I was looking over Wompus's post where he moved the "Y" stepper motor outside the case and put it in line with the drive rod, and this got me thinking...

Has anyone considered using 2 stepper motors to drive the "Y" belts directly?  There are at least a couple of ways of mounting the motors.

One would be from the outside of the frame with the shafts sticking inside (would require motors with longer shafts).  Another way would be to make some kind of mounting bracket (plastic or metal) that would let us mount the motor on the inside back face, with the shafts pointing towards the left and right side.

The big question is whether or not we could drive the 2 motors from the same output port. 

Hopefully some of the electronics wizards here might be able to chime in on this.  Can the stepper drivers handle 2 motors?  And this question pertains to both boards, Sanguinololu and Printrboard.

For future reference about this approach see here:
http://www.soliforum.com/topic/15753/so … tor-drive/

I will add to that: Since HBOT has a single belt, belt tensioning has less of an impact on the system (perfect circles). The bad thing is the X axis flexing out of square. As the distance between ends increases, the linear rods have less chance of being able to keep the axis square to the Y axis with fast movements. A good compensation for this is using linear rails instead of rods.

A COREXY has the benefit of using the tension of the two separate belts to counter-act the problem with keeping the X axis square, but requires both belts to be perfectly tensioned to each other.

I prefer HBOT on the merit that it is easier to setup. You can use proper design to keep your axis square, and the single belt makes taking things apart repeatedly much easier. You dont have to tighten the belt at the ends either, if you put oblong mounting holes for the motors so you use the motor to tighten the belts (like the Y motor on the SD).

Here is a good illustration of an HBOT:
http://www.thingiverse.com/thing:161000

Great info. Makes sense as to why h bot is a bit more popular at the moment. One other small thing, coreXY can be tensioned in the same way (with the motor mounts being elongated that is.

CoreXY and H Bot are slightly different. They look the same, but have slightly different belt paths. Each have their benefit.

CoreXY has two shorter belts, all four ends attach to the x gantry.

H bot only has one, and so only two ends are attached to the gantry.

CoreXY's belts are shorter, so in theory it should have less backlash, but the belt has to cross over (like in the video) for it to work. I personally prefer coreXY over h bot, but that's just my opinion.

I will add to that: Since HBOT has a single belt, belt tensioning has less of an impact on the system (perfect circles). The bad thing is the X axis flexing out of square. As the distance between ends increases, the linear rods have less chance of being able to keep the axis square to the Y axis with fast movements. A good compensation for this is using linear rails instead of rods.

A COREXY has the benefit of using the tension of the two separate belts to counter-act the problem with keeping the X axis square, but requires both belts to be perfectly tensioned to each other.

I prefer HBOT on the merit that it is easier to setup. You can use proper design to keep your axis square, and the single belt makes taking things apart repeatedly much easier. You dont have to tighten the belt at the ends either, if you put oblong mounting holes for the motors so you use the motor to tighten the belts (like the Y motor on the SD).

Here is a good illustration of an HBOT:
http://www.thingiverse.com/thing:161000

CoreXY has two shorter belts, all four ends attach to the x gantry.

H bot only has one, and so only two ends are attached to the gantry.

CoreXY's belts are shorter, so in theory it should have less backlash, but the belt has to cross over (like in the video) for it to work. I personally prefer coreXY over h bot, but that's just my opinion.

HBOT is a single belt.

Did you look at my link?

I think it's a matter of perspective. I tried to look up H-bots, like http://www.youtube.com/watch?v=dVMA883Quok, but that's two belts not one. It's just two belts acting in unison. 

At least, if you're making this claim, please help me understand the difference between an Hbot and the link I sent [the CoreXY one] I am having trouble understanding

EDIT: Below it is clarified [I am wrong]

http://corexy.com/theory.html

Uhhh...  you might want to get a second belt just in case.

My bad. Firmware calls it COREXY. What I am talking about is HBOT. Single belt.

sometimes it's cheaper to get one longer belt and cut it into two suitable lengths... depends on the type of belt

True enough.

C'mon guys.. where's the spirit of adventure?  And what's wrong with turning our machines into the ultimate Rube Goldberg machines just because we can 

Uhhh...  you might want to get a second belt just in case.

The idea behind this is that most of our problems with getting nice perfect circles is in the slight misalignment of the "Y" belts.  If one belt has more/less tension than the other or is not lined up almost perfectly, or the connecting shaft slides left or right, it creates an imbalance which contributes to the 'skewing" or the flat spots we see when we print circles.

Using 2 motors running in sync, that drive the belts directly without any additional interconnecting rods or extra drive belts, should in theory provide a perfectly synchronized, carriage.

I asked because I knew the answer. And the design is wrong for that answer.

Let me add to that: A solid shaft is more accurate than any possibility of breaking the system apart. There is no backlash or flexion from one end of the shaft to the other end. But that's moot. Your trying to solve the problem by changing something that is not even part of the problem. As adrian pointed out, perfect circles comes down to tension of the 2 Y belts (which your design does not address at all) and that also being tensioned in sync with the X belt (another part not being addressed by 2 Y motors). You can do all this work, and your problem is still completely unchanged.

You want a good solution to perfect circles? COREXY. A single belt driven by 2 motors for both X and Y axis. You have a single equalized tension in both directions, and your tension does not have to be perfect either (just enough to remove slack, but not enough to stretch). Another benefit is that the 2 motors are statically mounted to the frame, which means there is no motor adding weight to the X axis. Which means lighter carriage assembly, less torque, and less motor heat.

Hell, I have been thinking about doing this with all original SD3 parts. Use the Y and E motors for the CORE, and then use the X motor as the extruder with a Wades geared setup. Purchased parts would be 2 GT2 pulleys, 624ZZ bearings for idler pulleys, 625 pulleys for the X ends, GT2 belt (open), and a dozen LM8UU (get twice what you need to pick out the good ones). All of that is less than $20 from robotdigg. The wades would increase the steps/mm and the torque, and help get rid of that moire effect (although at the cost of speed, but you dont really need it on the extruder).

That has the potential of getting payback for the effort.

The twin z steppers aren't physically linked on the printrbot and it doesn't seem to be a problem.

Screws can't be backdriven, belts can. If you bump a dual belt system with the motors off... welp, see ya later!