Magic Screw for Makerbot
by VeryWetPaint, published
This is a 'kit' for designing or building magic screw threads that will (hopefully) build on a Thing-o-Matic and similar hobby printers. This should enable you to build a bolt and nuts so that one nut tightens normally while the other seems to turn the wrong way as shown in this video (this is the new model data):
The ancestor model did not build accurately on hobby-type 3D printers because (I think) the geometry had atypical overhangs that could foreseeably flatten out during extrusion. Instead of fighting those overhangs, this Magic Screw version tries to exploit them by building overhangs in straight lines between supported walls. It's just a conjecture (I don't know if I'm right) but it's illustrated in my blog at http://mysd300.blogspot.com/2011/12/magic-screw-for-makerbots.html
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Try building the three STL files as-is and see if they'll work exactly like the video.
If the model needs tweaking, or if you'd like to optimize the appearance of the trick threads, download and edit bwbolt.scad using OpenSCAD. Follow the instructions (comments) for some simple adjustments, or revise the whole thread profile as desired.
Be sure to share a derivative if you get a model that works better or looks better!