Double Helix Bolt - left and right-hand threads on one shank, two nuts (parametric)

A desk toy that looks like it shouldn't work. The bolt has a right-hand and a left-hand thread cut through each other, so it carries two nuts at once: one screws on clockwise, the other anticlockwise. Run them along the shank and they turn opposite ways over the same grooves and meet in the middle. The thread is a coarse 8 mm pitch with 45 degree flanks, so the bolt prints standing on its head without supports. Each nut has 0.45 mm of clearance round the thread. The left-hand nut has the tradit…

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