Dual "Y" drive motors (Page 1 of 2)

Also, "Y"? (why?)

I don't see the Y-axis being very long to have a problem driving it from one side.

Yes, you can run dual steppers, but, you are limited in that you still only have a 2 amp use able max on the Stepper Drivers unless you have special high current multilayer drivers, or a massive amount of active cooling. So your total torque is often not much improved vs a single decent stepper motor. It's more about the link rod than anything.

Running multiple drivers can introduce sync issues, so also has issues to consider.

Consider also this will increase current needed on the board as well, meaning you are really pushing any under spec/over driven connectors and tracks.And on a RAMPS board will put you over the 5 amp limit on steppers unless you replace the PTC fuse or pay attention to tuning your total current on all axis.

What about using smaller motors?

The Palolu stepper drivers (and the ones on the Printrboard) are rated at 2A.  The average NEMA 17 stepper is rated at 1.5A+, so obviously 2 of those would seriously overheat the stepper, but what about using 2 smaller, NEMA 14 stepper motors?  The average NEMA 14 stepper draws 1A or less, so having 2 of those should not overheat the driver, and having 2 of them should make up for the lower torque when compared to the NEMA 17.

I assumed the dual Y-axis would still be linked, as that way you would not really have a step issue. It seems that with two small unlinked motors you risk skewing without any clear benefit.

At a certain size, though, it can make sense. You would want some slightly high voltage rating/lower current steppers, and drive them with an increased voltage to overcome their inductance. But you would still want the two steppers on the same board. I don't know what the voltage limit is though on the control boards we use.

Our motor drivers are current-limited devices, so we can run a stepper that is rated to a higher voltage and get a corresponding increase in driven power. But higher voltage steppers at the same size are going to be higher inductance, which dictates how quickly you can step/run them for a given voltage.

Anyway, just throwing some stuff out there. overall seems everything is covered

Why?  Why not.  We make all kind of mods to our printers, and a lot of them are unnecessary anyway.  So why not this.

But since you asked...

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.

As for the complexity of it, it's not as bad as you make it sound.  2 NEMA 14 stepper motors (~$12/ea.), no rods (the belt sprockets are attached directly to the motor shafts), a Y splitter to connect the 2 motors. 

The only "real work" is getting the motors mounted, and even that is pretty simple.  I could get some aluminum angle from the hardware store, cut it drill it, bolt it to the back face of the SD frame, and bolt the motor to it.  Or print my own mounts.

I think that the only thing to truly worry about is the motors getting out of sync.  That would not be a good thing, but the only way that would happen is if one motor is failing for some reason.  Since both are driven by the same driver, they both get their electric pulses simultaneously, so it's not a matter of them getting out of sync because of some external problem (board or stepper driver related), only some internal problem in the motor.

If you look at the wiring schematic for the Sanguinlolu board, they show it with the optional splitter for dual "Z" motors.  I understand that "Z" motors don't work as hard as a "Y" motor, but again, as long as I stay under the 2A limit, why shouldn't it work?

So this is my long version to the question "WHY BOTHER?"

I'm not saying I'm going to do this anytime soon, but it's something worth exploring, if nothing else, get the brain cells engaged.

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

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.

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

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

HBOT is a single belt.

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.