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brandonh
Right, but then you've got to thread an m4 nut on slippery PTFE for 3 full inches, which seems like a bit of extra effort. If the nut was at the bottom of Hub Bottom, next to the hotend, and inserted first, you could eliminate the Bowden nut entirely, as well as chop 20 mm of plastic off of this part. Yes, you might need to thicken Hub Bottom to do that, and yes, it might be harder to remove the tube.
There's no way you could possibly thread the M4 nut onto the tube that far; it's barely possible to get the tubing all the way through the nut in my experience. However, that is enough to hold the tubing extremely tightly. I put nuts on both ends of a short section of tubing and couldn't pull them off again.
My GUS prototype actually has pushfits on both ends, because I had a bag of them and because I got into trouble with the force cone. It held the tubing perfectly well, but also caused increased friction and with all the crud we were dealing with at Maker Faire any additional friction was too much. I cut a short piece of tubing to slip into the hub, chamfered the top end to let the filament feed in easier, and centered it with a cut-down force cone. I'll probably stick with that since I'm much too lazy to reprint those parts unless I absolutely have to ;)