An assembly line is a line of factory workers and equipment that produce a product as it moves consecutively from station to station on the line until completed. Assembly line methods have become considerably more sophisticated since the first moving assembly lines were introduced in the automobile industry in the early part of the 20th century. Assembly line methods were originally introduced to increase productivity and efficiency by reducing the amount of manufacturing time required to produce a finished product. Advances in assembly line methods have the same objective—to increase throughput, or the number of products produced in a given period. While assembly line methods apply primarily to manufacturing processes, they can also be applied to other areas of business ranging from product development to management.
A look at the introduction of the moving assembly line in Ford Motor Co.'s Highland Park, Michigan, plant in 1913 and 1914 reveals some of the basic principles and objectives involved in the development of assembly line methods throughout the 20th century. As assembly line methods were introduced, manufacturing tasks became minutely divided and closely timed. Manufacturing became a highly mechanized process in which mass manufacturing was performed largely by unskilled workers. The assembly line cut down on human handling, and machines were designed to handle multiple tasks.
A key factor in the development of the moving assembly line was the mechanization of materials handling. Before power-driven conveyors were introduced to move materials in the automobile industry, they were commonly used in such industries as brewing, milling, canning, and meatpacking. The first power-driven conveyors in the Ford factory transported materials to individual workstations. Later they moved parts while workers worked on them.
Moving conveyors were first applied to Ford assembly operations in 1913 in the flywheel magneto assembly, part of the car's electrical system. Originally one worker took 20 minutes to assemble one unit. Ford's production managers looked at the operation and broke it into 29 separate operations that could be laid out along a moving belt. Assembly time dropped dramatically to only 13 minutes, then to five minutes after additional adjustments were made.
After Ford's success in applying assembly line methods to the flywheel magneto, as many manufacturing processes as possible were divided into a series of single work tasks that could be performed along a moving conveyor. By April 1914 Ford had introduced an electrically driven endless chain conveyor that moved the auto chassis down the line. This enabled Ford to increase production from about 475 cars in a nine-hour day to more than 1,200 auto assemblies in an eight-hour day. Ford tripled its production and reduced labor time per vehicle by nearly 90 percent.
The increased throughput at Ford's Highland Park plant required the installation of power-driven supply lines. Subassembly lines were laid out to feed into the main assembly line. When Ford built its famous River Rouge plant, automobile manufacturing became one continuously moving process, from the unloading of raw materials to the loading of completed vehicles onto railroad cars.
Ford's success with assembly line methods allowed Henry Ford to make good on his promise to build a car for the multitude. Manufacturing a single model, the Model T, Ford standardized the car's design, streamlined production, lowered costs, and made cars available to nearly everyone. Beginning in 1909, the first full year of assembly line production, the Ford Motor Co. increased Model T production from 17,771 vehicles to 202,667 in 1913. In 1924, the Model T's peak year, some 1.8 million cars were produced.
Ford's ability to mass produce the Model T inspired the revolution in business thinking that led to mass production and mass consumption of other goods. During the 1920s, for example, refrigerator production went from fewer than 5,000 units to almost 1 million. Radio production jumped from zero to 5 million units. The combination of standardization and assembly line methods not only resulted in a better material standard of living for many Americans, it also ultimately extended into the wartime production of U.S. industry during World War II, where the Allies gained a substantial advantage through the application of mass production methods.
Using modern assembly line methods, manufacturing has become a highly refined process in which value is added to parts along the line. Assembly line manufacturing is characterized by concurrent processes, or multiple parallel activities that feed into a final assembly stage. These processes require a well-planned flow of materials and the development of an advanced materials and supply infrastructure.
Just-in-time (JIT) manufacturing methods have been developed to reduce the cost of carrying parts and supplies as inventory. Under a JIT system, manufacturing plants carry only one or a few days' worth of inventory in the plant, relying on suppliers to provide parts and materials on an "as needed" basis. Future developments in this area may call for suppliers to establish operations within the manufacturing facility itself to provide for a more efficient supply of materials and parts.
Modular assembly is another advanced assembly line method that is designed to improve throughput by increasing the efficiency of parallel subassembly lines feeding into the final assembly line. As applied to automobile manufacturing, modular assembly would involve assembling separate modules—chassis, interior, body—on their own assembly lines, then joining them together on a final assembly line.
The recognized efficiency of machines performing multiple tasks has evolved into cell manufacturing. Cells of machines can be run by one operator or a multiperson work cell. In these machine cells it is possible to link older machines with newer ones, thus reducing the amount of investment required for new machinery. Cell operators can handle three or four tasks, and robots are used for such operations as materials handling and welding.
Team-style production is another development in assembly line methods. Where workers used to work at one- or two-person workstations and perform repetitive tasks, now teams of workers can follow a job down the assembly line through its final quality checks. Team production creates greater worker involvement and was adopted by Swedish automakers Saab and Volvo in the early 1980s. On the Sportster line at the Harley-Davidson, Inc. plant in Milwaukee, ten three-person teams follow the motorcycle through 20 assembly stations and its final check.
While assembly line methods are typically applied to high-volume runs, they can be just as successfully applied to one-of-a-kind products such as molds and dies. One shop improved the throughput and accuracy of the production of its molds and dies by breaking down operations into manufacturing steps and arranging for the steps to occur simultaneously. Detailed preproduction plans were made by a process engineering group, and the smooth production flow was coordinated by electronic communication tools. All of the employees were engaged in continuous training and skills upgrading, which included cross training in the various specialties involved in the production process.
Custom lines represent another innovation in assembly line methods. Rather than relying on a common assembly line, one automotive supplier that specialized in after-model-year parts such as hoods and fenders developed a system to make custom assembly lines that could be put together and dismantled in a short time. With floor space at a premium, the facility was designed so that nothing was attached to the floor; all of the machinery could either be lifted and carried, or it had wheels or forklift notches for easy movement.
Assembly line designers are able to use special software to make optimal use of workstations, minimize parts, and reduce production costs. Three basic types of software are available. One is called design-for-assembly (DFA) software. It is used to minimize the number of parts in a product. A second type of software evaluates the order in which the pieces are assembled to achieve greater economy. The third type is assembly system design software that helps designers select the best mix of automation and people in the assembly line.
As new assembly line methods are introduced into manufacturing processes, business managers look at the techniques for possible application to other areas of business. New methods all share the common goal of improving throughput by reducing the amount of time individual workers and their machines spend on specific tasks. By reducing the amount of time required to produce an item, be it an automobile, a new product, or a report, assembly line methods have made it possible to produce more with less.
[ David P. Bianco ]
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Samuelson, Robert J. "The Assembly Line." Newsweek 130, no. 24-A (Winter 1997).