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  1. #1
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    stepper motor questions?

    Hello,
    While doing a 3d carve into black walnut the other day, during the rapid the stepper motors for the Y axis would skip(?). I am using fusion 360 for design and code generation and it is the version where there is basically no rapids. The rapids move at the set feed rate while cutting. Anyway as it was doing the rapid it appeared the right side stepper would not move, randomly. Some times it was just fine then a loud grating noise. The left side (as looking from the zero end) was moving and the right side not moving. I do not know if it is just a alignment issue with the stepper, or maybe something else.
    A bit of history may help as well. I had a problem a while ago where the shaft broke off the Nema 23 motor and it was the same side that is now acting up. I replaced the motor but am unsure about the alignment, or how tight against the rack it should be. The Nema motor has a gear on the shaft which directly rides under the rack.
    Any thoughts or insights as to why this is happening?
    Thanks in advance for the help,
    rockaukum

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Re: stepper motor questions?

    If it works most of the time, then I would look for some crud in the rack. As far as pinion/rack engagement, about the only spec I can suggest is ''comfortably snug''. You might also look at backing the acceleration down a bit or reducing the ''rapid'' speeds a bit.
    Jim Dawson
    Sandy, Oregon, USA

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    Re: stepper motor questions?

    Jim,
    Thanks for the reply. I have seen something similar to what you suggested about tension between the rack and the pinion but was not sure so I had not tried that yet. I'll check the rack as well for debris however I'm not expecting to see much as the track is facing down and somewhat protected from the chips being tossed about. As for the rapids, It was kinda weird as when it "skipped" it happened mid run. With that in mind, doesn't the acceleration happen on the front side of the rapid? Anyway, I'll run a check on your suggestions in the morning.
    Rockaukum

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    Re: stepper motor questions?

    I'd check the pinion on the motor is tight. Sounds like a loose component. Peter

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    Re: stepper motor questions?

    Peter,
    Yes I'd think that as well. With that in mind, why did it not "miss steps" when the machine was under a load? Just thinking out loud here.
    Thanks
    rockaukum

  6. #6
    Community Moderator Jim Dawson's Avatar
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    Re: stepper motor questions?

    Yes, acceleration only occurs at the start of a move.

    Another possibility is that when running at high speed, the stepper is reaching the max RPM limit for the voltage applied. Stepper torque drops off rapidly as the RPM increases, and more voltage increases the torque at higher RPM. So slowing down your ''rapid'' speed might be helpful.
    Jim Dawson
    Sandy, Oregon, USA

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    Re: stepper motor questions?

    Hi Rock - if the pinion is held with a grub screw it can be an intermittent issue. It grabs it slips maybe and the other side can pull it back into place. I loctite all my pulleys and gears now as this has occurrred for me in the past. Was a hard one to track down. Had to pull drives down to discover it, The grub screw had worn a little groove in the shaft and it was slipping from one end of this groove to the other intermittently. was on a belt drive but I think same could apply here. Peter

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    Re: stepper motor questions?

    Peter,
    I checked the rack for any debris today. It was and is clean as it is hanging downwards. I did notice this (shown in photos) and it appears to be in the location when're the motor "skipped". You think this could be the culprit? I am heading out so when I get back later I will run the machine repeatedly at high speed across this area to see if I can reproduce the problem.
    Thanks for the help folks,
    rockaukum

  9. #9
    Community Moderator Jim Dawson's Avatar
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    Re: stepper motor questions?

    The picture doesn't look like a problem area. Maybe a little bump there, but I would not expect a skip. On the other hand, anything is possible. If that causes the gear to bind a bit right at that point, then the could overload the motor causing it to electrically decouple momentarily which would cause lost steps.
    Jim Dawson
    Sandy, Oregon, USA

  10. #10
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    Re: stepper motor questions?

    Is one rack installed backwards?

  11. #11
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    Quote Originally Posted by rockaukum View Post
    Peter,
    I checked the rack for any debris today. It was and is clean as it is hanging downwards. I did notice this (shown in photos) and it appears to be in the location when're the motor "skipped
    If the motors are sharing a power supply that tiny otherwise unnoticeable click could throw stepper into losing its “cog”. The single power supply is will push more amps to motor drawing (stepper not skipping) which feeds the cycle more.

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    Re: stepper motor questions?

    Hi Mem - The power supplies go to motor drivers not direct to the motors. Drivers are very well controlled and are current limited so unlikely to be a problem on the electronics side. To lose a step the motor has to experience more back torque then the drive torque the motor & driver can supply. This can be mechanical friction, an object. acceleration or decel overloads, cutting overload etc .... Peter

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    Re: stepper motor questions?

    The power supply is going to only put out so many amps. You have 4 motors drawing 4-5 amps each, as soon as the power supply reaches its capacity all motors will de-tune and not equally. Add to that as soon as one starts decel it is back driving voltage, lowering the voltage to other motors while still drawing amps.
    once a stepper looses its cog they become very weak, the other motors will start absorbing any available amps. The reason it is able to catch cog again is it is being jump started by the other x axis motor dragging it.
    Steppers that work in a coordinated motion really deserve a single power supply per motor.

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