My MK3s gets confused about where home is.  

RE: My MK3s gets confused about where home is.

the problem cycle appears to be. 
people buy a new printer, 
they use it until a problem occurs, and then complain...    Lol...  

The Prusa Mk3 does not have any end stop switches, it recognises the end of the axis by the motor current raising as the motor comes to a halt. unfortunately if there is excess friction  the control board erroneously recognises that point as the 'Home' end of travel, and it expects to have the full travel to the other direction. unfortunately the false Home means that you actually don't have the proper axis length available. and this is when you decide the firmware is faulty.
Have you tried lubricating the linear bearings,? 
other potential causes are
the motor pulley rubbing on the plastic parts. 
the cable bundle catching on something
the idler bearing failing, the idler wheel rubbing on the plastic parts and the ubolts being too tight

Regards Joan.

RE: My MK3s gets confused about where home is.

I was going to assume that the bearings were not greased during assembly, and are running dry....but seeing your belt hop and skip at the 24sec mark...there could be more to it.  I would start by checking for crashes in the history, and checking belt tension in the menu.  Also do a physical check of grub screws on the Y axis.

Its not a software bug.......lets discard that idea right away 😀

RE: My MK3s gets confused about where home is.

I HAD THE SAME ISSUE

I found the solution here in an other post --> Flat First: Drive gears must be tightened flat-first. So I did the following:

1)   I loosened both screws of the Y-Axis pulley completely free and moved the bed back and forth (must move smoothly).
2)   Then I aligned the flat set screw dead center with the flat on the motor shaft and slowly tightened the screw until it fully contacts the flat surface.
3)   Then I tightend the jam set screw, and torque it.

4) Checking the movement of the HeatBed - should still move smoothly

5) Restarted the calibration and did a first Testprint 😉 Yeaii it workes

RE: My MK3s gets confused about where home is.

@bf1942

Thanks for the suggestion. I did the flat first tightening technique during the build. 

I'm curious, what would be the symptom of your failure? Was the servo rotating loosely and the bed not moving into place?

RE: My MK3s gets confused about where home is.

@joantabb

Joan,  Thanks for the reply.  The insights in the end-of-axis detection method are valuable. At mark 12sec in my video, I can see the bed moving forward and then stopping. It's probably detecting that it moved "far enough" due to friction as you mention.

BTW, the cycle you describe is quite natural as you learn to use a new tool and discover difficulties in that process. Although, I thought I was asking for advice and not 'complaining'. My 'hunch' of it being SW related is because I saw the Y-axis resetting to 0 when I accessed the "move axis" menu. But I see that I can be electromechanical as well following your explanation.

kr, Gerard.

RE: My MK3s gets confused about where home is.

@gerard-maas

cheers Gerard. 

I used 'Complain' as a bit of shorthand... hence the Lol that followed it...

sometimes we can convince ourselves that one thing is happening, when it's actually something we hadn't considered...

it's good to learn... 

welcome to the hobby!

regards Joan

RE: My MK3s gets confused about where home is.

Thanks all for the feedback. I did a bit more testing. There's an area, like 75% down the Y-axis that has considerably more resistance than the rest of the axis.

That resistance is stronger when the servo is active. With the machine powered off, the Y-axis is fairly smooth, yet with that tension bump around the 75% mark to the back.

I checked the stats and my Y-belt tension is 297.  (The knowledge base says OK between 240 and 300)

The prints are really, REALLY good (I come from an ANET A6).  I only have issues between prints.

I'm printing these bearing holders to discard the U-bolts as the source of the problem:  https://www.thingiverse.com/thing:3558209

Any other points I should check?

Thanks!

RE: My MK3s gets confused about where home is.

@gerard-maas

to much tension and then the motor sliped 

RE: My MK3s gets confused about where home is.

The tension bump to the back 75% of you y-axis indicates an issue with one or more of the following:

1. possible debris/obstruction blocking the printer y-axis movement of the heatbed towards the rear of the printer. Disconnect the power, remove the print plate and slowly turn the printer on its right side. Then check for obstructions that may inhibit moving heatbed towards the rear of the printer.   There may be something near the belt holder on the bottom of y carriage or a misplaced cable that is blocking movement.

2. y-axis belt alignment near the y stepper motor at the rear of the printer.
   Check that the belt is still in correct alignment.
3. y-axis smooth rod alignment; they must be parallel in all three dimensions.
   Check that the rods are fully pressed down into their 3d printed mounts and the mounts are flat on the front side Aluminium extrusions. are parallel in the z-plane.  
   Check that your frame sits completely flat on its feet without rocking on a flat surface. You may need to re-square the frame as per the MK3s y-axis assembly instructions with the rod mount screws slightly loosened.

4. y-axis linear bearings on the rods. These need to sit parallel and flat and be only slightly and evenly tensioned down by the u-bolds and their retaining lock nuts.
   Remove the heat bed and loosen the u-bolts slightly.  Check that the bearings are undamaged and that the u-bolts were evenly tensioned, etc. and installed as per the online assembly manual. If a bearing looks to be even slightly crushed/damaged then you'll have to replace it. A bearing that rattles when it moves probably has missing balls and will need to be replaced.

RE: My MK3s gets confused about where home is.

@jonathan-lin

y-axis belt alignment near the y stepper motor at the rear of the printer.
Check that the belt is still in correct alignment.

Thanks for the advice. This is a good candidate. I remember having a hard time to make the belt come out perfectly straight from the printed belt holder under the bed. The belt tends to lean to the side of the driver gear (It would not stay in the middle of the two "walls" in the servo gear).

I don't know how to improve on this one. When pushing the belt into the holder, it has the tendency to get slightly skewed and it's hard to correct. 

I'll try to redo that part over the weekend and report back.

Thanks! Gerard.

RE: My MK3s gets confused about where home is.

@joantabb @nilok @bf1942 @jonathan-lin Over the weekend, I did further triage, using your guiding advice. 

After disassembling the bed, I found the cause of the issue: the bearings were moving fine in one direction but added a lot of friction in the other. The dust guard was dry and would prevent the bearing to go smoothly backward.

I lubricated all of them and took the opportunity to replace the U-bolts by this Thingiverse design: https://www.thingiverse.com/thing:3558209

(see attached photo)

On the reassembly, I had more trouble. Now, the bed would move with regular "friction bumps", like 5 across the whole length of the Y-axis. 

Releasing the servo gear would remove the issue, but when I was tightening it up again, the problem would repeat. It was as if the gear was off-center.

I detached the servo from the frame and installed the gear without belt tension. When I reassembled the servo in its place again, the complete bed was super-smooth. It would seem that attempting to tighten the servo gear with the belt tension engaged would result in a misalignment of the gear.

After this bit of sweating, the bed is running super smooth. I calibrated everything again and I'm getting amazing first layers. See attached pic.

Thanks a lot for your advice. It was your combined knowledge that helped me solve this problem.

RE: My MK3s gets confused about where home is.

That's Good Gerard

I am pleased that you sorted the issues. 

regards Joan