Not printing top layers, print head appears to be stuck in a loop

[Clived72 0]

Help! After struggling to get a large print started (Ultimaker 2+) I finally got it adhered well and printing nicely. It's been running smoothly for about 26hrs now but has just started to act up. I think it's on layer 266 out of 272, so just about to start the top layers (5 according to my default cura settings) but now the head has just started travelling forward and back between two points with the odd detour thrown in. It's clearly not supposed to be doing this and it's not laying down any filament. I've tried pausing and restarting but that same happens again, and I've tried removing and reloading the filament incase it was jammed. It wasn't, it came out nicely when loading but then continued with the weird travel. I'm loathe to abort the print at this late stage so does anyone have any suggestions? Is there any way I can reslice just the top 5 rows and have the printer print them on top of my current print? Thanks

[gr5 2,024]

Yes but it may take you an hour or so.  I guess if you really want to do that then make sure the heated bed stays hot because as soon as it cools down the part will pop off the bed.

 

continued...

[gr5 2,024]

So to continue a print you have to edit the gcode.  The first two (two!!! not one!) posts on this topic are excellent.  The G92 is critical.  You want to set the E value to whatever is the last E command that you delete such that the new E commands only move the extruder a little bit and so the UM2 doesn't try to extrude many meters of filament.

 

HOWEVER, there is a feature in Marlin that if you do an unreasonable extrude (I think it's over 1 meter?) it will be ignored so maybe you can skip the G92 step occasionally but I'd definitely do that step.  That's why the first post doesn't mention G92 because he just got lucky.  The E values actually reset every 10 meters or some such limit so it's possible that the E value will be less than the ignore value and then you have a disaster if you skip the G92 command.

 

[gr5 2,024]

By the way - before doing any of that - I'd load the gcode from the SD card back into cura to see if the weird movements are in the gcode.  Cura has a feature to display gcode files graphically.  It could be a bad block on the SD card or a bad slice result from cura.

[gr5 2,024]

Another option is to set the Z value of the part in cura so it is mostly below the glass, slice it, print it and glue the resulting top to your existing print.  This will be kind of ugly of course.  There is a setting in cura preferences (not in the normal settings) that when checked drops all parts flat on the glass so you have to uncheck that before messing with the move feature and changing the Z value.

[gr5 2,024]

There is also a feature called "lift head" in cura.  Make sure that is not checked.  If it was then that is probably what is happening on those last few layers.  Hover over that feature and read the description.