OpenSCAD generating seamless, friction-fit lens gears for follow focus. If you just want something you can print, I have made 100 .STL files with internal diameters between 50-100mm, which can currently be accessed on dropbox
More information and instructions on my site: http://jaymis.com/2015/11/3d-printable-seamless-friction-fit-lens-gears-for-follow-focus/