Extruder Linearity

Phaedrux · 19 Jul 2018, 06:38

@dc42 thanks. I'll give that a try first.

I understand from some of the work shown by @deckingman that higher microstepping can actually give better extrusion accuracy in certain situations. And if any axis could benefit from it, I would think it the extruder.

T3P3Tony · 19 Jul 2018, 12:08

If noise is not a consideration I assume that "real" microstepping is preferable to interpolation?

deckingman · 19 Jul 2018, 12:32

@phaedrux said in Extruder Linearity:

@dc42 thanks. I'll give that a try first.

I understand from some of the work shown by @deckingman that higher microstepping can actually give better extrusion accuracy in certain situations. And if any axis could benefit from it, I would think it the extruder.

Only if you are using a mixing hot end where one filament could be extruding as little as 1% of the total. I wouldn't imagine there would be all that much to gain by using higher than (say) x32 micro-stepping or 0.9 degree motors and 16x. It's easy enough to check. Just look at the extrusion amount you get for small segments in mm, then divide your steps per mm that to see how many micro-steps you get. IIRC the numbers I was looking at for a small 0.5mm segment gave an extrusion amount of around 0.025mm. Which at 415 micro-steps per mm (16x for a 3:1 geared extruder like Bondtech BMG or E3D Titan) is about 10 micro-steps which ought to be fine. But if you want to extrude 1% of that with a mixing hot end, then it's only 0.1 micro-step and ain't gonna work.

Phaedrux · 19 Jul 2018, 17:48

@deckingman yes those are the specific circumstances I eluded to, thank you for summerizing then far more accurately and concisely than I could have.

I guess another way to think about it isn't that microstepping is providing more accuracy, but rather that in this instance there aren't enough steps to capture the movement slices. Higher stepping means more step pulses and each step can capture a smaller movement slice so nothing gets lost to rounding.

In my case with a single extruder I don't think this applies to me.

deckingman · 19 Jul 2018, 18:55

@phaedrux said in Extruder Linearity:

@deckingman yes those are the specific circumstances I eluded to, thank you for summerizing then far more accurately and concisely than I could have.

I guess another way to think about it isn't that microstepping is providing more accuracy, but rather that in this instance there aren't enough steps to capture the movement slices. Higher stepping means more step pulses and each step can capture a smaller movement slice so nothing gets lost to rounding.

In my case with a single extruder I don't think this applies to me.

Exactly so. But do the maths. My typical usage case is with 0.3mm layer height and 0.5mm nozzle diameter. With smaller layer heights and nozzle widths, the extrusion amount for a given segment length will be less, and there may be a case for using say 32x or 64x micro-stepping in order to capture those small movements. Bear in mind that 0.9 degree motors @ 16x micro-stepping will give the same steps per mm as 1.8 degree motors @32x micro-stepping. But 1/16th micro-step will have more torque than a 1/32 micro-step all other things being equal (like the torque rating of the motors).

Phaedrux · 19 Jul 2018, 19:46

@deckingman Yes I'll have to dig into it a little deeper. I do print at low layer heights down to 0.05 at 0.4 extrusion width.

wilriker · 20 Jul 2018, 07:39

Reading all of this forms the question in me: is there a desirable minimum number of steps/mm for the extruder? Or even better have a calculator that can provide a recommendation based on layer height and nozzle size/extrusion width.

deckingman · 20 Jul 2018, 09:32

@wilriker said in Extruder Linearity:

Reading all of this forms the question in me: is there a desirable minimum number of steps/mm for the extruder? Or even better have a calculator that can provide a recommendation based on layer height and nozzle size/extrusion width.

What I was concerned about was that with very small extrusion amounts (i.e. when using mixing ratios of 1%) then the amount of material to be extruded was less that one micro step at the 16x setting that I was using. So the desirable micro-stepping would be that which allows the smallest extruder move to be accomplished. I did do a spread sheet but it's specific to my printer/layer height/nozzle diameter. Looking through some old gcode files, I found that for a 300mm carriage move, the extruder move was about 15mm so it kind of follows that the extruder moves are about 5% of the carriage move. I used that 5% as a basis and calculated the extruder move for a realistically small segment move of 0.5mm giving me 0.025 mm of extrusion. Then of course with a mixing ratio of 1% of that, it moves the decimal point two places to the left. It's easy enough then to compare that amount of movement with what 1 micro-step will give at different micro-stepping settings.

For a more universal calculator, I'd say that you've talked yourself into a job

wilriker · 20 Jul 2018, 09:49

@deckingman said in Extruder Linearity:

For a more universal calculator, I'd say that you've talked yourself into a job

I saw that one coming. So, I guess I gotta do what I gotta do then.

wilriker · 20 Jul 2018, 13:09

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?

jeanmarc_Scaled · 20 Jul 2018, 14:47

You could re-slice with Slic3r, then use the flow calculations from the manual: http://manual.slic3r.org/advanced/flow-math

wilriker · 20 Jul 2018, 14:51

@jeanmarc_scaled Thanks! I will definitely look at this.

As all slicers do basically the same (don't hit me ) this might already explain where this strange multiplier is coming from.

But still, if anyone else has an idea I am willing to hear that. Especially since I followed the source code trail in CuraEngine on how this amount is calculated - but clearly I missed something.

burtoogle · 20 Jul 2018, 17:36

@wilriker said in Extruder Linearity:

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 · 20 Jul 2018, 17:44

@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 · 20 Jul 2018, 18:24

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 · 20 Jul 2018, 18:37

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

wilriker · 20 Jul 2018, 19:06

@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 · 20 Jul 2018, 20:35

@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 · 20 Jul 2018, 21:29

@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 · 20 Jul 2018, 21:53

@dc42 oops - typo