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Thread: Runout, threads, concentricity.

  1. #1
    Insider PBSteve's Avatar
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    Runout, threads, concentricity.

    Just for the sake of keeping things organized.

    Quote Originally Posted by Florypb505 View Post
    Wow you know that clears so much up. I never really looked for how the m2 bolt was made but it makes total sense now why it doesnt slide nicely and why so many people have problems with those sail orings leaking, I personally have problems with the bolt not cycling when its first aired up in any weather and with the lpr set crazy high but once it breaks and starts moving free it is fine all day.

    I tossed my m2 bolt in my little china mill and stuck a dial indicator on the side of it, measured at the top to see the machines runnout, then where the threaded section was then all the way down at the tail. Its not concentric by 1/32" and mine isnt one of the markers that has tail sealing problems. For relative measurements I grabbed an old first gen image 1 matrix bolt and at the tail the runnout was about 1/3rd of the m2 tails runnout.

    I am posting up this video, it is set to only show for people who have the link

    https://youtu.be/aB7YiGE_k-8

    I'm curious do you guys have a concentricity tolerance at dye? Any paintball company? I dont know if its so out because of how the threads are made or if the machine is pushing to fast without enough stock support but something is making it off pretty drastically. I am going to try and straighten the tail of my bolt and see the difference in drag because of it, maybe solve my gas up issue. It is surprising none the less as I have run a dial indicator on a few ul backs and they have always been dead money in my experience so its a little shocking that a turned piece would be that far out

    It has me curious, when I get some time, I am going to try and measure up some more bolts to see how straight companies are keeping things.

    I'm sure there is an acceptable threshold for concentricity but 1/32" seems pretty excessive if we are looking at high end markers to have better performance than the lower end models.


    I bet the DSR bolt engine has less drag because that bolt would be easier to keep concentric.
    Well, for one thing threads aren't as automatically aligning as some people make them out to be. The rest of the mate matters too, most notably the surfaces the thread is mating against. If you're just bottoming on a blind thread you'll probably have a bad time.

    Also one thing I've noticed is if it's a shorter run (<1k) is that some shops won't thread things while they're still on the lathe. I suspect that if it's a standard size thread and they're not single-pointing them, they'll take them off the lathe and do the tap by hand. I've received a couple threaded parts with threads that have just massive concentricity errors on the threads because it was pretty clearly done by some dude with a power drill and not much attention to detail, and it doesn't matter what the rest of the mate looks like it's going to align like dogshit.
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  2. #2
    Insider PBSteve's Avatar
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    Speaking of which, does anyone know what a thread alignment callout would look like? Surely there's a "runout" measurement you can do for threads? I just want to make sure they're chucking up the tap on the lathe instead of doing the drill thing mentioned above.
    Last edited by PBSteve; 09-25-2017 at 10:16 PM.
    Comfort the afflicted and afflict the comfortable.
    I work for the company building the Paragon...once we figure out a name

  3. #3
    Idk about the callout, but would it be cheaper to:
    1) have a specified thread concentricity requiring it to be done on a lathe
    2) specify a looser fit thread with a tapered mating surface to provide concentricity
    If they're about the same cost, I'd think it be more reliable to have a tapered mating surface, since that essentially requires it be cut on a lathe and preserve concentricity. Although you lose some holding force with looser threads.

  4. #4
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    Yeah. Don't have the clearance for the mating surface here, it has to fit through a bushing and there are reasons not to go down a thread size.

    What would be a good concentricity number where a shop would assume it should be done on a lathe, but not so tight that it's ridiculous? 0.01"? Probably just leave a note?
    Comfort the afflicted and afflict the comfortable.
    I work for the company building the Paragon...once we figure out a name

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    Steve, have you seen the way it is done on the BL VIS engine? There is a little nub on the male threaded part and an oring is pushed down inside the female threads. The little nub can contact the rear wall. Also, the DSR does have the tapered mating surface and is pretty comparable.

  6. #6
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    Yeah I've seen them both, can't do it without going to a smaller thread which I have already done, and really don't want to go smaller.
    Comfort the afflicted and afflict the comfortable.
    I work for the company building the Paragon...once we figure out a name

  7. #7
    The OP here mentions a concentricity tolerance of 0.0005" and it seems like it's on a ground part, so it's probably too much. But 0.01" seems loose enough to accomplish on a drill press (0.01" concentricity over a 0.5" length gives 2.29 deg, which seems like a lot).


    OP:
    We are quoting some parts with 1/4 -28 2A (male) threads and 8-36 2A Left hand male threads. There is a callout that the treads are to run true with centers on the end of the part (other features are also called out to run true with the centers) within .0005". Is there a such a thing as a ring gage that would have a coincentric OD so that we could check the part between centers? I am going to ask my customer how they will check it, but trying not to look un prepared.
    responses:
    I'm planning on doing it between centers in a CNC lathe. Glastonbury Southern Gage is quoting me a master threaded ring gage with a ground concentric OD for checking this. They say they do it regularly, within .0002 concentric.
    Class 2A/B thread fits usually have enough clearance that .0005 concentricity is overkill. Also, referencing centers isn't the way things are done anymore, normally it's referenced to features on either end (bearing diameters, for example). I'll bet this is an old print, drawn before ANSI Y14.5 dimensioning and tolerancing became common. At least, this has been my experience. Suggest that clarification is in order.
    source: http://www.practicalmachinist.com/vb...ricity-211011/

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