I am in the process of getting some equipment ready to automate my focusing. I have a Moonlite focuser installed on my 8" f4 newtonian and it has a shaft diameter of 4.8mm. I will be attaching a NEMA17/27:1 Stepper motor to it for fine focusing adjustment, the Stepper has a shaft diameter of 6mm. I cannot seem to locate a shaft coupling suitable. I can find 5mm to 5mm or 5mm to 6mm, I may be looking in the wrong places. Would I have to drill out the hole to make it suitable or is there another option out there? I have attached images of the Shafts and dimensions.
Originally I had the idea of using a 2:1 toothed pulley and belt arrangement, but directly coupling the Stepper to the Focuser looked like a better option. I would have gone with the Moonlite stepper motor, but new cost on these is a little higher than the price I want to pay, not too sure there would be that many around on the second hand market.
Hi Astroman,
You're correct that a 5 to 6 mm ID shaft coupler with screws that tighten onto the coupler body won't hold on the 4.8 mm shaft end (pic 1), but a 5 to 6 mm ID coupler with grub screws that pressure tighten directly onto the shaft will give you a couple of mm flexibility (pic 2). I'd be surprised if a 5 mm ID coupler wouldn't hold on a 4.8 mm shaft. I have Moonlite, Starizona & Skywatcher couplers that fit onto slightly smaller focuser shafts in this manner. Plenty available on AliExpress for a couple of $.
The flex shaft couplers are the way to go - allow for imprecise shaft alignment to some extent too. If need be simply drill it to suit, then use the grub screws.
Thanks guys, wasn't sure if I should drill it out or not, I'll order a few and see which ones work, I know they aren't that expensive. hopefully I can bundle them up with some other parts I need also to save on postage...
