Internal Screw Thread

1. Create white helix with one revolution.
2. Copy white helix to make black helix.
3. Make blue and red sections.
4. Sweep1 red section about black path, use Roadlike Top.
5. Sweep1 blue section about white path, use Roadlike Top.
6. Array the two swept surfaces to make copies.
7. Join all the copies.
8. BooleanDifference them with purple block.

- Bryan Rhino Newsgroup