Odd first layer and crash detection on layer 2
Hello.
I’ve had a prusa Mk3 for over a year now and recently noticed a very odd pattern forming during first later prints. The extruder seems to create a zig zag pattern as it runs diagonally to make the first layer. After layer 2 it detects a crash part way through. This crash is always in the same location and I have watched to confirm it does not actually crash. It simply stops, registers a crash and when it attempts to return to printing the print has shifted over. I will attempt to attach a video mid print as well as a photo of the first layer. Any help is greatly appreciated 
RE: Odd first layer and crash detection on layer 2
Hi Nicholas,
the Red image shows evidence of Live Z being a touch too low (too big a negative number)there are lots of factors at play here, so you will notice that the left hand edge is smooth, then the middle portion becomes a little rippled, (like the sand at the seaside, when the tide goes out...)
this rippling is caused by the nozzle being too close tot he bed, the extruder pushes the correct amount of filament out of the nozzle to fill the expected space, but that space is actually smaller than expected... so the extruded filament 'squidges out' sideways, as there is already cooled filament on the build plate from the previous line, the squidged filament tends to squidge away from the cooled filament, then on the next pass the nozzle finds more existing filament in the way, and sort of piles the new filament up on the edge of the old filament, leading to the beginnings of the ripples,
Your grey patch seems to be starting to demonstrate this trait... as the zig zagging is caused b uneven filament deposition...
what is additionally confusing is that there is a small area down the left hand side of the red picture, which looks OK.
I believe this is caused by Linear Advance, which adjusts the flow of filament at the beginning and end of sections of filament deposition to try and make the model quality better, (previous to linear advance extruders fed the same amount of melted filament at times when the X and Y axis motors were accelerating and decelerating... which used to cause over extrusion in these areas.)
It is likely that the nozzle is stalling against one of the first layer ripples, when it is trying to lay down the second layer . make sure you are not running the printer in Stealth mode...
Are you using 7x7 mesh bed levelling at the beginning of your prints, or are you still using the 3x3 mesh bed levelling?
If you are not using 7x7 mesh bed levelling, then it's probably going to be a benefit to turn it on...
7x7 mesh bed levelling senses 49 points on the bed at the beginning of a print compared tot he 9 sense points used previously. and as such tends to give a more even first layer. (You may need to load the latest Firmware in order to be able to switch on 7x7 mesh bed levelling from the LCD Screen Menu!)
I hope this helps, regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Odd first layer and crash detection on layer 2
@joantabb
Hi Joan,
Is the 7x7 mesh bed leveling available on the Mini ?
Doesnt appear in the desciption of the new features in the 4.0.3 firmware update, mine auto does the 3x3 leveling.
Has it always been there, just i didnt know ? If this is the case where can i switch it on ?
(my Mini is currently in the middle of a 16hr print, otherwise i would try to find it.
Cheers
S
RE: Odd first layer and crash detection on layer 2
@joantabb
I will try that. Thanks
RE: Odd first layer and crash detection on layer 2
I would like to find out where this option is. I have been surprised how few options there are on the Mini and wonder if i have overlooked some kind of “expert” settings....
RE: Odd first layer and crash detection on layer 2
There is no other than 4x4 bed leveling on MINI. OP wrote good detailed description of a problem, received good answers - just in a wrong forum section 😀 but hey many people who frequent here also have other printers.