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.
Math is good, Mach is better... I would suggest getting a set of calipers and go to the settings page in Mach. There is a button that says 'Set steps per unit'. Find a piece of wood and put a sharp & pointed bit in your router. Bring the spindle over the wood and jog the Z axis down until you make a tiny point in the wood, then raise your spindle so the bit is not touching the wood. Use the "Set steps" utilty (note- you need to click in the "How far box to enter a value). After the spindle moves, bring your spindle down and make another mark in the wood- use your calipers to measure the distance so you can input back to Mach. Try doing this a few times using small and large values to see if things are set correctly (you might want to make notes of your current steps per unit setting BEFORE you do this so you can revert/compare.
... 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. .
BUT- did it start out at the exact same place and drift during the cut? If you stop and re start the program does it begin from the same place?
... 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.
Not totally clear on this? Way off when you came back to Zero? Did it come back to the correct Zero position and then only give you 23.32 when you asked for 24.00?
... until I had what seemed like perfect repeatable 24" travel. But a day later, without having ran any more parts, it was different.
In what way? Do you mean that went to a G0 X0 Y0 Z0 ok and then when you asked for a G0 X24 it went to X23.32?
...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?
In theory, Yes.
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?
Or like Brett already said, something is slipping somewhere. I know you mentioned lead screws but how are these coupled to your motors? Direct drive or are you using pulleys & belts to turn your screw? If so, maybe a loose pulley or belt?
...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?
Unless this is a case of missing steps, which can be determined by the way Zarzul explained, It doesn't really seem like this is a computer/Mach problem since it is not repeatable in a consistent way. It really seems more mechanical than software- otherwise it would seem that the computer by way of issuing the exact same commands over and over would render the same result (error or otherwise).
We haven't seen your G-code, I suppose there could be something funky (like a scaling command) in there that would cause this type of thing but, again, it would be repeatable.
Sid
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