Cannot set Home Point  

RE: Cannot set Home Point

There are troubleshooting guides that describe how to fix this on the Prusa web site. I suggest you refer to them and follow the guidance provided.

Here's a starting point: https://help.prusa3d.com/article/fja7hbb22y-xyz-calibration-error-messages-mk-3

ps: generally, shifts are due to lack of lubrication causing the bearings to seize during fast travel and print moves - according to the bearing manufacturers, the bearings should be packed with grease before installation - you might want to discuss this with Prusa tech people.

RE: Cannot set Home Point

I did quite a lot of searching for a solution before I decided to post this issue. The trouble shooting guide was of no help and I got very little information from the web. I am hoping someone has had the same issue and has some solid solutions.  This printer has been running probably 90 hours before the issue became apparent. Also according to the instructions the bearings are pre lubed. Before I installed them I checked to make sure that was the case. I am thinking maybe the problem centers around the Einsy board or perhaps a faulty stepping motor.   

RE: Cannot set Home Point

Sure, bearings are prelubed; and I have a bridge for sale in Brooklyn.   Bearings come shipped in a preservative anti-rust oil that is not intended for lubrication.  Prusa is - well - not providing the best info available. 

Shifting axes is almost always sticky bearings.  Though FW3.8 has a known bug that causes shifting, too.  But since your problem "appeared" after some use, its probably not experiencing the FW bug.  https://github.com/prusa3d/Prusa-Firmware/search?q=shift&type=Issues

Bearings will dry out once they are out of the plastic bag (that preserving oil thing again). When they are dry, the balls will begin seizing on the rods, eventually scoring them.  You can usually feel this by moving the extruder left right a few times and feel them grabbing. Same for the bed. 

If you look at the menu and find the "belt tension" numbers, anything less than 260 is indicating a likely problem (Prusa manual misleads again, the numbers represent motor drive current, not belt tension).  But they are handy for finding high friction. Lower numbers mean more friction.  Well lubed printers are 280 or higher.  A few drops of machine oil on the rods can help until you get time to pack the bearings properly (if they aren't already damaged).

The bed should move front to back when you lift the front of the printer a few inches. The extruder should slide side to side if you lift a side. And the moves will be stop to stop, no grabbing or stopping along the way. X is usually a bit "stiffer" than Y, and takes a higher angle. 

None of this is very scientific. But they are fair tests of bearing friction.

As for others having the same issue: yes, many. And it has almost always been cured by adding grease to the bearings.  Though if your printer were brand new, U-bolts being too tight would explain Y crashes, and overly tight Extruder case screws would explain X crashes.  And then there is the github posts with software defects and folk who down rev the firmware and everything starts working well again.  

The Einsy and motors are fairly reliable.  The most common failure there is broken wires, and they manifest in a slightly different way.  Though, without a photo of the shifts you are having, maybe. But I suspect not.

RE: Cannot set Home Point

Thanks for the input. I will plan on dismantling the unit and grease in the near future. I did inspect the rods and there are no indications of wear or scratches. However, since my last post, I did determine that by loosening the " X-carriage-back" component a bit solved the problem. I guess I was causing additional compression of the bearings which increased drag. And I think this drag was faking out the calibration causing the x axis to think it was at the far left edge when in fact it was a few inches short of the true edge. This is just my supposition since I don't see any end stops that would allow this edge to be identified. Anyway, everything is working now and I have done a number of large prints since this fix.

That said, I think there is a design flaw in that rear cover. In order to keep the bolts loose enough to prevent the problem there is a small gap between the cover and the extruder (half a mm). The instructions say tighten  with "a reasonable force". Whatever that means. I have since redesigned that cover adding that extra half mm to the mating surfaces so that it can be tightened flush to the extruder.

RE: Cannot set Home Point

The extra friction on any of the bearings leads to a motor stall. When a motor stalls, the printer is pretty much unaware of how many steps were lost; and the default recovery is to continue (unless crash detection is enabled, which causes the printer to halt).

This affects the printer because the printer has no means of knowing where it is except counting steps from home; and once a stall has happened - X and Y location in space is unknown - and can't be known until the printer is once again told where home is (0:0:0).   So it starts at 0:0:0, moves to 600:400:10, then in a move to 800:900:10 stalls.  The actual stall was 27 X counts. But the printer only knows it is supposed to be at 800:900:10, but is actually at 773:900:10.  So it is lost for the rest of the print, or until homed.

RE: Cannot set Home Point

I greased the bearings with Super Lube. Reinstalled everything with no change. The solution came down to one thing. Too much pressure on the X axis bearings. After greasing and reassembly, the issue was only solved by keeping the "x bracket back" loose. The manual says tighten with reasonable force. Can anyone define reasonable force? So I kept backing out the 4 bolts that hold the back on until the print head slid easily on the bars. Loosening the bolts ended up leaving a half mm gap between the back cover and the extruder body. I then reprinted the back cover after adding that half mm to the flat surfaces. These are the mating surfaces. Then I was able to tighten the back cover until there was no gap. The extra thickness relieved the additional pressure on the bearings.

Why are the bearings so sensitive to pressure? Who the heck knows. I see no drawback to the changes I made since the upper bearings are held on by wire ties. The lower bearing could be affected by the additional gap because the back cover is the clasp that hold it in place, but after assembly I could not measure any wobble or movement in the lower part of the assembly. Calibrates correctly now and prints are on the money.

RE: Cannot set Home Point

@maionaze

Hi, I think I may have your EXACT problem!

I built a #Prusa i3 mk3s kit, & for some weeks now, it has performed flawlessly. But yesterday, the x-axis moves away from home, & perhaps 2/3 the way across, there is this terrible noise, forcing me to turn the power off.

Re-setting up the machine, everything appears fine, till I reach the z-axis extrusion shape calibration point, when it does the SAME thing, at that same position making a grinding like noise. tipping the machine on it's side, I allow the extruder to fall under it's own weight, & it stops at the same point.

Having read what you've said here, I oiled the 3 bearings on the extruder, & it falls under it's own weight, to THAT EXACT point. re-oiling liberally, it falls a little further. I read the kit instructions, & I remember the kit said the bearings were PRE-Greased.

I'll try the oiling again, but fear now I might have to buy new bearings!

RE: Cannot set Home Point

Christine, the solution was not greasing or oiling the bearings. Try loosening the "x-axis back cover"  and see if you can move the x axis freely by hand. Make sure the power is off when you do that. That was the solution that worked for me. Read my final posting for the details.