Think you've tuned your rotation_distance properly?

[charlespick]

@koconnor

periodic swings in positional accuracy, but do not change the average position over long distances

This is very interesting and something that I did not think of. Let me see if I can get my hands on a larger micrometer. ASU has to have one somewhere… I have to think about it but I think placing another print 16mm forward and back to cover 48mm and thus more than one full rotation should also tell if this is a concern.

@koconnor

It is possible that an axis skew could result in a systemic position error over long distances

This is a good point. I checked my bed skew before calibrating my setup but I did fail to mention that…
For the record my xy plan is only skewed about .003 degrees so even if I hadn’t checked for this it wouldn’t have influenced it much.

@koconnor

I suppose it’s possible that a belt could be tensioned so tightly that it alters the average belt pitch

As I eluded to before, it doesn’t take that much change in belt tension to change the rotation distance. Simply retightening my belt was enough to move my rotation distance from 39.79102314 to 39.9063. You are right that the average pitch in the belt teeth is a primary factor but the other important factor is the diameter of the hobbed gear that interfaces the belt to the motor. Tightening the belt at all will change the diameter at which the belt meshes with the gear, just like how tightening an extruder influences how the filament interfaces with the gear.

@koconnor

Thus, it is the belt pitch (average distance between belt teeth) that determines the position over long distances

Again, you are forgetting that the diameter (and thus the circumference) of the drive gear plays an equal role as the teeth in the belt. If the gear has double the circumference, it will double the rotation distance. What I’ve found is that tightening the belt ever so slightly shrinks the “interface diameter” which is a term that I might have just made up but it’s the middle of where the teeth end up. Loosening the belt will cause the teeth to not mesh as tightly and increase the diameter and rotation distance.

This is part of a long saga I have in trying to beat machines like the Stratasys F170, not a complete guide to dimensional accuracy by the way. I want to be clear that jakep_82’s concerns about slicer adjustments have merit. But rotation distance isn’t something that’s specific to a printer for it’s life, and it’s certainly not specific to just any model of printer. This is why commercial 3d printers have hardware for calibrating their dimensional accuracy built in, typically a laser based system. Older printers like the uPrints would run a calibration procedure every night. Modern Stratasys F123’s do less often, although I’m not sure exactly how often anymore because Stratasys wants to be a butt about covering up everything.