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Thread: tpi to M

  1. #1
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    tpi to M

    i am used to calculate in mm so i try to convert some measurements
    but i find diff specs on tpi . for a new muzzle brake i need to change 3/4x24 tpi to m18x1
    i find a lot of different minor dia.specs on the 3/4x24 ?
    i do not have the barrel so i can not measure it.
    ore is it not possible an i need a adapter ?

  2. #2
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    Quote Originally Posted by johan teughels View Post
    i am used to calculate in mm so i try to convert some measurements
    but i find diff specs on tpi . for a new muzzle brake i need to change 3/4x24 tpi to m18x1
    i find a lot of different minor dia.specs on the 3/4x24 ?
    i do not have the barrel so i can not measure it.
    ore is it not possible an i need a adapter ?
    You need an adapter. I don't believe there are any standard threads that are the completely the same in SAE and metric.

    GsT

  3. #3
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    Johan, 3/4 24 = .750 x .0416 per revolution , 18 mm = .708 x .03937 per revolution it will work fine when returned and threaded

  4. #4
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    Quote Originally Posted by geo.ulrich View Post
    Johan, 3/4 24 = .750 x .0416 per revolution , 18 mm = .708 x .03937 per revolution it will work fine when returned and threaded
    that is wat i want to do return to .708 . but will all the 3/4 24" be gone or are they deeper then .708 dia ? i think there will be still some old threat marks there but not to mutch to affect the m18x1

  5. #5
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    Quote Originally Posted by johan teughels View Post
    that is wat i want to do return to .708 . but will all the 3/4 24" be gone or are they deeper then .708 dia ? i think there will be still some old threat marks there but not to mutch to affect the m18x1
    There will definitely be a bit of thread left at .708. Probably not enough to do any harm, but it may not look pretty. If I was doing it, I would probably sacrifice the length of the existing threads and start fresh. If it is to be a permanent brake installation with the brake installed with Loctite it likely wouldn't do any harm to thread metric over the old threads. One risk is to have another gunsmith take it apart sometime in the future and tell the customer that the previous gunsmith did a sloppy job.

  6. #6
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    Minor dia. is .691. turn to .708 there will be .0085 left per side of vee. when you pick up thread pick up at center of length there is only .002 difference in pitch at 1" long. you will cut .001 off either side of vee at each end of thread if 1" long. it will be fine and not really noticeable...

  7. #7
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    Cutting metric threads on a US pitched lathe has the same problem.
    You really need change gears in some different sizes than usual to pull it off.

    If the threaded run is short you can pull it off for the most part, but a long
    thread engagement with threads cut on a metric lathe is not going to work
    if the mate is cut on a US (imperial) lathe.
    Cutting threads for a shallower engagement and shorter length lets a lot of errors get past.

    Most threaded engagements rely on only a very few turns (around 3) to carry most of the load.

    It shows up when computing the loads required for pre-stress of high strength threads.

    It also shows up in computing the strength of threads in softer materials.
    Like aluminum.

  8. #8
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    exactly ^^^

  9. #9
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    I've got a Time Precision action which takes a threaded tenon near on 1.5" long and 18tpi IIRC

    I've toyed with the idea of (IF I ever change out my gears for a metric cut) threading up a barrel metric to see can I get 'er to tighten up on both ends.......

  10. #10
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    It will work just fine.

  11. #11
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    Quote Originally Posted by geo.ulrich View Post
    Minor dia. is .691. turn to .708 there will be .0085 left per side of vee. when you pick up thread pick up at center of length there is only .002 difference in pitch at 1" long. you will cut .001 off either side of vee at each end of thread if 1" long. it will be fine and not really noticeable...
    Exactly what I think as well. Pick up what is left of the old thread in the middle and have at it.

  12. #12
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    One of the things we used to do at one of my previous employers
    was use opponents equipment in testing US equipment.
    Everything from guns to radar sets.

    It was a well paying niche business.
    In some cases we even altered the equipment enough it no longer could be easily identified.

    We did not want anyone to easily figure out exactly what we had.

  13. #13
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    I have taken Panda Barrels, chucked them up and established a 16 tpi over the original 18 tpi to fit in a Remington action. If you didnít know what had been done, you would not even notice.

    Just machined a spacer to make up the difference in the tenon length.

  14. #14
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    One of the problems is the threading shaft pitch on the lathe.
    A 0.10 pitch drive is a real bear to alter to metric with gears.
    Keep in mind that gears only come in whole numbers of teeth.
    So you must go all the way to tooth engagement to maintain metric threads on an imperial lathe.

    And some are just not going to line up unless you go up and down in gear pitches trying to get the last bit of tolerance for long thread engagement.

    We finally ended up just purchasing a metric lathe.
    It saved a lot of labor in both the tool room, machine shop, and engineering.

    Of course we than had to have machinists trained to operate a metric lathe.
    The half nut engagement is significantly different on most metric lathes than US threading lathes.
    Thank God one of the older VERY skilled guys had bothered to stay up to date.
    He trained everyone else until we had enough folks to deal with the work load.

    And of course we then had a layoff of at least a third of the guys we trained as business took a downturn.
    At least all of them quickly found jobs (that wanted to keep working) with their extra training.

    Some at decent raises. Twenty years of experience and US and Metric trained? How about 30% more.
    Making them harder to ever hire back.

    The buyout of our parent company by Raytheon did NOT help things.
    There was not a single person in Raytheon ckeared to even know about some of our contracts and technology.

    We did manage to scare the cr@p out of them when we demonstrated jamming techniques that had not encountered or dreamed of.
    Some downright simple exploiting defects in 30 year old designs they still had a market for overseas.
    Luckily we had enough engineering folks we knew what changes would allow their often very old designs to compensate.
    Redesign anything?
    They did not have anyone available that truly understood the older designs.
    Just how to produce copies.

    An Engineering Services Company vs. a production missile company.

  15. #15
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    How come the US hasn't embraced the metric system?

    It seems to make more sense and a whole lot easier. The Auto industry has done it to some extent so why not just do it. 10's are a heck of a lot easier than 64 ths! Even England seems to have gone metric from what I've seen on TV!

    Pete

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