After that first try with the little mug, and after fixing the X problem, I decided that I will calibrate my Rapman before going any further.
Calibration will ensure that the overall dimensions of the printed object will be in respect with my original CAD file.

Calibrating the Rapman is just a question of finding the correct print head XY speed (Feedrate) and extruder motor speed (Flowrate or extrusion rate) that will give the correct wall thickness in respect with the "Layer thickness" and "Perimeter width over thickness", both in "Skeinforge - Carve". The speed parameters are in "Skeinforge - Speed".

Here is the theory :

  • the faster the print head will go, smaller will be the extruded filament
  • the faster the extruder motor will go, bigger will be the extruded filament (more plastic extruded)

Here is a test where you can see it is not just a theory :

The red square shows the weaker object (thin wall) while the green one shows the strongest (thick wall).
Along the blue line the objects are quite the same thickness.

So in order to calibrate your Rapman :

  • print a "single wall" object (deactivate Skeinforge - Fill) 
  • mesure the wall thickness with a caliper
  • for 0.4mm "layer thickness" and 1.8 "perimeter width over thickness" you should get 0.72 (0.4 x 1.8)
  • if your wall is thiner, increase your flowrate (or decrease your feedrate)
  • if your wall is thicker, decrease your flowrate (or increase your feedrate)

Personally I prefer to tweak the Flowrate rather than the Feedrate (print head speed) that I leave at the default 16mm/s.

Now that mine seems calibrated let's try to print a more complicated object...