Extruder Linearity

burtoogle

OK, created a spreadsheet to calculate minimum microstepping factor for a given line width, layer height, filament diameter, flow rate and segment size. To get reliable extrusion lengths I sliced a small disc in Cura with varying line width and took their source code as a basis for how they calculate how much the extruder should move to get the required amount of plastic pushed through the nozzle.

My only problem is: somewhere in Cura there seems to be a multiplication factor of slightly more than 57.1% that I cannot find anywhere. I have no clue where it is coming from but it is consistent between line widths of 0.4, 0.45 and 0.5mm.

E.g. for a line width of 0.5mm and a layer height of 0.3mm and a filament diameter of 1.75mm it would take 0.10913mm of extruder movement to extrude 1mm of filament through the nozzle. But instead Cura will write 0.06237mm.

Has anyone any idea? Maybe @burtoogle?

Maybe, I do. Let's see now. The line cross section is 0.3 x 0.5 and it is 1mm long so that is 0.15 mm^3 of plastic that needs to be extruded. The filament diameter is 1.75 so that has a cross sectional area of PI x 0.875^2 = 2.405... Therefore, you will need 0.15/2.405 mm of filament to pass through the extruder which comes to 0.06236... Makes sense?

wilriker

@burtoogle I did the same calculations in my spreadsheet. I just now realized that I had a pair of parentheses missing when calculating the cross sectional area of the filament and this evaluated to a wrong value.

wilriker

OK, error fixed, some formatting, some short instructions and here it is:

EDIT

This has been moved to microstep-calculator

Below only for reference but outdated

Microstep_Calculator.xlsx v1.01

I had to convert this with LibreOffice into Excel format to be able to upload it here. This might lead to slight formatting inconsistencies... Alternatively use Microstep_Calculator.ods in my Dropbox.

Instructions

You only enter values in the cells that are bordered in green. Most of these will probably be self-explanatory but still I describe all of them:

Enter your extrusion width
Enter your layer height
Enter your flow rate - or mixing ratio. Whatever applies.
Enter your filament diameter
Enter a length for a print move. This should be less or equal to 0.5mm. You can look at a sliced GCode file and search for very small moves or just take a guess here. Sometimes this value can also be adjusted in the slicer to never be smaller than this (Slic3r has such a setting).
If you are unsure leave the default 0.2 in there.
If you want to be extra safe replace it with 0.1 - smaller values should only be used if found in sliced GCode.
Enter your extruder steps/mm for 16x microstepping

All other values update automatically.

Check the table on the right side. It will give you information on what amount of steps/mm you will have at which microstepping-factor, how many mm of filament will be pushed into the hotend by the extruder with one microstep and (the most important) last column: how many microsteps are required to extrude the filament for the move selected in step 5 above.
If this value is less than 1 the cell will be marked red, green otherwise.

You should choose a microstepping-factor with a green value in the last column because otherwise for such a move no extrusion might happen at all which would lead to under-extrusion and loss of print quality.

@T3P3Tony or @MSquared: I originally wanted to upload an .ods file as I made this spreadsheet with LibreOffice but the forum does reject this filetype. Maybe this can be added to the whitelist, too?!

Phaedrux

@wilriker That's great. Just to clarify, are you getting the segment size from the E distance on a print move?

wilriker

@phaedrux I think "segment size" was a purely chosen label. What this means is the length of a print move. Everything is calculated based on how Cura calculates the extrusion amount or better the Ennn parameter of a G1 move.

I confirmed that Cura is actually using my calculated values by checking multiple GCode files sliced with Cura and checked the relation between move distance and extrusion distance on print moves. This usually only varies around the 3rd to 4th decimal so is pretty stable.

I will add a small How-to to the post above and will probably next week add a wiki page.

EDIT: Instructions added to the post above.

deckingman

@wilriker One more thing you could add is the maximum attainable extruder speed for the chosen micro-stepping setting. You'll likely come up against this limit for retraction moves, and possible filament loading and unloading, as these tend to be much faster than normal print moves. IIRC DC42 said that for these extruder only moves, somewhere in the region of 20KHz would be the maximum. To check, one can do M122 and for earlier firmware look at maxreps, with later firmware, look at Hiccups. I can tell you that with 256x micro-stepping on my machine, I can retract at 1,800mm/sec with max reps of 40 but not 3,600 which has max reps of 744. But if I set micro-stepping to 128x, then I can retract at 3,600 mm/sec without any problems (max reps down to 40).

dc42

@deckingman said in Extruder Linearity:

IIRC DC42 said that for these extruder only moves, somewhere in the region of 20KHz would be the maximum.

It's more likely that I said 200kHz.

deckingman

@dc42 oops - typo

deckingman

One other thing to bear in mind is that with high micro-stepping, you do lose torque. At least I seem to be when exploring the upper melt rate of what the hot end is capable of. It might be a problem if your extruder motor(s) is/are a bit borderline to start with.

Edit. Scratch that. Losing torque at high micro-stepping probably is the case but in this instance I had been testing Capricorn tubing and it had slipped back out of the heat sink a few mm. This left a gap for molten filament to get into and cause a partial blockage., which was why the extruder was struggling.

wilriker

@deckingman I will add max and desired extruder speed to it, that's a good idea! Not sure if I try to do this before Monday, though.

I think there is another typo in your post as I doubt you actually retract with 1,800mm/second than rather per minute, don't you? But then retracting in less than one millisecond could considerably reduce print time when doing lots of retractions.

What I also want to do is to transform the spreadsheet into a small web page that @dc42 is hopefully interested in integrating into reprapfirmware.org.

deckingman

@wilriker Yes you are right. Should have been 1,800 mm/minute (30mm/sec). Same with 3,600 mm/sec which should have been 3,600 mm/min. I blame it on a lack of caffeine

wilriker

@deckingman Oooh, no more cold coffee?! That's bad! That would be like out of tea for me.

deckingman

@wilriker Just a temporary hiccup. The Tassimo machine has been working overtime to rectify the situation.

wilriker

See https://forum.duet3d.com/topic/6156/guide-how-to-select-extruder-microstep-setting.

Phaedrux

@wilriker good stuff. I look forward to playing around with that.