Corexy belt tension and acceleration values.

[Timothy Zalusky]

Hello All,

I've been working on my own corexy using all misumi linear components (except for some chinese ball screws), and I just got it fully operational yesterday.

Now to tune it to make it as awesome as possible!  I have the belts pretty tight, but I was wondering how tight they should be compared to a normal cartesian machine.

Any tips there?  From what I read through searches, plucking a belt should be close to a low E string on a guitar, is that for a shorter belt span, or, a main, full span?

Also, what kind of acceleration values are you guys using?  I set it to 3000 mm/s^2 and it seemed to be fine.

-Tim

[Ryan Carlyle]

Pluck pitch depends on both span length and belt type (specifically, mass per unit length). I wouldn't bother trying to get a particular note, so much as ANY note -- if it gives a "plunk" noise at all, it's probably in the right ballpark for tightness. 

Nominally, a 6mm GT2 belt is rated for 6 lbs pre-tension. That's not all that much. But that's also spec'd for power-transmission service, where working load adds significant tension on top of the preload, and the belt is continuously rotating for extended periods. In 3D printing, we want the preload to be the majority of the tension, and have acceleration loads be smaller. So I think it's justified to preload higher than spec. There's really no official guidance though. 

As for whether CoreXY needs different belt tensions from regular Cartesian, I would say no. The longer belt spans mean belt stretch is slightly magnified, but not THAT much. In rough terms, each belt run is twice as long, but you also have a 1.41:1 pulley reduction inherent in the CoreXY mechanism, so stretch is only 41% larger for the same acceleration. And belt stretch is rarely an issue in any regular-sized printer to begin with unless garbage nylon-core belts are used. 41% more than "negligible" is still probably negligible.

[Ben Holmes]

Just out of curiosity since I was researching it for a different project and haven't found the answer yet.  What kind of tool could be used to measure the 6 lbs of pre-tension in-line?  The only tools I've been able to find cannot measure tension in-line.

Ben

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[Ryan Carlyle]

There are pluck pitch calculators where you need to know the belt mass per length, and deflection testers. With deflection testers, you basically push on the center of the belt span with a simple spring-loaded force scale, and measure the force when the belt makes a certain triangle. (E.G. 10:1 span/deflection ratio -- it's specified by the belt OEM with their force scale doohickey).

[William Cook]

That 1.41:1 ratio is the square root of 2 I assume? Because the motion of a motor produces diagonal movement?

[Ryan Carlyle]

Yep. The belt/idler arrangement produces a weird-ass compound pulley with a sqrt(2) reduction ratio (and of course 45 degree rotated output). That's one of the big reasons, in my opinion, why CoreXYs produce better print quality than other Cartesian-gantry printers: the motors are "geared down" to a more favorable drive ratio. The reflected inertia from the gantry seen by the motor is cut in half by that sqrt(2):1 pulley reduction. Plus you obviously aren't slinging X or Y motors around. 

[Jason B]

I'll add that if you've done the math for a particular span pitch for your belt but you lack a tension meter, you can use a cheap microphone connected to your PC and record the pluck in something like Audacity.  It's a free sound recorder that can give you the frequency your belt produces.  

But really, you don't need to be that accurate for a printer.  Just make sure both sides are as equal as your MK I ear can detect and you'll be fine.

[Ryan Carlyle]

I wouldn't say "both sides are equal" so much as "both sides have some tension and the gantry is squared up." CoreXY belt tension affects gantry skew, and if your gantry is slightly out of square (eg printed Y carriages not perfect) you can use differential tension to fix that. 

[Timothy Zalusky]

I engineered in some compliance into my gantry, and yeah, tension really affects the skewing of the gantry.  I've got it pretty good now, but I would like to put an indicator on it just to make sure.

[Ryan Carlyle]

The easiest thing is to make sure your frame is bang-on square (which is should be in any case), move the gantry bridge to a hard-stop against the frame, and make sure you have equal gaps between the frame and each end of the bridge.

A lot of people on G+ say they have issues with CoreXY belt tensioning, but I really don't know why. It seems very simple to me. 

[Whosawhatsis]

It depends on how you design it, and your order of operations in assembling. You could, for instance, use one long belt and tension it with the other side loose against the effector. Drive the Y axis forward or back to a hard stop to make sure it's square, then clamp the belt in place to keep it that way.

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[Joseph Ecker]

In addition to Ryan's suggestion, I used a tape measure to measure diagonally from corner to corner on the top of the printer to eliminate any parallelogram-type alignment.  I had been printing lots of big stuff and found out that one diagonal was about 1/8 to 3/16ths shorter, even though the gantry bridge was flush against the frame.  doh!