My cnc router has the rod rails. They are junk compared to proper linear rails. In saying that, for what the machine was originally intended to be, a wood work router to make wood things to +-0.05mm or +-0.002 inches, it does achieve that. Now that I want better than 0.01mm 0.0004 inches, there is no way it is capable of that. I will need a cast iron/steel machine to obtain that sort of accuracy. So got my thinking cap and plan C working part time. Just so many options, as to which is best is not easy. Robotic arms are making great inroads to position accuracy and repeat-ability. It maybe that my next venture is in this direction.
Neil
For cue making, heavy duty aluminum extrusion frame with profile rails should be more than fine. .002" accuracy is actually super-good as far as routers are concerned; that's C5 ballscrew accuracy, and that does NOT come cheap, especially at the lengths needed. .0004" would land you in C1 territory at a workable length, and that screw would likely cost as much as your lathe and all its tooling combined! These screws from C5 down to C0 are precision ground, not rolled. By definition their tighter tolerances give them anti-backlash properties, but for higher loads a double nut is usually used, tensioned against each other. For woodworking needs, a single nut would work just fine.
The most common screws used in CNC routers are C7 rolled, which have a stated lead accuracy of .005", though because of the rolling process more clearance (axial play) is built in to overcome deficiencies in the rolling process. Any claims that a single nut solution provides anti-backlash properties I feel are misleading. Here the preferred configuration is a double ballnut. The lead error could be mapped and corrected via the control software, but you'll need some pretty good measuring equipment to make this worthwhile.
Also note that chasing .002" accuracy on a product, wood, that can move three to five times that amount, even within a span of a few hours, may be asking a bit much. Part of the artistry of working with wood is understanding how the wood moves and working around that.
Also, many manufacturers will state a resolution for their machines. This is not a mechanical resolution, but an electronic resolution based on the step size of the motor/drive system. If your machine uses servos, the step size can be very, very small depending on the encoder. With steppers, there is an inverse relationship between step size and motor performance. So just because a machine uses steppers and the step resolution is set to 10x, does NOT mean you'll achieve the .0005" positional accuracy that is claimed.
