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Thread: HPT and MPI: still viable and necessary or outdated bureaucracy?

  1. #71
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    that is a huge oversimplification and does a disservice to the discussion

  2. #72
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    Let me pick out a few things.

    First
    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.
    Next
    Quote Originally Posted by rob_s View Post
    I don't think you can lump those two together, or even compare them here.

    In the first case, KAC redesigned the bolt lugs and corresponding barrel extension as well as extended the gas system all to reduce bolt fatigue...
    I'm not picking on Rob. Rob is very knowledgeable, continues to seek knowledge, shares what he learns and takes his lumps. But these two posts illustrate my point.

    HPT checks for weakness in the part regardless if it comes from a poor design, poor quality or wrong material.

    It was once posted that comparing the KAC bolt & a standard bolt is apples & oranges as the KAC has been redesigned when it was pointed out that KAC as a high quality manufacturer does not perform HPT/MPI testing. As we're talking about QC, it does compare directly. I don't have any inside information, but I can guess what happened based on my experiences with solving QC issues at McDonnell-Douglas. They identified a certain process as being part of the problem. The problem they identified was a weakness in the original bolt design. They made changes to the design, which improved their QC and found as a result, they no longer needed the Quality Assurance process of HPT/MPI to verify it. This worked as their bolts don't fail.

    Until it's discovered what is leading to this rash of bolt failures, it cannot be determined if the HPT test is of value.

    If "Worse Tier" companies simply started manufacturing their bolts to pass HPT/MPI and to meet dimensional tolerances, it doesn't mean they "get it" or their bolts will be of acceptable quality.

    It takes more than just passing the required tests to produce a quality part. Colt has the advantage of not only knowing how to get their bolts to pass the tests, but have the experience and knowledge how to make them good enough to satisfy their customers. That last is what gives them an edge
    Last edited by MistWolf; 05-17-11 at 20:02.
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  3. #73
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    that's a good point that cuts to the heart of the issue. Until we know why MPI was instituted it is very difficult to judge its value. All we have in anecdotal evidence.

  4. #74
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    Quote Originally Posted by orionz06 View Post
    What is "always pass", and is this before or after a proof test?

    In response to the rest:

    Think of it this way, if you build houses for a living, a tree house for your kids should not be an issue. Are you gonna trust that you are that good and let them climb up into it, or are you gonna head up and jump a few times first?

    One side of the argument suggests that if you are good, you are good, spend more time making the cuts look nice and measuring. The result will be a good tree house, and the verification IS needed.

    The other side suggests that the parts will still need measured and still need to be the right size, but no way in hell are we letting the kids jump up into an unverified tree house.
    Dont know how else to answer the "always pass" question but i would imagine/hope they mean after the proof load. Although it was not specified.

  5. #75
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    Quote Originally Posted by TXBob View Post
    that's a good point that cuts to the heart of the issue. Until we know why MPI was instituted it is very difficult to judge its value. All we have in anecdotal evidence.
    Someone needs to phone a friend and get failure rates. That would be more telling.

    Quote Originally Posted by Dirtyboy333 View Post
    Dont know how else to answer the "always pass" question but i would imagine/hope they mean after the proof load. Although it was not specified.
    I would hope as well, but there are some who just MPI and it is likely all bolts will "always pass".

  6. #76
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    Forgive me if I am wrong here but unless I am mistaken MPI is a visual inspection process. It requires a trained person who is paying attention to see the flaws that are highlighted by the dye.

    In writing PFMEAs a 100% visual inspection has a Detection rating of what a 5 or a 6 (I don't remember it has been a while) so to be "World Class" and limiting your RPN to 125 or less your Severity and Occurance would have to bee REALLY REALLY low to make that threshold.

    There are other methods out there for finding metal flaws that are quite a bit more reliable in my experience. I have to agree with Rsilvers here, I would rather spend the money and determine what the Critical Characteristics are to proper bolt functioning and check that.

    After all, since ISO/QS/TS methodolgy has in the last 50 years created vehicles that will last 300,000 miles or more versus vehicles that were basically shot after 100,000 miles using simple design/processing criteria. Surely we could use the same methodology to get similar performance from a machine as simple as a rifle. If we were willing to spend the time effort an money to do it.

    Reading some of these responses, some of you guys would go into seizures if you knew what type of manufacturing controls were in place on the brakes on your cars.

  7. #77
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    Ah finally a topic I can add some knowledge to. To start I am certified to ASNT SNT-TC1 as a Level II in Mag Particle, Penetrant, Ultrasonics, Radiography and a variety of other visual and specialized NDT. I have 9 years of experience in the industry...

    There is so much misinformation on this post it is ridiculous!

    Very simply, MPI is a test that is for surface and very shallow subsurface flaw in materials. Only DC current MPI will detect subsurface flaws.

    As far as it's usefulness for flaw detection in AR bolts...I think it is very relevant for a couple reasons.

    First, the design of the AR bolt creates sharp corners. These sharp corners are known as stress risers. These stress risers are more or less the area when stressed that the failure will propagate from.(ie the cam bolt hole and lugs)

    Second, when a bolt is proof tested...it has seen stresses that exceed it's normal operating range. While i don't know offhand the material, tensile strength or yield strength of the AR bolt...the fact is that it has seen higher stress than operating range. These stresses may come close to the allowable stress of the material at temperature.

    On another note...I would hate to be the technician on the Mag bench looking at these because of their complex design. Hope this helps

    Edit...some research shows that the material is Carpenter No. 158. A case hardened alloy mold or forged steel. I would suspect that irregularities in case hardening(heat treat), casting quality(not relevant if forged) and improper machining would be sources of failure. (Which HPT/MPI would be perfect to check for initial flaws in manufacturing)
    Last edited by 12gauge; 05-17-11 at 23:48.

  8. #78
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    Quote Originally Posted by rsilvers View Post
    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.
    SO WRONG! I assure you that a MTR(material test report) does not mean shit after you machine and proof test your bolt. Even if the material was inspected(typically ultrasonics) it means nothing more than the material was sound before you machined it and overpressure tested it.

  9. #79
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    To build on my previous post.

    I don't have any experience in firearms manufacturing, but I do have extensive experience in brake rotors and I can give some of my experience with it. Chances are, many of you are riding on brake rotors that I was in charge of setting up the manufacturing on. If you are driving or riding in a Ford Edge, Lincoln MKZ, Ford 500, Ford Escape, Ford F150, Ford Explorer, Ford Sport Trac, Nissan Titan, Toyota Venza, Dodge Dakota, Ford Limo, Chrysler Pacifica, Chevy/GMC full size Truck/SUV or a Dodge Magnum/Chrysler 300, your ability to stop is dependent on my ability to develop a reliable manufacturing method. (Scary hunh?)

    Now each customer was slightly different but the basics were the same.

    Of all the dimensions required to make a rotor only between 12 and 20 of those dimensions were checked 100%. Most of those dimension had nothing to do with safety but rather noise and vibration. Most of the other dimensions were demonstrated to be statistically stable and normal and > 1.67 Cpk and were checked by sampling or only 1/year.

    Brake rotors are either gray or damped cast iron. Being cast they can develop porosity (voids) which, when present, can cause a catastrophic failure. You apply the brakes, and the calipers pull the plates off the "top hat". Then you literally have no brakes at that point. Pretty scary right? Bet that is something you would test for 100%? Nope. We sampled the incoming material using an acoustic test and certified all the lots on initial production. After the 1st 3 months we went to skip lots, after that no testing other that veryfing the material certs coming from the supplier.

    But 100% of all the rotors that left that plant were checked for balance and lateral run out. Because that is what we could not demonstrate a CpK of 1.67 or greater even though the worst thing that could happen to one of you with a brake rotor with a LRO of 21 microns is that you might have a little brake shudder at slow speeds. That is what was measured because those were the dimensions we couldn't control.

    Applying this to rifles. Surely the guys who are working with the designs know what dimensions are critical to the performance of the bolt and if those dimensions (whatever they may be) would be what I would want to spend my time and money on.

    Maybe it is voids/cracks in the material after manufacturing, but based on my experience with American casting/forging companies of today, I find that unlikely. Now 50 years ago, maybe that was more of a problem that it is now, but automotive suppliers at least are pretty ****ing fantastic. Now offshore companies....

    Anyone afraid to drive to work tomorrow?

  10. #80
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    Just as a side note, I toured a few firearms manufacturers in Europe recently, and none appeared to be doing any MPI. They were all proof testing in accordance with various laws. Everything from the high dollar shotguns and rifles to certain blank firing pistols.

    edit: Some had some really cool examples of weapons that had failed the proof testing. These varied from "relatively minor" to "who put the hand grenade in that Thompson?"
    Last edited by 87GN; 05-17-11 at 23:17.

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