Confused about xy calibration

I’m a little confused about measurements from my x and y movements.
I’ve been perfectly happy with my prints so far. Having done the e steps calibration, adjusted flow rate, and various other settings, prints have been good, and dimensionally close enough for my liking. But trying to do some prints with tight tolerances, especially cylindrical shapes, I’ve noticed things could be better, so thought I’d check the xy calibration.

What’s confusing me though is that measuring the movement of the tool head and the bed, it seems they are moving slightly more than they should. Both axis are about the same, a 140mm movement actually measures 140.3mm (+/-0.1mm).

But, on a 20mm calibration cube, the x and y dimensions actually come out smaller than they should, around 19.8 and 19.9mm.

Am I not right in thinking, if they move more than they are supposed to, then parts would come out bigger than expected?

Or am I looking at it wrong?

We have been discussing this here…

Yes, I’ve seen that (and posted in there too). Didn’t see anything there that cleared this specific question up though, and thought it warranted is own thread.

Thing is, if I go by the measurements of the linear modules (which I’m sure I’ve read somewhere is a more accurate way to do it) then I need to decrease the xy steps. But if I go by the measurements of the test cube, I need to increase the steps.

you have to get it mechanically correct first, then you have to tune your profile. But a 0.2% inaccuracy isn’t bad. Probably not worth chasing at first.

Well at least I believe that is the correct way to go for the end goal of the most accuracy that you can get. You can always tune out allot of issues in software. But if the hardware isn’t accurate. Then you always battling an unknown battle.

We haven’t been able to come to a clear cause yet, which is why there is no defined answer, and why it’s really still the same discussion. Much more precision testing is needed, but the time and resources involved may not be worth it.