MakerBot Industries

Another kind of feed rate

I’ve been a MakerBot operator for about 18 months now, and only just now am I really coming to understand Skeinforge’s nomenclature. ¹  Dave Durant did an excellent job of explaining the most important settings and terminology for Skeinforge in his series on configuring Skeinforge.  “Flowrate” is the rate of plastic exiting the extruder’s nozzle.  With a little math and science the flowrate is a function of the extruder motor speed and the volume of filament.  “Feedrate” is the rate of speed of the XY platform.  There are several different methods proposed on Thingiverse for configuring Skeinforge, and most of them involve solving for the proper flowrate or the proper feedrate for a given profile.

When using a MK5 or other DC motor powered extruder, there’s a very narrow range of reasonable flowrate settings.  The MK5 DC motors had a maximum speed of 255 PWM and a functional minimum of around 230-240. ²  Much less below 240 or so and the motor just wouldn’t turn.  The MK6 stepper motor extruder, on the other hand, has a much wider range of speeds – from around about 0.0 to up to about 3.0 RPM or so. ³

What values, then, is a 3D printer operator supposed to modify to get the best profile?

If you’re using a MK5 or DC motor extruder, I would recommend calibrating Skeinforge as you hold the DC motor speed at 255 and the maximum reliable flowrate you are able to obtain.  You’re going to want to adjust the other variables that are easier to control – the feedrate⁴ , the layer thickness⁵ , and thread width⁶ .  Since I like choosing my resolution and there’s not much choice in flowrate, I like to vary the feedrate since it’s easier to control.

If you’re using a MK6 or other stepper extruder the flowrate, feedrate, layer thickness, and thread width are all easily adjustable.  I prefer to choose the layer thickness and thread width, hold the feedrate at a level I know my robot can handle without problems, and vary the flowrate.

Here’s the interesting thing about solving for flowrate: the nozzle size ends up not being super critical.  As long as you’re choosing sane numbers for layer thickness and thread width, the amount of plastic going in will always equal the amount of plastic coming out.  While a 0.5mm nozzle would be capable of up to about 0.36mm thick layers and 0.75mm wide threads⁷ , these numbers might not make sense for a 0.4mm nozzle⁸ but could still work for a 1.0mm nozzle.

What settings do you like to vary when you’re configuring Skeinforge?

1. Photo courtesy of MadBuster75 [↩]
2. My understanding is that a PWM of 255 equates to about 2 RPM [↩]
3. A few disclaimers are in order.  You can set your RPM in the control panel to whatever you want and try to extrude – with no guarrantee it’s actually going to extrude at the mathematically calculated flowrate.  Additionally, the maximum effective RPM will be a function of the nozzle aperture. [↩]
4. XY platform speed [↩]
5. The thickness of each layer as it is laid down.  0.25mm/layer to 0.36mm/layer are reasonable ranges [↩]
6. This is the width of the filament as it comes out of the extruder nozzle. [↩]
7. Since they’re getting squished as they’re laid down [↩]
8. Since it would be tough to squeeze a 0.75mm thick layer out of a 0.4mm filament [↩]