Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Hi,
My Core One comes from a MK4S upgrade kit, and I’ve found a mechanical mismatch in the Z axis.
The two lead screws reused from the MK4S rotate exactly the same number of turns as the new lead screw included in the Core One kit, but their trapezoidal nuts move about 2 mm less over the full travel.
To compensate, I had to add a 2 mm spacer at the lower left Z calibration stop so the bed is level near the top of the travel.However, as the bed moves down, the tilt appears progressively, reaching about 2 mm out of level near Z = 0 on the MK4S screw side.
Small prints are fine, but tall prints show increasing layer issues as Z increases.This suggests a different effective pitch or cumulative tolerance between screws.
I’m attaching two photos of a bubble level showing the bed level at the start and at the end of the travel.
Has anyone else seen differences in lead screw pitch or effective travel between MK4S and Core One screws?
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Oh yes... Here's the original thread about this. The link goes to the post where @pavel-brych has identified the root cause.
Prusa support will send you a replacement lead screw or two, but it takes some back & forth to convince them. Your pictures of the spirit level will be helpful; that's a demonstration they have asked me to provide (in a video, showing the change while the bed is travelling upwards).
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Are the lead screws replaceable? I always thought they’re fixed to the Z motors ?
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
They are fixed. When Prusa agrees to provide remplacements, you get the complete replacement units: lead screw, motor and cable. It's a bit of a hassle to swap them on a fully built printer -- front and side skirts around the base plate need to come off so you can pull the motors out towards the bottom.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
They are fixed. When Prusa agrees to provide remplacements, you get the complete replacement units: lead screw, motor and cable. It's a bit of a hassle to swap them on a fully built printer -- front and side skirts around the base plate need to come off so you can pull the motors out towards the bottom.
Jesus Ive just read the beginning of that thread and think I might have that issue ! I was batch one also.
Sorry to be a pain mate but is there a tldr to find out if Im affected? I have decent callipers but thats about it regarding measuring tools.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
is there a tldr to find out if Im affected? I have decent callipers but thats about it regarding measuring tools.
Calipers are very suitable in terms of accuracy, but tricky to use here because it is so easy to miscount the threads. And depending on the size of the calipers it may be difficult to fit them in. A nice measuring method has been proposed later in that thread:
Move the bed all the way up, cut strips of paper a tad shorter than the free length of the lead screws and ~ 5 cm wide. Fold a paper around each lead screw and secure it in place, e.g with paper clips from behind. Then copy the thread pattern onto the paper by rubbing with the side of a pencil -- like we used to copy coins as kids.
This will give a distortion-free picture of each lead screw's thread. (More accurate that photos.) You can either measure them manually, after counting off groups of 10 turns of the thread. Or the fancy version, put all strips next to each other on a flatbed scanner, scan them and measure the picture in your favorite imaging software.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
is there a tldr to find out if Im affected? I have decent callipers but thats about it regarding measuring tools.
Calipers are very suitable in terms of accuracy, but tricky to use here because it is so easy to miscount the threads. And depending on the size of the calipers it may be difficult to fit them in. A nice measuring method has been proposed later in that thread:
Move the bed all the way up, cut strips of paper a tad shorter than the free length of the lead screws and ~ 5 cm wide. Fold a paper around each lead screw and secure it in place, e.g with paper clips from behind. Then copy the thread pattern onto the paper by rubbing with the side of a pencil -- like we used to copy coins as kids.
This will give a distortion-free picture of each lead screw's thread. (More accurate that photos.) You can either measure them manually, after counting off groups of 10 turns of the thread. Or the fancy version, put all strips next to each other on a flatbed scanner, scan them and measure the picture in your favorite imaging software.
Cheers, I'll try the quick method with the spirit level first. If I video that will it be enough to convince support?
If I am affected, is it just 1, or 2 motors/leadscrews that will need replacing ?
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
Cheers, I'll try the quick method with the spirit level first. If I video that will it be enough to convince support?
If I am affected, is it just 1, or 2 motors/leadscrews that will need replacing ?
The spirit level test is not very sensitive, but it gives you a first idea. You will need to check sideways and front-to-back changes to figure out which screw is off. Personally I prefer the direct measurement of the screw pitches.
You will typically have two identical screws and one with a different pitch. Problem is that Prusa only has one part number; they can't send you screws with a specific pitch.
They sent me one replacement at first, and of course it turned out to have the exact pitch of the odd one out I already had -- so I now had two of each pitch. I contacted support again, asking for one more, which I expected to match either one pair or the other. But that time round, they insisted on sending me three new screws, all taken from the same shelf.
RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
I noticed someone in that thread uploaded a tool which should be a pretty simple, very quick check to see if I'm affected.
Honestly I'm amazed that this hasn't been picked up on more. Could be a hell of a lot of affected machines out there....quite staggering that there are still new printers being shipped with the problem.
I guess people won't notice it unless they're printing tall parts where dimensional accuracy is critical, the vast majority of the parts I print are quite small/flat, but with my warranty window drawing to an end its best I get on this straight away.
Cheers for the pointers !