Toast falls face down. Your car always breaks after the warranty period. A 3D print only fails after it is has been printing for 12 hours. Those things might not always be true, but they are true often enough. Another pessimistic adage is “no good deed goes unpunished.” [Shippey123] did a good deed. He agreed to make a 3D printed mask for his friend to give as a gift. It was his first print he attempted for someone else after about four months’ experience printing at all. After 20 hours of printing, he noticed the head was moving around in the air doing nothing — a feeling most of us are all too familiar with. But he decided not to give up, but to recover the print.
Luckily, he’s a CNC machinist and is perfectly capable of reading G-code. The first thing he did was to shut everything down and clear the head. Then he rehomed the printer and used the head to determine what layer the printer had been working on when it failed. He did that by moving over a hidden part of the print and lowering the head by 100 microns. Then he’d move the head a few millimeters in the X direction to see if the head was touching.
Turns out the printer was probably on the 40mm layer when it froze, but to play it safe, he decided to resume printing at 33.8mm. For this kind of print, being off by 200 microns would hardly be noticeable.
In general, G-code produced by a slicer has a preamble that sets certain parameters like temperatures and speeds. Then it has a bunch of move codes like G1 and G0. Typically, there will only be one G0 line per layer that has a Z parameter. This is the line that changes the layer.
In this case, it was layer 168 that had the line:
G0 X143.779 Y205.684 Z33.8.
The only thing he had to do was delete from the top up to and including that line. Then he had to script a little preamble that would set everything up without crashing into the existing print. Essentially he did the normal startup script but moved the head higher than the print from the home position. Then he moved to the correct spot and lowered the head to 33.8 mm before resuming the script:
G0 F3000 Z35.8 G0 X143.779 Y205.684 G0 Z33.8
The thin layer height probably helped. If you were doing very thick layers, you could easily have part of the makeup layer droop. Then again, it might be worth a try.
As you might expect, though, the process did leave a little line around that layer. After the print completed, he used a hobby knife to trim the line and then sanded with fine sandpaper. The end result looks pretty good.