From: osan@cbnewsb.cb.att.com (Mr. X)
Newsgroups: rec.guns
Subject: Re: Metal Working
Date: 17 Dec 92 12:20:12 GMT
Organization: Twilight Zone


In article <9212152245.AA06808@spsgate.sps.mot.com> rigg@ssdt-tempe.sps.mot.com (Dana Rigg) writes:
#It would be most helpful if someone knowlegeable in metal working
#could briefly define the various methods used to make metal firearm
#parts.  I often see the terms "investment-casting", "forged",
#"stamped", ... when describing the quality of workmanship that goes
#into a gun.

	INVESTMENT CASTING:  A casting process by which a disposable pattern
	(usu. of special wax, but may be of nearly any combustible material)
	is encased ("invested") in a special plaster-like compound (e.g.
	Crystabolite), air evacuated, and allowed to set up within a stainless
	steel cylindrical flask.  The mold/pattern is then put into a special
	oven which slowly heats said mold to relatively high temperatures
	(typically > 900 deg. F) over a 24-48 hour period.  At some point the
	pattern completely burns out of the investment, leaving a hollow
	cavity which is a VERY precise impression of the pattern (so much
	so that even finger prints will show up in the casting!).  The flask
	is removed from the oven and immediately placed into the casting
	apparatus which may operate on vacuum, cetrifugal force, or a 
	combination of both.  Molten metal is then poured into the apparatus
	and is forced into the hot cavity by the forementioned means.  While
	still hot, the flask is immersed into cool water.  The violent 
	contraction due to mass heat transfer causes the investment to break
	apart, leaving the flask and the new casting.  

	The advantages include precise forming of material, minimal waste, 
	little or not machine work necessary, the ability to produce shapes
	that cannot be produced by other means, and low cost in comparison to
	many other mathods of manufacture.

	The main disadvantage is a relatively poor grain structure and pattern
	in certain applications.  This can be mitigated by processes like
	solution heat treating, precipitation hardening, artificial aging,
	nitriding etc., but this will, naturally, increase expense and will
	probabl not produce a part as strong as might be made by other means.

	FORGING: A process where a piece of metal, either hot or cold, is
	forced into a steel cavity under high pressure, causing the metal to
	literally flow into the shape of said cavity.  This describes the bulk
	of forging processes.  One may also forge metal by hammering it into
	shape.  E.g., many hand made knives are forged by hand with a hammer
	on an anvil.  Gun barrels are often forged by piercing a piece of
	hot metal with a long steel rod called a mandrel, which is probably
	round in cross section.  The metal, so skewered, is hammered into
	a long shaft of essentially even proportions in cross section.  The
	mandrel is removed and the barrel blank is then ready for turning,
	grinding, boring, rifling, lapping and finishing.

	Main advantages to forging are high strength due to good flow of
	the metalic grain such that it follows the cross sectional outline
	of the forcing medium (hammer/anvil or die set).

	Main disadvantages are relatively high cost due to expensive dies
	and/or time intensive nature.  Also, forging places very real limits
	on the shape of the part to be made.  Forgings usually require
	comparatively large amounts of machining in order to bring the parts
	to final shape.  Tolerances are very low.

	METAL STAMPING/FORMING: A process where sheet metal (generally defined
	as any rolled flat metal stock less than 1/4 inch in thickness) is
	first blanked and then formed to shape.  Blanking involves dies that
	have a male and female part, the male fitting exactly into the cavity
	of the female (how sexy :)).  sheet metal stock is placed between the
	dies which are forced together.  The shearing action punches out a
	metal part in the profile of the female cavity.  The blanked parts
	may then be shaped by any of several methods into their final form.
	Such parts are said to be stamped or formed.  Parts made from sheets
	thicker than about 3/32" (my best guess) are more often referred to as
	having been formed, rather than stamped, but the terms are more or
	less interchangeable.

	Advantages include low expense for high volume production, potentially
	highly efficient use of materials,  high strength, particularly in
	appropriate applications, ease and speed of fabrication.  When combined
	with other methods, stamping offers extremely high versatility. E.g.,
	the slides on the P22x SIG combat pistols are a combination of formed
	and forged parts.  The slide consists of three pieces, two which are 
	welded together, and the third (the bolt) which is pinned to the rest
	of the structure.  What you get is an extremely strong part that 
	requires a very small amount of machining, yet is accurately made and
	comparatively very easy to fabricate.
	
	The main disadvantage that I see is inapplicability to low volume
	production.  Dies are very expensive (generally $20K and up for
	any non-trivial piece) and their cost can rarely be justified for
	runs of, say, less than several tens of thousands or even hundreds 
	of thousands of parts)
#
#How do you tell the difference between forged, cast, and stamped parts when 
#looking at a gun?

	You have to know what to look for.  Experience dealing with these types
	of things is really the only sure way to learn, but there are a few 
	hints that may help, though not always so.  Castings will often be left
	with raw surfaces where no metal to metal contact is to made.  The
	Ruger P85/89/90 series have investment cast frames and slides.  Look
	at the surface of the side of the slide and you will see the bare
	casting, essentially as it came out of the mold.  Politics aside,
	Ruger appears to have taken the science of high volume precision
	investment casting to new heights.  I am truly impressed with their
	operation.  As to how strong the castings are, I have no data, though
	I trust they are probably quite strong, especially for the given
	application.  

	On the other hand, a forged slide that left bare surfaces untouched by 
	machine tools could well look very similar to that of the Ruger.  What 
	would probably give it away in this case would be the amount of 
	machining.  Since the hollow inner surfaces of the slide could not
	likely be forged as finely as the castings are, the entire inside
	of said slide would probably be at least rough machined.  A forged
	slide will almost certainly be "heavier" than a casting or a stamping.
	Many hollow areas in a casting will almost always be fulkl of metal
	in a forging since it is too expensive to machine the material away 
	with small, specialized tools.

	The formed SIG slide is a dead giveaway at the back.  If you look at
	the opening around the firing pin, you will notice that the shear
	marks from the blanking dies have not been taken out.  The fact that
	the bolt is a separate piece, pinned into place is another tip off that
	either this is a formed piece or a casting (which fooled me when I
	all to casually looked at my P220 when I first got it).  Chances are
	slim that a casting would be designed like this, but this one cannot
	know if they are not familiar with processes and machine design.

	There are many other processes, such as sintering, milling, turning
	etc.  To learn about these, I would suggest getting a machine shop
	text, or perhaps better yet, a freshman/sophomore text for mechanical
	engineering students for introductory courses (usu. called something
	along the lines of "Materials and Processes" or some such).  Such
	texts are great for broad learning of the basics of materials science,
	production methods and machine design.

	I hope I have been able to answer your questions to you satisfaction.

	-Andy V.
		avs@somerset.att.com