Metal cutting is a kind of machining method that uses the cutter to cut the metal blank layer by layer and make the workpiece get the required shape, size and surface roughness. Metal cutting includes two methods of benchwork and machining - benchwork is a machining method that workers use hand tools to cut. It is flexible and convenient to operate and widely used in assembly and repair. Machining is accomplished by means of machine tools, including turning, planing, milling, drilling and grinding.
1) Turning: turning is the technological process of machining workpiece with turning tool on lathe. The lathe is suitable for cutting all kinds of rotating surfaces, such as inner and outer cylinder or conical surface, and can also turn the end face. Many axle parts and gear blanks of automobile are processed on lathe.
2) Planing: planing is a technological process in which a planer uses a planer to process workpieces. Planer is suitable for processing horizontal plane, vertical plane, inclined plane and groove, etc. The matching surfaces of the cylinder block and the cylinder head, the transmission case and the cover are all machined by planer.
3) Milling: milling is the process of machining workpiece with milling cutter on milling machine. Milling machine can process bevel, groove, even gear and curved surface and other old milling can be widely used to process various automotive parts. The cold stamping dies of automobile body are all milled. The CNC milling machine controlled by computer can process the workpiece with very complex shape, which is the main machine tool for modern machining.
4) Drilling and boring: drilling and boring are the main cutting methods for machining holes.
5) Grinding: grinding is the process of machining workpiece with grinding wheel on grinder. Grinding is a kind of finish machining method, which can obtain the workpiece with high precision and roughness, and can grind the workpiece with high hardness. Some automobile parts after heat treatment are finished by grinding machine.
Heat treatment is a method to change the structure of solid steel by reheating, heat preservation or cooling, so as to meet the use requirements or process requirements of parts. With the increase of heating temperature, holding time and cooling speed, different microstructure changes can be produced. When a blacksmith immerses the heated steel into water for rapid cooling (known as quenching by experts), the hardness of the steel can be improved. This is an example of heat treatment.
The heat treatment process includes annealing, normalizing, quenching and tempering. Annealing is to heat the steel piece, keep it warm for a certain time, and then cool it slowly together with the furnace, so as to obtain fine and uniform structure, reduce the hardness, and facilitate the cutting process.
Normalizing is to heat the steel, take it out from the furnace after heat preservation, and then cool it in air, which is suitable for refining low carbon steel. Quenching is to heat the steel parts and cool them quickly in water or oil after heat preservation, so as to improve the hardness.
Tempering is usually the subsequent process of quenching. After quenching, the steel parts are reheated and cooled after heat preservation to stabilize the structure and eliminate brittleness.
For many auto parts, it is necessary to use heat treatment processes such as surface high frequency quenching, carburizing and cyaniding to keep the toughness of the core and change the structure of the surface to improve the hardness.