There are two distinct assembly processes involved, one for unibody design cars (the most popular type today) and another for body on frame vehicles (most all trucks, and most rear wheel drive cars).
In the unibody design, the body weld shop is the first stage, in which the major body panels - the floor, the roof and the side panels, are first tack welded within a fixture. Later, after the body is released from the framing fixture, "respot" welds are applied, likely 1500 or more, and most by robot. Reinforcement brackets and supports for components are then welded and the body becomes very stiff. Unibody cars have a lot of heavy supporting parts which require arc welding as well, usually wire fed MIG welders are used, either by robot or by manual means.
Then bolt on components, including the doors, the trunk lid and the hood are assembled, using special fixtures to maintain proper clearances for a good fit.
The next major stage is that the body is selectively metal finished by disc wheel abrasives to eliminate any defects caused by dirt within dies. It takes a special skill to do this job, even to notice the potential defects which will become glaringly obvious after painting.
The next stage is phosphate coating, which cleans all the die oils and any dirt and applies a texture to the metal for painting.
Next stage is prime painting, which is almost always done by dipping in a long, deep bath unit and use of an electric current to "plate" the primer to the metal. The primer paint is baked in ovens running about 200 degrees Celsius.
The next stage is topcoat paint and again, most of this has been automated with either reciprocating beam sprayers or robotics. As there are usually multiple color possibilities, multiple paint pipes run to the spray booths, and back to keep the paint circulating. Multiple layers of topcoat are applied, each one having to be set up to allow a "flash off" time to elapse between coats. Thereafter the topcoat is baked.
Thereafter a similar process is done using "clear coat" paint, and baked again. Thus, three baking stages for the body - primer, topcoat and clearcoat.
Body framing, welding and painting consume about two-thirds of the cars total time in assembly. But, the last third of the time in system, the "general assembly" stage, the "bolts and nuts" assembly, occupies most of the human labor.
The interior is first assembled in a logical order - floor carpets, windshields and door glass, heating and air conditioning, pedals, headliners, lighting, instrument panels, steering columns. The last stage for the body interior is generally the seat installation.
The next stage is the power train installation. Before the body is finished, in parallel time, the engine has been "dressed" with wiring, fuel injection system, and accessory drives - generator, air conditioner, power steering pump. It is then mated to a supporting structure, called a "cradle," mated to a transmission and further work is done to install the exhaust pipes, drive shafts, front and rear hubs (or solid rear axle, in some cases), brakes, springs and shock absorbers. These are set up in a special fixture to support these components. The fixture will later raise these up under the body, and workers install the "engine cradle" and the attached components to the body, working below the body which is now supported from overhead on a moving conveyor.
The final assembly stage is where the wiring is connected, fuel tank installed, radiator is installed with connecting hoses, all the fluids added, the wheels are installed and the bumpers, grille and external lights assembled.
Now the car can be started and is tested using a dynamometer to check acceleration, transmission function and brakes. The steering alignment is later done on rollers with the engine spinning the wheels.
Any assembly error corrections are done usually in stalls, similar to an auto garage, and then the car is shipped.
The body on frame vehicles are built in a similar fashion, with these differences:
The body is simpler, less parts to weld, as the attaching surfaces for the engine and suspension are part of the frame, not the body.
The frame acts as the assembly fixture, and the engine/transmission unit is directly assembled to the frame, along with the front hubs, rear axle, suspension components, chassis wiring, steering and brake parts and the fuel tank. This comprises the "chassis." This work can be accomplished more easily, standing above the frame instead of under the body as with unibody cars.
Thereafter the body is lowered to the finished chassis, and a similar process is employed as with unibody cars.
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