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I have worked with just about every configuration of router out there. So far, I haven't seen a lot of difference in accuracy between the rack & pinion vs. ball screws. There are also some that have a timing belt drive, but I have no experience with those.
For the tabletop type routers the most common is ball screws and stepper motors. I have never seen one that didn't have a ball screw on the Z axis. On some of the wider routers, I have seen ball screws on both sides of the gantry. My 4 x 8 is all ball screw.
If this is for home hobby use then open loop steppers are a good choice. Accuracy is not as good as with closed loop servos because it is possible to lose steps and then the controller has no idea where the tool bit is at. For hobby work this is not normally a problem. It is possible to close the loop on steppers, and operate them like servos. This is what I'm doing on my machines, but it is relatively expensive to do so.
If your router is going to be used for commercial work or you need high accuracy and higher speeds, then I would recommend going with hybrid steppers or servos for increased accuracy and higher performance. I just did an upgrade on a 5 x 10 Multicam router for a customer and installed Teknic Clearpath SD servos. I found those the be a nice little unit. Direct bolt in for a NEMA 34 motor, and they take a step & direction input just like a stepper, and have the drive built in. I think they also have a NEMA 23 motor size. They close the loop internally.
Looking forward to seeing you new toy...er, tool
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