SoM print failures and how to prevent them

[the_digital_dentist]

I took apart the extruder and figured out why the last two attempts to print wheels failed in such an interesting and spectacular way:

There were three issues:

1) The pinch wheel pressure on the extruder was set too high and it was squishing the filament, making it easy to bend.  This is the filament that I pulled out of the hot-end:

2) I used volumetric extrusion when I sliced the file because the wheels will get printed a lot using different colors and diameters of filament.  Volumetric extrusion allows the gcode to be reused without reslicing.  I set the filament diameter in the slicer to 1.128379 mm.  Whoever started the last two failed prints didn't know that you have to enter the actual filament diameter (marked on the spool) at print time.  The machine thought the filament diameter was 1.128379 mm but it was actually 1.793 mm, hence the gross over extrusion- see the prints in the first picture.

3) There is a design flaw in the extruder- a gap of 5-6 mm between the filament pinchwheel/drive gear contact point and the entry to the guide tube that steers the filament down into the hot-end.  The gap allows the filament enough room to buckle.

When the print started the over extrusion started.  Eventually the hot-end couldn't keep up with the extruder which continued to push on the flattened filament, which then buckled and wrapped itself around the drive gear before breaking.  The free end found its way out of the extruder below the pinch wheel.

The fix is easy- when you are going to start a batch of wheels, power up the printer, enter the filament diameter, then select the file to print (more wheels.gcode).  I left a post-it with exact instructions on the printer today.