PRESSED METAL, a broad term which includes that class of merchandise made by the process of bending, shaping and form ing sheets, strips, plates or bars of metal in either hydraulic or mechanically-driven presses. Steel so treated is referred to as "pressed steel" or "stampings." While brass, aluminium, zinc and various alloys are all formed in substantially the same manner as steel and with similar equipment, this article will be largely confined to pressed steel.
The greater portion of the work is done cold and pressed steel is generally assumed to mean the cold forming of sheet steel in power-driven presses. Castings and forgings constitute the classes of material that are most generally replaced by pressed steel. Pressed steel is different from both castings and forgings in that, while either a casting or a forging may have a solid section of varying degrees of thicknesses, except in rare cases, pressed steel is a flat or hollow form, such as a shell, the walls of which are of equal thickness throughout. While by special processes and by specially constructed tools it is possible to make a wall of varying thickness, it generally follows that if the sheet of steel which is used as raw material is in. thick, the walls of the finished pressed steel form will also be substantially in. thick. Cold forming of steel first became industrially important through its adoption by bicycle manufacturers. When bicycles became popular, pressed steel replaced forgings and castings because of the need of pro ducing a lighter vehicle at a lower cost.
The next great advance in the use of pressed steel was by the automobile manufacturer. In 1929, apart from units such as the engine, front axle and a few minor parts, most automobiles were made almost entirely of pressed steel. (See MOTOR CAR BODIES.) Other manufacturers, perceiving the advantages of pressed metal over castings and forgings, changed their methods so that pressed steel now goes into many manufactured products. It is impossible here to give a complete list but a few of the better known products are beds, alarm clocks, cooking utensils, gas and electric ranges, lighting fixtures, electrical conduits, switch boxes, push button plates, office furniture, filing cabinets, concrete forms, car wheels, hot water heaters, gasolene pumps, washing machines and type writers. Wherever a large number of individual units are de
manded for mass production, pressed metal has, in the majority of cases, been utilized.
The equipment used for the manufacture of pressed steel carries the general term of presses. There are two general classes known as hydraulic and mechanically-driven. The majority, however, are belt-driven power presses of varying size. The press furnishes the power and the means of holding the "tools" or punches and dies that do the actual forming of the metal. These "tools" consist of an upper or male section, which is known as a punch, and the lower or female section, which is known as a die. A piece of steel is held over the lower section (the die) and the upper section (the punch) is brought down on this by the power press, thus cold forming the steel into the desired shape. The majority of pressed steel parts are made by several successive operations. A certain shape of flat blank receives the first operation in one set of tools, it is then passed along to a second press carrying a second set of tools and is further formed or drawn, and so on through various stages.
It is usually necessary to heat-treat the steel between these various operations as cold pressing makes steel harder and tougher and without heat-treating the steel would not withstand the strain to which it is subjected. Soft, well-annealed steel is ductile, while steel that has been worked in the tools is tough and hard. The tensile strength of a piece of steel can be increased as much as 3o% by a single drawing operation. Holes are punched and edges trimmed smooth and round by specially designed tools. While forgings are heavy and castings are both brittle and heavy, a pressed steel part, when properly manufactured, is both lighter and stronger, has a smoother surface and requires less finishing or machining. It has replaced wood in many cases.