View Full Version : MPI Tested
Did a search and couldn't find any info on MPI tested bolts and barrels, sooooo...
What is it?
What does it do??
Do I want it???
Thanks!
All I know: its good and LMT does it, yes you want it. Probably someone here may ilustrate the tech details about MPI.
MPI stands for Magnetic Particle Inspection. It is a type of NDT process (Non-Destructive Testing) to check for flaws, cracks, etc in metal/conductive parts.
Basically, the part to be inspected is magnetized, and exposed to magnetic particles (either suspended in air or liquid), which are attracted to any cracks or fishures in the part. Any "reading" (high concentration of particle attraction) beyond nominal or acceptable limits and the part is usually disposed of. This may sound sort of vague, due to the fact that different parts have different acceptable standards, and different companies have different criteria for acceptance.
For example, there are small cracks that form at the base of the lugs on an M16 bolt after it is proof fired. The degree and location of these cracks is compared to the government inspection standard / quality assurance provision, which in turn determines whether the part is acceptable. Assuming the part is correctly proofed and then final-finished (phosphate), it is marked "MPI" to signify acceptance.
Hope this helps,
Clint
It is something you want in both your barrel and your bolts. To my knowledge, LMT, Colt and special ordered CMT (Stag Arms) are MPI tested.
For barrels, LMT, Colt and BCM are HPT and MPI tested. There maybe others, but I can't think of any.
The thing to be careful of is that some companies MPI a couple barrels out of a batch, but go ahead and mark every barrel as being MPI tested (bad juju in my book).
The other big burning question is what are these companies exit criteria for a flawed barrel or bolt? I think some companies allow minor flaws in their products.
The only company I can say for sure that has a zero flaw acceptance policy is BCM.
C4
Leaveammoforme
05-26-16, 16:34
MPI stands for Magnetic Particle Inspection. It is a type of NDT process (Non-Destructive Testing) to check for flaws, cracks, etc in metal/conductive parts.
Basically, the part to be inspected is magnetized, and exposed to magnetic particles (either suspended in air or liquid), which are attracted to any cracks or fishures in the part. Any "reading" (high concentration of particle attraction) beyond nominal or acceptable limits and the part is usually disposed of. This may sound sort of vague, due to the fact that different parts have different acceptable standards, and different companies have different criteria for acceptance.
For example, there are small cracks that form at the base of the lugs on an M16 bolt after it is proof fired. The degree and location of these cracks is compared to the government inspection standard / quality assurance provision, which in turn determines whether the part is acceptable. Assuming the part is correctly proofed and then final-finished (phosphate), it is marked "MPI" to signify acceptance.
Hope this helps,
Clint
Interesting.
I win by the way.
I win by the way.
For the necropost of the century?
For the necropost of the century?
I agree but if he didn't do it IG would have canned the thread!
Pretty much. Or at a minimum the decade.
For the necropost of the century?
I can't keep up with who does what, we need a chart! :sarcastic:
You can MPI any ferritic metal anytime you want. I've been MPI certified since 1998. I should start an MPI service for M4C.
I would even do a liquid penetrant for no extra charge!
Dionysusigma
05-28-16, 09:08
https://www.youtube.com/watch?v=qpgcD5k1494
Video explaining the process very well.
No cracks are allowed around the lugs or the bottom of the extractor slot around the lugs.
No cracks are allowed around the cam pin hole.
No cracks are allowed around the extractor pin hole.
No cracks are allowed on the face of the bolt.
.
.
.
Incidentally, MPI isn't going to do much if done without HPT being done first...
The MPI doesn't have anything to do with the HPT. Myself and some actual smart people think that the HPT does more harm than good pertaining to bolt life.
Remember, the MPI is only valid at the time of inspection. It's just a method that allows a person to see some surface discontinuities easier. That's it.
so you are suggesting that, if we had the time, facilities, and bolts to test, you would expect to see Colt and BCM bolts fail at a slightly increased rate when compared to bolts that do not undergo HPT? I hear where you are coming from, logically, but I'm skeptical. I just don't think that people's experiences over the years corroborate that point of view. Granted, i have no science to reference, but i think, over the decades, Colt bolts would have been exposed as slightly less reliable. Thats simply not the case.
elephantrider
05-30-16, 04:51
so you are suggesting that, if we had the time, facilities, and bolts to test, you would expect to see Colt and BCM bolts fail at a slightly increased rate when compared to bolts that do not undergo HPT? I hear where you are coming from, logically, but I'm skeptical. I just don't think that people's experiences over the years corroborate that point of view. Granted, i have no science to reference, but i think, over the decades, Colt bolts would have been exposed as slightly less reliable. Thats simply not the case.
I doubt that this has been tested on a wide enough scale say definitively one way or the other. A true test would require everything to be identical, with the exception of HPT, across a large number of guns.
Some high quality mfgs. are spec-ing bolts without the HPT. Apparently they believe there is a positive difference.
The MPI doesn't have anything to do with the HPT.What is HPT?
What is HPT?
High pressure test.
USMC_Anglico
05-30-16, 07:00
MPI and Dye tests are common in engine building machine shops, hell most industries where things might come apart under load. I might just take my bolts in and run the tests for shits and giggles. Would be an interesting sample test of 3.....
The MPI would likely work ok, but the penetetrant (it's penetrating oil) would likely make a mess in the phosphate finish.
You could clean your bolt and look at it with a magnifying glass and be real close to MPI. The real purpose of MPI is that it makes surface discontinuities easily viewable.
so you are suggesting that, if we had the time, facilities, and bolts to test, you would expect to see Colt and BCM bolts fail at a slightly increased rate when compared to bolts that do not undergo HPT? I hear where you are coming from, logically, but I'm skeptical. I just don't think that people's experiences over the years corroborate that point of view. Granted, i have no science to reference, but i think, over the decades, Colt bolts would have been exposed as slightly less reliable. Thats simply not the case.
Don't take my word for it. Ask Knights Armament about it...
The MPI doesn't have anything to do with the HPT. Myself and some actual smart people think that the HPT does more harm than good pertaining to bolt life.
Remember, the MPI is only valid at the time of inspection. It's just a method that allows a person to see some surface discontinuities easier. That's it.
Sorry, they are related....
HPT stresses the parts and will open or expose any flaws that might exist....
MPI without first a HPT will not show anything, unless there was a flaw in the base stock, which isn't likely if the stock if quality. If there is a weak spot or high stress concentration, it will not fail until it is loaded.
Like you said MPI is only good at the time of testing, so if the part has never been highly loaded, what are chances of showing a weak spot that will open up under load?
Answer - none.
The MPI would likely work ok, but the penetetrant (it's penetrating oil) would likely make a mess in the phosphate finish.
You could clean your bolt and look at it with a magnifying glass and be real close to MPI. The real purpose of MPI is that it makes surface discontinuities easily viewable.
MPI fluids will not hurt you finish in any way....
Oh, and with MPI you can detect a crack 1 μM wide, you are not going to see that with a magnifying glass....
Sorry, they are related....
HPT stresses the parts and will open or expose any flaws that might exist....
MPI without first a HPT will not show anything, unless there was a flaw in the base stock, which isn't likely if the stock if quality. If there is a weak spot or high stress concentration, it will not fail until it is loaded.
Like you said MPI is only good at the time of testing, so if the part has never been highly loaded, what are chances of showing a weak spot that will open up under load?
Answer - none.
Most of the threads/info I've read discussing HPT and MRI discount the one-shot HPT as essentially useless in disclosing faults that MPI wouldn't reveal without a HPT. There is/was a guy who went by Keith J on TOS who posted quite a bit on this, his posts gave me headaches.
It is argued that HPT won't reveal anything that wasn't already there. Since, as Samuse stated, MPI will generally only reveal EXISTING discontinuities, some believe that the HPT is unnecessary. I'm not sure either way, as I feel HPT could possibly reveal deeper material integrity issues that short of x-ray or ultrasonic testing one wouldn't know existed.
It is also important to understand that any form of NDT does not make a bolt "better". It only makes it more expensive. That is why many companies batch test as part of a first article inspection only. The thought being if material and processes are to spec and locked down, MPI of every piece is not value added.
Sent from my iPhone using Tapatalk
It is argued that HPT won't reveal anything that wasn't already there. Since, as Samuse stated, MPI will generally only reveal EXISTING discontinuities, some believe that the HPT is unnecessary. I'm not sure either way, as I feel HPT could possibly reveal deeper material integrity issues that short of x-ray or ultrasonic testing one wouldn't know existed.
It is also important to understand that any form of NDT does not make a bolt "better". It only makes it more expensive. That is why many companies batch test as part of a first article inspection only. The thought being if material and processes are to spec and locked down, MPI of every piece is not value added.
Sent from my iPhone using Tapatalk
I'll agree with the bold statement.
But, to further explain my above statements - my experience with NDT is that under heavy loads tiny flaws that may go undetected are enlarged so that detection is easier.
Given the fact that some places can sell a bolt for the low price of $40-$60, and this with HPT and MPI, I find the omission of such testing pointless penny pinching....
I'll agree with the bold statement.
But, to further explain my above statements - my experience with NDT is that under heavy loads tiny flaws that may go undetected are enlarged so that detection is easier.
Given the fact that some places can sell a bolt for the low price of $40-$60, and this with HPT and MPI, I find the omission of such testing pointless penny pinching....
I won't disagree with that, but is the MPI on these cheap bolts to a test spec? Also, NDT is subjective to the interpretation of the tester, and also the rejection criteria of the customer. Is it a spec'd, certified, and stamped process from a lab? Or just an employee that "MPI's" every part?
My experience with NDT is largely welding related. Even with destructive testing, a certain amount of inclusion, lack of fusion, etc is allowed and is cumulative. I don't know necessarily if it is the same with AR bolts, but wouldn't be surprised either way.
We know Colts bolt is tested per an outlined spec, as it is part of a TDP. If all bolts are tested to the same criteria, I'd agree with the penny pinching statement. But as I'm sure you know, there are now "Lean" experts whose sole existence is to pinch pennies. So as always, caveat emptor.
Sent from my iPhone using Tapatalk
MPI fluids will not hurt you finish in any way....
Oh, and with MPI you can detect a crack 1 μM wide, you are not going to see that with a magnifying glass....
No the penetetrant won't hurt the finish, but that phosphate will hold a lot of penetetrant and it's going to bleed a lot and mask any smaller indications. That's why I said he could do pretty good with a magnifying glass.
I've performed and been involved with a lot of NDE and it's just not all that reliable. It's mostly just documentation to relieve liability. Good manufacturing processes are way more important.
MPI isn't going to do much if done without HPT being done first...
This isn't necessarily so. MPI without HPT will show if there are cracks that are beyond allowable limits from the manufacturing process. HPT only shows if those cracks have grown past allowable limits after one over pressure event. The disadvantage to HPT testing is that each bolt has a finite number of over pressure events before they fail and each HPT reduces that number by one.
What is more important than HPT is that each bolt goes through proper shot peening to relive stress so the chance of cracking occurring is minimized
The loads on the M16/AR15 bolt design are such that the loading exceeds the yield strength in certain places on the bolt, so the bolt is subject to low cycle fatigue. There are only so many cycles, over-pressure or otherwise that the bolt will see be for developing a crack and have to be retired (failure).
Yes, the loading with a HPT cartridge will increase the cycle count by all of one, plus some fraction (to account for the fact that the load is higher than average), call it about 1.25 of a cycle of a normal load. However, fatigue life is not precise. There is no actual "number" of cycles all parts will see before developing a crack. The calculation of a 'fatigue life' is a statistical exercise to give a reasonable close estimate of the minimum number of cycles before failure given the expected loading.
Bolt life can be as low as 5000 or as high as 12,000 rounds, so you can see that there is a rather large range as to the number of cycles the bolt can see before failure, add to the count one high pressure test is insignificant. The total decrease in life from one HPT cycle only 0.000179 % of the total range, less than insignificant really.
For a manufacturer to claim that they are doing it to make their bolts last longer, is being disingenuous at best. One load at 25% above normal is not going to shorten the life any measureable amount and given the range of cycles a bolt can have, statistically, it is just as probable that high pressure tested bolts can last longer than non-HPT bolts.
For a manufacturer to claim that they are doing it to save money is at least honest, but I really don't thing the customer will ever see the savings given the price of HPT and MPI tested bolts, and extrapolating out the cost of manufacturing a bolt with testing, and comparing that to all the other non-tested bolts…
constructor
05-31-16, 09:55
The loads on the M16/AR15 bolt design are such that the loading exceeds the yield strength in certain places on the bolt, so the bolt is subject to low cycle fatigue. There are only so many cycles, over-pressure or otherwise that the bolt will see be for developing a crack and have to be retired (failure).
Yes, the loading with a HPT cartridge will increase the cycle count by all of one, plus some fraction (to account for the fact that the load is higher than average), call it about 1.25 of a cycle of a normal load. However, fatigue life is not precise. There is no actual "number" of cycles all parts will see before developing a crack. The calculation of a 'fatigue life' is a statistical exercise to give a reasonable close estimate of the minimum number of cycles before failure given the expected loading.
Bolt life can be as low as 5000 or as high as 12,000 rounds, so you can see that there is a rather large range as to the number of cycles the bolt can see before failure, add to the count one high pressure test is insignificant. The total decrease in life from one HPT cycle only 0.000179 % of the total range, less than insignificant really.
For a manufacturer to claim that they are doing it to make their bolts last longer, is being disingenuous at best. One load at 25% above normal is not going to shorten the life any measureable amount and given the range of cycles a bolt can have, statistically, it is just as probable that high pressure tested bolts can last longer than non-HPT bolts.
For a manufacturer to claim that they are doing it to save money is at least honest, but I really don't thing the customer will ever see the savings given the price of HPT and MPI tested bolts, and extrapolating out the cost of manufacturing a bolt with testing, and comparing that to all the other non-tested bolts…
So here's a question.
Lets say company XYZ that machines bolts for many in the industry offers bolts not marked "MPI" for $38 and they offer bolts marked "MPI for $39.
How do they place the bolt in a test jig and fire a proof load that cost $1.40 on it, then remove the bolt and inspect it then etch or stamp it for $1?
I was told by XYZ that the etch or stamp they offer is a convenience option then it is up to the company buying the bolts to test and MPI inspect then reject any that do not pass THEIR tests. XYZ does not test any bolts.
BTW, I'm not disagreeing to anything you have said just asking a question.
If HPT and MPI are done, somebody has to pay the technicians to do it, whether the actual people that machine the bolt from a hunk of steel or the marketing company that advertises and sells them, and that cost has to be rolled into the final price.
Is it possible that somebody would but "MP" marked bolts and not test them? sure, that's possible.
How much do you trust the guy selling them? I trust BCM, and DD to do what they say, and they manage to sell bolts at reasonable prices....
How much do you trust the $8/hr flunky that's passing all those bolts through an MPI station?
I'd rather have him/her/it testing bolts because maybe that'll keep them from messing up my hamburger!
constructor
05-31-16, 11:20
I believe in the track record. If a company has been in business a long time and doesn't have a lot of failures I would believe that over a new company that was selling bicycle parts last week and firearm parts this week regardless of the claims.
Some more on why magnetic particle inspection is of limited value without first subjecting the bolt to a high pressure test:
From MIL-S-13165, SHOT PEENING OF METAL PARTS (this information is also found in SAE AMS2430):
. . . When magnetic particle or dye penetrate inspection is required, parts shall be subjected to such inspection BEFORE peening. . . . (my emphasis)
Why do they recommend inspection prior to shot peening?
The shot peening process is, very simply, bombarding the surface of the part with a stream of small steel balls at high velocity. This will pound craters in the surface, the object of which is to leave a compressive residual stress on the surface which will reduce the part's susceptibility to fatigue. However, these small craters will also mask small cracks at the surface by beating them closed. Since there is no longer any actual gap on the surface, detection by MPI or FPI is limited (in the case of FPI, nonexistent).
While MPI can detect subsurface cracks it is possible, accuracy and reliability of such flaw detection is problematic.
Since high pressure testing loads the bolt into the elastic range (and in places, into the plastic range), any existing cracks will be re-opened for detection, especially those in the critical areas at the base of the lugs.
If manufacturers plan on only magnetic particle inspecting without first high pressure testing, they need to do this inspection prior to the shot peening process, not after peening and application of the final protective finish.
How much do you trust the $8/hr flunky that's passing all those bolts through an MPI station?
I'd rather have him/her/it testing bolts because maybe that'll keep them from messing up my hamburger!
I seriously doubt the guy at DD or Colt doing the MPI testing is only paid 8 bucks an hour....
Unless you have evidence to back up your claim, you're starting to resemble your avatar.
I seriously doubt the guy at DD or Colt doing the MPI testing is only paid 8 bucks an hour....
Unless you have evidence to back up your claim, you're starting to resemble your avatar.
15 years in NDE. Production level MPI is an $8+/hr job that is usually performed by new kids or people who can't pass a background check.
You could mag a bolt in literally 10 seconds if you took your time.
I used to work in a shop where we inspected cylinders for nuclear reactors and the MPI 'techs' were entry level kids who weren't old enough or couldn't pass a drug/background check to go on field jobs. Someone who knows what they're doing will set up the equipment and verify the procedure. Then they'll train/certify techs to do the actual work. I used to qualify procedures and train techs. I worked in aerospace, nuclear, petro-chem.
I'm not saying that NDE isn't worthwhile, but you have to take it with a grain of salt. It's dependent on the skill and quality of the individual performing the check.
I used to work in a shop where we inspected cylinders for nuclear reactors and the MPI 'techs' were entry level kids who weren't old enough or couldn't pass a drug/background check to go on field jobs.
Well that makes me feel good in kind of a Homer Simpsonish way.
Well that makes me feel good in kind of a Homer Simpsonish way.
The radiographers and ultrasonic guys were Tier 1 operators!
I believe in the track record. If a company has been in business a long time and doesn't have a lot of failures I would believe that over a new company that was selling bicycle parts last week and firearm parts this week regardless of the claims.
Hey man, bike manufacturers produced the first real airplane ....
Sorry, totally off topic. Fascinating info, for sure.
I'm always a bit surprised at 2 things: 1, that xray and ultrasound can be used for NDT. How so? Given the issues we have in the medical field with shadowing and radiation not penetrating hard objects (bones, teeth, implants). Does NDT just really increase the energy to non-safe for human levels?
How much "NDT" is allowed before it's considered destructive? The life cycle shortening of HPT was mentioned above, but ... is there an acceptable level of stress? Ie, a 1% predicted decrease in longevity? I assume it's different across different things (ie, a bolt, versus, say, an airbag).
Thanks for the SME's inputs.
Hey man, bike manufacturers produced the first real airplane ....
Sorry, totally off topic. Fascinating info, for sure.
I'm always a bit surprised at 2 things: 1, that xray and ultrasound can be used for NDT. How so? Given the issues we have in the medical field with shadowing and radiation not penetrating hard objects (bones, teeth, implants). Does NDT just really increase the energy to non-safe for human levels?
How much "NDT" is allowed before it's considered destructive? The life cycle shortening of HPT was mentioned above, but ... is there an acceptable level of stress? Ie, a 1% predicted decrease in longevity? I assume it's different across different things (ie, a bolt, versus, say, an airbag).
Thanks for the SME's inputs.
UT in welding anyway, isn't the same as in the med field. We are looking for defects, in an almost binary fashion. It either exists or it doesn't. Instead of a sonogram image on a screen that is produced by echoes, it's a graph on an oscilloscope that returns the presence of some type of discontinuity. Honestly, it takes an operator that is quite skilled to understand the size/type/location of a defect beyond the fact that's it is either there or not there.
RT, like UT, is different than the med field. Instead of a large machine (typically), a radioactive "pill" is used to expose the film, with the work piece between the two if I remember correctly. For my purposes, these methods of NDT are fairly specialized, and I use a lab service if they are spec'd.
As far as what amount of NDT...
HPT may indeed decrease the life cycle of a piece, but that reduction is beyond nominal. It does not damage it beyond its designed useful part life. It either fails or it doesn't. NDT is just that...non destructive. If the piece is rendered useless, in most cases it is considered destructive testing.
Hope this helps. It's been several years since I covered these in school, so obviously if anyone sees any mistakes or if there have been changes since then, I'm sure they'll correct me. :)
Sent from my iPhone using Tapatalk
15 years in NDE. Production level MPI is an $8+/hr job that is usually performed by new kids or people who can't pass a background check.
You could mag a bolt in literally 10 seconds if you took your time.
I used to work in a shop where we inspected cylinders for nuclear reactors and the MPI 'techs' were entry level kids who weren't old enough or couldn't pass a drug/background check to go on field jobs. Someone who knows what they're doing will set up the equipment and verify the procedure. Then they'll train/certify techs to do the actual work. I used to qualify procedures and train techs. I worked in aerospace, nuclear, petro-chem.
I'm not saying that NDE isn't worthwhile, but you have to take it with a grain of salt. It's dependent on the skill and quality of the individual performing the check.
Our facility has the same requirements for NDT technicians as it does for the line technicians and component rework technicians, and are the same pay-grade. We are a aircraft overhaul and component overhaul facility.
(We don't allow people who can't pass a drug test/background check to work here at all....)
Yes, dipping a bolt in a particle bath and magnetizing it in five directions and doesn't take long, probably take a bit longer that 10 seconds to do it right, though.
EDIT:
The way x-ray is used in flaw detection is based on the simple fact that when a neutron "light" is shined on a part, the neutrons penetrate the object. The intensity coming out the other side is dependent on the thickness and type of the material, so voids or inclusions will show up as changes in the neutron intensity.
Ultra-sound works the same as SONAR, sound will reflect off changes in density in an object, such as cracks, voids, or inclusions.
15 years in NDE. Production level MPI is an $8+/hr job that is usually performed by new kids or people who can't pass a background check.
I used to work in a shop where we inspected cylinders for nuclear reactors and the MPI 'techs' were entry level kids who weren't old enough or couldn't pass a drug/background check to go on field jobs. Someone who knows what they're doing will set up the equipment and verify the procedure. Then they'll train/certify techs to do the actual work. I used to qualify procedures and train techs. I worked in aerospace, nuclear, petro-chem.
I am rather surprised you allow unqualified personnel to do something as critical as MPI. We, and every vendor I have ever worked with has adhered to ASNT recommendations on qualifying personnel of 12 hours of training and 280 hours of supervised MPI experience before cutting them loose on their own.
I guess it really does boil down to: How much do you trust your vendors to do the right thing. I also is the reason why we require on-site inspection and a review of training records and processes before we qualify a vendor to supply us parts . . .
Integrity: At the Heart of the Matter
The nondestructive testing profession has suffered over the years because owners or managers have sent poorly or incompletely trained people to perform services for clients who have not always been savvy enough to recognize the difference between certified technicians and unqualified “warm bodies.”
They were trained and supervised accordingly. A class with a written and practical demonstration and all shop hours are supervised.
I worked for a couple of other companies doing NDE in petro-chem and we used to train up techs on MPI, PT, and UTT (thickness) in about 20 hours.
I was a lot more critical and usually did the NDE myself when preparing to weld on in-service piping. It usually flowed gasoline, Jet, hydrogen, stuff like that!
They were trained and supervised accordingly. A class with a written and practical demonstration and all shop hours are supervised.
I worked for a couple of other companies doing NDE in petro-chem and we used to train up techs on MPI, PT, and UTT (thickness) in about 20 hours.
I was a lot more critical and usually did the NDE myself when preparing to weld on in-service piping. It usually flowed gasoline, Jet, hydrogen, stuff like that!
In-service pipe...NO THANKS!
Good friend of mine had every stitch of clothing he was wearing blown off while welding ISP. That and his ears were the only injuries he had, bizarrely enough. This was in Alaska in the 80's/early 90's.
Sent from my iPhone using Tapatalk
In-service pipe...NO THANKS!
Good friend of mine had every stitch of clothing he was wearing blown off while welding ISP. That and his ears were the only injuries he had, bizarrely enough. This was in Alaska in the 80's/early 90's.
Sent from my iPhone using Tapatalk
Yeah it can be hairy. Never had a real incident, but but had a couple 'oh shitters! I oversaw a lot of in-service welding when I worked for the inegrity group for a pipeline company. Fun times in South Texas. Had a couple welders and helpers killed by wetbacks, but no lost time injuries.
Yeah it can be hairy. Never had a real incident, but but had a couple 'oh shitters! I oversaw a lot of in-service welding when I worked for the inegrity group for a pipeline company. Fun times in South Texas. Had a couple welders and helpers killed by wetbacks, but no lost time injuries.
Damn. That should be an episode of dangerous jobs...
Sent from my iPhone using Tapatalk
Don't take my word for it. Ask Knights Armament about it...
Knight dodging the truth.
Knight dodging the truth.
There might be something to what they do, nobody makes a longer lasting bolt.
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.