From scratch, understand the processing technology of seven auto parts

A car is composed of more than 10000 parts and components. Each part must be processed and formed through different processes before it can be assembled on the car. Today, let’s take a look at the processing technology of seven auto parts.

1. About casting

Casting is a production method in which molten metal is poured into the mold cavity, cooled and solidified to obtain products.

In the manufacturing process of various parts, such as the cylinder body and the brake drum, many parts are made of cast iron, accounting for about 10% of the total weight of the vehicle body and the rear axle.

Sand mold is usually used to manufacture iron castings. The raw material of sand mold is mainly sand, which is mixed with binder and water. The sand mold material must have a certain bonding strength so that it can be molded into the required shape and resist the scouring of high-temperature molten iron without collapse. In order to mold a cavity in accordance with the shape of the casting in the sand mold, a model must be made of wood, which is called wood model. After the hot molten iron is cooled, the volume will be reduced. Therefore, the size of the wooden mold needs to be increased according to the shrinkage rate on the basis of the original size of the casting, and the surface to be machined needs to be thickened accordingly.

Hollow castings need to be made into sand cores and corresponding core wood molds (core boxes). With a wooden mold, you can turn the cavity sand mold (casting is also known as “sanding”). When making sand mold, we should consider how to separate the upper and lower sand boxes to take out the wooden mold, and also consider where the molten iron flows in and how to fill the cavity to obtain high-quality castings. After the sand mold is made, it can be poured, that is, molten iron can be poured into the cavity of the sand mold. During pouring, the temperature of molten iron is 1250-1350 degrees, and the temperature is higher during smelting.

2. About forging

Forging is widely used in automobile manufacturing. Forging is divided into free forging and model forging. Free forging is a processing method in which a metal blank is placed on an anvil to bear impact or pressure and formed (it is called “forging” in the market). The blanks of automobile gears and shafts are processed by free forging.

Model forging is a processing method in which the metal blank is placed in the die bore of the forging die and formed under impact or pressure. Model forging is a bit like the process in which dough is pressed into the shape of biscuits in the mold. Compared with free forging, the shape of the workpiece manufactured by die forging is more complex and the size is more accurate. Typical examples of automobile die forgings are: engine connecting rod and crankshaft, automobile front axle, steering knuckle, etc.

3. About cold stamping

Cold stamping or sheet metal stamping is a processing method in which sheet metal is cut or formed under pressure in the die.

Daily necessities, such as aluminum pot, lunch box and washbasin, are made by cold stamping. For example, to make a lunch box, you first need to cut out a rectangular blank with four rounded corners (experts call it “blanking”), and then press the blank into the concave die with a punch (experts call it “drawing”). In the drawing process, the flat sheet becomes box shaped, its four sides are bent vertically upward, and the materials at the four corners are piled up and wrinkles can be seen.

Auto parts processed by cold stamping include engine oil pan, brake base plate, auto frame and most body parts. These parts are generally formed through blanking, punching, deep drawing, bending, flanging, trimming and other processes. In order to manufacture cold stamping parts, stamping dies must be prepared. The die is usually divided into two pieces, one of which is installed above the press and can slide up and down, and the other is installed below the press and fixed. During production, the blank is placed between two stamping dies. When the upper and lower dies are closed, the stamping process is completed. Stamping processing has high productivity and can manufacture parts with complex shape and high precision.

4. About welding

Welding is a processing method of joining two pieces of metal by local heating or simultaneous heating and pressurization.

We often see workers holding masks in one hand and welding tongs and electrodes connected with wires in the other hand. The welding method is called manual arc welding, which uses the high temperature generated by arc discharge to melt the electrodes and weldments and make them join.

Manual arc welding is not widely used in automobile manufacturing. Spot welding is the most widely used in automobile body manufacturing. Spot welding is suitable for welding thin steel plates. During operation, two electrodes apply pressure to the two steel plates to make them stick and merge. At the same time, the bonding point (a circle with a diameter of 5-6 Ω) is heated and melted by current, so as to be firmly bonded. When two body parts are welded, one point shall be welded every 50-100 ° on the edge to form discontinuous multi-point connection between the two parts. Welding the whole car body usually requires thousands of welding points. The strength requirements of the solder joint are very high. Each solder joint can bear 5kn tensile force, and even tear the steel plate, but the solder joint cannot be separated.

The common gas welding in the repair workshop is to use acetylene combustion and oxygen combustion to produce high-temperature flame, so that the welding rod and weldment are melted and joined. This high-temperature flame can also be used to cut the metal, which is called gas cutting. The application of gas welding and gas cutting is more flexible, but the heat affected zone of gas welding is large, resulting in deformation, metallographic structure change and performance degradation of weldments. Therefore, gas welding is rarely used in automobile manufacturing.

5. About metal cutting

Metal cutting is to use cutting tools to cut metal blanks layer by layer; A machining method that enables the workpiece to obtain the required shape, size and surface roughness. Metal cutting includes fitter and machining – fitter is a machining method in which workers cut with hand tools. It is flexible and convenient to operate and is 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. Lathe is suitable for cutting various rotating surfaces, such as inner and outer cylindrical or conical surfaces, and can also turn end faces. Many shaft parts and gear blanks of automobiles are processed on lathes.

2) Planing: planing is the technological process of machining workpieces with planer knife in planer. The planer is suitable for processing horizontal plane, vertical plane, inclined plane and groove. The mating surfaces of cylinder block and cylinder head, transmission case and cover are processed by planer.

3) Milling: milling is the process of machining workpiece with milling cutter on milling machine. Milling machines can process inclined planes, grooves, and even old milling such as gears and curved surfaces. They are widely used to process all kinds of automobile parts. The cold stamping dies of automobile body are processed by milling. CNC milling machine controlled by computer can process workpieces with complex shapes. It 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 technological process of machining workpiece with grinding wheel on grinder. Grinding is a finishing method, which can obtain high precision and roughness workpieces, and grind workpieces with high hardness. Some automotive parts after heat treatment are finished by grinding machine.

6. About heat treatment

Heat treatment is a method of reheating, heat preservation or cooling solid steel to change its organizational structure to meet the use requirements or process requirements of parts. The heating temperature, holding time and cooling speed can make the steel produce different structural changes. A blacksmith immerses heated steel parts in water for rapid cooling (known as quenching by experts) to improve the hardness of steel parts. This is an example of heat treatment.

The heat treatment process includes annealing, normalizing, quenching and tempering. Annealing is to heat the steel parts, keep them warm for a certain period of time, and then cool them together with the furnace slowly, so as to obtain a finer and uniform structure and reduce the hardness, which is conducive to cutting.

Normalizing is to heat the steel parts, take them out of the furnace after heat preservation, and then cool them in the air. It 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. The quenched steel parts are reheated and cooled after heat preservation to stabilize the structure and eliminate brittleness.

For many auto parts, high-frequency surface quenching, carburizing, cyanidation and other heat treatment processes are needed to retain the toughness of the heart and change the surface structure to improve the hardness.

7. About assembly

Assembly is to use connecting parts (bolts, nuts, pins or buckles, etc.) to connect and combine various parts into parts according to certain requirements, and then connect and combine various parts into a whole vehicle.

Whether the parts are combined into components or the components are combined into the whole vehicle, they must meet the mutual cooperation relationship specified in the design drawing, so as to make the components or the whole vehicle achieve the predetermined performance. For example, when assembling the transmission to the clutch housing, the center line of the transmission input shaft must be aligned with the center line of the engine crankshaft. This centering method is not adjusted by the fitter (fitter) during assembly, but guaranteed by design and processing.

If you visit an automobile factory, the most attractive thing is the automobile assembly line. In this general assembly line, a car drives off every few minutes. Take China’s FAW Jiefang Truck assembly line as an example. This assembly line is a 165m long conveyor chain. The car moves to each station along with the conveyor chain and is gradually assembled. There are conveyor suspension chains around it to continuously transport the engine assembly, cab assembly and wheel assembly from each workshop to the corresponding stations on the assembly line.

First place the frame at the starting position of the conveyor chain (bottom facing up), then install the rear axle assembly (including leaf spring and wheel hub) and front axle assembly (including leaf spring, steering knuckle and wheel hub) on the frame, then turn the frame over to install the steering gear, air reservoir and brake pipeline, oil tank and oil pipe, wire and wheel, and finally install the engine assembly (including clutch, transmission and central brake) and connect the upper drive shaft, Then install the cab and front panel. At this point, the car can drive off the assembly line.