Does anyone remember seeing a novel idea by 'Bob' over at CNCZone about a year ago, where instead of a dedicated 4th axis he built a rolling pinion system along a rack mounted to the bed, parallel to the long gantry axis, so that translation along this axis caused the stock to rotate. I can't find the post now, can't remember Bob's real name, but he provided the rack and pinion patterns, and some photos of projects that I thought looked pretty good. It was about 3" of motion per full rotation.
I'd be interested to see if anyone else had successfully implemented his idea.
WarpDriver, StandingWave, Topo and gadgets available at PaulRowntree.weebly.com