K.L. Davis
07-29-06, 12:40
This word is getting thrown around a lot lately... let's take a look at what it implies and means.
First off, the term billet is most often improperly used... but like many things, it has been used for so long that the original meaning has become somewhat lost with the consumer market.
The term "billet" really refers to an rectagonal shape that is two squares in proportion -- it is commonly used to refer to the place where a military member sleeps and has been extended to include the general assignment that comes with the space.
However, the term is used in the world of making and machining metals -- first off, the term "billet construction" is more accurate as "machined from billet" or perhaps easier understood as "stock removal"... buts let's see how we arrive at that point.
Dealing primarily with aluminum, as that is mostly what we are talking about with the ARs -- nearly all refined metals start life as a casting... I know, we are all told that castings are evil and bad, but the fact is that nearly evey part of your rifle is made from a casting in one way or another.
The molten metals are poured into ingots, these are the first shapes and are generally very large -- even the aluminum ingots can weigh as much as 10 tons.
These ingots are rolled into plates... these are large, thick plates that can be further cut into more managible stock or billets. These billets are often then forged or extruded into more precise shapes for finish machining.
I am not going to get into the line between forging and extruding... but forging is primarily where a piece of metal is worked into a shape through hammering, upsetting, rolling or pressing -- in the most common (and simplest) terms, a piece of metal is placed in a shaped die and then hammered or forced to fill that shape through mechanical pressure. A good example of forging is the traditional hammer and anvil shaping of a knife blade.
During forging, the metal can be solid or heated to be softer... Casting is where the metal is fully molten and then "poured" into the shaped die, where it freely flows into the all the areas of the shape (hopefully).
Extrusions are where metals are forced under great pressure through a hole... and in turn take on the shape and dimensions of that hole -- a lot of you probably had a childhood toy that forced a type of dough into little star shaped ropes, via a sort of "toothpaste" squeezing.
Both extrusions and forgings require that a mold or die be made for the part... this can be an investment of a few thousand to several hundered thousand dollars -- for this reason, these techniques are traditionally associated with more "mass production" parts.
Machined from billet is simply where the raw billet is not preshaped, it is thrown on a table and machined into the final part... somewhat like the old joke that to make a life size statue of an elephant, you just get 200 tons of granite and chip away everything that does not look like an elephant.
Machined from billet parts are often associated with a high level of quality, as in the past the part would have had a machine operator oversee each part of the manufacture and actually turn the hand wheels on a knee-mill for each dimension -- today, CNC machines reduce that amount of time and labort involved greatly, but the technique is still the most labor intensive.
Which is better? It is impossible to make one statment as to which is better, it depends too much on what the purpose of the part is, the particulars of how it was actually made and finished and what the most sought after end trait is. Each technique has distinct advantages, or they would not still be around.
Forgings - These tend to offer good grain struture, but can require additional stress relieving and treating -- keep in mind that the part was literally beaten into submission. Forgings can be as strong or stronger than machined from billet parts, but may be subject to problems with dimensional stability.
Machined From Billet - These parts are often very precise in dimension and usually very strong and stable, the grain structure is uniform but does not "flow" with the shape of the design. Billet parts can see problems with fatigue cracking and are only as good as the person, machine and tooling that made them -- one common problem with billet parts is that the raw stock may not have had its grain structure aligned so as to compliment the finished part.
Extrusion - Extruded parts offer great finish shape and are often very stable. The amount of finish machine work can very greatly, based on the shape of the orignal extrusion... some are nearly finished on extrusion and others are little more than a "billet" that more closely matches the shape of the finished product -- extruded parts generally have proper grain direction and form.
That is a real quick and simple look at things... we could get all off in the weeds with the different aspects of cogging, blooms, scalping, sheeting, blah, blah, blah... I just wanted to try to clear up things a little.
Some terms get lost in the frey at times... I am sure that some of you remember buying "Mag" wheel for your van in the 70s (the love machine?), but they were actaully aluminum, not magnesium and we all knew that -- just a case of term having been used incorrectly for so long that we all just accepted it for what it was -- marketing hype.
First off, the term billet is most often improperly used... but like many things, it has been used for so long that the original meaning has become somewhat lost with the consumer market.
The term "billet" really refers to an rectagonal shape that is two squares in proportion -- it is commonly used to refer to the place where a military member sleeps and has been extended to include the general assignment that comes with the space.
However, the term is used in the world of making and machining metals -- first off, the term "billet construction" is more accurate as "machined from billet" or perhaps easier understood as "stock removal"... buts let's see how we arrive at that point.
Dealing primarily with aluminum, as that is mostly what we are talking about with the ARs -- nearly all refined metals start life as a casting... I know, we are all told that castings are evil and bad, but the fact is that nearly evey part of your rifle is made from a casting in one way or another.
The molten metals are poured into ingots, these are the first shapes and are generally very large -- even the aluminum ingots can weigh as much as 10 tons.
These ingots are rolled into plates... these are large, thick plates that can be further cut into more managible stock or billets. These billets are often then forged or extruded into more precise shapes for finish machining.
I am not going to get into the line between forging and extruding... but forging is primarily where a piece of metal is worked into a shape through hammering, upsetting, rolling or pressing -- in the most common (and simplest) terms, a piece of metal is placed in a shaped die and then hammered or forced to fill that shape through mechanical pressure. A good example of forging is the traditional hammer and anvil shaping of a knife blade.
During forging, the metal can be solid or heated to be softer... Casting is where the metal is fully molten and then "poured" into the shaped die, where it freely flows into the all the areas of the shape (hopefully).
Extrusions are where metals are forced under great pressure through a hole... and in turn take on the shape and dimensions of that hole -- a lot of you probably had a childhood toy that forced a type of dough into little star shaped ropes, via a sort of "toothpaste" squeezing.
Both extrusions and forgings require that a mold or die be made for the part... this can be an investment of a few thousand to several hundered thousand dollars -- for this reason, these techniques are traditionally associated with more "mass production" parts.
Machined from billet is simply where the raw billet is not preshaped, it is thrown on a table and machined into the final part... somewhat like the old joke that to make a life size statue of an elephant, you just get 200 tons of granite and chip away everything that does not look like an elephant.
Machined from billet parts are often associated with a high level of quality, as in the past the part would have had a machine operator oversee each part of the manufacture and actually turn the hand wheels on a knee-mill for each dimension -- today, CNC machines reduce that amount of time and labort involved greatly, but the technique is still the most labor intensive.
Which is better? It is impossible to make one statment as to which is better, it depends too much on what the purpose of the part is, the particulars of how it was actually made and finished and what the most sought after end trait is. Each technique has distinct advantages, or they would not still be around.
Forgings - These tend to offer good grain struture, but can require additional stress relieving and treating -- keep in mind that the part was literally beaten into submission. Forgings can be as strong or stronger than machined from billet parts, but may be subject to problems with dimensional stability.
Machined From Billet - These parts are often very precise in dimension and usually very strong and stable, the grain structure is uniform but does not "flow" with the shape of the design. Billet parts can see problems with fatigue cracking and are only as good as the person, machine and tooling that made them -- one common problem with billet parts is that the raw stock may not have had its grain structure aligned so as to compliment the finished part.
Extrusion - Extruded parts offer great finish shape and are often very stable. The amount of finish machine work can very greatly, based on the shape of the orignal extrusion... some are nearly finished on extrusion and others are little more than a "billet" that more closely matches the shape of the finished product -- extruded parts generally have proper grain direction and form.
That is a real quick and simple look at things... we could get all off in the weeds with the different aspects of cogging, blooms, scalping, sheeting, blah, blah, blah... I just wanted to try to clear up things a little.
Some terms get lost in the frey at times... I am sure that some of you remember buying "Mag" wheel for your van in the 70s (the love machine?), but they were actaully aluminum, not magnesium and we all knew that -- just a case of term having been used incorrectly for so long that we all just accepted it for what it was -- marketing hype.