Need help understanding idler placement in a CoreXY belt path.

[Nick Foley]

I'm setting up my first CoreXY printer (crude 3D model on OnShape here), and am trying to wrap my head around the ideal placement of the idler pulleys. In a world where belts had zero thickness, it seems like the goal would be to have all pulleys equally spaced as the four vertices and two midpoints of a rectangle. Since we are forced to adjust that scenario to prevent belts rubbing on each other, it seems like the motor pulleys need to be spaced apart furthest, the opposite side idlers need to be slightly closer together, and the gantry idlers closer still. 

As the idlers deviate from a rectangular plan, though, aren't we introducing XY coordinate space distortion over the bed because of the triangular belt paths? Do these distortions cancel in a way that isn't immediately obvious? Are they negligible? Are they corrected in the firmware? How critical is it that the idler placement is perfectly symmetrical across the perpendicular-to-gantry axis? 

[whosawhatsis]

The runs of belt that attach to something that moves (and thus are variable-length) need to run parallel to that direction of movement so that their length varies linearly with distance traveled. Runs of belt between two fixed points, such as the motor pulley in the front and the idler pulley in the back on the same side (or however you orient them), or between fixed idlers where the belts cross, don't need to be parallel to anything because their lengths are constant.

On Sunday, February 7, 2016 at 10:06, Nick Foley wrote:

I'm setting up my first CoreXY printer (crude 3D model on OnShape here), and am trying to wrap my head around the ideal placement of the idler pulleys. In a world where belts had zero thickness, it seems like the goal would be to have all pulleys equally spaced as the four vertices and two midpoints of a rectangle. Since we are forced to adjust that scenario to prevent belts rubbing on each other, it seems like the motor pulleys need to be spaced apart furthest, the opposite side idlers need to be slightly closer together, and the gantry idlers closer still. 

As the idlers deviate from a rectangular plan, though, aren't we introducing XY coordinate space distortion over the bed because of the triangular belt paths? Do these distortions cancel in a way that isn't immediately obvious? Are they negligible? Are they corrected in the firmware? How critical is it that the idler placement is perfectly symmetrical across the perpendicular-to-gantry axis? 

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[Nick Foley]

Thanks, that makes it clear. I was hoping for a slightly more compact layout, but I now see that isn't possible (without software correction?).

[John Gelnaw]

On Sunday, February 7, 2016 at 1:06:25 PM UTC-5, Nick Foley wrote:

I'm setting up my first CoreXY printer (crude 3D model on OnShape here), and am trying to wrap my head around the ideal placement of the idler pulleys. In a world where belts had zero thickness, it seems like the goal would be to have all pulleys equally spaced as the four vertices and two midpoints of a rectangle. Since we are forced to adjust that scenario to prevent belts rubbing on each other, it seems like the motor pulleys need to be spaced apart furthest, the opposite side idlers need to be slightly closer together, and the gantry idlers closer still. 

As the idlers deviate from a rectangular plan, though, aren't we introducing XY coordinate space distortion over the bed because of the triangular belt paths? Do these distortions cancel in a way that isn't immediately obvious? Are they negligible? Are they corrected in the firmware? How critical is it that the idler placement is perfectly symmetrical across the perpendicular-to-gantry axis? 

Ryan Carlyle posted a nice summary over in the 3D Printer Tips/Tricks Group.

[whosawhatsis]

You can't correct for it in software. The failure mode is a change in belt tension. You would need another actuator to even attempt to compensate for that.