Thanks Sid. I initially calibrated the screw by using the math method outlined in Mach setup, basing it on it being in THK catalog as 20 mm lead screw. I did same process for Z axis, a 2 mm lead, and it works fine. The x being "close" I didn't notice any problem until I ran a parts program. Then it seems like if I finish the parts program, leaving the part in place, and simply ran it again, it didn't line up perfectly. Close, but maybe 1/64 off, more at the end of the program than beginning. That presents a big problem for running a second program with a finish pass.Using Vcarve Pro for CAM.
So I rechecked, by zeroing the X axis, issuing G0X0, G0X24 commands, measuring and marking the results. That's when I saw I was way off, getting something like 23.32" of travel rather than 24. I did this several times with results identical to my steel ruler and 49 year old vision. So I did a math conversion based on the units I used for motor config, and reran it, doing this several times with smaller and smaller adjustments until I had what seemed like perfect repeatable 24" travel. But a day later, without having ran any more parts, it was different. Now if it were simply a problem of the wrong number of units for motor config, the axis would go precisely to a specific spot for the G0x24 command, and precisely back to 0 for the G0X0 after, repeatably with perfect precision, if I wasn't losing steps, right? But what I"m seeing is it goes as close as I can measure to 24" of travel each time, but if I run it a dozen times (g0x0, G0X24) the points it goes to start to drift a tiny bit, and then a larger bit, in one direction, both points being off in the same direction, still very close to 24" apart. This means missed steps, right? I've checked the axis by decoupling the motor and it seems very smooth to me. Decoupled, you can actually push the whole router-Z -axis-assembly along the X axis rather easily, and if feels good. My machine is fixed gantry, moving Y axis table.
Since I used a different driver for the Z axis, maybe I should try attaching the z axis that and see if I get similar problem. Or simply try different microstep settings. I"m running it at 1/25th. I put it higher than I would have for my resolution needs (woodworking) because it (X axis) was very grumbly at lower microstep settings at lower speeds. Is this a clue to my problem?