The shaft of the drill motor is from the very first drill motor I ever took apart (circa 2004). I didn't have a decent set of screwdrivers so ended up chewing up the reverse threaded screw and couldn't get the chuck off. So I took a hacksaw to the shaft which got the chuck off but took the threaded portion of the drill shaft with it. So I drilled into that, tapped it and fitted a short piece of M8 bolt to use as a shaft.

When it came to making the lifter, I just drilled an 8mm hole in the end of the threaded rod from the car jack, slipped it over the piece of M8 bolt and held it in place with an M4 grub screw. This didn't hold it very well though as the grub screw would occasionally come loose, or if I moved the lifter blade below horizontal (to lift up the front of the robot if stuck) it would pull the threaded rod off the drill shaft.

So I cut a bit off the threaded rod until flat-ended again and drilled a 6.5mm hole into the end of it this time. I tapped it with an M8 thread and then screwed it on to the piece of bolt/drill shaft. Secured again with a grub screw which held up fine, but this time the piece of bolt was unscrewing from the drill shaft, so I re-cut the thread to freshen it up, applied some Loctite to it and fitted a 4mm grub screw on that section of it too. So now there is a grub screw holding the one end of the M8 bolt onto the drill shaft, and another holding the car jack threaded rod to the other end.

A bit convoluted I know, but since I've done that (and secured the two grub screws with Loctite too) I have had absolutely no problems with the connection at that point and it has lifted robots without any hassle