And
What would you use to draw it?

I was thinking about what to use for drive belts and have tried string, bowden cable, toothed belts and screws but I thought that this:

http://www.ebay.co.uk/itm/2309588040...%3AMEBIDX%3AIT

would do the job really well and extremely cheaply.

I bought some and it is easily strong enough to shovel a head and carriage about so I now need to print some pulleys for it which is where my problems start.

I have to have a circumference that is an integer number of pitches, the balls are around 2.3 mm, the rod is 1mm so the pitch is 3.3mm or so. On the face of it it is a trivial problem.

I thought of setting up a great big spread sheet for all possible diameters but the arithmetic is not too easy.

Screenshot from 2014-11-21 18:28:27.jpg

I think there should be an easy way in Openscad but I am not familiar enough to use it. That picture is done in Freecad but it is less than ideal and involves loads of working out for each repeat.