Printer measures a calibration point twice while bed leveling
 
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Anachronist
(@anachronist)
Estimable Member
Printer measures a calibration point twice while bed leveling

Lately I have noticed when my printer (MK3S+MMU2S) is doing its 7x7 bed leveling at the beginning of a print run, on the 9th calibration point it pauses, the printer acts like it's thinking to itself "hello, what's this? Let me measure that again." The print head lifts up a bit and then goes back down, and it measures a few samples of that point again before moving on.

The last time this happened about a year or so ago, I found a microscopic fragment of filament string debris under the bed, and clearing it out fixed the problem.

This time, however, there's no evidence of any debris. I cleaned the steel sheet, made sure the heat bed is clean, made sure there's no stray nut or screw stuck magnetically underneath it (that's happened before). I can't think of anything else to do.

This pausing doesn't seem to hurt anything. The leveling completes, and it prints just fine. I am just wondering why this behavior has started occurring recently, and whether I should be concerned that my PINDA is failing or something.

 

Posted : 22/05/2021 10:59 pm
jsw
 jsw
(@jsw)
Noble Member
RE: Printer measures a calibration point twice while bed leveling

It's been called 'Double Tapping' and it happens occasionally.  I believe it's when the machine get a value that's somewhat different from what it's expecting and goes back to double-check.

I've found that mine usually has one, sometimes two double-taps when the bed is heated to ABS temperature, but none when heated to PLA or PETg temperatures.

Posted : 22/05/2021 11:17 pm
Anachronist
(@anachronist)
Estimable Member
Topic starter answered:
RE: Printer measures a calibration point twice while bed leveling
Posted by: @jsw

It's been called 'Double Tapping' and it happens occasionally.  I believe it's when the machine get a value that's somewhat different from what it's expecting and goes back to double-check.

I've found that mine usually has one, sometimes two double-taps when the bed is heated to ABS temperature, but none when heated to PLA or PETg temperatures.

That's good to know. Thanks.

It's happening all the time for me, consistently, at just one calibration point. I noticed it started a couple months ago. I always use PLA, so I have no idea what has changed over the past year to cause this to start happening.

Anyway, it sounds like I shouldn't be concerned about it.

Posted : 22/05/2021 11:21 pm
cwbullet
(@cwbullet)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling

@jsw

I concur.  Double tapping happens and is not rare.  

--------------------
Chuck H
3D Printer Review Blog...
Posted : 22/05/2021 11:23 pm
Anachronist
(@anachronist)
Estimable Member
Topic starter answered:
RE: Printer measures a calibration point twice while bed leveling

After reading this post, I began to wonder if the double tapping is happening because the PINDA probe is warming up due to its close proximity to the bed during calibration, and its temperature compensation is shifting. I want to see what happens if the printer warms up with the print head very close to the bed, to warm up the PINDA probe at the same time. I typically let the bed warm up with the head raised well away from it. Just a thought, something to try.

Posted : 23/05/2021 12:05 am
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(@)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling

Double tap is due to (according to Prusa) an unstable surface under the printer. They recommend stabilizing the table top or bench. And there are a couple of posts where people have done this (bricks under the printer) and it has helped.

Posted : 23/05/2021 4:16 am
Anachronist
(@anachronist)
Estimable Member
Topic starter answered:
RE: Printer measures a calibration point twice while bed leveling

@tim-2 - my printer is on an extremely heavy desk, and it sits on sorbothane vibration damping feet. I don't believe an unstable surface is causing it in my case, particularly since the printer has been in the same configuration for over a year and has only recently been exhibiting the double tap.

Posted : 23/05/2021 5:02 am
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(@)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling

@anachronist

I've seen it on my heavy table, too. I added bumpers and had to make sure the table was firmly set against the wall. I moved the printer to a built-in cabinet countertop and haven't had it happen since.

Another thought, it might be time for new bearings ... they do wear out and do let the bed wobble. Here's what I do when the wiggle gets out of hand and I don't have time to swap them out.

Posted : 23/05/2021 5:17 am
Anachronist
(@anachronist)
Estimable Member
Topic starter answered:
RE: Printer measures a calibration point twice while bed leveling

@tim-2 - I am not sure what I am looking at there in the picture. It looks like big rubber bands tied to the bearing on one side and and the 3030 aluminum on the other side?

You do have a point about my bearings though... about 6 months ago the printer aborted itself, reporting a head crash, but the print head hadn't crashed. Rather, the heat bed bearings had overheated and seized up. It was fine again after cooling down. Prusa support told me to lightly spread some of the bearing grease on the rails, and I haven't had a problem since. But given that incident in the past, I wouldn't be surprised if those bearings are wearing out. On the other hand, if I try to wiggle the bed side to side, the whole printer moves, so the table seems pretty firmly positioned.

Posted : 23/05/2021 5:35 am
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(@)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling

@anachronist

One large rubber band to preload the bearings to prevent rattle. The play is palpable when the bearings go. I see more bed rotation than any side to side play.

This post was modified 6 months ago by --
Posted : 23/05/2021 6:49 am
Clarmrrsn
(@clarmrrsn)
Honorable Member
RE: Printer measures a calibration point twice while bed leveling

Double tapping began happening on my new mk3s+ after about 4 weeks, I found 2 things at the time that cured it permanently.

1. My Y Axis had a very minor bit of play vertically, it was not like that at install so wonder if some amount of bedding in had occurred.

I checked all bearing holders and re-nipped them up, and also readjusted the rods for parallel. 

2. I had around a.. 3 to 4mm variance across the bed, I levelled this to within. 02mm.

I cannot be entirely sure what was the root cause (more likely the Y axis play)

There is a blog on mesh bed levelling that explains the process, I will see if I can find it.

 

Tank you very much!...
Posted : 23/05/2021 7:08 am
Clarmrrsn
(@clarmrrsn)
Honorable Member
RE: Printer measures a calibration point twice while bed leveling

https://blog.prusaprinters.org/prusa-tech-insider-1-mesh-bed-leveling_30817/

Here you go

 

Tank you very much!...
Posted : 23/05/2021 7:10 am
Marvin
(@marvin)
Eminent Member
RE: Printer measures a calibration point twice while bed leveling

I've started to get double taps since the the last firmware upgrade. Can't find anything in the release notes about changes in the bed leveling though.

(MK3S+ so SPINDA)

Posted : 23/05/2021 9:22 am
jsw
 jsw
(@jsw)
Noble Member
RE: Printer measures a calibration point twice while bed leveling

My surface is just as stable when the printer is heated to ABS temperature as it is when heated to PLA/PETg temperature.  Temperature appears to be the only variable here.

Posted : 23/05/2021 9:40 am
Baklin
(@baklin)
Reputable Member
RE: Printer measures a calibration point twice while bed leveling

What fixed the double tapping for me was running XYZ calibration again after any changes I did.

 

Like when I had the bed off and re did the bed leveling with the nylock mod. Get it all up to how I want it.

And than as a last step, get the bed up to my most used print temp and run XYZ calibration again.

I think this does it for me as the printer sees that last calibration as the new baseline and it will not deviate to much from that.

 

If I just do the bed leveling with the nylock mod the level is to much changed compared to the last XYZ calibration that it sometimes needs to re probe some points.

 

Even without the nylock mod there will be sometimes small shifts that can confuse the bed leveling and need a re probe of some points.

Posted : 23/05/2021 9:58 am
cwbullet
(@cwbullet)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling
Posted by: @tim-2

Double tap is due to (according to Prusa) an unstable surface under the printer. They recommend stabilizing the table top or bench. And there are a couple of posts where people have done this (bricks under the printer) and it has helped.

This is my understanding also.  

--------------------
Chuck H
3D Printer Review Blog...
Posted : 23/05/2021 2:27 pm
cwbullet
(@cwbullet)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling
Posted by: @jsw

My surface is just as stable when the printer is heated to ABS temperature as it is when heated to PLA/PETg temperature.  Temperature appears to be the only variable here.

True, but the higher temp introduced higher variability.  Especially if the printer is not in an enclosure.  

--------------------
Chuck H
3D Printer Review Blog...
Posted : 23/05/2021 2:29 pm
cwbullet
(@cwbullet)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling

@baklin

Very true.  I am hesitant to install a bed modification because I hate to create another point of variability.  

--------------------
Chuck H
3D Printer Review Blog...
Posted : 23/05/2021 2:30 pm
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(@)
Illustrious Member
RE: Printer measures a calibration point twice while bed leveling

Seriously - it is the phase of the moon that causes double taps. 

https://en.wikipedia.org/wiki/Lunar_effect#:~:text=A%20study%20of%2013%2C029%20motorcyclists,moon%20compared%20to%20other%20nights.

Posted : 23/05/2021 5:48 pm
jsw
 jsw
(@jsw)
Noble Member
RE: Printer measures a calibration point twice while bed leveling

One other data point that I just remembered.

This was several months ago.  I replaced the stock nozzle with a tungsten-carbide .04 nozzle and then did a re-calibration of the Pinda position using the highly-precision feeler gauge that Prusa supplied with the printer, AKA a cable tie.

I immediately tried a test print and found that it was double-tapping at each sample point.

I aborted the print and did a complete calibration, the one where it forces the Z to bang into the top and then jukes around a bit.

After the re-cal it went back to the no double taps at PLA temperature but one (sometimes two) at ABS temperature.

Posted : 23/05/2021 6:02 pm
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