RE: Skewed XY-Plane (Z Rods, Heatbed)
After what I can only assume must have been lengthy deliberations at Prusa HQ, I finally got new motors today. As far as I can see, their spindles finally do have the same pitches. Between the right and left front spindle, I now measure 0.4mm difference (see first post). Probably good enough.
This whole spindly "experience" took me way longer than assembling the kit...
RE: Skewed XY-Plane (Z Rods, Heatbed)
Just adding in here that it appears that I have a similar XY skew as well. Measuring the base at z=275 I'm coming up with all measurements being equal, but when raising the bed to z=0, there is a X axis skew between the front screws of 1.4 mm.
This photo is X=-2, Z=0. Y remains constant at 10.
While this is at x=252, z=0 
I had the mk4s>CoreOne upgrade kit, but it originally started as a mk4 several years ago.
RE: Skewed XY-Plane (Z Rods, Heatbed)
Hi,
Decided to check the pitch of the Z lead-screws on my Core One batch 2 Kit, and I got similar results to previously presented in this thread.
Procedure
- Paper strip with center line marked out, tape to fasten and limit measurement area to 150mm
- Attach Strip to lead screw
- Pinch tape on one end with fingers
- Lighly trace out the threads with pencil away from pinched side
- Photograph (flat bed scanner would have been better) and count pixels and threads
Results
L = Left front Screw
R = Right front Screw
C = Center Rear Screw
Sanity check by aligning the threads and miss-match in pitch after 21 turns is clearly visible
RE: Skewed XY-Plane (Z Rods, Heatbed)
Nice job on the measurements. Make sure to notify Prusa via chat.
I might do the same trick to get accurate measurements!
RE: Skewed XY-Plane (Z Rods, Heatbed)
Petulf, now that's a clever way to take the measurements!
RE: Skewed XY-Plane (Z Rods, Heatbed)
Very smart.
That's quite a difference for tall prints.
RE: Skewed XY-Plane (Z Rods, Heatbed)
What’s the latest status on this? I cancelled my conversion order over this issue; doesn’t look like much progress has been made since.
MK4S/MMU3
RE: Skewed XY-Plane (Z Rods, Heatbed)
I upgraded an MK4 from late 2021 with one of the Batch 1 upgrade kits . Using @petulf's measuring method (very satisfying for its simplicity), both front screws have 100 ridges in 200mm. In my instance the kit motor seems to match the MK4 ones.
I did use this mod to level up the left front of the frame. I think that's an artifact of the modular frame design.
FWIW, printing 50mmx200mm cylinders near the three lead screws match within a few hundredths of a millimeter on mine.
RE: Skewed XY-Plane (Z Rods, Heatbed)
It took long time and many emails with Prusa support, but here is the conclusion for me:
I was sent multiple motors to test over time. Some of them had the same error, some were better but not perfect and finally the last one is perfect and the problem is completely solved.
I returned the bad ones to Prusa for quality control. They measured the pitch and got same result as me (about 0.5 % error, 1 mm per 200 mm of travel). According Prusa, this is still within allowed tolerance.
So the result is mixed - Prusa support was helpful and eventually helped me to resolve the issue, but did not acknowledged that the pitch is out of spec.
RE: Skewed XY-Plane (Z Rods, Heatbed)
>> (about 0.5 % error, 1 mm per 200 mm of travel). According Prusa, this is still within allowed tolerance.
Please tell me this is a joke.
RE: Skewed XY-Plane (Z Rods, Heatbed)
(about 0.5 % error, 1 mm per 200 mm of travel). According Prusa, this is still within allowed tolerance.
Unless there is a specification which Prusa has made available to their customers before the purchase, I would dispute that statement. (And I am pretty sure no such specification has been published.)
This deviation would translate into 1.6 mm deviation over the full travel height of the bed, and hence more than 1 mm skew in a full height, full width print. That seems excessive to me.
RE: Skewed XY-Plane (Z Rods, Heatbed)
If I read this correctly, the 0.5% error could well be within the limits Prusa put in their contract with the motor manufacturer, as such not a wrong statement in itself (I wouldn't like it but I guess it is a question of cost). The assumption is that the motors delivered to the end customers should be out the same production runs (aka do not deviate from each other, aka have the same error), and this is where the problem is: quality control between a pallet of motors at the Prusa warehouse and the packaging of the motors pairs in the kit.
My opinion only, and not meant to start another flaming war with someone that has specific industry knowledge (just saying as I've seen similar discussion go bad :D)
RE: Skewed XY-Plane (Z Rods, Heatbed)
If I read this correctly, the 0.5% error could well be within the limits Prusa put in their contract with the motor manufacturer, as such not a wrong statement in itself (I wouldn't like it but I guess it is a question of cost). The assumption is that the motors delivered to the end customers should be out the same production runs (aka do not deviate from each other, aka have the same error), and this is where the problem is: quality control between a pallet of motors at the Prusa warehouse and the packaging of the motors pairs in the kit.
That makes good sense to me. A 0.5% deviation from the nominal thread pitch which is the same on all three motor axes is much more tolerable. And it is easily compensated by scaling the whole print in the Z direction if one does a large-scale print with critical dimensions.
Core One kits are most likely to be affected by the "lead screws from different batches" problem, I think. Obviously the MK4S conversion kits, where two old lead screws are combined with a new one from the upgrade kit. But also the full kits, which include a pre-packaged box with the MK4S complement of motors, plus an extra Z motor and lead screw -- so not all Z motors are picked from the same shelf at the same time.
RE: Skewed XY-Plane (Z Rods, Heatbed)
not exactly my idea of a very interesting job but I would love to know that someone is picking the motors and grouping them right. Wouldn't have fixed the kits (were a new motor is added to two older ones) but at least all the full kits and factory assembled ones would be free from this issue.
If I read this correctly, the 0.5% error could well be within the limits Prusa put in their contract with the motor manufacturer, as such not a wrong statement in itself (I wouldn't like it but I guess it is a question of cost). The assumption is that the motors delivered to the end customers should be out the same production runs (aka do not deviate from each other, aka have the same error), and this is where the problem is: quality control between a pallet of motors at the Prusa warehouse and the packaging of the motors pairs in the kit.
That makes good sense to me. A 0.5% deviation from the nominal thread pitch which is the same on all three motor axes is much more tolerable. And it is easily compensated by scaling the whole print in the Z direction if one does a large-scale print with critical dimensions.
Core One kits are most likely to be affected by the "lead screws from different batches" problem, I think. Obviously the MK4S conversion kits, where two old lead screws are combined with a new one from the upgrade kit. But also the full kits, which include a pre-packaged box with the MK4S complement of motors, plus an extra Z motor and lead screw -- so not all Z motors are picked from the same shelf at the same time.
RE:
After dragging my feet for a few months, I finally contacted Prusa support about the different lead screw pitches in my Core One. Everything worked out fine eventually, but it took a while:
The first support technician seemed unaware of this issue. It took a lot of back and forth to convince him that this was not a case of sporadic step loss due to friction in the Z drive. And he did not trust the screw pitch measurements as discussed in this thread, but insisted on seeing a video where a spirit level travels upwards on the heatbed and shows the change in tilt. A rather insensitive method, but I did end up buying a small spirit level and making that video to keep him satisfied.
He did agree to send a replacement motor & leadscrew, but had no way of selecting one with a specific pitch -- as one would expect; only one standard part number is available at Prusa. And of course the replacement I received had exactly the same pitch as the "odd one out" I already had in my printer... So I now had two leadscrews each with two different pitches; not very useful.
I contacted support again and suggested that they send one more motor & leadscrew, in the hope that it would match either one of the two pairs I already had. But this time support preferred to send three motors from the same lot -- throwing in six new trapezoidal nuts for good measure. All were "right side" motors with the long cable, which makes sense to me: They literally wanted to grab them all from the same shelf. (I offered to return the replaced motors, but they did not take me up on that yet.)
Swapping the motors took a bit longer than I had expected. Best done after removing the front door and top lid, moving the heatbed to medium height and the Nextruder to the back, and laying the printer on its back. (Provide a relief for the WiFi antenna!) The bottom profiles on the front and sides need to come off to be able to pull out the two front motors; so you have to remove the display, a dozen screws and rivets, and the two front rubber feet. Pro tip: Install the new trapezoidal nuts correctly right away, with the flat side on top; otherwise you get to repeat the whole exercise once you realize that the heatbed can't move quite high enough... Don't ask... 🙄
Anyway, I now have a printer which can move the heatbed up without gradually tilting it in the process. Big thanks to Prusa for providing spare parts so generously!
RE: Skewed XY-Plane (Z Rods, Heatbed)
It sounds like the spirit level is a simple and graphic way to demonstrate the issue. Glad this worked out.
RE: Skewed XY-Plane (Z Rods, Heatbed)
It sounds like the spirit level is a simple and graphic way to demonstrate the issue. Glad this worked out.
It would have been a quick check if I had owned a spirit level short enough to fit onto the heatbed...
But its sensitivity is not as good as one might think. I bought the best compact level I could find, specified at 0.5 mm/m sensitivity. The tilt of the heatbed over its full travel range is 1.5 mm/0.25 m, so more than an order of magnitude more. The effect could indeed be seen on the spirit level, but it was marginal. No idea how those guys specify their level's minimum sensitivity; you probably need a microscope to watch the bubble move.
Various "spirit level" apps for my smartphone were not sensitive enough to show any effect, by the way. I tried a few before I headed out to buy a proper level. -- In this particular application, measuring the height of the three heatbed mounting points with a machinists ruler is certainly more accurate and quantitative.
RE: Skewed XY-Plane (Z Rods, Heatbed)
So over the weekend I built my Core One. I had 2 issues: 2 of the 3 trapezoid nuts were so tight that the motors skipped steps and the bed went tilted. You could feel it when screwing them on the motors. These were the 2 nuts which were already on the motors in the Motor package. PPL putting them on the motors should notice that there is something wrong. Good thing is I have lots of Mk3, 3.5, 3.9 and Mk4 in the office and there were always spare Trapezoids in the kits. So I gathered 2 spare nuts and swapped them.
Next thing I noticed is the issue above. The heatbed moved along the Z axis while printing flat surfaces. I also noticed that the nozzle had a different distance to the heatbed when moving manually. So I printed 3 cylinders, each 260mm high. The rear and right were 260mm, the left was 261mm. Then I ordered 3 new motors, when I have fixed this issue I will deal with support (I have built another core one for my buddy, I will check it also in the next weeks).
It is a real bummer that there is such poor quality at the leadscrews. Especially with CNC machines or 3d printers such tolerances are unacceptable. Yes, it's only one milimeter at 260mm or 0,1mm at 26mm, still too much. I am used to draw my objects with the measurements I need and till now everything came out exactly as intended. That is also the reason I never used the Mini in our print farm, too many ways for some measurements to be off. We use the printers for mechanical parts and it's essential that they are on spec.
Of course I will stay with Prusa, because the printers are great ( and if I am really pissed I could drive the 340km from Vienna to Prague and act like a Karen 😉 ), and I know every of our printers screw by screw (I always buy kits, and that is the reason I find most problems instantly) but this could be an issue for first buyers....
Please improve your QC !
RE: Skewed XY-Plane (Z Rods, Heatbed)
I had exactly the same problem with one of my trapezoid nuts. Way too tight, same solution, used a spare. There seem to be some sort of bad batch around as otherwise we would have had more reports in the forum about this.
the problem with the motors is not, imho, the tolerances, is the mixing of old mk4 and new motors. Were all from the same production batch, no issues (as I suspect is the case when a new core one is assembled by prusa, as they get all 3 motors from the same box) but for some that got two different tolerances it creates a problem. Motors have been exchanged by support already though...
RE:
Since the motors have different cable lengths (or at least different laser print on them), they are obviously from different batches. Therefore I ordered 3 motors all with the long cables. Hopefully I need only one because I prefer 260mm to 260mm 😉
Let's see when the motors arrive (probably Friday or Monday)....