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Stripped grub screw in a Williams Optics Star71 refractor. Help!


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2 minutes ago, vlaiv said:

When I researched DIY motor focuser - recommendation was to put it on fine focusing shaft due to precision needed. One of comments in the thread that I linked to - also pointed that out.

However, ZWO EAF has rather decent resolution - of 5760 steps. Even if one revolution of coarse knob is 10mm of focuser travel - that is still 1.7µm per step. More than enough (critical focus zone is in tens of µm for fast systems).

Aha.  Consider me less confused than I was before :)

James

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3 minutes ago, vlaiv said:

I think it will work without issues.

Thanks Vlaiv

Just to confirm, sorry to keep repeating, as I will end up keeping it if you confirm?

When just imaging and electronic focusers are concerned, a rough and grindy focuser is not really a big issue?  That would be just an issue for manual focus and visual?

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5 minutes ago, JamesF said:

If the grub screw is cross threaded (and I think that's the most likely state of affairs now), it may well be pretty much impossible to move, hence the stripped head.  Drilling it out may be the only solution to remove it.

However, if you fit the EAF to the coarse side then you could just drill another hole for access to tighten the coupler and ignore the grub screw.  Unless of course it fouls the coupler, in which case it will probably need to be removed or ground off anyhow.

James

Thanks James

Yes, I think the grub screw protrudes enough to foul the coupler

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1 minute ago, oymd said:

Thanks Vlaiv

Just to confirm, sorry to keep repeating, as I will end up keeping it if you confirm?

When just imaging and electronic focusers are concerned, a rough and grindy focuser is not really a big issue?  That would be just an issue for manual focus and visual?

I'm tempted to say that it won't be big issue - but I can't be 100% sure as I don't have experience to answer that question.

I would rather someone else confirm that for you.

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6 minutes ago, Richie092 said:

I have a star 71 and my EAF is on the course side

Excellent. 
 

If it’s nit too much trouble, do you mind please sending me some pictures?

PM would be very helpful

Thanks a lot!!

 

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How to remove grub screws without a workshop full of tools and the skill to use them?

I would be looking at Torx bits to remove that grub screw.
Find one which is a tight fit but which will just go in.
Pliers will provide the pushing force and torque. [Turning force.]

An Allen [hex] key is just that. A hex with sides parallel to the hole.
Which provides very little torque to an already rounded hex socket.
The Torx bit [or screwdriver] has a star shaped profile.
Which provides more grip at the tips.

You could try gluing a disposable hex key in the hole with superglue.
Then turning it once well set. Apply glue to the tip of the hex key. NOT the screw!

Allen [hex] keys are highly variable in quality.
Some are mild steel mush. Others last for decades of daily use.
Same goes for Torx bits and screwdrivers.

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One LAST thing please  

if I am to re-grease the focus flat shaft and worm in the focuser, which type of grease should I use?

I have 2 tubs. 

One is white lithium grease I use on my bikes which is quite thin, and the other tub is thick brown lithium grease. It’s much thicker and more viscous. 
 

Which one should I use?

Edited by oymd
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7 hours ago, inFINNity Deck said:

Perhaps you could first give it a try with a screw extractor set?

Nicolàs

An expensive investment for one screw. I bought a cheaper set and broke one the first time I used it.
I have a lifetime of experience with tools and mechanical items. I have been cutting threads for decades.
Not all that is called "Dormer" means quality. Taps on eBay[UK]are sold as "Dormer."
They look as if the name is hand engraved by a child. The quality of the item was pitiful.

A better alternative to a difficult screw removal would be a new focussing knob.

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