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.
Modern Machining Techniques
Machining techniques for the removal of material have evolved into specialized techniques allowing for processes that would be technically difficult or too expensive to achieve using traditional methods.
Some of the newest techniques:
- Electrical discharge machining (EDM) is a process in which the tool (an electrode) and the workpiece are oppositely charged with a nonconductive dielectric fluid between them. A spark arcs across the fluid from the electrode to the workpiece, melting an area of the surface of the workpiece, which is then flushed away in the fluid.
- Laser machining involves the optical focusing of a laser beam, producing high temperatures, vaporizing the metal to be removed.
- Electrochemical machining (ECM) is similar to traditional electroplating, but in reverse. The process uses an electrically conductive fluid and direct current to remove material.
- Chemical machining or etching involves the masking or protecting of areas not to be machined, then exposing the remaining areas to a chemical etchant.
- Hydrodynamic machining uses a water jet combined with abrasives that can cut complex patterns without the generation of heat.
- Ultrasonic machining is a technique that combines ultrasonic sound waves with a slurry of abrasive material for the controlled machining of a surface.
- Electron beam machining (EBM) is a technique used in the aerospace industry to cut any metal or nonmetal with a high level of precision. It requires a vacuum and shielding due to the x-rays produced by the electron beam.
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