Racking on dual Y-axis

OKShapeO
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Racking on dual Y-axis

Post by OKShapeO » Tue Oct 22, 2013 4:31 pm

I've just put my Shapeoko together and it's racking badly on the Y-axis. I built it once from a kit according to the specs, had this problem, so decided that rather than sort it out, I'd put a few mods in first. They were to move the Y motors outside the end plates, move the idlers down and invert the belt so it has more contact area on the pulley, replace the MXL with GT2, and add a simple belt tensioning mechanism. Done all this, and I'm still having the same problem.

The z-axis and x-axis both work fine over their whole range. I've checked the frame and gantry for squareness and as far as I can measure they look spot on. If I disconnect the motors, the gantry runs back and forward freely end to end, with no play. If I run it with either left or right motor on its own, it's fine. As soon as I connect up the second motor, it racks, quite visibly within just a few mm of travel. It looks as though one motor is leading, and the other struggling to keep up. I've tried halving the steps/mm from 80 to 40, but it makes no difference.

I've checked the wiring very carefully. It's as in this post: http://shapeoko.com/forum/viewtopic.php ... 7530#p7530 ie serial wired. Suspecting there might not be enough current, I've tried increasing it on the grblshield. Turning the pot more than a very little (20-30 degrees) in either direction stops it working completely. Too much and it just thunks without moving, too little and it sounds like it's gone straight into thermal overload. So basically that's not really adjustable at all as it stands.

I've really ground to a halt on this and would appreciate any help anyone can give. At the moment I suspect either the grblshield or one or both of the motors but have no idea how I might check this.

OKSO

Improbable Construct
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Re: Racking on dual Y-axis

Post by Improbable Construct » Tue Oct 22, 2013 5:14 pm

This may be a silly question...
Did you remember to swap one pair of the wires on one of the motors? So that they spin opposite directions?
If so are you sure that you have a pair swapped?
I would also double check all of my connections. Could easily be a loose wire.
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OKShapeO
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Re: Racking on dual Y-axis

Post by OKShapeO » Tue Oct 22, 2013 7:00 pm

Thanks for the suggestions.
Not a silly question at all, but of course I did check. And remember both motors work fine individually and both turn in the correct direction. I would guess that if they were turning in opposite directions I'd get a lot more than racking!
I have also checked the connections and they all seem to be correct and well made. I also rewired it when I added the mods yet the problem persisted, which implies that's not the cause.

OKSO

cvoinescu
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Re: Racking on dual Y-axis

Post by cvoinescu » Tue Oct 22, 2013 7:56 pm

More suggestions, some of them silly:
  • One motor is 200 steps-per-rev, the other is 400;
  • The motors have different current and/or voltage ratings. Use identical motors if connecting both to one driver;
  • You have them wired incorrectly (remember, they may turn if wired wrong, but with much less force);
  • One or both pulleys are slipping on their shafts;
  • Try wiring the two motors in series rather than parallel. That limits the maximum speed, but gives you much more torque at low speed, and does not require extra current. It's a good solution when you're limited by thermal overload of the driver.
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twforeman
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Re: Racking on dual Y-axis

Post by twforeman » Wed Oct 23, 2013 1:17 pm

Another silly question: Do you have a fan cooling the GRBLShield?

The fact that increasing the current makes it shutdown is why I ask. I have dual motors wired in parallel and I can turn the pot all the way up. I don't know if I can make any axis shutdown due to thermal overload, but I haven't tried very hard.

Wiring the motors in parallel works great for my machine, you could give that a try.
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OKShapeO
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Re: Racking on dual Y-axis

Post by OKShapeO » Wed Oct 23, 2013 2:41 pm

OK, I decided to be methodical and follow the advice exactly - there's no advice sillier than some of the mistakes I've made in the past...

Step 1. Rather than just examining the wires I went round and tugged every single one hard - result [blushes bright scarlet] there were not 1 but 2 loose wires, one on the working z-axis, one on the y-axis! The problem is with the connector block mounted on the z-plate - it's very difficult to see what you're doing when wiring up the lower connectors. Anyway, I connected them properly, and tried again. Result - no better, possibly worse.

Step 2. Check the motor specs. Result: 3 x 42BYGHM809, 1 x SM42HT47-1684A. You guessed it, the odd one out is a 200 steps/min motor, the others are all 400 steps/min, and the odd one is on the Y-axis. So that looks like the problem. Thank you Inventables, who supplied the odd one out as an upgrade to the Shapeoko kit, without so much as a mention of a possible change in spec. Perhaps there should be a note somewhere in the wiki about this? I might have spotted it but, while the 42.. motors have '0.9 degrees' on the identity plate, the SM.. motor has only the part number so I just assumed it was the same. Well, you would, wouldn't you?

Presumably there's no reason why I can't just put the 200 step motor on the z-axis and use its motor for my second Y?

cvoinescu - the motors are wired in series, thanks to your earlier posts on this topic. I'm not bothered about the speed, it's only for occasional use, so I really want reliability and accuracy above all else.

twforeman - no cooling yet, but as I could never run it on that axis for more than about 10mm, and it's a rather chilly unheated room, I don't think that's a problem. While the lack of much apparent adjustability seems a bit odd to me (I can understand turning it down giving problems, but not up), in the light of the other issues, I don't think it's worth worrying about heating and adjustment until everything's fixed. I have an eye on it though.

Thanks everybody for the suggestions - most helpful.

OKSO

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Re: Racking on dual Y-axis

Post by WillAdams » Wed Oct 23, 2013 4:49 pm

I've added a note on the wiki.

Yes, swapping the odd motor out to the Z-axis seems the right thing to do --- you'll need to re-calculate / re-configure the Grbl settings, but that should be the only bobble.

For cooling, things like really weird if one runs the machine when steppers are going in and out of thermal shutdown --- bright side is it's an easy fix and means one is close to having everything up-and-running.

Looking forward to hearing of your success!
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cvoinescu
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Re: Racking on dual Y-axis

Post by cvoinescu » Wed Oct 23, 2013 11:13 pm

Glad you found the problem. Yes, use the 200 step-per-rev motor on the Z.
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etxbkst
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Re: Racking on dual Y-axis

Post by etxbkst » Thu Oct 24, 2013 1:46 am

I feel silly asking this, but what exactly is racking?

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Re: Racking on dual Y-axis

Post by WillAdams » Thu Oct 24, 2013 2:01 am

In this instance, racking means the gantry is being twisted by moving unevenly at either end.
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