What your seeing is mach rounding the movement to the nearest step your machine is capable of making. Take for instance the 3.0035"....another step up or down of the motor, would have put you beyond the 0.0035 length, therefore, 3.0035 was simply the closest that could be achieved.
Thank you for your prompt answer Rich, but I respectfully disagree. Perhaps I need to offer more details to allow you to appreciate my assessment.
First is the fact that I can manually step the X axis to the any number I choose, so in this example I can step it all the way back to 3.000" after its initial commanded movement.
Second, mechanically the resolution is 0.0001" per step, but obviously not the accuracy. My screw is Roton #5932, not a particularly accurate one, but ok overall. It's a 0.200 inch per revolution which combined with my 200 steps per turn stepper motor, plus 10 microsteps per step, gives me 10'000 microsteps per inch.
So, I can physically move the table at increments of 0.0001", that's just a fact. I cannot however place the table where I want it using an X#.#### command because the spiral screw is not consistent throughout its length (my assessment).
This is where the idea of mapping the screw comes in.
Now, I've seen a few people mentioning it, but never anyone actually doing it, so I wonder at this point if it is an actual feature of Mach3, or not. If it is I would like to take advantage of it, as it would allow my somewhat inaccurate screw to achieve much better positioning, at least as accurate as my mill's DROs can measure. Who wouldn't want that?!