Minimum extruder feed rate

I just noticed that the "60" and "120" buttons for doing manual extrusion seem to feed at exactly the same rate. I tested with manual commands (i.e. "G1 E3 F120"), and it seems like the slowest feed rate possible is 150mm/min. It is hard to tell exactly what the feed rate is, but everything at 150 and below produces the same pitch sound from the stepper motor, and the move lasts for the same amount of time and the same rotation of the extruder gears. I can tell no difference in the commands "G1 E3 F1" and "G1 E3 F150".

Is there a minimum feed rate, and why? Where is it controlled? Or have I just got the whole thing wrong somehow?

Looking at some random g-code files, the slowest F value I can see for an extruding move is 300. That was obviously when doing something pretty slow, but I can imagine really low feed rates being necessary when using a smaller nozzle and lower layer height (I'm using the default 0.5mm nozzle and 0.24mm layers).

Crude example on how to find the lowest feedrate used:

cat *.g | grep -e "G1 .*E.* F" | sed 's/.* F\([0-9.]*\).*/\1/'| sort -n|head -n 1

Edit: Forgot to mention, I am on 0.65i-dc42 firmware / 0.95 web-ui.

Edited 1 time(s). Last edit at 07/01/2014 12:09PM by jstck.

Good observation! I just checked the firmware source code, and the minimum extruder-only feed rate is 2.0mm/sec, or 120mm/min. I believe this value is the same across all Duet firmwares, including RRP's 0.78 test version. Combined XY and extruder moves will use whatever feed rate is needed to match the XY move.

I can easily reduce the minimum extruder-only feed rate if you think it would be helpful. Alternatively, I can change the web interface so the buttons are e.g. 2, 4 and 8mm/sec instead of 60, 120 and 240mm/min.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

While I don't really have a need to extrude things slower (at least manually), I don't quite see the point of having a lower bound on feed rates or move speeds. Is there a mechanical reason for it?

In any case, there is little meaning in having buttons with values lower than the minimum possible rate, and I personally do like mm/sec better than mm/min.

Funny as I was calibrating my G92 feed ratio thingy I used the WI controls to feed back and forth.
Trying the different speeds I could not notice any difference. But thought it was just me not being able to notice the difference.

This is fixed in 0.78. The config.g has minimum extruder speeds, set with M566 X600 Y600 Z30 E60 (from here [github.com] ). So extruder minimum feed rate is set at 1mm/s.

Ian
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