Marlin Motion Related Configuration.h Settings for MakerFarm i3v

[printbus]

THEORETICALLY ACHIEVABLE FEEDRATE VALUES

We can apply the DEFAULT_STEPS_PER_UNIT values and the rep rap wiki achievable motor step rate of 16,000 steps per second to come up with a notion of achievable feedrates. The achievable feed rate = ( achievable step rate ) / (steps per mm).

THE X AND Y AXIS
We've already affirmed the value of 80 steps per mm for the X and Y axis. The achievable feed rate = ( 16,000 steps per second ) / ( 80 steps per mm), or 200 mm per second. The stock firmware from MakerFarm sets the DEFAULT_MAX_FEEDRATE for X and Y to 250 mm/sec. This suggests the 16,000 steps per second value is an understatement, that the MakerFarm value is eating into margin possibly factored into the achievable step rate, or that the MakerFarm value of 250 mm/sec is a bit excessive.

I've briefly ran my printer as high as 350 mm per second feed rate on X and Y, but the motors get more and more susceptible to skipping steps as the rate goes up.

THE Z AXIS
Using 4000 steps per mm for the Z axis, the achievable feed rate = ( 16,000 steps per second ) / ( 4000 steps per mm), or 4 mm per second. Stock MakerFarm firmware has DEFAULT_MAX_FEEDRATE set to 2 mm per second for Z.

Some other limitation evidently comes into play here. I've briefly ran my Z axis at values between 2 and 5 mm per second, but the best I thought I could do reliably without motor skipping was about 2.2 mm/sec. So, unlike the X and Y axes, Z can't obtain the achievable feed rate. These tests weren't very scientific though - I only tested them by applying some drag using a finger rubbing on one of the rod couplers on 2mm movements. EDIT: A later post concludes that the motors on my printer are what is likely limiting the motor rotation speed.

THE EXTRUDER
In calibrating the extruder value in DEFAULT_STEPS_PER_UNIT, I think everyone ends up with a value higher than 841. For simplicity, I'll use a notional value of 900 steps per mm in an example. The achievable feed rate = ( 16,000 steps per second ) / ( 900 steps per mm), or 17.8 mm per second. Stock MakerFarm firmware uses a value of 22 mm per second for Z in DEFAULT_MAX_FEEDRATE.

I haven't figured out what to do with my experimental testing on the extruder feedrate. Extruding in free air (not printing), I started seeing the extruder and extrusion getting wonky by about 5 mm per second. For example, at slow speeds the extrusion flow stayed consistent at about my 0.4mm nozzle size. At higher speeds the extrusion jumped to being much wider at around 0.9mm measured when cold. Again, I'm still pondering this.

I did find a source that suggested the extruder value in DEFAULT_MAX_FEEDRATE mainly comes into play in retraction. In that regard, seeing the achievable max feed rate calculate to 17 mm/sec might explain why the 30mm/sec retraction rate in the MakerFarm Slic3r configuration files has been found by some to be too high.

CONCLUSIONS
Without anyone jumping in to prove otherwise, the data so far suggests DEFAULT_MAX_FEEDRATE {200, 200, 2, 17} would be more appropriate than the {250, 250, 2, 22} provided by MakerFarm. The 200 mm/sec for XY and 17 for the extruder are based on the calculated achievable feed rate values using reprap suggestions. The 2 mm/sec for Z is based on the fact that testing indicates the achievable feed rate isn't possible here and that one can't get much more than that with at least some of the motors MakerFarm has used. The data here also suggests a slicer retraction speed setting of 17 mm/sec or less.

With a baseline in hand for DEFAULT_MAX_FEEDRATE, let's take a look at MANUAL_FEEDRATE in configuration_adv.h. The MakerFarm defaults for this is MANUAL_FEEDRATE {50*60, 50*60, 4*60, 60}, with values in mm per minute. IMO, the 50 mm/sec rates for X and Y seem slow, but I find the above 200 mm/sec rates are too fast for safe use with the LCD. 100 mm/sec or 100*60 mm/min seems comfortably fast. DEFAULT_MAX_FEEDRATE limits the high end, so there's no reason to set the Z rate higher than 2 mm/sec or 2*60 mm/min. Another personal opinion, but I think the 1 mm/sec extrusion rate is slow, but that the 17 mm/sec value from DEFAULT_MAX_FEEDRATE is faster than we need for extrusions requested from the LCD. 5 mm/sec or 5*60 mm/min fits better. So, MANUAL_FEEDRATE {100*60, 100*60, 2*60, 5*60} is suggested.

FOLLOWUP COMMENT: We later discuss how the sustainable extruder feed rate, when extruding, is driven by the size of the filament. The 5*60 value suggested here is at or near the upper limit for 1.75 mm filament. For the same extruded volume, the feed rate using 3mm filament would be slower. A value of 1.5*60 to 1.7*60 would likely be a better fit for 3mm filament. The 4*60 rate in the MakerFarm defaults is not a rate that can be sustained for a long time using 3mm filament.

With at least Cura or Cura Engine, we can leverage these settings into updates there. Travel feed rate, Z axis feed rate, and manual extrusion rate can be set to match these, adjusting for seconds vs minutes in the units as appropriate. For lack of better guidance, I also set manual retraction rate to the manual extrusion rate.