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Posted: 12/8/2005 6:30:01 PM EDT
Can some one please explain these four? I know just enough to be dangerous. ;)

More specifically, how they related to firearms and strength. A web site where i could read would be great.

I have been googling, but no web site really breaks it down.

Here is how this started... in the Rob Arms XCR thread, it has been noted that the XCR is made of extrusions... and so is the FN Scar as well. Some are not happy with this.

But then i thought... "isnt the AK 47 made of Stampings, and isnt that weapon the pinacle of reliability and toughness?"

thank you.
Link Posted: 12/8/2005 6:43:28 PM EDT
In a nutshell:

Forging refers to the process where a piece of metal (typically heated, but can be done to a cold metal too) is manipulated into a particular shape by way of hammers 'smashing' it. This process is supposed to make the metal more dense.

Casting involves melting metal and then pouring it into a mold. Compared to a forging, castings typically aren't as strong becaus porosity can be introduced into the metal. Manufacturing something via casting is typically less expensive than forging.

Extrusion refers to heated metal being pushed through a die. The aluminum railings for windows sliding doors are frequently made using this process.

Stamping describes a process where a flat sheet of metal is pressed into a particular shape.

As far as the AK goes, the early ones were made from machined billet.

Hope this helps.
Link Posted: 12/8/2005 6:44:15 PM EDT
Forging is the strongest. It is made by pounding steel or aluminum into the rough shape of the product and then finishing it with machine tools.

Extrusions are made by passing hot, ductile material, in this case aluminum, through an opening that will give it its shape. Bricks are also made this way.

Stampings are made by taking sheet metal and pounding it against a form, then bending it to its final shape. I don't belive the AK needs to be shaped before it's bent, but I know the HK does.

Castings are made by pouring molten metal into a mold. This doesn't work well with aluminum, although it makes OK pistol frames if you use steel.
Link Posted: 12/8/2005 6:47:32 PM EDT
the AK comment has not so much to do witht the the tensile and yeild strengths of the steel used, which is inherently stronger that aluminum anyway, but due the fact that the AK is manufactured with lower tolerances , so more stuff can go wrong before it totally fails.
Link Posted: 12/8/2005 6:48:55 PM EDT
modern casting technology is such that it is nearly competitive with fogings and still far cheaper.
Link Posted: 12/8/2005 6:49:04 PM EDT
A little OT but what the hell...

Was wathing history channel or something on plumbing I think...or copper....anyway...they were making copper plumbing pipes and the way they did it was drop a red hot ingot of copper into a a very long thin "bath" and by some mechanism the ingot was pulled to like 20-30 feet...right then you had your copper pipe...was pretty cool....

thats extrusion.
Link Posted: 12/8/2005 6:51:49 PM EDT
[Last Edit: 12/8/2005 6:52:09 PM EDT by Combat_Jack]
It seems to me that using lower tolerances would only make a gun reliable if it were designed to be loose. If I had a 1.0 inch pin to fit a 1.0 inch hole, and the tolerances on each were, +/-.05 I may or may not run into some trouble, whereas a tight part on an AK would be like throwing a hot dog down a hallway.
Link Posted: 12/8/2005 6:53:29 PM EDT

Originally Posted By lokt:
modern casting technology is such that it is nearly competitive with fogings and still far cheaper.



While it can't be JUST as good, there is no downside to using it in applications like 1911 frames. It still sucks for aluminum recievers, IMO.
Link Posted: 12/8/2005 6:54:11 PM EDT
Castings are made by pouring liquid metal in a mold. These can be very strong, especially when produced with advanced techniques, but are generally relatively low in the possible strength for the alloy being cast. Tooling costs are generally low.

Extrusions are sort of a continous casting process. For firearms these are probably all aluminum extrusions. A die is made with the cross section you want and the metal is forced through it to make a continuous length, then cut into pieces and maybe finish machined. These parts are often very low strength, but can be pretty good, for aluminum. This is how storm door frames are made, for instance. This is usually cheap for large quantity parts.

Stamped parts are punched out of sheet metal. Parts are generally just two dimensional, they may have simple features like tabs. Tooling is expensive, but parts are really cheap in quantity.

Forgings are essentially three dimensional stampings. Tooling and production costs are high, but the parts are usually very strong and high quality.
The steering parts of your car are probably forged, because it's a big problem if they fail.
Link Posted: 12/8/2005 8:17:24 PM EDT
[Last Edit: 12/8/2005 8:52:45 PM EDT by AeroE]
There is an even simpler distinction.

Extrusions, forgings, and stamped sheet metal are all wrought materials, meaning they have been worked mechanically to shape them which usually brings along improved mechanical properties.

Castings are not wrought, they are typically used without additional mechanical forming (squishing, pounding, or rolling), hence their grain structure hasn't been improved and their mechanical properties (strength, ductility, and so on) suffer.

Castings have larger variations in grain stucture and it's disorganized, and their are more likely to be other flaws such as voids, all factors that cause castings to have mechanical performance worse than wrought materials.

Modern castings using HIP processes, or more exotic processes such as centrifugal casting gain some performance improvement, but they still can't compete with wrought materials in high performance structural applications.

Castings are teh suxxor when your life is on the line. They are okay for cheap crap used around the house.
Link Posted: 12/8/2005 8:46:51 PM EDT

Originally Posted By AeroE:
Castings are teh suxxor when your life is on the line. Theyare okay for cheap crap used around the house.



Cast explosives are OK... (thinking Claymore)


j/k
Link Posted: 12/8/2005 8:53:38 PM EDT

Originally Posted By Tango7:

Originally Posted By AeroE:
Castings are teh suxxor when your life is on the line. Theyare okay for cheap crap used around the house.



Cast explosives are OK... (thinking Claymore)


j/k



That all depends on your location in that case.
Link Posted: 12/17/2005 11:42:04 AM EDT

Originally Posted By lokt:
modern casting technology is such that it is nearly competitive with fogings and still far cheaper.



You have to have the process tightly controlled though - or you will wind up with voids, porosity, and generally shitty castings.
Link Posted: 12/18/2005 6:05:29 PM EDT

Originally Posted By jchewie:

Originally Posted By lokt:
modern casting technology is such that it is nearly competitive with fogings and still far cheaper.



You have to have the process tightly controlled though - or you will wind up with voids, porosity, and generally shitty castings.



Link Posted: 12/18/2005 6:38:44 PM EDT
Something missed with the AK (and G3) is that the stamped types still have a forged and machined trunion that holds the barrel, and has the recesses for the bolt lugs. This trunion is riveted to the stamped body of the receiver. Therefore, this trunion takes the extreme pressure of firing. The stamped body of the receiver simply guides the bolt/carrier and holds the mechanics. Thus, the stamped parts never see substantial 'pressure'.

Also, the AK does not have loose tolerances as a rule, but designed in loose clearances as 'tolerance' implies error. Headspacing requriements are no less specific than any other type. This being said, the loose clearances do allow for a degree of manufacturing error that would cripple other designs. I suspect that if one were to carefully evaluate a Polish or Soviet AK, one would find little 'out of spec'.
Link Posted: 12/18/2005 6:43:03 PM EDT

Originally Posted By Combat_Jack:
Forging is the strongest. It is made by pounding steel or aluminum into the rough shape of the product and then finishing it with machine tools.



I've seen a lot of forgings that would argue with you.

Generally speaking, you are absolutely right.

But you better hope your engineer did his homework, because parts can be forged
too much and become very weak compared to other options.

Link Posted: 12/18/2005 7:07:39 PM EDT
just a note: forging does not make the metal denser. However, as mentioned, the mechanical properties are very much inproved over casting.

Applications that require high strength and good fatigue resistance are forged (generally speaking). Aircraft engine parts that rotate are forged (blades, rotating seals, and disks), although some turbine blades are single crystal "grown".

If the application involves having the part spend the majority of its life in a highly stressed condition, with stresses as a large percentage of yield, then forging is the only way to go. Cast parts are (again, generally) susceptible to failure by cracking, with the crack unzipping the part leading to catastophic failure.

Don't mean to scare anyone, but jet engines fly around all the time with cracks in the critical parts. However, the cracks are small, and their growth rate is understood, and these parts are "life limited" in that they are replaced on a set schedule. The air force gets a lot longer life out of jet engines by having spent the money to understand when particular parts are likely to develope cracks, and how long the cracks can be allowed to grow to and still provide safe operation.

Cast parts are used where the stresses (or cycles) are low because they can be substantially cheaper to manufacture than forgings, due to casting close to final shape, reducing or even eliminating the machine-to-finish-shape steps.

For an AR lower, cast is problably okay, the part sees little stress (I mean, it IS aluminum in the first place, right?), but most people prefer forged for the sense of (maybe misplaced) security.

Think about it, they make the lowers out of carbon fiber too, right? And the matrix there is epoxy, which is about as brittle as it comes. This is possible because the stress (and the zero-to-max stress cycles) for these parts does not come close to the expected fatigue failure life for the design.
Link Posted: 12/18/2005 7:35:54 PM EDT
Link Posted: 12/18/2005 7:37:43 PM EDT

Originally Posted By home_with_kids:

Think about it, they make the lowers out of carbon fiber too, right? And the matrix there is epoxy, which is about as brittle as it comes. This is possible because the stress (and the zero-to-max stress cycles) for these parts does not come close to the expected fatigue failure life for the design.



The reinforced plastic AR lowers I have handled are molded from short fibers in nylon matrix, and I'm not so sure the fibers are carbon, not that it matters with this construction. Personally, I wouldn't buy one because of the low bearing strength at the pins.
Link Posted: 12/18/2005 7:44:12 PM EDT
.
Link Posted: 12/18/2005 7:45:05 PM EDT
AeroE said:
The reinforced plastic AR lowers I have handled are molded from short fibers in nylon matrix, and I'm not so sure the fibers are carbon, not that it matters with this construction. Personally, I wouldn't buy one because of the low bearing strength at the pins.

I did not know this (the nylon matrix). I just assumed it was a standard graphite/epoxy. I too am leery of applications with composites that have a lot of concentrated load points.

I wonder how light you could make an aluminum upper/lower with a serious wieght reduction program based on stress analysis. I would bet it would come pretty dang close to the weight savings of the "carbon" design.

Have PO, will analyse......
Link Posted: 12/18/2005 8:08:23 PM EDT
[Last Edit: 12/18/2005 8:11:24 PM EDT by Willis1fit]
most of these guys are pretty much right. Not trying to piss any of you off but it seems like home_with_kids has the best idea of what's going on. I am a mechanical engineer and I just got through taking the extra classes to get my materials science certificate. I also just got out of a graduate course in machining and forming which dealt entirely with the properties and pros and cons of operations such as forging and extruding. As to those who you have heard with concerns about the extruded parts on the weapon. These extrusions should be more than adequate to handle any stresses involved with any of those weapons. 2 factors influence extrusion strength. normal and planar anisentropy. This is basically the inconsistencies or "degree of randomness" of the grain orientation. All of these properties are taken into great consideration in any extrusion operation to ensure that they will handle the stresses that will be applied without crack propogation.

second....to the question about stamped ak's
while it is true that stamping is not the strongest metal forming process it is by far strong enough. If you look at the design of the receiver its under no real stress, especially no real stress in tension. One of the first things they teach us is design and build so that the material is in compression because it is when it is in tension that cracks form and subsequently lead to failure.

basically the question is not whether they have been made by the "strongest" process but instead has it been made by a "strong enough" method.

Probably more than you wanted to know. Now that I look at it, I hope it didn't sound snooty

Willis


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