XL runout sensor cannot actually do anything

[3D Printing Tips and Tricks]

The runout sensor is a metal ball pushing against the filament. No filament and the ball is pushed into two contacts which closes the circuit.

The problem is each extruder uses a long ptfe guide tube and the ending tip for just about all spools is deformed or bent. As such the filament can’t feed into the tube. The sensor never gets a chance to get the run out. You can’t get to the ending tip until the spool is just about empty.

Whoops!

[Neil MacGregor]

What's your general workflow for handling the end of a spool? 

I have a Mk3, which has a slightly different runout sensor, I believe it's a ball moved aside by the filament & a hall-effect sensor sensing the change in position.  It doesn't have a guide-tube like the XL.  It has a small 3-printed plate covering the electronics of the runout sensor, with a hole in it, matching the hole in the extruder below it containing the ball, with the drive-gears just below it.

Filament path in blue.  Grey area above the gears contains the ball + sensor. Pink is the 3D-printed block fixing everything in place.  The plate isn't pictured, here, it would be a hat atop the pink:

Each brand of filament is different, but yes, the end of a spool is inevitably a blob, or bent, or both, or !still attached to the spool!  The Mk3's extruder design handles blobs with aplomb.  But a bend almost always means a failed print  - the bent end has to go into the plate and past the sensor, but when the drive-gears reverse a bent-end back out of the plate, sometimes the bend causes it to "hit its head" on a tiny misalignment of the hole in the plate and the hole in the block.  This is a design flaw, but it's simple enough to drill the hole in the plate a little sloppy, or reprint it with a bigger hole.

Also, timeouts (20 minutes?) will cool the extruder and the bed, if the operator doesn't act quickly enough to change filament. That will also ruin a print - as previously described, it cracks loose off the PEI build plate when it cools!

All of which is to say: "the operator is to remain present during operation", and that is doubly important when a spool nears the end of roll.

I've thrown away enough failed prints to have developed this operational workflow: snip the end of every roll at a 45 degree angle before it disappears into the extruder-block!

This is a case of the human adapting to the machine.  Should we perfect the machine to the lazy habits of the human who can't schedule his life around waiting for the end of the spool? Or should the human prematurely snip the end & swap spools early, before going to bed?  The fantasy is "upload object, push button, ignore the machine until it's done".  The reality is dramatically different!

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[3D Printing Tips and Tricks]

Not every plastic spool will provide access to the end tip. IME none have. Properly trained Engineers understand how important it is to know all the requirements and constraint's for a system to meet its goals. It’s plain to me that Prusa doesn’t!

For a  run-out sensor to work it has to be situated before the guide tubes and be able to sense the tip distortions. Prusa failed to do that.