Coupler problems for Focuser Automation

[Astroman]

Hi all,

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.

Help appreciated.

[multiweb]

Drill one side of the coupling slightly bigger to suit.

[AstroApprentice]

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 $.

[LewisM]

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.

[Astroman]

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...

[Russj]

Try these sites,

Stepper Motor Couplers
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_sop=15&_nkw=jaw+coupling&rt=nc

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR0.TRC 0.H0.TRS0&_nkw=Jaw+Shaft+Coupling+Spider+Flex ible+Coupler&_sacat=0

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2322090.m570.l1313.TR0.TRC 0.H0.Xjaw+plum+coupling+shaft++spid er+coupler.TRS0&_nkw=jaw+plum+coupling+shaft++spide r+coupler&_sacat=0

[AndrewJ]

Gday Andrew


4.8mm is close as dammit 3/16", so if necessary, get a 3/16" coupling, then drill one side out to 6mm.
or
https://www.amazon.com/Stainless-Steel-Screw-Shaft-Coupler/dp/B012DV70YC


Andrew
Ed

just noted the shipping cost ( but there are other vendors )

:jawdrop:

[Imme]

I've used a little heatshrink tubing around a shaft to make it a little larger yet still keep the shaft centered in the coupler.....worked a treat