CNC Machining, QA/QC, And the World of Good Enough

[noonesshowmonkey]

The recent threads about two different rifles of two different price points being put through the ringer brought up--to my mind at least--the issue of how the modern manufacturing era that we currently live in and enjoy influences the quality, durability, and reliability of firearms.

To repost:

[The performance of a PSA rifle over 5k rounds] is a strong testament to the fact that CNC machining takes so much of the guesswork and feel out of building a firearm, and exposes the genius of the AR-15 pattern: that it is a series of parts which, when made to specifications, can be assembled and built and function. The only significant variable--when the manufacturer is producing mil-spec, within tolerance parts (which the above mentioned advances in high quality, high precision CNC machining makes far more reliable and easy)--is going to be QA/QC.

This is not to understate the value of QA/QC, or the smaller details that make up a KAC or DD or BCM rifle vs. that of PSA, but rather to solidify the notion that the AR-15 is an eminently producible firearm.

How much of this view is true? What are the realistic controls and acceptable variability of various CNC machining operations, and how do those compare to the operating tolerances of the AR-15? If a CAD guided 5-axis CNC machine can produce results to within the 10^-4m to 10^-5m with pretty incredible repeatability, and the specifications of those parts are clear and have been tested and are known to function (ie the TDP), then can't we expect just about any shop with a decent CNC machine to produce parts which are in spec, and thus functioning?

And if this is true, then where do we need to spend our time and our money? MPI testing seems a very worthy thing, as any high resolution imaging will show most (if not all) gross irregularities within the material itself. From there, the issue is mostly one of QA/QC: throwing a micrometer onto the part to ensure that it isn't irregular.

So, what are those parts which need the most attention? What are those parts for which there is absolutely no compromise?

Clearly, the Bolt Carrier Group is a strong candidate. The BCG already represents the least homogeneous part of the AR-15, the place where the largest variety of materials are present, where the vast majority of wear and tear occurs. Even so, if the correct materials are used, can we not produce a reliable BCG with a startling level reliability and repeatability?

Next, barrels. The barrel is probably the place on the AR-15 where the most amount of time, money, and effort should be spent, and I'll willingly cede the ground that whatever mixture of special sauce produces a sub-moa barrel is worth the investment, and that brand can mean a great deal here.

Past these two places, what are we buying but a few lumps of aluminium that have been CNC machined to nearly identical specifications, and produced at levels of precision far beyond the operating tolerances of the rifle itself?

[Kdubya]

The fact that both those threads were locked is evidence that some refuse to accept this reality. There is no "secret sauce" when it comes to building a servicable and reliable AR. With modern manufacturing, any company with the desire and commitment can put out a consistently functional rifle. Heck, there are guys milling their own lowers in their freaking garages that go onto run without any issues. Yet, some still would have us believe that companies, in spite of having the capital for precise production lines can't possibly get things right. And base the opinions solely upon the roll mark. Regardless of the progress we've seen, there are still many who claim that mfg A can do no wrong and mfg B can do no right.

We can't ignore that decisions are driven by cost, and some mfgs will find savings through departure from the mil-spec. But, does that really matter for things like barrel steel, linings, bcg material, etc? As long as the materials used were properly constructed and processed, it's generally meaningless.

Now, when we move beyond essential functionality and reliability, there are some who've found a better way; as you elude to with the sub-moa barrels. And that's what's truly great about the times we live in as AR owners. Overall, rifles have become so reliable that we can now nitpick at the little improvements we'd like to see. We all have different desires and opinions on what would make our specific rifles better, and are quite fortunate that there are so many options being produced to fill those demands.

Personally, I agree BCGs and Barrels are a good place to spend time and money. For me, triggers and coatings are others. Yet ammo is probably at the top of the list. At face value, because most rifles are generally reliable, we'd be best served to focus on practice and training. Tinkering with a rifle to improve its functionality is largely unnecessary, and is more about preference. Most would be much better served leaving the rifle alone, and instead work on improving the guy holding it. On a more technical note, in regards to ammo, there is always room for opportunity. Any round can reach out to 5-600 yards, but advancements can be made to increase the performance at those ranges. Better powders, projectiles, etc. Same goes for CQB applications; and everywhere in between. I'm sure others' desires will vary.

[Eurodriver]

Sigh.

It has little to do with tooling.

It has little to do with materials.

It has even less to do with a chart.

It has everything to do with attention to detail. Everyone puts out a bad product once in a while, but not every company has a "where is my order" thread; not every company puts out canted FSBs; not every company sends parts kits out missing parts.

Some do. If you want to put your life in the hands of that company why on earth would you feel the need to get on the internet and brag that your life is worth less than the $500 it would cost you to go with a company that doesn't?

And If you say that your rifle isn't for those purposes, then you quite simply don't belong on M4C.

[titsonritz]

It has everything to do with attention to detail. Everyone puts out a bad product once in a while, but not every company has a "where is my order" thread; not every company puts out canted FSBs; not every company sends parts kits out missing parts.

Not every company need BS parts (like relieved buffers or offset retainer pins) to compensate for their lack quality.

[JC5188]

All other things being equal, assembly is the most important part of manufacturing. Period.

You can take a bunch of engine parts, kit them up so that they are identical in quality and config, and I'll guarantee you that a guy who builds engines professionally will get better results than a shade tree mech with a chiltons manual.

Most people who argue the "better than" or "just as good" angle have very little manufacturing knowledge, or they would understand this.

That's not to say that "vendor a" can't make a rifle the equal of "vendor b". But skilled, experienced labor costs more, and if you introduce non value-added steps like NDT and QA into the process, it drives the cost up. That's just a fact.


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[Eurodriver]

All other things being equal, assembly is the most important part of manufacturing. Period.

You can take a bunch of engine parts, kit them up so that they are identical in quality and config, and I'll guarantee you that a guy who builds engines professionally will get better results than a shade tree mech with a chiltons manual.

Most people who argue the "better than" or "just as good" angle have very little manufacturing knowledge, or they would understand this.

That's not to say that "vendor a" can't make a rifle the equal of "vendor b". But skilled, experienced labor costs more, and if you introduce non value-added steps like NDT and QA into the process, it drives the cost up. That's just a fact.


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BUT THEY USE FN BARRELS

[MistWolf]

What "Good Enough" REALLY means is "Within Spec". If it's not within spec it's not good enough

[MegademiC]

Cost is
Raw materials
Chemistries
Tempers
Coatings
Proper dimensions
Proper use and knowledge of how parts interface to produce proper dimensions
Assembly
Qa/qc
What the specs allowed to go out the door.
Material tracking
1000 other influences.

You can dig deep into each category listed.

For instance, if your acid supplier is questionable, and sends you crap, and you don't check your stuff, you just made a bad batch of recievers. Will you scrap them? If you attempt to salvage, how do you ensure quality is kept? What methods are in place to ensure procedures are followed? On and on, this all adds cost.

The attention to detail the guy building in his garage has, is probably not the same as the guy building 100 rifles a day.

[JC5188]

BUT THEY USE FN BARRELS

Lol....that they do.


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[noonesshowmonkey]

You can take a bunch of engine parts, kit them up so that they are identical in quality and config, and I'll guarantee you that a guy who builds engines professionally will get better results than a shade tree mech with a chiltons manual.

Most people who argue the "better than" or "just as good" angle have very little manufacturing knowledge, or they would understand this.

As a man who has spent quite some time assembling / building in the aerospace industry, and then performing meticulous QA/QC, I can indeed understand and agree with this.

It has everything to do with attention to detail. Everyone puts out a bad product once in a while, but not every company has a "where is my order" thread; not every company puts out canted FSBs; not every company sends parts kits out missing parts.

Some do. If you want to put your life in the hands of that company why on earth would you feel the need to get on the internet and brag that your life is worth less than the $500 it would cost you to go with a company that doesn't?

This, and the above quote about assembly being crucial to the turning out of a good product, while very true, are not necessarily directly addressing my question re: the production of individual parts. At no point am I advocating for or against a given manufacturer, or a price point, nor am I attempting to create a false equality where there is none.

But, taken as individual parts, and simpler sub-systems (an assembled upper isn't exactly a demonstration of rocket surgery), parts produced out of known materials to known specs are, in point of fact, the same. The TDP defines the specifications, and those specifications have been proven to function reliably. If they call for 9130 or Carpenter 158 or 6061 or 7075 or 4150 or 4140 or if they call for balsa wood, those specifications, if met, produce a part that works.

Assembly is indeed where a great deal of voodoo comes into things. A BCG, for example, can be assembled or disassembled, with minimum handtools in a matter of minutes. There's a few pins, a couple of springs, some washers, and... well... that's just about it. Those are items that we know how to make, and have been making for years, and can now produce at tolerances far beyond what was available at the outset of the AR-15 pattern. There may be an art to staking, but there's a lot of mojo and voodoo and street magic used to describe hitting a punch with a hammer to displace some material. Maybe I am making light of the process because I've done it a few times, I don't know.

What "Good Enough" REALLY means is "Within Spec". If it's not within spec it's not good enough

This is, in fact, kind of my point.