3mm Airtripper Extruder

This derivation of the Airtripper Bowden extruder (version 3.0) centers more parts and holes and parameterizes filament size, tolerant at least between 1.75 and 3mm. Default settings are for a 3mm filament with an MK7 drive gear (10.56mm effective diameter). The Bowden tube outer diameter is presumed to be 6mm. The mounting holes are also parametric and are set up here with grooves for 3mm bolts, good for attachment to beam extrusions.