Humans and Machines
Cars are built by a combination of humans and robots (programmed by
humans) on an assembly line. Some cars are totally built by hand,
basically limited production cars, classic cars, or something
special. Some others are totally built by machines. Quality checks
are normally made by humans. All cars are built through an assembly
line process, some on the assembly line, some with the parts going
to the car instead of the car going to the parts.
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
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
The next stage is phosphate coating, which cleans all the die
oils and any dirt and applies a texture to the metal for
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
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
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
The body is simpler, less parts to weld, as the attaching
surfaces for the engine and suspension are part of the frame, not
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.