Friday, 17 June 2011

Prusa and Wade Firmware Changes for Gen 3

I’ve now got the three axes of my Prusa up and working with some temporary wiring, woo! Time for some calibration, now when I first set up my electronics I was doing it with a standard Sells Mendel in mind. To get the Prusa to work correctly I’ve got to make some changes to the configuration file of the firmware.

The first one is the z axis steps per mm, now I think that as the z axis is now directly driven rather than via a pulley and belt, this needs to be altered. The pitch of M8 rod is 1.25mm, so for every full turn of the stepper motor it should produce 1.25mm of travel. My steppers have a step angle of 1.8° (for a full step), now I think Gen 3 works in “half” steps, so for one full turn of the stepper I have (360/1.8)x2 = 400 half steps. Therefore the z-axis value should be 400/1.25 = 320 steps/mm.

The second one is the x and y axis, as I am using newer design pulleys (9 teeth rather than 10) I need to change the steps/mm. The belt pitch is 5mm, so 9x5 = 45 mm circumference. Therefore the steps/mm should be 400/45 = 8.889. (Interestingly the value given in the firmware is 8.9307 I wonder if it allows a little extra).

Thirdly is the extruder steps per mm. I am now going to be using a Wade’s extruder rather than Adrian’s. This has a gear ratio of 39/11 rather than Adrian’s which has a ratio of 59/11. I’ve had a go at calculating the steps/mm using the equation given on the wiki. For my set up, assuming a pinch wheel diameter of approx 5mm, nozzle of 0.5mm, and 3mm filament size, I make it: ((400x(39/11))/(pi x 5))x((0.5/3)² = 2.508 steps/mm.


  1. If you are going to use skeinforge 40+ and volumetric 5d - which comes highly recommended - you have to calibrate your extruder according to the filament going in, not out.
    Read more here:

  2. Thanks Nudel, that's a good point I missed. Does that mean it is not possible to switch between RepRap host and ReplicatorG without making firmware changes?

  3. Yep, I worked on that formula to give me an accurate extrusion rate, when I entered that number in, and tried to do some extrusions, it was cmpletely whacked, and a guy on the RepRap forum guided me to a forum that wasn't too clear on the new calibration algorithm.
    This new way doesn't seem to make sense, but i'm gonna try it on Tuesday night.

    Thanks Nudel for the link.

  4. Nice work on the Prusa, Im collecting parts right now to put mine together.