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Diagonal fixing screws holes threads get stripped

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#1 JP-Astro

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Posted 17 May 2019 - 08:17 PM

I've had my SW Mak 127 for less than a month and used it a dozen times. I used a stock diagonal that came with it. When changing eyepieces I tightened 2 screws at the top of diagonal's barrell. So the knurled head screws were screwed in and out while tightening each new eyepiece.

Today I found that one of the screws stripped the inner thread in the diagonal barrell and the screw now falls off. I didn't expect it to happen (too soon?). What have I done wrong and how common is this failure?

I can tap a new thread in a different location or I may install a bushing in the stripped thread hole and re-tap the thread there but I don't know what should be my strategy to avoid this in the future? Should I not tighten the knurled screws when installing eyepieces in the diagonal barrell? What are they there for then?


Edited by JP-Astro, 17 May 2019 - 08:18 PM.


#2 ngc7319_20

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Posted 17 May 2019 - 08:52 PM

What is the diagonal made from?  Cast aluminum?  Plastic?  If it is cast aluminum, this stuff does not take threads very well, especially fine threads.   You could try to re-tap to larger / coarser thread.  And replace the stock set screws with nylon thumb screws.

 

I can't tell how "tight" you are making the screws.  If your eyepieces have a safety undercut, the screws don't need to be tight at all.  They just need to engage the undercut to prevent accidents.


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#3 junomike

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Posted 18 May 2019 - 06:45 AM

Rare for this to happen with a metal Diagonal.

I'd go the re-tap route (both)  and use thicker screws.


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#4 JP-Astro

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Posted 18 May 2019 - 07:32 AM

What is the diagonal made from?  Cast aluminum?  Plastic?  If it is cast aluminum, this stuff does not take threads very well, especially fine threads.   You could try to re-tap to larger / coarser thread.  And replace the stock set screws with nylon thumb screws.

 

I can't tell how "tight" you are making the screws.  If your eyepieces have a safety undercut, the screws don't need to be tight at all.  They just need to engage the undercut to prevent accidents.

Yes, the diagonal top barrell that accepts eyepieces looks like it's either the cast aluminum or maybe machined aluminum tube.

Both screws were somewhat wobbly in their threads right from the start but I didn't expect they'd strip the threads so soon.

 

Are there replacement top barrels for diagonals made from more robust metal? Where can I get one?


Edited by JP-Astro, 18 May 2019 - 07:32 AM.


#5 spacemunkee

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Posted 18 May 2019 - 08:37 AM

+1 for retapping slightly larger and using some nylon screws. Maybe retapped a bit tight so the nylon kind of taps itself into the new hole.

And if and when they strip, just pop in some new ones and not be retapping things.
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#6 SteveG

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Posted 18 May 2019 - 12:58 PM

Yes, the diagonal top barrell that accepts eyepieces looks like it's either the cast aluminum or maybe machined aluminum tube.

Both screws were somewhat wobbly in their threads right from the start but I didn't expect they'd strip the threads so soon.

 

Are there replacement top barrels for diagonals made from more robust metal? Where can I get one?

No, they are almost all made of aluminum, and your issue is not uncommon. As Mike & Matt mentioned it's best just to drill the hole out and tap it for a larger screw. I do this do my diagonals & barlows even if they weren't stripped to begin with. I use the black nylon thumbscrews sold by Scopestuff.com. A 1/4-20 nylon screw is much larger and will hold the eyepiece more secure without marring the barrel. A tap & handle is available at any hardware store for very low cost. I also use an oil for tapping aluminum threads, but it's probably not necessary.


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#7 NMBob

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Posted 18 May 2019 - 01:06 PM

I replaced most of my OEM diagonals with Televue diagonals. The screws thread into a tube with lots of threads, instead of just a hole with just a few threads. Some other brands have thicker metal eyepiece tubes, or a thicker ring around the top for the screws to go through. They will hold up better.


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#8 JP-Astro

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Posted 18 May 2019 - 07:19 PM

Thank you all for your very useful replies! I now know I'm not alone on this and I know what to do.



#9 kevint1

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Posted 19 May 2019 - 07:30 PM

Have you contacted Skywatcher ???



#10 Migwan

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Posted 19 May 2019 - 08:01 PM

This happened to me on a 2" to 1 1/4" adapter.  Steel and aluminum just aren't a good mix.  After tapping that hole out and using a larger thumbscrew, I put candle wax on all such thumbscrews.   They all snug up better than they ever did before.  I highly recommend it as preventive.  

 

jd


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#11 JP-Astro

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Posted 20 May 2019 - 12:20 PM

Have you contacted Skywatcher ???

The SW headquarters, you mean? - Not yet. I'm sure the SW president will be disappointed.

(Just kidding :-))

I might call the local dealer I got it from and report the issue to them. Maybe they'll swap the diagonal top EP barrel?

Thanks - good idea, however as others reported this issue should be addressed on a regular basis.

I agree - the thickness of aluminum walls even though enlarged at the location where screws go in still doesn't look secure to hold up well.

I doubt they are going to redesign the barrell anyway.


Edited by JP-Astro, 21 May 2019 - 06:28 AM.


#12 fgraus

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Posted 20 May 2019 - 01:16 PM

You could use some JB weld to build up a thicker area and then drill and tap. More threads will be less likely to strip out.



#13 havasman

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Posted 20 May 2019 - 01:54 PM

+1 for nylon screws, but tap a good clean hole

 

They're also a better solution than brass compression rings for good retention w/o marring. If anything strips it'll be the screw and they're cheap. I have a few at the bottom of my small astro tool kit that goes with me for remote sessions. Have not yet needed one.


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