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.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
I'm trying to put some numbers to the layer errors, but my brain isn't up to it at the moment. Maybe later. I did disable my motors and turn the RF Z screw to see how far the bed could tilt before anything seemed to bind or tighten up. Mine seems to have a usable window of 10 mm! That seems like plenty to accommodate any errors, but I don't think it's that simple. The window didn't seem symmetric around the level point, so it might be that only a couple mm is enough to cause binding. I'll try to look into this further.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
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??
think there may be a misunderstanding about where the error comes from.
The deviation is not related to layer thickness or percentage per turn.
At the start of the print, the firmware creates a topography mesh that is correct at Z = 0.
The issue is that one side of the bed has an accumulated error of about 2 mm over the full travel.
This means that as the print reaches higher areas, the nozzle positions the layer according to the initial mesh, but the actual surface of the part is progressively lower — about 1 mm at mid height and up to 2 mm at maximum height.
This geometric offset is far beyond what the system can compensate and eventually causes deposition and interlayer adhesion issues on tall prints.
So this is not a percentage layer error, but a cumulative positional error.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
think there may be a misunderstanding about where the error comes from.
The deviation is not related to layer thickness or percentage per turn.
At the start of the print, the firmware creates a topography mesh that is correct at Z = 0.
The issue is that one side of the bed has an accumulated error of about 2 mm over the full travel.
This means that as the print reaches higher areas, the nozzle positions the layer according to the initial mesh, but the actual surface of the part is progressively lower — about 1 mm at mid height and up to 2 mm at maximum height.
This geometric offset is far beyond what the system can compensate and eventually causes deposition and interlayer adhesion issues on tall prints.So this is not a percentage layer error, but a cumulative positional error.
Sorry Albert, but I think you are mixing up two separate aspects of this deviation.
Sure, the total dimension of a large printed object will be distorted, by up to 1.5 mm if you max out the build volume. But you keep referring to layer adhesion problems. And as the layers are printed one by one, the incorrect lead screw pitch will only affect each layer's thickness by the small percentage you have measured, i.e. 0.7% of the nominal layer thickness. I would very much expect that this makes no difference at all to the new layer's ability to stick to the layer below.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
@jurgen-7 So support have dispatched a replacement motor for me, I have absolutely zero doubt I'll end up with 2 in tolerance and 2 out of tolerance 😂 I'll cross that bridge when I come to it.
Quick question, as I'm still trying to get my head around which one is out of tolerance...I lack the artistic skills to trace the threads haha
OK so here is the bed visualization after calibrating z through the menu, and doing a full bed probe at Z:0
So basically what I can gather from this image, the front right (green) and rear center (also green) are around where they should be if those 2 lead screws are matched, its the front left (blue) lead screw with the issue, hence thats the one thats out of tolerance?
RE:
So basically what I can gather from this image, the front right (green) and rear center (also green) are around where they should be if those 2 lead screws are matched, its the front left (blue) lead screw with the issue, hence thats the one thats out of tolerance?
AFAIK, for PRUSA printers, X=0, Y=0 is the front left corner, in the image you show, it's in the rear right, so rotated by 180°.
I don't know what reference your tool uses, just wanted to point this out.
If it is indeed rotated 180°, then the left lead screw would be the faulty one, which moves too far
RE:
Quick question, as I'm still trying to get my head around which one is out of tolerance...I lack the artistic skills to trace the threads haha
OK so here is the bed visualization after calibrating z through the menu, and doing a full bed probe at Z:0
So basically what I can gather from this image, the front right (green) and rear center (also green) are around where they should be if those 2 lead screws are matched, its the front left (blue) lead screw with the issue, hence thats the one thats out of tolerance?
I don't think the bed scan will tell you much about the lead screw differences. Even if all lead screws have perfectly matching pitch, the printer's base plate (which provides the reference plane for the Z calibration) may be tilted vs. the CoreXY mechanism (which defines the probing plane during mesh bed scanning). That tilt will directly show up in your bed scan.
You would either have to measure the difference in bed tilt between the bottom and top Z position, using a sensitive spirit level. Do this twice, with the level oriented in X and Y direction -- or maybe it's clearer if you take three measurements, with the level oriented along the connecting lines between pairs of lead screws. If the tilt along one of the connecting lines does not change at all while the bed travels upwards, then these two lead screws are in agreement.
Or try the "attach paper strip, rub off with pencil, measure the copied thread pitch with ruler or caliper" approach. That provides a direct measurement of the screw pitches and is more sensitive than measurements in tilt angle. (At least for my eyes and spirit level..)
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
So basically what I can gather from this image, the front right (green) and rear center (also green) are around where they should be if those 2 lead screws are matched, its the front left (blue) lead screw with the issue, hence thats the one thats out of tolerance?
AFAIK, for PRUSA printers, X=0, Y=0 is the front left corner, in the image you show, it's in the rear right, so rotated by 180°.
I don't know what reference your tool uses, just wanted to point this out.
If it is indeed rotated 180°, then the left lead screw would be the faulty one, which moves too far
It's my own tool, ignore the numbers mate, the actual orientation is correct the numbers are inverted.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Or try the "attach paper strip, rub off with pencil, measure the copied thread pitch with ruler or caliper" approach. That provides a direct measurement of the screw pitches and is more sensitive than measurements in tilt angle. (At least for my eyes and spirit level..)
On second thought, you should do this anyway. How else are you going to check out the new lead screw you will receive, to figure out whether and where to install it? You don't want much trial and error there; replacing the motor & lead screw is a tedious operation.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Or try the "attach paper strip, rub off with pencil, measure the copied thread pitch with ruler or caliper" approach. That provides a direct measurement of the screw pitches and is more sensitive than measurements in tilt angle. (At least for my eyes and spirit level..)
On second thought, you should do this anyway. How else are you going to check out the new lead screw you will receive, to figure out whether and where to install it? You don't want much trial and error there; replacing the motor & lead screw is a tedious operation.
Yeah I definitely want no mistakes, I only want to do this once...Ive spent the last 15 minutes searching my house for a pencil 😂
I read your post about swapping the motors, so did you do the motors one at a time ? and is it necessary to take off the trapezoidal nuts?
Im still a bit baffled on how Prusa has let this situation occur. All this time I've been under the impression that something on my printer was mechanically skewed.
RE:
I read your post about swapping the motors, so did you do the motors one at a time ? and is it necessary to take off the trapezoidal nuts?
After Prusa generously sent a full complement of new motors from the same batch, I swapped all three at once. I approached it like this:
- Drive the bed to medium height. So you can get to the screws which hold the trapezoidal nuts, as well as the screws which hold the motors to the base plate.
- Remove the door.
- Switch the printer off and put it on its back. Make sure to place some support (wood, book, ...) so the printer does not rest on the plastic WiFi hood.
- Remove the front and side skirts from the base plate; they would get in the way of removing the front motors. Maybe it is possible and preferred to remove all four skirts as a whole and keep the adhesive feet attached; I did not try that.
- Then swap the motors, one by one:
- Remove the two M3 screws which hold the trapezoidal nut; rotate the nut upwards. (You can't remove it yet.)
- Remove the four M3 screws which hold the motor flex mount to the base plate.
- Pull the motor & lead screw out halfway; rotate the trapezoidal nut all the way up and remove it; pull out the motor entirely.
- Remove the four M3 screws which hold the flex mount to the motor.
- Install the new motor by reversing the above steps; rinse and repeat for the other motors if applicable.
- Connect and tidy up the motor cables (potentially longer now); attach skirts and adhesive feet. I have been wondering whether it is important to keep the stepper motor cables away from the display cable: My display had been acting up and would only work reliably in slowed-down mode; after the motor swap it worked in fast mode again.
RE:
think there may be a misunderstanding about where the error comes from.
The deviation is not related to layer thickness or percentage per turn.
At the start of the print, the firmware creates a topography mesh that is correct at Z = 0.
The issue is that one side of the bed has an accumulated error of about 2 mm over the full travel.
This means that as the print reaches higher areas, the nozzle positions the layer according to the initial mesh, but the actual surface of the part is progressively lower — about 1 mm at mid height and up to 2 mm at maximum height.
This geometric offset is far beyond what the system can compensate and eventually causes deposition and interlayer adhesion issues on tall prints.So this is not a percentage layer error, but a cumulative positional error.
Sorry Albert, but I think you are mixing up two separate aspects of this deviation.
Sure, the total dimension of a large printed object will be distorted, by up to 1.5 mm if you max out the build volume. But you keep referring to layer adhesion problems. And as the layers are printed one by one, the incorrect lead screw pitch will only affect each layer's thickness by the small percentage you have measured, i.e. 0.7% of the nominal layer thickness. I would very much expect that this makes no difference at all to the new layer's ability to stick to the layer below.
I see your point, and I agree that if this were a uniform pitch deviation across all screws, the effect would mostly be a small scaling error in Z, with each layer being However, in this case the deviation is not uniform — it’s differential between the screws — so the issue is not just a percentage change in layer thickness.
Because the bed is supported by three lead screws with slightly different effective travel, the plane of the bed does not maintain the same orientation as it moves through Z. The mesh is generated correctly near Z = 0, but as the bed moves, the plane gradually tilts relative to the nozzle.
So the nozzle is no longer printing at the intended distance from the actual surface of the part. The error becomes progressively larger with height — not as a per-layer percentage, but as a geometric offset between the expected plane and the real one.
That’s why small prints look fine, while taller prints start to show deposition inconsistencies: the system is no longer operating within the distance tolerances it expects.
So I think we’re essentially describing two different models — a uniform pitch error versus a changing bed plane — and in this case the behavior matches the latter.
I appreciate the discussion — it helped clarify the distinction
RE:
Albert -- either you or I have a major mental block here. 🙄 Let me try and explain my understanding one last time:
Let's say the pitch of the front left lead screw is too steep by 0.75%. Then every single layer that gets deposited will have its nominal thickness of, say, 0.2 mm on the right side of the bed, and 0.2015 mm on the left side.
Yes, when depositing the final layer of a large tall print, the nozzle (while on the left) will be 1.5 mm higher above the bed than it should. But the prior layer was already (1.5 - 0.0015) mm higher, so there is just that minimal, incremental deviation of 0.0015 mm which the final layer will have to bridge. There will be no adhesion issues.
I am running out of ways to describe this. Maybe someone else can chime in and explain either to you or to me how this works, or can figure out where we keep misunderstanding each other.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Hi, Jürgen,
That’s actually the key point in my setup.
All three Z motors are wired in parallel through a splitter and driven by a single driver, so they always receive exactly the same step and direction signals and therefore rotate the exact same number of turns.
Because of that, there is no possibility for the firmware to compensate or correct any differential mechanically - the system is purely kinematic.
If one lead screw has a slightly different effective pitch, identical rotation does not produce identical linear displacement, so the bed cannot move up as a perfectly parallel plane.
Instead, the bed plane develops a progressive change in inclination as a function of Z travel.
So the relevant effect is not a tiny per-layer thickness variation, but a growing tilt versus distance, which creates an increasing geometric offset between the nozzle’s expected position and the real surface as height increases.
That progressive tilt with distance is the core of the issue, and I think this is simply where our interpretations diverge.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Hi Albert, it was really your concern about layer adhesion which threw me off. As long as we are talking about a linear distortion of the overall object shape, we are on the same page.
As an aside, it would actually be possible to compensate for this skew in firmware -- even with all three motors wired in parallel. The software-based bed leveling correction already moves the bed dynamically during the print, as a function of X and Y. It would "only" need to be enhanced to move as a function of X, Y, and Z: As the print progresses, there would be a gradually increasing contribution which compensates for the tilt caused by the screw pitch difference. It would be difficult to calibrate though: It can't rely only on automated bed probing, but needs to know the screw pitch difference in addition.
But let's not go down that rabbit hole. It looks like Prusa has decided not to go for software compensation, but rather provide replacement screws to the limited number of users who notice this flaw.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
My assumption is that the actual layer thickness is determined by the distance from the nozzle to the previous layer. Closer and the material ends up a bit wider and thinner. Further and the material still fills the gap, but the track is a bit narrower. So, at any given height, no matter what the leadscrew error is, the error between the last layer and the nozzle position is a couple percent at worst, which is all that matters, so long as you don't look at the overall dimensions. 😉
[I come from a precision metric world and may never accept that we use 0.2 mm layers, not 200 microns. My nozzle is 400 microns, even if yours is 0.4 mm.]
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
[I come from a precision metric world and may never accept that we use 0.2 mm layers, not 200 microns. My nozzle is 400 microns, even if yours is 0.4 mm.]
I much appreciate that you don't measure it in "thous". 😉