Fixing eccentric drive pulleys?
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Hi all,
I'm building a large CoreXY machine - about 800mm x 800mm, and I'm struggling with belt tension issues. In particular, I notice that the stepper motor pulleys are slightly eccentric - they're secured onto the motor shaft with 2x grub screws. When you move the print head by hand you can feel alternating loose and tight spots, and you can visually observe the motor pulley moving slightly eccentric. In industry, we'd use taper lock shaft bushings to maintain concentricity, but I doubt they exist at this size. My next thought is maybe I can buy the blank gear tooth extrusion (teeth only, solid bar, no other features) then turn down a shaft that's supported in bearings, and driven by the stepper motor via a coupling - however, I haven't found the extrusion available anywhere.
Any suggestions?
Thanks
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Either from SDP-SI or Misumi if you are in the US.
Both carry the extrusion stock as well.
If you really want taperlock or similar, you would need to go up to something like a 3mm pitch belt with a 34 tooth pulley.
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That would be an issue with an oversized bore more than the screws, I'd think. Yeah, taperlock would be cool, but a centered bore .0005" or .001" over the shaft diameter should run well.
Would it be easier to buy pulleys with an under-sized bore and bore/ream them to fit the stepper shaft?
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I've found the pullies can be easily deformed by over-tightening the grub screws. If you have a new pulley, try installing it while paying close attention to the amount of force on the grub screws.
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I recently ran some tests on my corexy printer and came to the conclusion that one or both drive pulleys are either drilled off center, shifted due to grub screws, or the motor shaft(s) are bent. I measured the effect by mounting a digital gauge to read the displacement of the Y axis bearing blocks as the extruder carriage moved in X.
I wrote a blog post on it here: https://drmrehorst.blogspot.com/2020/12/x-axis-wobble-in-ummd.html
I'm going to order drive pulleys from Misumi and rerun the measurements.
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I asked a related pulley question and came to the same conclusion, buy pulleys from industrial suppliers. I like your method though since it gives quantifiable data. I just used an engineer square propped up to see the barest sliver of light and if the light disappeared as a tooth passed or the light grew to be more than a sliver I thought I could judge eccentricity. Turns out almost all of my pulleys actually have a U shaped profile in the tooth such that if you lock down calipers in the middle, you can't slide them to the edge of the pulley, and not at a consistent stopping place either.
