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Wobbly pulleys - total dealbreaker?

Posted by jkeegan 
Wobbly pulleys - total dealbreaker?
April 12, 2010 06:32PM
Hey,

I've been building my Mendel for a few months now and I finally filed down all three of the axis stepper motors and fitted them with drive pulleys. The repstrap I was using is now in someone else's possession, so unless I call in a favor or two, I'm stuck with the parts I have, so I wasn't able to instead go with some of the alternative pulleys (such as the one with the trapped nut and a set screw).

So the problem is, despite my best efforts, the drive pulleys on all three stepper motors definitely wobble. When setting up the X axis belt, if I had it on too tight, it wouldn't even move (because it would wobble to its lowest position during tightening, holding the motor in one position). I eventually fit the belt much looser, and it does move the carriage, but moving the carriage feels a bit wave like, with it going a bit faster, then slow, then faster, etc.

So my question is, is this a *total* dealbreaker, and I should stop assembly now until I've resolved that (by calling in favors for the alternate pulleys and perhaps purchasing new stepper motors)? Or is it possible that I might be able to get the whole thing printing "somewhat ok" but good enough to print out the alternative drive pulleys myself?

Has anyone tried pushing forward with wobbling pulleys? What were their results?

And lastly, has anyone actually been able to take a wobbly mendel pulley and somehow correct it somehow?

Thanks. Once I'm actually up and running, I expect to be on here far more regularly. I've kept my head down for the most part, just trying to get to a working Mendel.


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
April 12, 2010 08:03PM
The original drive pulleys are a bit difficult to get right sometimes. Usually you have to drill them out a bit and that is hard to get that straight too. I wouldn't worry about it to much. Try moving them further down the shaft so that the belt rides closer to the motor relative to the pulley. It will be more concentric at that point.

I most definitely wouldn't worry about z, in fact take your worst motor/pulley and put it there.

If you still have problems contact me, I'll try to get you a set printed.
With wobbling drive pulleys It'll be pretty impossible to get the x, y axis precision to 0.1mm. It's not a dealbreaker, as most parts will print fine, but it'll be notieceable.
Re: Wobbly pulleys - total dealbreaker?
April 13, 2010 06:41PM
I purchased drive pulleys specifically to avoid that kind of issue. Mcmaster has some that matched my motors and belts perfectly. The only drawback is they have 10 teeth instead of 8. (I lose a little torque and gain some speed) They cost me about $7 each. So for about $25 you might be able to solve your problem. They drive my machine super smoothly.

Also someone posted that they were willing to send anyone a set of the drive pulleys in the forum somewhere.

cheers,
josh
Re: Wobbly pulleys - total dealbreaker?
April 21, 2010 03:16PM
Also if McMaster isn't easily available for you, like it is where I am I made a version you can get laser cut in acrylic locally or online with much higher tolerances then a RepRap can produce. These have been working perfectly for me so far although they require gluing to the shafts:

[objects.reprap.org]

/\ just make sure the spacing is right for your belts [it can all be adjusted in the open scad model], i have it set for the standard McMaster Carr belt specs on the drive pully stl.. Also you'll need a good 3 or 4 per pulley to get the right amount of height for your belt depending on material thickness.
Re: Wobbly pulleys - total dealbreaker?
April 21, 2010 03:33PM
So the other day I purchased the McMaster pulleys that jupdyke mentioned here, figuring they were only $7 each, and if they let me print even slightly ok I could then print out these if I really want to.

jupdyke, did I imagine this, or did I see (either in source code somewhere, or in a post of yours) that there was somewhere in software to actually specify the diameter or circumference of the pulley being used, so minimize software problems like having parts printed too big because of increased speed? Have you successfully printed things with these 10-tooth pulleys? Are they accurate?

Thanks everyone. Now I just have to worry about my leadscrew problem and I'm close to good to go!


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
April 23, 2010 05:34AM
I am using the makerbot firmware and ReplicatorG, so I don't know exactly where it is. But in ReplicatorG you can define your machine and part of that is how many steps is equal to one millimeter. I assume that the same can be done with the RepRap software. But I don't know exactly where. I am working on getting the RepRap software up and running as well.

cheers,
Josh
Re: Wobbly pulleys - total dealbreaker?
April 23, 2010 06:56AM
You set it up in the RepRap Preferences dialog window (click on the Preferences button from the RepRap Console window) and then set the proper values for: XAxisScale(steps/mm), YAxisScale(steps/mm) and ZAxisScale(steps/mm).


Bob Morrison
W├Ârth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse
Re: Wobbly pulleys - total dealbreaker?
April 23, 2010 07:14AM
I think if you are using g-code the host preferences are ignored and you have to recompile the firmware. The moves are sent in mm so it has to be the firmware that converts it to motor steps.


[www.hydraraptor.blogspot.com]
Re: Wobbly pulleys - total dealbreaker?
April 26, 2010 03:04PM
So just to follow up..

I know now why I had such a problem fitting the drive pulleys onto the stepper motors. The stepper motors I had had the wrong diameter shaft!

I'd originally purchased 4 of the Kysan stepper motors from MakerBot.com (which are fine). I'd incorrectly thought they didn't have enough torque, so on a whim, along with another purchase, I bought 4 "unknown" stepper motors from sparkfun.com (where someone had guessed they were NEMA 17 based on the spacing, etc). When it finally came time to file down stepper shafts, I did the actual calculations and found that they were almost the same torque, but the sparkfun ones (MERCVRY MOTOR, SM-42BYG011-25 51/2009) had sliiightly more torque, so I used them.

Well, when the McMaster pulleys mentioned above came in, I removed the printed pulley from a sparkfun motor and tried putting on the McMaster pulley.. The shaft was too big! I then tried the unused Kysan motors, and it fit like a glove.

Doh. Now I'll bet they would have gone on easily. I read the line about lightly using a mallet, and went with it. Grrr. Anyway, all is good (with respect to this anyway), now I'm using the Kysan MakerBot.com motors, with the McMaster pulleys (until I can print out some of these for the hell of it).

Thanks!


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
April 26, 2010 11:38PM
Note: The Nema17 spec doesn't specify the shaft diameter.

Given you're ordering some bits from McMaster (which puts you in the USA), I'd guess that the Sparkfun motors had a metric 5mm shaft, whereas the Kysan motors have an imperial 3/16" shaft.

3/16" = 4.7625mm

If you've got a pair of callipers about, you could measure this and know for sure.
Re: Wobbly pulleys - total dealbreaker?
April 27, 2010 11:13AM
Bingo, that's exactly it. As soon as I read your comment I knew that was the difference, but I measured it with my digital caliper to confirm.


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
April 27, 2010 11:33AM
(I updated the x-axis page to warn people about this)

Thanks again everyone!


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
May 13, 2010 06:33PM
Agh...

So I went to remove the pulley from my Z-axis stepper motor so I could file down the shaft, so the set screw would have a flat to push against..

..and the set screw seems stripped.. I can't remove it.. The pulley is stuck to the stepper shaft, which would be fine if I thought it wouldn't slip, but I think it will.

Agh.. The set screw seems to be 1/16".. I'll try getting another 1/16" hex driver, and I bought some ez-grip fluid after reading about it on the net. If I can find a left-handed drill bit that's that small, I'll buy one.. I don't think I'll be able to cut slots on the head to make a flathead slot.. Agh..


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
January 03, 2011 12:28AM
For anyone finding this thread down the road..

I recommend against buying those 10-toothed McMaster pulleys unless you've got electronics that do microstepping. A huge amount of the pain that I've been through can be attributed, in my opinion, to using these pulleys. They'll let you print some things, but they'll prevent you from getting decent quality. Switching to printed 8-tooth pulleys (printed by someone else) changed my print quality drastically. I'm using the 3rd gen makerbot electronics on my Mendel, which does not do microstepping, and it's clear from my test prints that the resolution wasn't good enough to not cause problems overlapping with infill overlapping perimeters, etc.

Don't make the same mistake I did.


--Jeff Keegan (Just Another RepRap blog)
Re: Wobbly pulleys - total dealbreaker?
January 03, 2011 02:58PM
The stepper kinda has microstepping, in the form of half steps, which the gen3 electronics have enabled by default.
Assuming the McMaster 10 tooth pulley with a diameter of 0.637"
That is approx 0.127 steps/mm

What are you getting for yours Jeff?
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