Sheet-Metal Stamping

the production of semifinished articles, parts, and finished products from sheet stock by forming under pressure.

Sheet-metal stamping originated many centuries ago as a method of producing household utensils, ornaments, and weapons of uniform shape and size. The technology and manufacturing methods of sheet-metal stamping advanced in the second half of the 19th century in connection with the mass production of parts of weapons, vessels (kerosine lamps), and other items. Improvements in technology resulted in a marked increase in the rate of production and the operating efficiency. In the early 20th century, sheet-metal stamping played a prominent role in the automotive industry (especially in the production of motor-vehicle bodies); in the 1930’s, in aviation and the shipbuilding and appliance industries; and in the 1950’s, in the aerospace industry.

Parts produced by sheet-metal stamping have high strength relative to weight and are distinguished by the efficiency of their shape. Through the use of highly plastic materials, sheet-metal stamping makes possible the production of both thin-walled parts of complex shape and large durable parts that cannot be produced by other processes (for example, the process can be used to produce hands for wristwatches or truck spars 5 m long). Sheet-metal stamping of parts, in conjunction with welding, makes possible the production of one-piece assemblies of virtually unlimited size (in railroad-car construction and shipbuilding).

Ribbon, strip, or sheet metal is used as the stock in sheet-metal stamping; the stamping is usually cold. If the material being worked is of low plasticity, or if the power of the equipment is insufficient, hot stamping is used. The main operations in sheet-metal stamping are separation and forming. As a result of the separation process, the part to be formed is separated during movement of the material according to a preset pattern. The separation process includes the cutting-off, punching, piercing, trimming, notching, deburring, and cleaning of the part. In the forming operations, the shape and dimensions of the part are changed. The material is shaped but does not fracture. Among the forming operations are bending, twining, winding, expansion, reduction, beading, drawing, and molding of relief.

Sheet-metal stamping is done in dies that usually consist of stationary and movable halves containing the working parts (the matrix mold and the punch). A part is formed from the sheet metal when the two halves come together. The halves of the die are secured in the press. The stationary half is on the table, and the movable half is in the slide block (the working mechanism). The working parts of the die are made from tool steel. In small series production of parts from aluminum or other soft materials, tool steel is replaced by various other materials (plastics, pressed wood, and so on).

To reduce production costs in small-scale production of parts from sheet metal (particularly large parts), the matrix is made from iron, steel, or concrete, and the punch is replaced by water or another fluid in a container located on the matrix above the sheet stock. Detonation of a powder charge in the water creates pressure on the sheet stock and shapes it according to the matrix. This is called explosive forming.

Another method of stamping, in which the action of an electrical discharge on water is transmitted to the stock (electrohy-draulic forming), is also used. The stock may be stamped in a die that has one working part (matrix or punch). In this case the discharge of high-voltage capacitors, which results in a strong and rapidly changing magnetic field, is used to create pressure on the strip (the method of electromagnetic forming).

The precision of parts manufactured by sheet-metal stamping (in most operations) is of the third or fourth class (Soviet standard). Some operations, such as deburring and special methods of punching, piercing, drawing with ironing, and calibration, provide precision of the second class. In cold sheet-metal stamping the surface quality of the part is maintained in most cases. Thus, in stamping from cold-rolled materials the surface roughness of the product is of the sixth to eighth class. The specific strength characteristic of products produced from rolled metal is not decreased by stamping but rather increases during certain forming processes as a result of the strengthening of the material that accompanies the processes.

The cost of parts is determined by the cost of the material used and the expenditures for production. In sheet-metal stamping, material cost accounts for 80–85 percent of the cost of the part; production costs account for the remaining 15–20 percent. The variety of stamping methods, the use of dies of various designs (specialized and all-purpose), and the use of various materials to produce the dies make profitable the production of the same articles by sheet-metal stamping in any quantity.

Sheet metal stamping is a highly productive process. For example, presses with a force of 1 meganewton (100 tons-force) and manual feed of the strip stock have a production rate of 600–800 units per hour. With mechanical feed, the rate increases to 3,000-4,000 or more.

Sheet-metal stamping makes possible the reduction of complex production procedures to simpler processes (the blows of a press), produces uniform precision of the stamped parts, and makes possible the production of parts with a small number of operations and changes and with low production costs. In some cases, the material not only retains but increases its strength. In addition, the cost of stamping equipment is low. Parts and finished products produced by methods of sheet-metal stamping are used in various branches of the national economy, such as instrument-making, shipbuilding, motor-vehicle and aircraft construction, and the clock and watch industry.

The further improvement of the sheet-metal stamping process requires greater mechanization and automation of small series production, the use of automatic lines of presses in mass production, an increase in the durability and ease of replacement of dies, in the effective use of stamping equipment, and the speed of forming operations, and a reduction in the consumption of metal.