Core XY belt paths
October 13, 2018 01:23AM
I'm building a core xy printer based on the fabotatum design (no crossing belts) as in the image below. It's coming along fine but I was reading some threads here about how to adjust the tightness of the belts. I think digital_dentist showed some images that got me thinking about something.

I've always been of the assumption that for this design all the belt paths must be either perpendicular or parallel to the x/y carriage. But after seeing some of the images of how people were suggesting doing the belt tightening, I'm not sure that's necessarily true.

So my question is which belt paths (if any) are required to be perpendicular/parallel to the carriage? If possible I would like to use the drive pulleys that carry both belts (upper ones in the image) and move them outword to tighten the belts, but would that cause any issues?


Re: Core XY belt paths
October 13, 2018 01:42AM
Checkout the excellent writeup that the_digital_dentist made about belt paths.

CoreXY Mechanism Layout and Belt Tensioning

-os3dp
Re: Core XY belt paths
October 13, 2018 01:55AM
In your case, you won't be able to use the dual idlers as tensioners. You'd have to use two stationary idlers and two moveable idlers.
A simple rule is: all belt segments that change length have to be 'true'.
Re: Core XY belt paths
October 13, 2018 03:22AM
Quote
os3dp
Checkout the excellent writeup that the_digital_dentist made about belt paths.

CoreXY Mechanism Layout and Belt Tensioning

-os3dp

Thanks for that link. That's a good writeup. @o_lampe, thanks for your response also. I think what I can do is make the motor mounts moveable in a forward/backward direction. That would allow me to adjust the tension without affecting parallelism.
Re: Core XY belt paths
October 13, 2018 06:27AM
The sad truth is, a lot of people lay out the belt paths incorrectly and then wonder why print dimensions are off sometimes but not others, or never realize that there's a problem and just accept that varying belt tension is a "feature" or the mechanism. It isn't. In a properly laid out system, the tension will not vary with extruder carriage position. In this diagram, segments labeled A-H must be parallel to the guide rails for proper operation of the mechanism.



The diagram above isn't the only possible layout for the mechanism. I saw one design in which the motors were placed in the M segments. That should work fine, too, but will require extra pulleys to ensure sufficient wrap around the drive pulleys. The diagram above is probably the optimal in terms of minimizing the number of pulleys. Every pulley you add increases moving mass and friction.

This diagram shows all the places you can adjust tension without screwing up the motion of the mechanism:



I recently built a large (about 1.8 x 1 m) corexy mechanism for a sand table that put the belts on the same level and twisted/crossed them in the M segments. I didn't leave it that way for long- the belts rubbing against each other at the twists/crossover were grinding rubber off the backs of the belts, and the belts kept climbing and riding on the pulley flanges. I converted it to stacked belts and all problems were solved.





Edited 1 time(s). Last edit at 10/13/2018 06:50AM by the_digital_dentist.


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