RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
IMO, it's certainly desirable that the leadscrew pitches match, but a total dimensional error should be the only effect. The error is distributed over the travel distance, so there's never a problem layer-to-layer. It's not going to crash or shift, at least I don't think so. The system also has enough flex or slop so that a couple mm difference shouldn't cause significant lateral tension, friction or binding of any sort. That's easy enough to test by manually rotating one screw to see how far you can go before it binds. These are rolled leadscrews, not ground, and I suspect they have a wider than might be desired tolerance, especially batch to batch. Being a fast multi-start screw, it doesn't take much error in the screw to cause a position error of a mm or so over a long distance. What would really worry me is if there were a single rotation "drunken screw" problem, because that would cause a local issue between layers.
RE:
Hi jürgen,
I respectfully disagree that this only affects dimensional accuracy. In a system where the bed moves away from the nozzle, a pitch discrepancy creates a dynamic skew.
I did not mean to say (and did not say, I think) that it is just a constant scale factor. "Dimensional error" is a broader term in my understanding. I fully agree, the effect will be a linear distortion of the shape.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
IMO, it's certainly desirable that the leadscrew pitches match, but a total dimensional error should be the only effect. The error is distributed over the travel distance, so there's never a problem layer-to-layer. It's not going to crash or shift, at least I don't think so. The system also has enough flex or slop so that a couple mm difference shouldn't cause significant lateral tension, friction or binding of any sort. That's easy enough to test by manually rotating one screw to see how far you can go before it binds. These are rolled leadscrews, not ground, and I suspect they have a wider than might be desired tolerance, especially batch to batch. Being a fast multi-start screw, it doesn't take much error in the screw to cause a position error of a mm or so over a long distance. What would really worry me is if there were a single rotation "drunken screw" problem, because that would cause a local issue between layers.
I respectfully disagree. A 0.7% lead error (2mm over 275mm) is a hardware defect, not a minor tolerance. The critical issue is that Mesh Bed Leveling is static: it maps the bed at the top, but that map becomes physically invalid as the bed descends and the plane tilts. If one side is 2mm lower than what the mesh expects, the nozzle will progressively lose its reference, leading to severe layer bonding issues and flow inconsistencies as it approaches the deviated side in tall prints. Furthermore, forcing a rigid system to absorb a 0.35° tilt creates constant lateral stress and friction on the rails and POM nuts that no 'flex' should be expected to handle
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
If one side is 2mm lower than what the mesh expects, the nozzle will progressively lose its reference, leading to severe layer bonding issues and flow inconsistencies as it approaches the deviated side in tall prints.
But it's only less than 1% of deviation! So each layer will be 0.2015 mm thick instead of 0.2 mm. What problems do you expect??
Furthermore, forcing a rigid system to absorb a 0.35° tilt creates constant lateral stress and friction on the rails and POM nuts that no 'flex' should be expected to handle
As Conrad had argued, it is not that rigid...
Just to avoid misunderstandings: We are not saying "it's fine, users should just live with it". (At least speaking for myself. I did not consider it fine, and have convinced Prusa support to send me replacement lead screws.) We are just saying that this cannot cause any layer adhesion problems, and is very unlikely to cause jammed Z drives or other strain-related problems. Incorrect lateral positions of the trapezoidal nuts are much more likely to cause significant stress in the Z drive.