So the next thing to sort is the fitting of a suitable pinion to the SEM motor shaft. The original stepper motor had a nice accurate 16 mm shaft, and a 20 tooth 1.5 mod gear fixed by 5 mm key and a grub screw, with the grub screw trapped by a circlip. I would have liked to re-use this gear, if only for economy, but the SEM motor shaft is a rather under sized 5/8" resulting in a 4 thou slop.
I had considered that perhaps Loctite would bridge the gap, but better sense prevailed and I bought a replacement with a 'pilot bore' from HPC gears. Loctite say up to 1.5 thou gap and mine would be 2 thou circularly. Incidentally let me pour praise on HPC. A phone call at 15:30 on Friday resulted in the postman bringing me their parcel on Saturday morning - how fast is that ! They also were able to provide some drive pulleys and a belt that I need for the encoder, but more of that later.
So I need to mount the gear in a collet, bore it to the rather arbitrary motor shaft diameter, and slot a key way to match the 1/8" key in the shaft.
I've also decided to fit a single lip oil seal to the shaft of the motor, as the gears run in an oil bath.