Two nuts travel opposite directions on the same thread! As seen on YouTube! http://www.youtube.com/watch?v=o-fPsvqjqZI

Update 27 Oct 2011: I've uploaded a new trick nut triplescrew_backward_floor.stl which eliminates superfluous slack and reduces the overhangs to where they might be buildable on a Makerbot.

The YouTube video inspired Thingiverse user msruggles to design his own "Screwy Screw" thing. The STL files below are the originals, which were used to make the video, which inspired msruggles to make Screwy Screw, which inspired me to upload the STL files below...oh gosh, I'm completely lost!

Don't know how it works? Obviously there's a trick to it, but I encourage you to try to figure it out before you "spoil" the fun by downloading the files. Try designing your own!

More discussion on my blog at http://mysd300.blogspot.com/search/label/screw

All units are millimeters.

1. Choose the small bolt triplescrew_bolt40.stl or the long bolt triplescrew_bolt60.stl. To help camouflage the trick thread choose a transparent build material for the bolt, if possible.

2. For the nuts, build both triplescrew_backward_floor.stl and triplescrew_forward2.stl. I think dark colors work best for the nuts.

3. Thread the nuts onto the bolt by turning one clockwise and the other counterclockwise, unless you live in a Commonwealth Nation in which the second nut must be turned anticlockwise.

Additional observations: •All parts have been confirmed buildable in LOM (on a Solido SD300) and FDM (on a Dimension 768) but your mileage may vary. •In addition to the obvious trickery, these threads have an unconventional sinusoidal profile. msruggles used a more conventional 'vee' profile for Screwy Screw. I'm not sure which will work better for hobby FDM printers. •The forward and backward nuts have the same travel per revolution (ie: they move the same distance but in opposite directions) but the backward threads on bolt and nut have 3 starts each. The forward threads on the bolt have 1 start while the forward nut has 2 starts to enhance its resemblance to the backward nut.