elmom/MCAD — pull request #21 by hyperair (2014-05-13 18:50:48+00:00)
Title: Add internal option for gear()

This option allows for the generation of internal spur gears. Internal mode
adapts the meanings of several options for use on internal gears:
- backlash is negated for larger teeth, as we're subtracting the gear from a
cylinder.

- clearance is added to the addendum instead of dedendum, which
pushes out the outer circle instead of pushing in the root circle.

- rim_width is added to outer_radius in order to get the containing cylinder's
radius.

- flat renders the gear shape in 2d, ready for extrusion and subtraction.