How can I calculate volumetric speed for Slic3r auto speed?

I have noticed that Slic3r offers a speed setting called "auto speed" meant to give a constant filament pressure at the extruder, which I believe could eliminate filament grinding issues at higher printing speeds.

According to the tooltip in Slic3r, auto speed is calculated from two parameters:

• Maximum speed
• Maximum volumetric speed

Maximum speed speaks for itself, but how can I calculate the maximum volumetric speed of my print?

Auto speed is calculated from maximum volumetric speed in mm3 per second. If you normally print at 80 mm/s, your extrusion width is 0.5 mm and you are printing 0.2mm high layers, your volumetric speed would be 80 * 0.5 * 0.2 = 8 mm3/s, which is the volume of plastic extruded by your printer every second when printing at that speed (not accounting for any die swell).

• You are right about the terms: I have updated the post to reflect this. I also found a blog post discussing the math in context of extrusion width, for those interested: extrudable.me/2013/11/03/… Jan 14, 2016 at 20:36
• Technically not an answer... he asked for the maximum one, you gave him how to calculate the "usual one" he already tested.
– FarO
Apr 9, 2018 at 8:53

There's no fixed maximum volumetric speed that works for everyone, there's simply too much variables to account for. By using @Ian Williams explanation you can convert from volumetric to regular speeds but you still need to test what speed works best for your setup.

Just a few of the other variables affecting how fast material can come out consistently: temperature (nozzle & heat brake), extruder motor power, path friction between extruder and hotend, material compressibility, fluidity and glass transition temperature, ..

There are interesting topics on RepRap forums, like this one: