Making a Crayford focuser smooth and silky

[topdude]

Hi, if anyone has a Crayford and feels it could be a bit smoother this might be of interest

I was cleaning up a used Crayford i recently bought and being a bit of a tinkerer, stripped it down to see how it works.

Cleaned everything up and re-assembled and it was worse !!!

Now it was slipping on low tension and jerky on higher tension.

So, did a bit of Googling and came up with this guide to improving a GSO Crayford.

GSO Focuser

Well, it worked and now it moves smoothly whatever the tension

Apologies if this is common knowlege but it might be useful to someone.

[skye at night]

That IS useful...

thanks..

However i have just upgraded the GSo focuser on my scope to a Moonlite, so have also spared myself the hassle if not the expense..

Sometimes you have to be brave and just see how stuff works to get the best out of it.

Steve

[MishMich]

Thanks.

I tried this.

Took it apart, and there were three grooves worn along the focuser tube, two for the bearings and one where the brake connects. I used a small flat grind-stone and ground out the two grooves from the bearings, and ground down the flat section where the focuser shaft runs till the shallowest groove was gone, leaving an even surface across 95% of its surface. I removed the plate holding the bearings, and this was chipped, and one of the bearings' runners appeared slightly corroded. My guess is that only three of the bearings were actually making contact with the focus tube. I ground out the chipping, just to be sure it was smooth and not coming into contact with the tube, and used fine paper to rub off any tarnish & corrosion on the wheel of the bearings, and applied lithium grease. I ground out the channel in the tension plate though with the focuser shaft runs, the plastic coating flicked out at some point. I greased the bottom of the 'V' shaft, as well as the plastic spacers that go on the shaft to separate the knobs from the tube. I then polished up as per instruction using some vehicle cutting polish.

I put back the tension plate and bearing plate, and repositioned the 4 hex nuts and two screws to ensure the tube had a firm connection with the the rolling shaft, and then put back the two thumbscrews for tension and brake, and put it back on the telescope - this time with the knobs going cross-wise rather than lengthwise. There is now enough room to twiddle the knobs even! Well, it all looks and feels better - but I need to test it loaded up outdoors at night.

M.

[topdude]

Well done, sounds like you had quite a bit of work to do. Luckily mine was not so worn so grease and car polish was enough.

Satisfying though isn't it

[MishMich]

Yes, it is satisfying not to have to spend out on something when what I have looks like it could be perfectly functional (I prefer to recycle that replace) - it didn't take too long, and I'm not sure the grinding was absolutely necessary, but there were three well-worn grooves there, so I thought I'd do the full job while I had it out.

I was looking at retro-fit microfocusers, and the only one I can find is the GSO one, but mine doesn't have the screw between the brake and tension screw to attach it to. I noticed in their disassembly instructions that the little plastic sheath is quite important. So, in hindsight I guess I should have paid more attention to that bit flicking off (it looked like a crust of dried gunge TBH, so guess it was past it anyway). I'll going to have to cut a replacement out of an old blister pack, take the focuser apart again, and replace what was there with that.

Who knows, maybe I'll feel brave enough to remove and clean the mirror - just found an advert for a polymer gel that you can apply to optics, leave it to set, and then peel it off, taking any dust etc. with it.

M.

[topdude]

And i thought i was keen

[MishMich]

Obsessive, I'm afraid.

Reassembled with the DIY plastic bush between the focus-knob shaft and the tension plate. Replaced and readjusted the 4 alignment nuts to ensure the tube is sqare. Will let it bed in and see how it works in use.

M.

[Dangerous-Dave]

Very useful. The CF on my Megrez 90 has been grumbling for a while. I might pluck up the courage and give it a go after reading this. Is there a specific type of grease you should use or will the multipurpose lithium stuff do?

[topdude]

I think any grease would do. I also think the smoothness was helped by the application of the car polish on the plastic bushing which pushes the focusing shaft up against the tube.

[blinky]

Can I just say..... Dont take apart one of the William Optics Linear Power Focusers - as they have a nice linear ball race in them. I never knew this until I heard the sounds of 48 tiny steel balls dropping to my living room floor!

[Rob]

Sorry to dig up an old thread here.. I've spent a few hours stripping my GSO style focuser down to stop it from slipping. I've gone in with the oil stone to flatten further the flat. One thing I've noticed is the v plate to which the focus wheel shaft sits in does not have a teflon shim??.. is this usual?.

I've cleaned everything up and put back together.. seems to have worked, but I now have a squeak when I rack in & out!!. how?.

Any thoughts.

Cheers

Rob