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Thread: Locking lug math help

  1. #1
    Gunco Member Warwickben's Avatar
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    Default Locking lug math help

    Looking at building a .50bmg bolt action using a interrupted thread style locking lugs.

    Looking at a Barrett bolt action gave me the idea.
    I figure it would be a lot easier todo then cutting a normal 2 or 3 lug setup

    I'm stumped on trying to figure out what size acme thread rod and how many threads to use.

    Any help would be great.

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    Chief Administrator 7.62x39's Avatar
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    Back before he became a gentleman of leisure Winn was a structural engineer of some sort (we all remember the infamous toothpick experiment lol) He might be able help you out.
    Let's see if we can get him to chime in here.
    .

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    GuncoHolic Sprat's Avatar
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    great idea

    please go to the new member section and introduce yourself
    Sprat and sprat1 are one and the same.

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    gunco irregular moleman's Avatar
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    I know you mentioned in your other thread that you'd found acme thread and nuts in 4140, do you know what the RC is as it will make a difference.

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    Indian Admin Winn R's Avatar
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    Quote Originally Posted by 7.62x39 View Post
    Back before he became a gentleman of leisure Winn was a structural engineer of some sort (we all remember the infamous toothpick experiment lol) He might be able help you out.
    Let's see if we can get him to chime in here.
    Well -- I was a little younger then and a whole lot stupider.

    In answer to the thread starter's question, It's all dependent upon the steel type and hardness. And not only of the bolt material but the receiver as well. There's a reason the ar uppers are $1000 plus and still report problems.

    Not meaning to be a smart ass, but if you've got to ask you shouldn't be doing it.
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


    "Never attribute to malice that which can be adequately explained by stupidity." Robert J. Hanlon

  6. #6
    TRX
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    Hi Warwick! Glad to see you here...

    The usual web resource that comes up is Varmint Al's Eclectic Web Pages Look under the "Rifle Action Stress" and "Bat Action Analysis" pages.

    I haven't come across any printed-on-paper references; I suspect that, historically, designers and manufacturers used "test to destruction" first, then "that looks about right."

    Putting "strength of Acme threads" into Google turns up a bunch of hits, probably a lot more useful than two-lug stress analysis.

    McMaster-Carr has 4140 Acme threaded rod and nuts in their catalog. If you use a separate Savage-style bolt head and turn the nut round and trap it between the barrel and a step in the receiver you could save a lot of machining.
    Perge, scelus, mihi diem perficias.

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    Gunco Member Warwickben's Avatar
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    I thought I posted in the new member thread , guess not will do tho. Been looking around for some time about block back Ak's etc thanks to 7.62x39 sending me links .

    I've found a few online vendors that will cut the threads out of what ever stock I want . Plus I'm friends with a few machine shops in my neck of the woods that told me they'd thread some stock for me too. I'd just cut the threads my self I just lack the tooling to cut non standard threads at my personal shop . Also the company I run a machine shop for don't allow any one to work on projects after hours any more.

    I was gonna send it out to be heat treated .
    Figured the heat treating and material specs for the Maadi would be good.

    I figured using a interrupted thread would save on machine ops/setups on a manual mill. Basically not having to off set the bolt and extension on a indexing head to round the corners . Just need to mill sections out at x degs.

    That said , some are probably thinking he just said he's friends with shops. Threading and tapping some stock would take a lot less time for my friends todo for me then writing a program up and doing it for me on one of there Cnc mills . Don't want to ask to much of my friends shop if you know what I mean .

    Also figured if I can get help with figuring out the math it's a lot easier to build this style of a bolt vs the Maadi style so it would be a win win for other builders.
    I've just been scratching my head on the math for some time .

  8. #8
    TRX
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    In the 21st century there may be fancy "modeling" software that can do it... but the shear equations you'll find in Marks' or Kent's Engineering Handbooks, or any of my pre-WWII engineering books, differ so much that I am firmly convinced they're all codified rules of thumb from empirical testing.

    The problem is, you're looking at two hairy strength-of-materials problems in the same part - "edge effect" (or "notch effect") and impact. You can fuzzywuzz your way around the notch calculations, but last time I was trying to find anything concrete about impact effects, the handwaving from the textbook authors kept blowing the pages around.

    Back in the old days engineers would size their lugs by "best practice", ie, looking at what other engineers had gotten away with. Then they'd build a prototype and test it to destruction. On the now-defunct garagegunsmithing board, member Heathydee designed a rear-locking bolt action, cobbled up a proof-of-concept prototype, and kept loading it with oversized bullets and ridiculous charges of pistol powder. If I remember right, the locking lugs never did fail...

    Of course, you have people who stress out over whether various Mausers can handle a .308... and then you look at the bolt and trunnion of a .308 AK, and not only are the bits less than half the size, the bolt doesn't even turn completely onto the lugs in the trunnion when it goes into battery... and they work just fine, thankyouverymuch.

    You can calculate your ass off, but the only real way to know is to proof test it. For three hundred years know, proof has trumped math every time.
    Perge, scelus, mihi diem perficias.

  9. #9
    Gunco Member Warwickben's Avatar
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    Quote Originally Posted by TRX View Post
    In the 21st century there may be fancy "modeling" software that can do it... but the shear equations you'll find in Marks' or Kent's Engineering Handbooks, or any of my pre-WWII engineering books, differ so much that I am firmly convinced they're all codified rules of thumb from empirical testing.

    The problem is, you're looking at two hairy strength-of-materials problems in the same part - "edge effect" (or "notch effect") and impact. You can fuzzywuzz your way around the notch calculations, but last time I was trying to find anything concrete about impact effects, the handwaving from the textbook authors kept blowing the pages around.

    Back in the old days engineers would size their lugs by "best practice", ie, looking at what other engineers had gotten away with. Then they'd build a prototype and test it to destruction. On the now-defunct garagegunsmithing board, member Heathydee designed a rear-locking bolt action, cobbled up a proof-of-concept prototype, and kept loading it with oversized bullets and ridiculous charges of pistol powder. If I remember right, the locking lugs never did fail...

    Of course, you have people who stress out over whether various Mausers can handle a .308... and then you look at the bolt and trunnion of a .308 AK, and not only are the bits less than half the size, the bolt doesn't even turn completely onto the lugs in the trunnion when it goes into battery... and they work just fine, thankyouverymuch.

    You can calculate your ass off, but the only real way to know is to proof test it. For three hundred years know, proof has trumped math every time.
    I normal work that say . But I normal have a idea where to start .

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