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Modeling multi-chamfer *round* threads?

Question asked by Hunter Hunter on Dec 6, 2014
Latest reply on Dec 8, 2014 by Hunter Hunter

I'd like to 3D print cups that screw together using rounded threads. The threads are essentially a circle swept along a helical path, i can create this easily with a sweep (shown below)

 

theadcup1.JPG

 

In order for the cups to fit within each other, I have to compensate for the shell thickness by chamfering/tapering the cup by a little more than the wall thickness. It also helps if the very bottom has a harsher chamfer so a sharp edge doesn't screw into the other cup (I would radius the edge at the 95mm plane, this would touch this inside of the lower cup)

 

Lofting these always seems to fail (self intersecting), and I'm not quite sure I even want a loft. It might work with a "sheared" helix as an edge guide curve but I don't know how to make that parametrically. An angled helix edge guide just makes uneven/improper threads. Flex-twisting an off-axis cylinder by a thousand+ degrees just results in a hot mess. Even just constraining to the helix is difficult /: Does anyone how to create this shape?

 

cupprob1.JPG

In the end I will have global variables for [wall thickness], [height], [diameter], and [screw pitch], so the model must remain parametric.

 

IMG_20141205_232234.jpg

 

I've printed a lot of these screw cups, but with improper threads and no bottom chamfer. This problem has been bugging me for over a year /: I'd really appreciate your help.

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