Mega Y expansion idea....

[Cameron]

Good point on the speed. Can't you change the steps from 1/16 down to 1/4 with jumpers under the stepper driver on the ramps board? Then it could go pretty fast. 40mm sounds pretty painful. That is definitely something to look into!

[printbus]

Good point on the speed. Can't you change the steps from 1/16 down to 1/4 with jumpers under the stepper driver on the ramps board? Then it could go pretty fast. 40mm sounds pretty painful. That is definitely something to look into!

Yeah, it would be 14 or 15 seconds to go from one end to the other on a 24-inch bed at a 40mm/sec rate, not factoring acceleration. Of course, it's unfortunate that Marlin and slicers don't let you set X and Y independently. If Y has to be set slow, X is forced to run that slow too. Before investing this kind of money, I'd ask OpenBuilds if they have suggested Marlin settings to use with the linear actuator. Will they give you a spec sheet for the motor or at least a part number for the motor that could be pursued for detail? Even then, I'd personally start by only purchasing the linear actuator for some experimenting. You'd probably be able to learn a lot with it being driven by itself with different weights fastened to the actuator plate.

Keep in mind that I'm not an expert in all this. Those that understand it all either have better things to do with their time or like to sit back and laugh at those of us trying to piece together what we can figure out. Perhaps there's a reprap forum on it, or the reprap IRC might be place to ask for references. Details that follow come from my pursuit of knowledge last year in the thread Marlin Motion Related Configuration.h Settings for MakerFarm i3v. That effort was based on Marlin as of the becdac fork in October 2014. I have no idea whether any of these fundamentals have changed in the recent updates to Marlin.

I learned Marlin has a limit on how fast it can generate step pulses. The upper limit is 40,000 steps per second. Sounds high. I believe this is total across all movements. It also uses a 10 kHz interval to "packetize" motor steps. As I understand it, Marlin will generate one step per interval up to 10,000 steps per second. After that, it puts two or four pulses in each interval in order to achieve the maximum step rate of 40,000 steps per second. Some feel, however, that the combining of steps per interval can lead to movement jitter since all steps aren't evenly spaced. After all, moving one pulse at a time is different than moving two or four steps rapid steps at spaced intervals. Because of this, I read some people suggesting 10,000 steps per second is a better limit than 40,000.

Let's go with 10,000. Start with the stepper driver configured for no microstepping. Most motors are 200 full steps per rev, so in theory (so far) you could achieve 50 revs/sec. 1/16 microstepping would give just over 3 revs/sec. OK, no microstepping sounds good. But that sacrifices position accuracy, so let's look at that. The 4-start, 2mm pitch ACME screws have a lead of 8 mm/rev. A normal stepper motor without microstepping is 1.8 degrees per step, or 200 steps per rev. Combining the two, this equates to 0.04mm linear movement per full step. Is that resolution OK? For comparison, the 20-tooth OEM pulleys in the MakerFarm design provide an XY axis resolution of 0.0125mm per step at 1/16 microstepping. Going with 1/4 microstepping with the linear actuators would get you back to that ballpark. 1/4 microstepping at the 10,000 step rate would give you 12.5 revs/sec, or 100mm/sec. Tempting, at least compared to 40mm/sec. That'd get you down to 6 seconds full-length travel time on a 24-inch Y-bed.

Ah, but the motor likely complicates this. I know that the data sheet for the Kysan 1124090 NEMA17 motors I use includes a graph showing pull-out torque vs step frequency. The pull-out torque reduces pretty linearly with the step frequency. So, the faster you want to go, the less torque you have to reliably work with. Scanning back through the Marlin Motion thread, I mention in post 12 that I was able to drive the Kysan with 1/4 microstepping at what should have been 10 revs/sec (8000 microsteps/sec), but only with minimal loading. In the Z-axis application, I could only achieve less than half that when attempting to lift the shared weight of the x-axis assembly.

Again, I'd start with purchasing the linear actuator by itself and play with it. If you go that route, there are non-printing gcode snippets in the Marlin motion settings thread that might be instrumental in helping you observe how the actuator performs zipping back and forth with different microstepping, acceleration, and movement rate settings.

It'd be a fun project.