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[Risolto] MK3: Misaligned PINDA, wrong auto home position...  

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Matthias M.
(@matthias-m-2)
Active Member
MK3: Misaligned PINDA, wrong auto home position...

Hi...

I recently built the MK3 printer from kit. The self test was successful.

But auto homing fails, because the PINDA does not stop above the circle on the heat bed (about 5 mm shifted in y direction). Therefore calibration fails: the first bed calibration point was not found.

The document „misaligned PINDA“ ( https://help.prusa3d.com/article/7W8CEH4CfU-p-i-n-d-a-probe-misaligned) describes my issue very good as „front shift“ but this document belongs to the MK2S printer. The solution there does not work with the MK3.

I did:
1) check the correct installation of the hot end/extruder unit (ok).
2) checked the moving y axis for any visuable problems (none for my eyes).
3) checked if the bearings are in right position (it seems so).
4) checked if the bearings are hold in position by the u holders (one showed a little tolerance +/-0,5 mm, tried to tighten the u holder)
5) checked belt status (about 240-250, ok?)

Now I do not know how to proceed. How can I deal with my „MK3 front shift“?

Thanks for any help!

Best regards
Matthias Maas

Postato : 14/08/2019 11:02 am
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(@)
Illustrious Member
RE: MK3: Misaligned PINDA, wrong auto home position...

240 seems very low (I believe the manual is wrong). 240 indicates severe friction on the axis.  Most users report 260 or higher, my readings are 290's.   The numbers do not indicate belt tension, they indicate friction to move the axis.  Lower numbers, higher friction.

Failure to pass a cal is often a cable or cable tie on the rat tail cable bundle coming off the extruder keeping the extruder from reaching the intended HOME position.  Also, often it's the axis drive gear not properly attached to the motor shaft.  The set screws must be tightened flat first as follows:

 

Flat First: Drive gears must be tightened flat-first.

1)   Start with both set screws fully loosened so the pulley is free to spin on the motor shaft.

2)   Align one set screw dead center with the flat on the motor shaft, slowly tighten the screw until it fully contacts the flat surface.

3)   Torque the flat set screw to spec.

4)   Now tighten the jam set screw, and torque it to spec.

Once tightened, never touch the flat set screw unless the jam screw is first fully loosened.

Why Flat First? Set screws have flat ends. If you tighten the jam screw first the set screw on the shaft flat doesn't fully contact the shaft, only one small edge of the screw surface is biting. Reversing torques can easily shift the shaft to a position the flat screw no longer contacts the shaft. This lets the jam screw wriggle loose. And after a while, vibration loosens the screws until the gear is free to rotate.

Questo post è stato modificato 5 years fa da --
Postato : 14/08/2019 7:02 pm
Matthias M.
(@matthias-m-2)
Active Member
Topic starter answered:
RE: MK3: Misaligned PINDA, wrong auto home position...

Thank you for answering.

I tried two things as you mentioned in your post:

1) I opened and then tried to tighten the screws of the y axis motor in the right position.

After this the y axis belt status increased to 255 or 260 (self test performed two times).

It could be necessary to reduce the belt tension? Should I remove the y axis belt out of the red belt holder and set it in again with less tension (for example one tooth less tension)?

 

Postato : 14/08/2019 8:03 pm
Matthias M.
(@matthias-m-2)
Active Member
Topic starter answered:
RE: MK3: Misaligned PINDA, wrong auto home position...

The second thing I was looking for:

2) can there be any problem with the cables coming out from the back of the extruder?

Unfortunately I cannot see a problem with the cables...

 

Nevertheless: Self test is OK. Calibration still fails. Home position still has (circa) a 5 mm y axis shift...

I did a third thing:

3) after starting the home position command I watched the y axis bearings. The y axis rod that has two bearings shows no strange behaviour. the y axis moves until the back bearing (of the two bearing) hits the red rod holder. I think this behaviour is correct and is needed to find out the real y position...?

hmh.

Postato : 14/08/2019 8:14 pm
Dave Avery
(@dave-avery)
Honorable Member
RE: MK3: Misaligned PINDA, wrong auto home position...

you can watch the status of thehome sequence via a terminal connected to the USB port (pronterface works well)

Postato : 14/08/2019 8:49 pm
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(@)
Illustrious Member
RE: MK3: Misaligned PINDA, wrong auto home position...

The PINDA won't "home" to the circle until after a calibration has completed; until then it is only guessing the location. 

Concentrate on passing the calibration.    Lots of threads here with solutions for getting XYZ to work; nearly all of them is a minor assembly issue. 

PINDA height is another assembly issue - especially if the cal can't find the first cal point.  Make sure the PINDA is between 0.8 and 1.2 mm higher then the nozzle tip.  Follow the assembly guide to recheck you don't have the PINDA too high.  Also verify the PINDA is changing state when a screw driver tip is placed near - red LED on top should wink out as the screwdriver gets close, and the extruder status on the LCD should show PINDA changing from 1 to 0 and back.

 

Postato : 14/08/2019 9:38 pm
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(@)
Illustrious Member
RE: MK3: Misaligned PINDA, wrong auto home position...

This cable tie position will cause a cal failure 99.9% of the time - it limits X-axis length by 4 mm. Reread the assembly instructions and ensure ALL cable tie ends are where the instructions say they should be.  The detail in the manual is important.

If anything on the rat tail touches the frame, Einsy case, or power supply, it must be fixed.  There needs to be a gap between the wire harness and everything else.

 

Also - people like pretty things and add fan guards - these also cause cal problems. Nothing should touch anywhere but the two sides of the extruder to the two supports. Direct contact. If anything blocks this, then X-axis is off.  Y-axis same: bearings hit rod holders, and bearings must be in the center of the frame hole they mount in, not off to a side.

Questo post è stato modificato 5 years fa 2 tempo da --
Postato : 14/08/2019 9:55 pm
Matthias M.
(@matthias-m-2)
Active Member
Topic starter answered:
RE: MK3: Misaligned PINDA, wrong auto home position...

Today I did:

1) Double checken extruder cables and tie ends: they are all in positions the assembly instruction suggestive (nothing found)

2) Decreased the tension of the y axis belt (one tooth less: now y belt has status circa 270).

3) The y axis bearings were still slightly moveable (+/- 0.5 mm): Tightened the u holders a bit. (Perhaps calibration fails because printer cannot find y position correct due to movable y axis bearings)

4) Checked the PINDA if working via menu Support->Sensor info: PINDA status toggles between 0<->1 when a metallic object comes near PINDA (seems OK)

After this I repeated Calibration XYZ - and it worked!

DEAR Tim-m30 AND David A-66...

THANK YOU FOR YOUR HELP!!!

Postato : 16/08/2019 2:10 pm
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