Archimedes screw – double thread

Fri, 11 Mar 2011

My first draft at an Archimedes screw in the previous post had a single spiral running down its length. 

After the feedback from the previous post it looks like the actual things, out in the wild, as it were, usually have two or more spirals. The screw pitch also seems to be much steeper at perhaps 45 degrees. I've been experimenting with double spirals. I've tried to create the spiral from short sections joined together. I marked out a quarter turn of spiral then, as I'm wanting it at 45 degrees on the shaft, I stretched it by 141% horizontally. (Pythagoras – the diagonal of a square is root two times the side length, square root of two is 1.41) 

I then stuck the two rows of parts onto the shaft and waited for the glue to dry. Once dry I tried to join all the parts together.

Hmmm. So it doesn't always work out as I'd expected. The result is really uneven with lots of weird overhanging edges and none of the parts lining up properly. I wonder if I need to sub divide the sections even more? Perhaps I need eight parts per revolution rather than four? Any ideas anyone?