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Thread: HPT and MPI: still viable and necessary or outdated bureaucracy?

  1. #31
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    Quote Originally Posted by d90king View Post
    Based on your scenario how can you be assured that every part you are using meets your own standards if you don't test to make sure? I'm certain that you don't want end user failures to determine the quality of your materials and manufacturing. Just because it worked in the past does not mean that it will work in the future even though it might be a good early indicator that you are using good materials at the start.
    Basically once the design is proven, then you can move away from certain tests. Just to give you an example of a more destructive test - drop testing....

    When you have a new gun design, you can drop test it many times. It may break. You change the design and do more drop testing. Say it passes this time. You now know that the design is strong and it passes a drop test. You don't drop test every one you make after that. And true, you never know for sure if the next one would have broke or not.

    I know this is not a perfect analogy because drop testing does physical damage, but my point is that, once every unit you test for a while does not break, then you should probably stop that test and spend time on things which are more likely to find bad parts.

    I am claiming that this is not 50 year ago, and AR bolts are to the point where you can test thousands of them in a row and not have any failures, so one is better off spending the dollars for that test on additional dimensional testing by upping the number of statistical samples you take which will increase the probability of locating bad parts (dimensional tests commonly finds bad parts, so it is a much more productive thing to spend time on).

    Why not do both? It could take 50 or 100 man hours to test everything there is to test on a rifle. Realistically, you can budget up to an hour or two at most, so you really have to spend the time on what finds the most bad parts. MPI does not find the most bad parts.

  2. #32
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    Quote Originally Posted by rsilvers View Post
    I can't follow what you are asking. What do you mean about "passing everything else?"

    What do you mean about "good bolts not being correct?" If you are asking if there are bolts which have passed MPI but are dimensionally incorrect, of course they exist and are probably not rare.

    Untested and tested bolts will break, after 5,000 or 10,000 rounds. You seem to be saying that only untested bolts break. All bolts break.
    "Probably" not rare? Could you please provide some empirical evidence that a credible company producing MPI/HPT bolts are passing a statistically significant number of dimensionally incorrect bolts?

    Yes, all bolts break. But when? Do you have empirical evidence to suggest that the likelihood of a non-test bolt and a tested bolt fail at the same rate? If there is a difference, is the difference in failure rates greater than or less than the rejection rate of companies that do MPI/HPT?

    If not, it sounds as though your entire argument is based on hearsay. Furthermore, I've not seen anybody suggest that companies shouldn't test for dimensional accuracy.
    Last edited by tylerw02; 05-17-11 at 13:03.

  3. #33
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    Quote Originally Posted by 5pins View Post
    Isn’t the point of HPT to reveal any defects that can be found with MPI? I don’t understand why you would HPT without MPI. To me that would be like filling your condom with water then not looking for leaks.
    You can inspect an HPT part visually and with gauging. AAC does it both visually and with gauging - 100% of the uppers.

  4. #34
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    Quote Originally Posted by mike_556 View Post
    Simple question for any mfg's that may be on here..when your parts are produced, are they not CMM'ed, or are they just batch tested? Tolerance stacking in certain military aircraft is no longer accepted as they were in the past--I would think in small weapon parts such as the bolt it would be fairly simple and inexpensive to verify the dimensions are in spec?
    If you think your favorite brand CMMs every dimension on every bolt - well, that is not going to be the case. That would be too expensive. And no one is demanding it. Do I think there are a lot of MPI bolts out there which are out of spec? Yes. I think there are a real lot.

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    Quote Originally Posted by dpaqu View Post
    With the money guys spend on there guns I would think a "premium" bolt with fancy hard to machine steel thats been cryo treated and MPI would sell well.
    I avoid things cryo treated. I don't believe in the Easter Bunny either. The exception is I will buy Krieger barrels.

  6. #36
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    Quote Originally Posted by rsilvers View Post
    Basically once the design is proven, then you can move away from certain tests.
    But the HPT/MPI is not about testing the design, it is about testing the quality of the material.

  7. #37
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    Quote Originally Posted by rob_s View Post
    But the HPT/MPI is not about testing the design, it is about testing the quality of the material.
    Exactly. You can't visually see metallurgical flaws.

  8. #38
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    Quote Originally Posted by rsilvers View Post
    I avoid things cryo treated. I don't believe in the Easter Bunny either. The exception is I will buy Krieger barrels.
    That I can agree with.

  9. #39
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    Quote Originally Posted by rob_s View Post
    But the HPT/MPI is not about testing the design, it is about testing the quality of the material.
    End of thread.

    All the dimensional gauging in the world isn't going to detect something like an inclusion in the steel.

    A mag-particle test will.

  10. #40
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    Quote Originally Posted by rob_s View Post
    But the HPT/MPI is not about testing the design, it is about testing the quality of the material.
    I thought thats what my post said but he snipped it out in his response...

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