ball screw - parametric OpenSCAD

##A parametric design of a ball screw in OpenSCAD

I tried my best to provide a fully parametric design of a ball screw system with a lot of freedom to choose own values, mainly radii of nut, screw, balls as well as pitch, #threads, windings and gap size.

A print of the design comprises four parts: the screw, the nut middle and the two nut ends. Use M3 countersunk screws (10-15mm) to screw one nut end to the nut middle (in a self-tapping way), then put in the screw and completely fill up all ball paths with balls - the open end facing upward. After that you screw the second nut end from the top and work the mechanism in a bit. Handle the screw with care and be aware that it must always remain completely within the nut - otherwise the balls will escape. Adding some silicon grease to the grooves of the screw helps for a silent and precise run.

###The code
The code heavily builds upon two of my favorite libraries [Naca_sweep.scad](https://www.thingiverse.com/thing:900137) and [ShortCu