I HIGHLY RECOMMEND THAT YOU USE THE NEW PUSHER DESIGN OVER THIS. Many improvements have been made, most of all is proper mounting and gear interaction.

https://www.thingiverse.com/thing:3134538

These are the internal pusher parts for the MHP 15. They need to be printed pretty high infill (65%+), and are oriented properly for maximum strength. Many, many thanks to those who are smarter and better designers than me for making an auto pusher possible at all.

https://www.thingiverse.com/thing:2829145

IMPORTANT: I have had a chance to work on my second copy, and I noticed the the bottom of the pusher piece was causing friction that could cause it jam. There's a straight spot on the bottom from (around the oval opening) that needs to be rounded out. Also of note is that the fit on the pusher gearbox isn't perfect, and I recommend gluing some 1/16" foam on the bottom (but under the pusher arm) to get it in there nice and tight.

Minor tweaks to the pusher, to improve the screw mounting, among other things. I added a stopper to the pinion gear, though it may not be necessary, as I had a jam from the pusher arm itself slipping off the gears. The stopper should keep it from moving too far off the ideal trajectory, but may not be necessary. The mounting bracket has been tweaked to fit better, as I missed a few spots that it didn't want to fit into after being printed.