Cars are built in very big factories, where people and machines make many cars every day, and a car is made of thousands and thousands of parts, and the number depends on how you count. A car has about 30,000 parts if you count every screw, nut and bolt, but if you count only the big pieces, such as the engine as one part, it has only about 1,800 to 2,000. What counts as a 'part' changes the answer: counting every fastener gives 20,000 to 30,000 parts, while counting only major pieces gives under 2,000. A car plant runs several workshops in a row to bring all of those parts together: a press shop that starts the process, a body shop, a paint shop and assembly halls. A BMW plant in South Carolina builds more than 1,500 vehicles every day.
Interchangeable parts were a prerequisite for the assembly line. In the early 19th century, machine tools such as the screw-cutting lathe, metal planer and milling machine, along with toolpath control through jigs and fixtures, made interchangeable parts a practical reality. Before Ford's moving line, workers built each car by hand while it stayed in one place, and Ford's moving assembly line first ran at his Highland Park plant in Michigan in October 1913. An assembly line, or progressive assembly, moves the unfinished product in a direct line from workstation to workstation with parts added in sequence. The moving assembly line was developed for the Model T and began operation on 7 October 1913 at the Highland Park Ford Plant, and it continued to evolve after that, using time and motion study. A rope and winch pulled the chassis of each Model T along a short track while workers added parts as it passed.
The line caused a paint bottleneck. It was so successful that only japan black would dry fast enough, forcing the company to drop the variety of colors available before 1914, until fast-drying Duco lacquer was developed in 1926. Other reasons given for Ford's all-black Model T include that black was cheap and durable, and that it saved time because production did not have to stop to change paint. The Model T was not always black, however, since early Model Ts came in colors such as red, green, blue and gray, and the black years ran from about 1914 to 1925. Early Model T bodies were painted with several coats of varnish, and each step had to dry for a long time before the next. In the black years the Model T's paints were really color varnishes, and more than 30 kinds of black paint were used at once.
The work on a moving line was monotonous. Ford's workers quit and missed work in large numbers, which was part of why he raised pay. Ford substantially increased his workers' wages to combat absenteeism and employee turnover that approached 400 percent a year, and a byproduct was giving workers the means to become customers. In 1914 Ford began paying workers $5 a day, more than twice what most carmakers paid. Historians do not agree on one reason for the $5 day, and accounts say it was to keep workers who might quit, to hire better and steadier workers, or both. Ford's high pay was later described as part of a 'virtuous cycle': when workers get more productive, wages can rise, and workers then have more money to buy products. Fordism rested on three principles: standardized products made by machines and molds, assembly lines using special-purpose tools so common-skilled workers could contribute, and higher 'living' wages so workers could afford the products they make.
Toyota's way of building cars rests on two ideas: just-in-time and jidoka. Sakichi Toyoda built a loom that stopped by itself when a thread broke, and this idea became jidoka. Jidoka is called 'automation with human intelligence': a machine or worker can notice a problem and stop work right away, so defects are not passed on. Toyota's just-in-time idea comes from a comment by Kiichiro Toyoda, and Taiichi Ohno later developed it into a system that controls overproduction. The Toyota Production System is a management system that organizes manufacturing and logistics, including interaction with suppliers and customers. Taiichi Ohno and Eiji Toyoda developed it between 1948 and 1975. The system addresses eight kinds of muda, or waste: overproduction (the largest), waiting, transportation, processing itself, excess inventory, movement, making defective products, and underutilized workers. Toyota's system was built over years by trial and error, and Taiichi Ohno is credited with building it.
Designing an assembly line is a well-established mathematical challenge called the assembly line balancing problem, and in the simple version the aim is to assign a set of tasks to a sequence of workstations. Assigning tasks to stations has two limits. A precedence graph says what must be done before a task can start, for example not putting in a screw before drilling the hole, and a cycle time restricts the task time at each station before the conveyor moves the piece on.
Big presses squeeze flat metal into the shapes of car parts such as doors and side panels. A press line can run up to 18 strokes a minute, as of November 2026. Stamping operations have distinct jobs. Stretching increases a blank's surface area by tension and is often used to make smooth auto body parts. Curling forms tubular profiles, such as door hinges, and hemming folds an edge over onto itself, and the edges of automobile doors are usually hemmed.
Hydroforming differs from stamping, in which flat sheet metal, as a cut blank or from a coil, goes into a press where a tool and die surface forms it into a new shape. Hydroforming instead uses high-pressure hydraulic fluid to press room-temperature metal into a die, and it shapes ductile metals such as aluminium, brass, low-alloy steel and stainless steel into lightweight, stiff, strong pieces. The car industry is one of its largest uses because it allows complex shapes in unibody structures. Hydroformed parts reached mass production in cars in the 1990s. The first mass-produced automotive component was the instrument panel support beam for the 1990 Chrysler Minivan, and the first mass-produced hydroformed engine cradle was for the Ford Contour and Mystique in 1994.
Die casting forces molten metal under high pressure into a mold cavity formed by two hardened tool-steel dies, which work much like an injection mold. Most die castings are made from non-ferrous metals such as zinc, copper, aluminium, magnesium, lead, pewter and tin-based alloys. By late 2019, press machines capable of die casting single pieces over 100 kilograms (220 pounds) were being used to produce aluminium chassis components for cars. Tesla uses giant machines to cast big parts of a car body from molten aluminum in one piece. Tesla says the front and rear sections of the early Model 3 each had more than 70 parts, and on the very similar Model Y each is now one cast part. Gigacasting trades fewer parts for more risk: one cast section replaces 70 or more pieces, but changes are very costly, a single flaw can compromise the whole piece, and the scrap rate is higher. Tesla developed a special aluminum alloy that lets it skip a heat-treating step after casting, as of November 2026.
Spot welding joins sheet metal with heat from electrical resistance, and the sheets are typically 0.5 to 3 millimeters thick, which covers most car body panels. Spot welding delivers its energy in a very short time, which lets the weld form without excessive heating of the rest of the sheet. The energy is matched to the sheet's material, thickness and electrodes: too little makes a poor weld, and too much melts excess metal, ejects it and makes a hole rather than a weld. Spot-welding guns come in two layouts. The C-type is rigid and is used for thick material and high forces. The X-type, whose electrodes move like scissors, is less rigid but reaches a far larger workspace, so it is common on thin, flat parts such as floor pans and roof panels.
A car's paint is put on in layers, not all at once. A paint shop builds up layers: corrosion coats, a primer, a base color coat and a clear coat, and at BMW's South Carolina plant a body gets five coats and the whole painting process takes about 12 hours. Primer is the layer that makes the surface smooth and even, so the color goes on well, and it levels the marks and defects left by the body shop. The base coat gives the car its color, and the clear coat on top protects the color from sunlight and from air that can dull it. Paint thickness is measured in microns. A car's primer is 8 to 38 microns thick, and the whole paint film is about 67 to 198 microns.
At the end of a car's life, recycling is a big business. About 86 percent of a vehicle's material content can be reused or recycled, as of November 2026. Steel from scrapped cars adds up to more than 15 million tons a year in the United States, as of January 2025. Old cars are recycled, and their steel is melted and made into new steel, which can become cans, appliances, buildings or new cars. Recycled steel is as good as new steel, and all new steel contains some recycled steel because scrap is a necessary ingredient. Cars are the most recycled thing people buy, and nearly all the cars that leave the road are recycled. In the United States, over 12 million cars are recycled in a year, as of November 2026.
Words to know
- interchangeable: swappable
- muda: waste
- hemming: folding edges
- hydroforming: shaping with fluid
