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MK4 skew correction  

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Kiddocteur
(@kiddocteur)
New Member
MK4 skew correction

Hello,

Prusa used to pride itself on having automatically square prints with XYZ correction with superpinda. In MK4, superpinda has disappeared, and XYZ calibration . My prints are no longer square. I contacted Prusa support and indeed there is no skew correction. What's a pity is that apparently you can't even use M852 in Marlin. I will now try to physically calibrate the printer. It's a big step backwards I think. 

If you have alternatives I'm interested!

Dimitri

Posted : 21/12/2023 2:15 pm
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Kiddocteur
(@kiddocteur)
New Member
Topic starter answered:
RE:

An ad that's missing today

Posted : 22/12/2023 11:38 am
miroslav.h4
(@miroslav-h4)
Honorable Member
RE: MK4 skew correction

This correction was introduced in the MK2 due to the construction of the lower frame. Because this frame was made using threaded rods, and the X and Z axis frame was attached to these rods only with screwed nuts that could be located at different distances on the right and left threaded rods, so the perpendicularity of this frame to the Y axis was not guaranteed. Therefore, there was XYZ calibration which detected this and compensated for this skew within small limits. For printers MK3 and higher, the perpendicularity of the frame is ensured by precisely cut extruded profiles that are exactly the same length on the left and right sides, and thus the perpendicularity of the X and Y axes is also ensured. Therefore, this calibration is not needed.

Posted : 22/12/2023 3:53 pm
Kiddocteur
(@kiddocteur)
New Member
Topic starter answered:
RE: MK4 skew correction

Hello, the XYZ correction with the superpinda was used on the MK3 which has almost the same frame as the MK4. Furthermore, aligning machined metallics parts, even with precision, with plastic parts leads to play, and therefore imprecision. My skewed prints confirm this (it's not major, but it's not satisfying either).  

 

Posted : 22/12/2023 4:22 pm
domble
(@domble)
Eminent Member
RE: MK4 skew correction

Surely the pinda prove could only ever do compensation for the z axis. X and Y is just the mechanics of the frame? And the mk4 doesn't use the pinda but the more accurate strain gauge system instead. 

I assume that the pinda or strain gauge probe levelling keeps the whole print the right distance from the print bed; if the print bed is mechanically bent then the print will match it perfectly?

Posted : 24/12/2023 5:39 pm
Bpendragon
(@bpendragon)
Active Member
RE: MK4 skew correction

 

Posted by: @domble

Surely the pinda prove could only ever do compensation for the z axis. X and Y is just the mechanics of the frame? And the mk4 doesn't use the pinda but the more accurate strain gauge system instead. 

I assume that the pinda or strain gauge probe levelling keeps the whole print the right distance from the print bed; if the print bed is mechanically bent then the print will match it perfectly?

Nope, the Mk2 and Mk3 (and all variants of those that had a PINDA/SUPERPINDA probe) had built in XY skew compensation. As part of the calibration you were supposed to take the print sheet off and there were 4 specific spots on the heat bed that the PINDA would find and very precisely measure their locations taking several minutes. Because the location of those spots was known with certainty relative to each other, the printer would then be able to calculate the skew based on the discrepancies between the known and measured values. Then it was able to apply those changes automatically. 

Posted : 31/01/2024 12:15 am
blauzahn
(@blauzahn)
Reputable Member
RE:

Posted by: @miroslav.h4

For printers MK3 and higher, the perpendicularity of the frame is ensured by precisely cut extruded profiles that are exactly the same length on the left and right sides, and thus the perpendicularity of the X and Y axes is also ensured. Therefore, this calibration is not needed.

I disagree. It depends on how accurate you want it to print, regardless whether it is on an MK3, MK4 or XL.

On the MK3/MK4, the mechanically machined parts are good but they are not the only parts which determine the tramming. The y-rods rest in printed holders which are positioned just by regular screws put through drilled holes with slightly larger diameter. The x-axis is suspended in several printed parts for the z-axis. Skew of the latter versus either x or y is not compensated at all, though Merlin has implemented some as mentioned by @Kiddocteur. The meshbed is ootb neither flat nor parallel to neither x nor y. I therefore modded my MK3S: The meshbed has the springmod. I also added adjustments to the y-rods in x and z and the z-rods in x and y as well as for both upper end stops in z.

Even at my newly arrived XL-kit, waiting for assembly, I still wonder how to make the core-xy with its single profile connection left and right (apart from the thin grid at the back) less weak. Even though the the frame might be superior to those on other printers, I consider the frame connections rather weak for the XL's size. Shear and torsional stiffness of the whole frame leaves room for improvement. All frame connections are of the same type. And neither side nor bottom panels improve shear stiffness as much as they could due to their weak attachments. The latter can be fixed easily but torsional stiffness is another story. I consider adding a sheet below its base frame in order to create a torsional stiffer base box as a start. Imagine you want to put the XL onto feet that damp a bit more than the simple rubber feet. Or ask yourself, what resonance frequencies do limit input shaping. I also wish the XL's core-xy belts were stiffer (e.g. 9mm wide). I was rather shocked when I measured the tool changed nextruder to weigh whopping 550g. I also hope that on the XL I will not miss the 4 spot compensation of my beloved MK3s+.

 

Posted : 31/01/2024 6:42 am
miroslav.h4
(@miroslav-h4)
Honorable Member
RE: MK4 skew correction

I would like to disagree here. Be aware of the class in which these printers are classified. They are intended for home printing for fun and education, and here their accuracy is more than enough. If you have higher demands on printing accuracy, this is not such a big problem. There is nothing to prevent you from having precise metal parts precisely embedded in other components made on the basis of plastic printed parts. It's just that you'll probably have to dig a little deeper into your pocket, and instead of shelling out some $800 for the MK4, you'll have to shell out significantly more money. Or you will have to choose a different class of printers for semi-professional and professional printing. There are many manufacturers of these printers. However, my remark about the price of such printers, which is not popular at all, also applies here. And you will be satisfied for sure. What is true here is that if I am a craftsman and the work will support me, then I will never buy tools for my work made for hobby use, because such tools are not intended for permanent load. For professional use, I also have to buy professional tools that can withstand the daily load. But I also have to pay regularly for such tools.

Posted : 31/01/2024 10:16 am
domble
(@domble)
Eminent Member
RE:

 

Posted by: @bpendragon

 

Nope, the Mk2 and Mk3 (and all variants of those that had a PINDA/SUPERPINDA probe) had built in XY skew compensation. As part of the calibration you were supposed to take the print sheet off and there were 4 specific spots on the heat bed that the PINDA would find and very precisely measure their locations taking several minutes. Because the location of those spots was known with certainty relative to each other, the printer would then be able to calculate the skew based on the discrepancies between the known and measured values. Then it was able to apply those changes automatically. 

Ooh didn't know that; so the Pinda was used to detect the edges of some special copper areas on the heatbed by moving sideways across them?  That's clever.  I've only used mini+ (which as far as I know doesn't do that with its pinda) and Mk4.

Never had to worry about skew with either printer; guess they're both reasonably good. 

Edit: oh - exactly like the video posted earlier explains.  I should read before I post!

 

This post was modified 4 months ago by domble
Posted : 31/01/2024 10:17 am
Kiddocteur
(@kiddocteur)
New Member
Topic starter answered:
RE: MK4 skew correction

I don't agree with your answer. The price has nothing to do with this discussion. I upgraded from mk3 to mk4, and nowhere is it written that you lose the super pinda and therefore the XYZ correction.When you upgrade, in people's minds you gain functionality and you don't lose any. I made this topic because no one had mentioned it to my knowledge, and I was surprised. Otherwise, I'm very happy with my mk4, and I've found a solution thanks to https://github.com/olistrik/goskew

Posted : 31/01/2024 10:36 am
miroslav.h4
(@miroslav-h4)
Honorable Member
RE:

You had to count on SuperPinda disappearing in MK4. Just read all about the MK4 and their features. And be aware that SuperPinda with MK3 was mainly about leveling the bed. The fact that it was also used to measure the printer at the beginning of the calibration remained after MK2. There, Pinda even measured XYZ at nine points during calibration, first at the points in the middle of each side and then refining the measurements at other points of the bed as well. You can see that this is so in the drawing of the MK42 bed, where all nine points are visible. Only four points remain for the MK52 bed. But that doesn't really matter, it looks like you solved the problem with the GoSkew script. And perhaps it is even more accurate than using Pinda, where the position was determined only on the basis of the average of several passes of Pinda over the measuring point. I was just wondering what skew you measured with GoSkew.

This post was modified 4 months ago by miroslav.h4
Posted : 31/01/2024 11:31 am
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