This is a design of the RepRap extruder drive that is compatible with the Darwin quickfit clamp system and the Ponoko lasercut kit. It will take either 16mm PTFE or heatsink based heaters. It is designed to be made with no lathing or machine tooling. An opto sensor may or may not be fitted to regulate rotation speed - I found that this was definitely required with an all-metal heater assembly. If you develop software to do that, we'd love to see it!