Short version: How fast are you setting speed/acceleration on the c-beam with the 8mm leadscrew, particularly the 500mm or longer ones? Long version: I just put a 250mm across the top of a 500mm, and turned my lathe into CNC. Unplugged, turning the lead screws by hand there is no end-play and it is smooth. Powered it up with LinuxCNC , and if I set acceleration to any setting faster than "arthritic turtle" both axes sound terrible and on the 500mm length the 8mm lead screw starts whipping side-side. I couldn't even run it long enough to test the accuracy because it sounded like it wanted to explode. (These are the new 8mm lead screws that now come pre-shrunk to save us the unbearable agony of sanding down the ends.) I've got other machines that you could run in a library and nobody would complain, but they are using a belt/pulley system. How fast have you guys been able to push the c-beam/8mm setup?