Thanks Pekka. I do have a big micrometer, I was thinking I'd have to make a bar up with it, but if I have a cal rod with it, I'll use that. In mounting, I've been very careful not to bind the scales in any way, but we'll see.
I think friction or binding error would show up as scale reader backlash, in other words not returning to the same point after a traverse and then reversal, vs calibration error which would show up as a one directional measurement error.
The main thing I'll be looking for, after calibration adjustment would be inconsistencies for traverses of varying lengths and their returns. That should show what the limits are.
I think I remember reading something somewhere that the leadscrews on these mills is actually a metric approximation of inches, and the inch dials will show some error, but I'll figure out -- will also check my DTI, out of curiosity.
"Chinese universal" leadscrew definately could be one possible culprit. I have one in lathe....dial has metric and imperial markings (that are not really that accurate) and later I found out that the leadscrew on the cross slide is not really metric, nor really imperial. I have been miking a diameter a lot and usually mount DTI onto cross slide to positively keep track on last few mm of diameter when I try make last few passes to hit the target and keep DOC fairly constant.
