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Thread: HPT and MPI: still viable and necessary or outdated bureaucracy?

  1. #41
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    Quote Originally Posted by tylerw02 View Post
    "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?
    I tested many bolts - and many were out of spec. They were marked MPI. I can't give you evidence - but just get a bolt drawing and test a lot of bolts and I bet you will find some bad ones.

    Quote Originally Posted by tylerw02 View Post
    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?
    Well, if almost no bolts ever fail MPI, then it would seem obvious that you would not be able to find a statistically significant difference in the bolt life of tested vs non tested bolts. That being said, there is evidence that the very process of testing bolts makes them last a few thousand round less long. So if you want to increase the average life of a bolt, I would say you should demand no testing.

    Quote Originally Posted by tylerw02 View Post
    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.
    I know people want their parts tested 100% for dimensional accuracy, but it seems like nearly everyone is assuming that is already being done. That is simply not the case. It is a question about how many parts are tested and what areas of those parts. The less time and money you waste on MPI, the more parts and areas on those parts you can do other verification on and the more test firing you can do.

  2. #42
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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.
    If all of your bolts are made from the same mill-run of C158, then you don't need to test each one. You can use certified material and/or test some of the steel from the batch. We have a private stash of C158. We know it is good.

  3. #43
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    Quote Originally Posted by polymorpheous View Post
    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.
    And what if you tested 250,000 of the parts, and none of them failed MPI? But also what if your CMM machine found that 1.8% of trigger parts were bad - but you could still not test 100% of trigger parts due to time/budget?

    Would you keep on testing MPI, or spend that same money on a second CMM to double the number of dimensions you could check on the trigger parts to further weed out the bad ones?

    Well, if the internet demanded that you do 100% MPI, then you may skip buying the second CMM and keep on doing the 100% MPI. But if you were truly quality oriented rather than marketing driven, you would do whatever weeded out the most bad parts.

  4. #44
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    My trade is as a welder.
    When a weld inspector uses a mag particle test on my welds, he is not looking for flaws in the weld process I have used.
    He is looking for flaws in the base and filler metals.

    If I had a business with a good reputation for putting out quality product, I wouldn't give up my quality control steps to raise my profit margin.

    Edit: Not only do "internet" end users demand their parts be HPT/MPI. But so does the DoD.

    Quote Originally Posted by rsilvers View Post
    And what if you tested 250,000 of the parts, and none of them failed MPI? But also what if your CMM machine found that 1.8% of trigger parts were bad - but you could still not test 100% of trigger parts due to time/budget?

    Would you keep on testing MPI, or spend that same money on a second CMM to double the number of dimensions you could check on the trigger parts to further weed out the bad ones?

    Well, if the internet demanded that you do 100% MPI, then you may skip buying the second CMM and keep on doing the 100% MPI. But if you were truly quality oriented rather than marketing driven, you would do whatever weeded out the most bad parts.
    Last edited by polymorpheous; 05-17-11 at 13:37.

  5. #45
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    Quote Originally Posted by polymorpheous View Post
    Edit: Not only do "internet" end users demand their parts be HPT/MPI. But so does the DoD.
    On the M16 and M4, yes. Not on the 416, Mk11, M110, M110C, etc.
    Last edited by rsilvers; 05-17-11 at 13:41.

  6. #46
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    Quote Originally Posted by rsilvers View Post
    On the M16 and M4, yes. Not on the 416, Mk11, M110, M110C, etc.
    So what's your point?

    My point is:
    This thread makes no sense.

  7. #47
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    My point is that it is in the M16 spec because it was created 50 years ago before people had enough data. And the M4 spec just carried it along. On new contracts for other guns, it is not being required.
    Last edited by rsilvers; 05-17-11 at 13:56.

  8. #48
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    Let me put it this way...

    I lay a beautiful multipass weld using ER-71 fluxcored wire.
    For the sake of time and profits, I failed to clean my welds fully after every pass.
    But my cover passes look great.
    The weld inspector comes along and does a visual inspection only. He gauges my welds for proper dimensions. Inspects for discontinuities and defects.
    Now underneath that pretty looking weld is slag inclusion. And lots of it.
    Something he can't see using a visual inspection.
    A magnetic particle test would.

    If I were a customer of this weld shop, and found out about this, I would never do business with them again.

  9. #49
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    NDT of welds is important because it often finds bad welds.

  10. #50
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    But you fail to take into consideration why the testing is done in the first place. (After so many members here have already pointed this out)

    You seem stuck on dimensional inspections, that is not what MPI is for.

    Not only that, but the M16/M4 is still the issue primary weapon of our armed services.
    Not the Hk16.

    Quote Originally Posted by rsilvers View Post
    My point is that it is in the M16 spec because it was created 50 years ago before people had enough data. And the M4 spec just carried it along. On new contracts for other guns, it is not being required.

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