Threads API and Tutorial?

**Issue by [fragmuffin](https://github.com/fragmuffin)**
_Monday Sep 18, 2017 at 05:45 GMT_
_Originally opened as https://github.com/dcowden/cadquery/issues/195_

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A tutorial on how to make a solid geometry thread would be very helpful.

I've (sort of) succeeded by using a few back doors in [this gist](https://gist.github.com/fragmuffin/266bee4babd9a25aea2d53c739144ee3) (more information on implementation there)
However, it takes a very long time to generate (~15sec), and it's not perfect.

Any advice on the best way to generate a thread?
Perhaps make it into one of the example scripts.