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SEMI-PRO SCOPE MOUNTING

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/16/2010 at 06:31
Jon A View Drop Down
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Originally posted by jonoMT jonoMT wrote:

The question about larger screw sizes had me wondering what the shear strength of a 6-48 screw might be. I couldn't find a min. diameter for a 6-48

Since 6-48 and 8-40 are not widely used in other industries, the common references containing allowables for bolts and screws typically don't have these threads listed.  But you are correct in that when the threads per inch is increased, the depth of the threads is reduced and the "stress area" is increased, so using numbers from more coarse threads yields conservative results.  When you look at 6-32 and 8-32 vs. 6-40 and 8-36, etc, the stress area is increased fairly proportionally--not exactly, but pretty closely--with the #8's having roughly 50% more area.  This varies a bit and depends upon the exact type of threads (MIL-S-7742, MIL-S-8879, etc) but you get the idea.  So let's look at your comparison.
Quote but since it's a considerably smaller thread pitch than a 6-32, which has a min. dia. of .1329" it would be safe to use that number:

Grade 8 bolt PSI: 150,000
screw radius: (.1329/2) = 0.06645
cross-section area: 0.06645 squared * pi = 0.01387
150,000 * 0.01387 = 2080.8 lbs.

I think you must have been looking at 8-32 here, as .025" deep threads wouldn't make for much of a screw.  The biggest stress area typically given for 6-32's in most texts is .0091in^2, with 8-32's coming in at .0140 (very close to your number).

Now the next thing is we are mostly worried about a shear failure.  150 KSI is the Tensile strength of a typical Grade 8 screw.  Shear strength is roughly 60% of that, about 90 KSI.  This leaves you with .0091*90,000= 819 lbs for a 6-32 and 1260 lbs for a 8-32.  With four on each base that's 3276 lbs for #6's and 5040 lbs for #8's. 

Again, that's just an example using 32 TPI fasteners so it's a bit conservative, the finer threads will be a bit stronger but the proportionality will be fairly constant.  Whether you need that extra strength is up to you.
Quote So you've got something like a ton of shear strength balanced against the recoil impulse, rifle and scope weight...and I won't pretend to know how that all interacts.

That gets complicated.     Wink    It can all be calculated if you have all the variables, but getting those is the problem.  For example, a big guy that doesn't give as much with recoil will be easier on the scope mounts than a small person who is pushed back by the recoil more.  Using lead sleds or the like can be even easier.  It's hard to quantify.  Also, while recoil is certainly a worry, so is rough handling.  Drop a rifle on a hard surface just right and you can put many times the amount of force on the scope mounts as even harsh recoil will. 

But naturally, for many cases it's an issue of a rapid change of momentum (impulse) where the heavier the scope, the larger the resulting force.  So what works well for 12 oz scopes might not be optimum for "38 ouncers."    Wink     Or in the case of a "scope down landing," it's the weight of the rifle that drives up force put on the mounts. 

Edit:  Harder, easier, oops.


Edited by Jon A - January/16/2010 at 15:14
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/16/2010 at 09:34
jonoMT View Drop Down
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That was some good info, JonA. FYI, the screw diameter I was using was for a 6-32 - the smallest I could find listed, which was .1329". (http://www.fastenal.com/content/product_specifications/SHCS.ALLOY.BO.pdf). Other references showed it as .131" and .1312". You were right that I was using the tensile strength figure (150,000 psi). I forgot the 60% rule. So I should have multiplied 0.01387 * 90,000 which would yield 1248 lbs. for shear strength.

Whatever the number may be, it seems that with the proper grade screws (i.e not the ones that came with the Trashco rings I bought ONCE) in combination with a base that has a recoil lug, there's probably plenty of strength in a reputable mount.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/16/2010 at 10:37
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This is where bedding bases and lapping rings, like cream, come to the top.  The better the contact, the stronger the installation will be when you need it most.  Do it slowly and thoroughly and you will be happy with the results.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/16/2010 at 15:13
Jon A View Drop Down
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Originally posted by jonoMT jonoMT wrote:

That was some good info, JonA. FYI, the screw diameter I was using was for a 6-32 - the smallest I could find listed, which was .1329".

I see, you were using the minimum body, or shank diameter.  That's usually what you would use for a nicely designed shear joint where fastener threads were not used in bearing.  In this case though, the shear failure will happen in the threads so you must use the minor thread diameter to calculate the area.  It makes a pretty significant difference.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/17/2010 at 00:43
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Originally posted by Jon A Jon A wrote:


Originally posted by jonoMT jonoMT wrote:

That was some good info, JonA. FYI, the screw diameter I was using was for a 6-32 - the smallest I could find listed, which was .1329".
I see, you were using the minimum body, or shank diameter.  That's usually what you would use for a nicely designed shear joint where fastener threads were not used in bearing.  In this case though, the shear failure will happen in the threads so you must use the minor thread diameter to calculate the area.  It makes a pretty significant difference.


I thought that the min. was the thread...shows how little I know.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 09:32
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very interesting
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 11:11
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However, keep in mind that on a typical 60-deg UN thread, as is used with most fasteners, when the fastener is under load, the first 2-3 threads underneath the bolt head are bearing the majority of the load, something like 70%, because the fastener stretches under load.  Beyond that, there is a rapid decrease in load bearing for each successive thread, such that increasing thread length at some point doesn't provide an increase in fastener connection strength.  This is the reason why ramp style threads such as the "Spiralock" thread design was created, to spread the load over a greater portion of the thread.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 12:19
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I learned something new.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 13:13
Jon A View Drop Down
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While there's some truth to that Ted, it's a consideration for tension failure of a screw.  The mode of failure we've been talking about above is shear--though in this case as well, the number of threads won't matter much.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2010 at 22:46
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Great article in the first post. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/09/2010 at 14:39
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My own personal favorite is the Sako Opti-lock system.  No frail screws or tippy bases to deal with and they also have the plastic centering rings to relieve tension.  One thing they do lack is the ability to swap inserts though.
 
I'l be mounting one soon and will re-read the article (only part of it sunk in so far) before and probably during my install to be certain that I understand and verify the install.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/02/2010 at 11:51
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A buddy and I have been debating whether it is important to center the scope "travel" for windage and elevation before mounting a previously used scope on a different rifle.  He claims that it makes no differance because the amount of reticle adjustment is compensated for when you have to make the appropriate adjustments to sight in the rifle.  In other words, even if the scope was previously adjusted almost all the way to the left of its total available windage "travel"; it would not matter when you put it on a new rifle since by adjusting the reticle for the new rifle you have accomplished what you would otherwise have done if the reticle was centered.  I disagree.  Who is correct ? 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/03/2010 at 14:15
Jon A View Drop Down
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If using mounts that are adjustable, such as the common Leupold style with adjustable windage, or Burris rings with the inserts, it is a good idea to center the travel in the scope and do your first rough adjustments when mounting with the mounts. 

If using non-adjustable mounts, it will make no difference.  The scope will end up where it ends up regardless of where the reticle was before you mounted it.  Don't waste your time.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/08/2010 at 09:39
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Unless you want to know how much adjustment range you have left...
If you don't need to know that, it is a waste of time.  
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/10/2010 at 00:32
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Great post - alot good info 
 
We always seem to have issues with scopes sliding in the rings, even with rifles with factory muzzlebrakes, I have an M25 mild 7.62 nato - I can't get cheapo bushnell to stay put - I have some QRW rings on order, cheapo Weavers are not doing it. 
 
I had a Win 70 classic rear bridge so badly off I wound up adding a stack of brass shim stock under the base till after hunting season was over, now to show how bad my luck is, Winchester rifle div sold to Browning at that point - about 9 mos later Browning assembled a new Win rifle for me. I am thinking it was an 1/8 + too low the rear bridge, must of been a Friday afternoon built rifle LOL
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/14/2010 at 13:21
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Good read.........no more spitting on the hands and grutting as you tighten them little screws
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/23/2010 at 21:52
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I JUST PURCHASED A REDFIELD 3x9 50 MM TO PUT ON MY SAVAGE 111 IN 7MM MAG. IM HAVING AN ISSUE MOUNTING THE SCOPE. WEAVER RINGS AND BASE'S I HAVE ARE TOO FAR APART AND THE FRONT RING IS ALREADY AN OFFSET RING. THE TUBE SEEMS TO BE SHORTER THAN ANY OTHER SCOPE THAT I HAVE. DOES AN ONE HAVE A SUGGESTION ON A BASES AND RINGS. I KNOW THE DNZ IS A GOOD OPTION.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/23/2010 at 22:18
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You could always go with a 1 piece picatinny style rail.  Then you can mount rings where ever you want to.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/16/2010 at 20:04
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Edited by koshkin - August/16/2010 at 20:08
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/29/2010 at 10:09
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Good read, thank you
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/06/2010 at 11:34
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Great Read. 

I love the Burris Signature rings and have them on all of my precision airguns and light caliber guns.  There is nothing like them for keeping your scope protected and keeping it in its place.  I am sold! 

I am changing scopes on my big game guns and practice guns and am wondering what the best base is to use with the signature rings?  Any suggestions on this?

Thanks,
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/09/2010 at 21:13
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Idaho;

I don't know the best bases.
If you like the Burris Signature rings, why not use the Burris XTR or DD bases?
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