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What is a Billet and Why an airgun doesn’t use it

August 23 2005 at 2:20 PM
Quackenbush  (Login DAQ)
YC

A billet is a semi-finished piece of metal and they’re not used in an airgun.

Airguns are made of finished metals; bar stock (round, square or rectangular) and tubing. All of which has been cold finished, or cold drawn, so as to have a good surface finish.

When molten metal is poured into an ingot mold, the metal is allowed to cool until it solidifies. While it is still hot and “plastic” it is rolled to shape. The rolling is just like using a rolling pin on biscuit dough. The roller presses down and smoothes the surface. On the hot ingot it is done from all sides to make a square or rectangular, in cross section, piece of metal with rounded corners, called a billet.

A billet is a semi-finished piece of metal that has been roughly worked to shape and will go on to other finishing processes. The billet will be re-rolled while hot to further bring it to the desired finished size. This hot rolling has “scale”. The scale is the decarborized metal that the carbon has been burned out of. While the metal is red hot, its surface is in contact with the atmosphere, which burns the surface. To get a good surface on the metal the final processes are done cold. When done cold, there is no scale produced and the surface finish is better. But cold rolling takes more time to do because you have to use more passes, through the rolls, to size the metal. The rolls, used for cold rolling, have to be run at a much higher pressure to form the metal in the cold state and the rolls have to be maintained with a high polish, so as to leave a good finish on the metal. The cold finishing of the metal is the expensive part to do, but the bar stock produced has a good surface, is dimensionally accurate and, on square and rectangular stock, has square corners.

Billets, the unfinished metal bars, are used where the manufacturers don’t want to pay the extra money for the finishing of the metal because it would not add value to the finished product. For example: forgers use billets because every surface of the metal is reshaped in the forging process. A race car crankshaft can be machined out of a billet because the surface finish of the billet has nothing to do with the finished product, so why pay the extra money for a good surface finish? Custom aluminum wheels are made of billet because the finished product in no way benefits from starting with good surface finish metal. So why increase the cost of the finished product by wrongly specifying that a cold finished metal would be your working stock?

A breech maker tried to impress people by saying that the breech was made from a billet. The breech was made from bar stock and it’s a mistake to believe that making a breech from a roughly formed piece of metal would make a better breech. But that’s what people claiming to make things out of billets are telling you. When I have students in the shop, one of the things I insist on is that they learn “the language”, to know the names of things and their use. If one doesn’t, they’re going to use the wrong terms and “impress” people with how much they don’t know.

Other terms: Bloom, slab, sheet bar are similar to a billet in that they are semi-finished mill products of square or rectangular cross section, hot rolled from ingots, but not finished rolled so they have rounded corners. The difference between them is their cross section area and their intended use.

 
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