Marlin Motion Related Configuration.h Settings for MakerFarm i3v

[printbus]

ESTIMATING A FEED RATE LIMIT ON EXTRUDER E-STEPS CALIBRATION

The background information leading to this has been puzzling. Some people have reported inconsistent results if they run at the Pronterface default of 100 mm/min feed rate during extruder calibration, yet I've known I could do calibration at the 300 mm/min default in Repetier-Host. But I've also realized my results also get more consistent if I back off from that. And then there's the "wonky" extrusion I see when I try to extrude into free-air above a 5 mm/sec feed rate, which correlates to 300 mm/min. As it turns out, all of these are likely tied to size of the filament being used and a notion of extrusion capacity for the hot ends. In other words, just how much volume of raw filament can the hot end melt and extrude?

It's not clear what if anything the software does with it, but Repetier-Host even has a setting for this - max volume per second, with a units of cubic millimeters per second. The comment on the setting says one should be able to reach 12 mm^3/sec. I've seen 10mm^3/sec mentioned as a typical capacity in various web posts, with others encompassing a range from as low as 8 to as high as 18. I haven't heard or found of a value specific to any hexagon hot end, what yet specific to a 0.4mm nozzle version of it like I have. As with a lot of other things on the printer, it was enlightening to compare these numbers to the volume of filament necessary to support things like the extruder calibration.

Let's assume a possibly optimistic goal of 12 mm^3/sec as the maximum amount of filament we can melt and extrude. The input feed rate that correlates to this would depend on the filament diameter. The area of a circle is pi*r^2. The volume of a cylinder like our filament feed is simply the area * length. For perfectly round and properly dimensioned 1.75mm diameter filament, the area of the filament end is about 2.4mm^2. Calculating over a one second window for a volume of 12mm^3, the cylinder length of filament would be (volume)/(area), or (12 mm^3)/(2.4mm^2), or 5mm. Hmmm. Right at the 5mm/sec point where I've noticed my free-air filament extrusion getting "wonky". And 5mm/sec extends to the 300mm/minute threshold I've seen in the calibration extrusion. This suggests that a 5mm/sec feed rate while extruding seems to be at about the limit or slightly beyond the limit of my 1.75 mm hot end and 0.4mm nozzle.

How about 3mm filament? Area of the filament end is again pi*r^2, or just over 7 mm^2, about three times the area of 1.75mm filament. The cylinder of filament to be extruded in one second for volume of 12 mm^3 would be (12 mm^3) /(7 mm^2), or about 1.7 mm. 1.7mm/sec is just about the same as 100 mm/minute, where some people have been reporting issues in the calibration. Were they people with 3mm feeds and not 1.75? IDK - I should have paid more attention, but that's sounding pretty likely.

Maybe as a conservative view, extruder calibration should be conducted at something like half these rates, or 50mm/min for 3mm filament and 150mm/min for 1.75 mm filament.

WHAT ABOUT OTHER CONSIDERATIONS ?

Should the E term in DEFAULT_MAX_FEEDRATE be changed to reflect this? I don't think so. DEFAULT_MAX_FEEDRATE should likely stay at the mechanical limit of the motor drive in support of retraction and replenish. We don't have to factor melting new filament for those movements. It's just obvious now that we can't expect to be extruding at that rate, or at least not for very long.

So the capacity of the hot end to melt filament will likely be a limit on printing speed. All you'd have to do is factor the volume of the extrusion (most places say to just calculate volume as extrusion width * layer thickness and ignore factoring rounding of the corners), and divide it into the hot end flow rate capacity to come up with a print speed limit. The key to this would be in having a solid number for the flow rate capacity. Variables I've read others mention include the nozzle size and temp, specifics of the filament (material, color), and effectiveness of the thermal junction between the aluminum block (the hot end heat reservoir) and the nozzle. One source declared that the higher surface area to volume ratio for 1.75mm filament allows it to support a higher hot end throughput than 3mm. I can logically expand this list to include more possible variables like the quality of the thermal connection between the cartridge heater and the aluminum block, whether or not the aluminum block is insulated, and if so, the quality of that insulation. The approach used to cool the hot end heatsink could make a difference (air volume & direction, nature of insulation vs. airgap above the heat break, etc.). The accuracy of the hot end thermistor...

Results from any volume testing I did would likely be of little value to anyone else. Thinking I had cross-threaded my hex hot end aluminum block, I replaced it with a different one that is 20mm square instead of the hexagon's standard 16mm block. This likely changed the characteristics of the heat reservoir formed by the block. Since the block is larger, I've replaced the cartridge heater with one 20mm long; this heater seems faster (more effective) than the original one. I used heatsink compound on the cartridge to block connection; I may have even put a small amount on the nozzle threads. The aluminum block is insulated in two layers of kapton, with portions covered by three or four layers. The hot end shroud has been customized to remove the bottom plate and to improve the amount of air forced over the hot end heatsink. I almost always print with a print cooler running. Most days I'm using 1.68mm diameter filament - shy of the 1.75mm "standard". All of these factors will likely matter.

Could there be a relationship between hot end flow volume capacity and acceleration and jerk settings? Maybe, at least to a small extent. By increasing XYZ acceleration and going with a higher jerk allowance, we're reducing opportunities for "slack time" where the hot end might otherwise be able to restore heat lost to extrusion.

Yes, smarts built into the heater temperature control loop can deal with a lot of the variables - at least up to some extent. Regardless, I'm starting to see how there's more to pushing a printer to the limit than I realized.

HOMEWORK AND INDEPENDENT RESEARCH

For the free-air extruder e-steps calibration, what limit on feed rate do you get if you assume the hot end can only handle 10 mm^3/sec?

As I understand it, some slicers attempt to keep extrusions to something like 0.4mm while others accommodate a width setting. Do you know what yours is? Have you attempted to measure it on a thin-wall calibration print? Assume a simple rectangle extrusion of width * layer height, and calculate an estimated print speed that equates to the hot end volume limit of your choice.

A thicker layer or a wider extrusion on the first layer would lead to a lower limit on the print speed. Come up with another print speed estimate based on what you think you're getting for an extrusion on the first layer.

REFERENCES

http://www.extrudable.me/2013/04/18/...ty-and-limits/ - provides a good overview of the issue and demonstrates how feed volume capacity goes up with nozzle temperature. States that 8-10mm^3/sec is the limit for a 0.4mm nozzle. Note that the stated volume test results are high since tests were conducted with a 0.65mm nozzle - look at the data for an understanding of how temperatures, etc. affect volume flow rate, not as a baseline that you should strive for.

http://forums.reprap.org/read.php?1,226728 - says 10mm^3/sec is typical

http://umforum.ultimaker.com/index.p...der-extrusion/ - sort of a followup to the extrudable me reference. One comment states 10mm^3/sec is the limit for a 0.4mm nozzle. Comment #8 touches on the 1.75mm vs 3mm aspect.

FOLLOWUP COMMENT: I've noticed the MakerFarm FAQ page describes the hexagon hot end as being capable of 200 mm/sec print speeds. That seems high in comparison to the sense I'm getting regarding my hexagon hot end. Unfortunately, MakerFarm doesn't elaborate on the test conditions where that print speed is possible. If the test conditions are a 0.30 mm nozzle and a 0.10 mm layer height, 200 mm/sec could be reasonable because of the smaller extrusion volume involved.