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LXD75: Worst Threads in Mounts

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#1 Boot

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Posted 29 June 2009 - 11:36 AM

The Meade corporation has broken a cardinal rule of machine design as far as their (my) LXD75 mount goes.

The rule is this:

Screws should be made of a softer material than that into which the threads for said screw have been tapped.

Why? Beacause if anything gets stripped, it is the easily-replaceable screw, NOT the body of the device.

What am I talking about?

The LXD75 mount is cast from rather soft aluminum (or something too soft...). The "secondary lock knob", on the other hand, is made of a much harder metal. The result is that the threads in the mount are stripping-out rather than the screw threads.

I will now learn how to tap threads.

I need to drill-out and tap some larger threads into the mount, then replace the secondary lock knob w/ a larger, matching SOFTER screw.

Am I the only one out here finding new ways to break things?



#2 Luigi

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Posted 29 June 2009 - 03:34 PM

>>>Screws should be made of a softer material than that into which the threads for said screw have been tapped. <<<

That not a rule I've hear of or seen applied. Screws have to be hard so the heads don't strip, much harder than the aluminum much equipment is made of. The only reason that a hole will strip is if the fastener was over torqued, the thread have corroded or galled, the joint was subjected to well over the intended load, or repeated disassembly/reassembly cycles have worn them out. If you do strip out threads, it's common to fix them using Helicoil inserts or similar. This generally produces a stronger thread than that in the native material. In some applications in engines and such, Helicoils are used from the outset to improve strength.

#3 Ed Kessler

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Posted 02 July 2009 - 10:04 PM

Use a high quality bolt with threads that are not rough or sharp, and some lithium grease.

#4 vegasMike

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Posted 03 July 2009 - 03:42 AM

Take a look at this vendor "Scopestuff". THey have heard your (and others) cries. http://scopestuff.com/ss_l7sa.htm

In the case of my previous LXD75, repeated tightening and loosening destroyed the threads. I bought a Vixen mount after that. Vixen, whom Luigi has poignantly called, "a casting shop", yet in my mind, a damn good one.

I wish you luck my friend.

#5 Eddgie

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Posted 03 July 2009 - 08:38 AM

I agree with Luigi. A screw softer than the aluminum mount would be too soft.

And if you made the mount out of steel to match the hardness of the screw, it would be a lot heavier.

If you DO want a harder thread, buy a Helicoil kit. When you drill out the old thread, use the Helicoil to go between the screw and the mount material. This way, when you screw things in, they screw into the helicoil.

If you ever strip the Helicoil, you can unscrew it from the hole and replace it.

The only downside is that there needs to be enough room in the metal to support the bigger hole required by the Helicoil.

And have no doubt about the strength. When professional shops find stripped scew holes on cars, this to me is what the BEST ones do to repair it. This way it allows you to keep the same size hole and tread of the original screw rather than having one hole that is bigger tread size than the other.

The kits are not all that expensive, and if you think this will be an on-going problem, the Helicoil will be THE way to solve it.

Regards.

#6 Eddgie

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Posted 03 July 2009 - 08:53 AM

Oh, and this. The problem is likely NOT the soft metal, but most likely rough threads on the screws chewing up the metal.

The solution to this is to get a threading die and chase the threads using a die. You can buy a die kit from harbor freigt.

Just oil the bold or screw and screw the die down the length of the screw or bolt a couple of times to uniform the threads and knock off any burrs.

ANY screw or bolt that has rough threads will damage just about any thread you put it in if the material is softer.

I bought an inexpensive set of taps and dies many many years ago and ANY time I encounter a bolt or screw that will be removed or turned frequently, I always chase the threads smooth. THIS is the key to not only eliminiating the problem you have, but getting these kinds of connections to work SMOOOTHLY! Having a screw or knob with dress threads greatly improves the smoothness when you DO have to turn or make an adjustment.

Good luck.

#7 Boot

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Posted 06 July 2009 - 05:11 PM

Thanks for the advice, guys. I've been away for a few days. But now that I'm back in town, I plan to hunt-up a Helicoil kit w/ the proper range of thread-sizes and go for it.

#8 gnowellsct

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Posted 08 July 2009 - 12:28 PM

I saw the head of an LXD75 crack open when it fell over onto soft grass, and got some criticism after saying it looked to me like pot metal, but whatever it's made of, billet aluminum it ain't, and it's considerably less sturdy than the Vixen line which may be cast but is good quality machining.

I think the Chinese mounts have their place but when you find that issues of build quality are begining to annoy you, it is probably time to be thinking about a Losmandy or a Vixen or some other step up.

regards
Greg N

#9 NewAstronomer

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Posted 08 July 2009 - 02:24 PM

I think the Chinese mounts have their place but when you find that issues of build quality are begining to annoy you, it is probably time to be thinking about a Losmandy or a Vixen or some other step up.


FYI for Vixen I think the Portamount is made in China, while Sphinx SX EQ line and above are made in Japan.

#10 GaryML

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Posted 08 July 2009 - 04:16 PM


I think the Chinese mounts have their place but when you find that issues of build quality are begining to annoy you, it is probably time to be thinking about a Losmandy or a Vixen or some other step up.


FYI for Vixen I think the Portamount is made in China, while Sphinx SX EQ line and above are made in Japan.


THe Vixen Porta series are Chinese. The Sphinx, Sphinx D, GP2, and GP-D2 mounts are all made in Japan. The HAL-130 tripod often sold with these mounts is made in China.

#11 gnowellsct

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Posted 08 July 2009 - 09:58 PM

I wasn't thinking of the Vixen Chinese stuff. GN

#12 Trebor777

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Posted 08 July 2009 - 10:30 PM

Here's a pic of my Atlas EQ-G. Just a cheap Chinese casting.

Attached Files



#13 gnowellsct

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Posted 09 July 2009 - 10:54 PM

Well Randy that's why I always wince inside when I see loads of folks urging someone on to the EQ6 and similar. I would rather have a non-go-to stepper G11 than a Skywatcher EQ6. In fact that is not theoretical, you don't see me selling the G11 to get an EQ6 with go-to. Seeing fractures on poorly cast material makes me cringe.

I think most people feel, however, that the go-to EQ6 is better to own than a non-go-to G11.

regards
Greg N

#14 jrbarnett

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Posted 10 July 2009 - 10:20 AM

How exactly would you do that with cast aluminum mount bodies? Screws are rarely made of materials softer than cast aluminum. Perhaps high-stress areas should use threaded stainless steel inserts into which the screws thread. But then you up the cost of manufacturing considerably on an otherwise budget-priced item.

Good luck fixing your stripped threads.

- Jim

#15 BCB

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Posted 12 July 2009 - 09:19 AM

When a thread in ANY part strips out, there's usually one of a few things to look for..

Biggest one: over torquing (tightening, which boils down to people having gorilla hands, and not knowing their own strength)..

Second : A low percentage of cut thread (thread depth, peak to valley) in the part. This comes from the tap drill selected being too large.. Standard thread cut is approx 65%. Less than that, and you begin to really run the risk of easily stripped threads from not enough thread "overlap/contact..

Third: Worn cutting tools.. IE: drills cutting oversize, worn taps..

Just a few of the majors to consider. Most people will NEVER admit to stripping out a tapped hole, because they don't know what it feels like when the threads give out and strip.. They usually fail after repetitive over torquing, and give way quite easily so it's usually hard to feel it let go..

#16 Eddgie

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Posted 12 July 2009 - 11:20 AM

Well, I don't know if it would raise the price CONSIDERABLY. I mean if they were going to do ALL of the holes, mybe it would raise the price $15 or $20. If they ONLY did the holes where a big load was going to be placed (like the Altitude bearing), it would only add a few cents.

Remember, they have to drill and thread the hole already.. To drill it a little larger and thread it with a little larger thread would cost almost nothing additional. Then to run in a Helicoil insert would cost them $2.00 plus labor (and let me tell you, labor in China is appallingly cheap).

#17 Eddgie

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Posted 12 July 2009 - 11:41 AM

Well, I am not so sure if this is JUST a casting issue. Castings can be quite strong.

This could also be a design issue in that when an Atlas is loaded with counterweights and before the scope it put on , the Counterweight shaft acts as a lever focusing all of weight through the fulcrum of the altitude bearing and the lug that the altitude adjustment screw bears on, and transfers ALL of that pressure to a point that is less than one half square inches in diameter.

Any flaw in the casting is going to show up, but even without flaws, there is a LOT of pressure on this one spot.

As much as people overload these mounts, I am surprised that we have not (YET) seen more of this kind of failure.

A casting might not have been the best choice here so that would make it a design issue.

It is true though that any flaw in the casting would make failure far more likely.

Regards.






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