before this error pops up, which is what made me suspect thermal cycling. But, that could (I think) be due to the motor working too hard as much as any problem with the current setting.
     
  14. Techvette

    Techvette Journeyman
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    I installed the motor a little while ago. Everything is working great so far. (Of course, now that I've said that...)

    The way I have my Z configured, the motor is supporting the weight of the entire stage, not just the router. A bigger motor like this makes sense.

    (More accurately, the leadscrew and AB block are supporting everything; the motor just keeps it from spinning and falling. )

    Thanks to everyone for the help. This would've taken me awhile to figure out on my own.
     
    #14 Techvette, Sep 15, 2018
    Last edited: Sep 16, 2018
  15. CNCMD

    CNCMD Veteran
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    I'm sure you will be fine. That motor you were using isn't going to be enough to hold. You should be fine now.
     
    Techvette likes this.
  16. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    One off-the-track thing that caught me a while back... Thought I was having Z skipping as the cut kept going deeper. Turns out I had somehow got some 6mm shank endmills in my tray (and the Dewalt has a 1/4 collet). Turns out that 0.35mm difference is enough that the collet didnt clamp on tight enough. The downward forces (Upcut endmill) basically slowly pulled the endmill out of the collet, and deeper into the material. :) Crazy one!
     
    GrayUK, Kevon Ritter and Techvette like this.

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