Hello,
I am finishing my my migration from Mach3 to Mach4. I just got done with calibrating my axis (without backlash) and noticed an issue. My calibration was done over 1". The mill is moving more accurately than I was hoping. The issue is when I move less than 1" there is a progressive error (not moving far enough). Once the machine gets to 1" it is perfect. I am unsure if the error continues on the other side of 1" but if it does, I would imagine the machine will be moving too far.

Anyone else run into this?

If you have the ability, calibrate it to a longer length.  The longer, the more accurate.  I use a dial indicator to calibrate axis on our machines, but I always check to be sure it moves further accurately.

@Cbyrdtopper, thank you for the response. Using a ruler I was able to see that the mill is off by 1/16" over 20". I think where I may have been going wrong is that I am not preloading the screw in the right direction before taking my calibration measurement. Today I will try to move the screw 0.05 (in the measurement direction) to zero then measure my 1" with the micrometer. Maybe moving back and forth between 0 and 1" was adding backlash to my calibration. Additionally I will look into taking the backlash out using the screw adjustments.

Getting the backlash out mechanically is absolutely the best way to do it.  Removing backlash in the software is just like a bandaid... it just masks the problem.
If you have a good quality ruler, use that for your calibration.  Get an extremely fine point tool in the spindle and move it from 0 to 20 and try that.  It should get you pretty close across 20" of travel.

I am out of the woods, thank you. This is the most accurately I have ever calibrated a machine. Super stoked!

What is also instructive is placing a dial indicator - a sensitive one, like a tenths indicator - and jogging in 0.001" increments. It demonstrates backlash and other weirdness immediately