Metal forging involves using great pressure and/or heat to squeeze metal into a particular shape in a die. Typically forged parts will require additional finishing operations. There is a significant possibility for the influence of subclass characteristics in forging operations not followed by finishing operations.
Typical variations of metal forging include the following:
- Cold forging or cold heading, in which heavy force is applied from the outside to a workpiece contained in a die with no heat involved. The metal is forced to fill the volume of the die and assume its shape. Bolts and rivets may assume their rough dimensions using this process.
- Hot heading or upset forging, which is similar to cold heading except that the workpiece is heated.
- Drop forging, which is very similar to cold forging except that the metal is heated prior to the forging operation and hammered into a die.
- Casting, which is a form of metal shaping done by pouring molten metal into a mold. It is possible that mold marks may be repeated in each casting as subclass characteristics if each mold is made from an original master mold or if the same mold is used repeatedly.
Typical techniques include these:
- Sand casting, which is a traditional technique in which a wooden form is embedded in sand in a two-part frame (a cope and drag). The cope and drag are carefully separated, the form removed, and the two sections rejoined. Molten metal is poured into the now hollow space through a sprue hole and the hardened metal shape is later removed.
- Investment casting (lost wax process, precision casting), in which a heat resistant ceramic slurry is coated over an expendable form made of some substance that will melt at low temperatures (e.g., wax, plastic). Once the ceramic coating sets at room temperature, the interior form is melted away, leaving a mold that will not melt when molten metal is poured into the mold.
- Blanking, which is cutting a large section of sheet metal stock into smaller pieces for another operation, such as drawing.
- Shearing, which is a form of metal separation accomplished partially by the slicing action of a dropping blade, followed by a clean fracture along the cut.
- Stamping, which is a technique for transforming sheet metal into a three-dimensional object in a large machine press.
- Drawing (deep and shallow), in which a blank of sheet metal is restrained at the edges and the middle section is forced by a punch into a die to stretch the metal into a shape.
- Swaging, which is a process for reducing the diameter of a tube, rod, or other object (e.g., a cartridge case in the manufacture of ammunition). The object is placed inside a die that exerts exterior radial force on the object, gradually compressing it and reducing its diameter. It is often performed using a steel mandrel to control shape and dimensions of the result.
- Extrusion, in which long straight metal parts can be produced or reduced in size by passing through successive dies. Wire is an excellent example.