Before Ford's moving line, workers built each car by hand while it stayed in one place. An assembly line, or progressive assembly, moves the unfinished product in a direct line from workstation to workstation, with parts added in sequence. By mechanically moving parts and the product, a finished product can be assembled faster and with less labor than having workers carry parts to a stationary product. Cars still move from station to station while workers and machines add parts to them. Cars are built in very big factories, where people and machines make many cars every day. In 1913 Ford's workers built cars on a line that moved: a rope pulled each car past the workers, and each worker did one job.

The assembly line did not begin with cars. Ford borrowed from older industries when he made his line, and writers point to meatpacking and brewing, where work already moved past workers who each did one job. The meatpacking industry of Chicago is believed to be one of the first industrial assembly lines in the United States, starting in 1867. Workers stood at fixed stations, and a pulley system brought the meat to each worker, who completed one task. Ford was not the first carmaker to use an assembly line. Ransom Olds built cars on an assembly line before Ford did, and Ford's moving line combined ideas from still assembly lines like Olds's. What was new in 1913 was a line that moved the car to the workers.

The moving assembly line was developed for the Model T and began operation on 7 October 1913 at the Highland Park Ford Plant. It continued to evolve after that, using time and motion study. After the 1913 trial with a rope and winch, Ford added a powered conveyor, and the line kept being rebuilt and improved. A rope and winch had pulled the chassis of each Model T along a short track while workers added parts as it passed. Ford first tried the line on a small part, since a magneto took 20 minutes to build before the line and only five minutes after it. The Model T line then kept getting faster, so that at first it cut the build time from 12.5 hours to under 6, and within a year to 93 minutes.

Ford's line combined standard parts with more efficient assembly. Together they raised output per worker and let Ford cut the cost and price of the Model T. Henry Ford wanted to make cars that ordinary families could buy. Ford's moving line cut the time to build a Model T from more than 12 hours to about an hour and a half. A new Model T cost $850 when it first came out, and later the price fell to $260, so many more families could buy one. About 15 million Model Ts were built before the car was dropped in 1927. The line changed what a car cost, and in that way it also changed which families could afford to own one, because a lower price put the car within reach of far more buyers.

Work on a moving line was hard on the people who did it. The work was monotonous, and Ford's workers quit and missed work in large numbers, which was part of why he raised pay. In 1914 Ford began paying workers $5 a day, more than twice what most carmakers paid. 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. Historians do not agree on one reason for Ford's $5 day. Accounts say it was to keep workers who might quit, to hire better and steadier workers, or both.

Toyota built a different way of making cars, and it 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. The other idea, just-in-time, is a different kind of idea, and it 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. Taken together, the two ideas describe a factory that stops when something goes wrong and that makes only what is needed.

Taiichi Ohno saw the supermarket as the model for what he wanted in the factory. A customer takes the desired amount of goods off the shelf, and the store restocks enough new product to fill the shelf space. Ohno developed kanban to improve manufacturing efficiency, and it takes its name from the cards that track production within a factory. One goal of kanban is to limit the buildup of excess inventory at any point in production. When material is consumed, a signal tells the supplier to produce and deliver a new shipment. Kanban uses the rate of demand to control the rate of production, passing demand from the end customer up through the chain. Toyota's system was built over years by trial and error, and Ohno is credited with building it, which makes kanban one part of a much larger system.

Casting has become a way to cut the number of parts. Tesla uses giant machines, which it calls Giga Presses, to cast big parts of a car body from molten aluminum in one piece. As of November 2026, 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. Voids can form inside big castings, and changes to the tooling are very costly.

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. Aluminium is lightweight and has high dimensional stability for very complex shapes and thin walls. Magnesium has an excellent strength-to-weight ratio and is the lightest alloy commonly die cast. Each metal suits a different job, with aluminium used where very complex shapes and thin walls are needed and magnesium where low weight and strength matter. Casting a big car section needs a special aluminum alloy, and Tesla developed one that lets it skip a heat-treating step after casting. A cast part is made by injecting molten aluminum into a die, cooling and ejecting it, and trimming it. It is hard to do well, because a single flaw can ruin the whole piece.

Spot welding joins sheet metal with heat from electrical resistance. Two shaped copper-alloy electrodes clamp the sheets together and concentrate a large current into a small spot, which melts the metal to form the weld. 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, and that brief burst lets the weld form without excessive heating of the rest of the sheet. This balance matters, because too little energy makes a poor weld, and too much melts excess metal, ejects it and makes a hole rather than a weld.

Laser beam welding is a second way to join a car body. It uses a concentrated heat source that allows narrow, deep welds at high welding rates, and it has a high power density, on the order of 1 megawatt per square centimeter. That results in small heat-affected zones and high heating and cooling rates. The method is frequently used in high-volume work that needs precision and automation, as in the automotive and aeronautics industries. Unlike electron-beam welding, it does not need a vacuum, because the beam can be transmitted through air, and it generates no X-rays. The process is also easily automated with robotic machinery.

Before the color goes on, a car body gets a chemical pretreatment. A phosphate coat protects the body against corrosion and prepares the surface for the E-coat. In the electro-coat tank a high-voltage current makes the body a cathode and the paint an anode, so the paint sticks to the metal surface, and the process uses the paint very efficiently, at about 99.9 percent. The primer then levels the marks and defects left by the body shop, and it protects the vehicle from corrosion, heat differences, bumps, stone-chips and UV light.

Modern automobile paint is applied in several layers with a total thickness of around 100 micrometers, or 0.1 millimeter. After preparation and primer, a basecoat is applied. Then a clearcoat forms a glossy, transparent coating that has to be able to withstand UV light, and it protects the color from sunlight and from air that can dull it. As mass production of cars made slow drying untenable, paint began to be dried in ovens. Today two-component paint is usually applied by robotic arms and cures in a few hours. A moving line, supply cards, giant castings, welds and thin layers of paint each solve a different problem, and together they turn metal into a car.

Words to know

• kanban: tracking cards
• jidoka: self-stopping automation
• Gigacasting: giant-part casting
• electrodes: current carriers
• inventory: stock on hand