Assembly is typically the final stage in discrete manufacturing and is often the most labour-intensive. Consequently, assembly tends to employ the highest number of factory workers. To illustrate the scale of such manufacturing systems, the 2020-2021 Occupational Outlook Handbook (U.S. Bureau of Labor Statistics, www.bls.gov) estimates that nearly 2 million workers in the U.S. alone are engaged in fabrication and assembly, with assembly workers constituting the largest portion of this group. The number of these jobs is expected to decrease by 5% between 2020 and 2030, although this forecast was made before the political push to reshore production following the supply chain disruptions of 2020-2021.
The number of assembly workers in China is substantially higher, though specific data is not readily accessible. For context, the U.S. Bureau of Labor Statistics reported that 99 million individuals were employed in China's manufacturing sector in 2009, compared to 14 million in the U.S. in 2020.
Assembly work is predominantly human-driven, and many assembly tasks—especially those with cycle times of less than one minute—are monotonous, repetitive, and occasionally hazardous. While automation can replace certain tasks, no technology remains as adaptable as human labour for many operations. The key challenge for manufacturers is to design and manage assembly facilities that achieve high performance by capitalising on the strengths of their workforce while providing meaningful career opportunities.
Humans have been involved in assembly ever since the creation of the first tool that required more than a single piece to construct. Apart from a few early exceptions, large products were traditionally assembled in one location by multiple skilled artisans. At the same time, smaller items were typically crafted by a single artisan with the help of a few apprentices in a small workshop. Examples of large products assembled in those times include ships, railroad cars, and locomotives, whereas smaller products include garments and shoes. The assembly factories of the First Industrial Revolution, which primarily emerged in the mid-1800s, brought these artisans together to work under the same roof.
This approach changed once again in 1913 when the assembly line was introduced at Ford's Highland Park factory.
Definition: Assembly is any manufacturing process that takes multiple, distinct items as input and produces a single output.
For instance, casings and screws are discrete parts; joining two casings with screws constitutes an assembly operation. Conversely, paint is a bulk item, and applying paint to a workpiece is considered fabrication, not assembly. When an electronic component is soldered onto a board, the assembly involves the component and the board; solder is a bulk item used in the operation but is not counted as a component. This distinction is essential because discrete components and bulk items are managed differently.
In manufacturing, assembly work is almost always repetitive. The only exception is in the production of custom products; however, even in these cases, the process is designed to take advantage of the similarities between different units, such as by assembling a common semi-finished product and customising it in the final stages. Repetitiveness enables assemblers to develop muscle memory and perform manual assembly operations precisely without conscious thought while still being alert to any irregularities. However, this repetitiveness can also make assembly work monotonous, particularly when the cycle times are short.
Assembly is primarily characterised by the activity of joining, which is considered the main task. However, additional upstream or downstream activities, such as handling, inspecting, and adjusting, are also necessary. Assembly activities can be categorised in the following ways:
Assembly activities can be divided into various levels, including subassembly and final assembly. A subassembly is a component that is assembled separately but designed to be integrated with other components to form a larger finished product. Subassemblies may be used directly in the final product or combined into higher-level subassemblies.
A bill of materials (BOM) documents the product's structure. The same components and subassemblies are used in an efficient assembly process across multiple higher-level subassemblies and final products.
Design decisions made earlier in the manufacturing process influence the ease of assembly and the quality of the finished products. A product that has not been designed with assembly in mind may include more fastener types than necessary, requiring different tools, or may combine subassemblies that are incompatible. As a result, assembly often becomes a catch-all for upstream errors. To address these errors, assembly processes must be highly flexible, and human workers are crucial in providing that flexibility. Although automation is advancing, assembly relies more on human labour than fabrication. Humans are particularly difficult to replace in processes that handle errors and anomalies. As a result, assembly process design must prioritise human operators and account for the diversity of the workforce in terms of size, age, and gender.
If you are interested in manufacturing design, continue reading about industrial process optimization.
The two key factors influencing assembly performance are part supply and the design of assembly work. Part supply is the most critical element in determining assembly productivity. A common complaint among assemblers, supervisors, and managers is, "If only we had all the parts we need, we could assemble everything without any issues!" This highlights the importance of coordination with logistics and purchasing departments to ensure smooth assembly operations. However, assembly teams cannot rely solely on suppliers; they must also focus on in-plant logistics, which is the final link in the supply chain and within their control.
In-plant logistics refers to how a part physically moves from the unloading dock to the assembler's hands. This process can vary—sometimes the assembler can reach for the part by simply extending their arm, while in other cases, they may have to walk 20 steps to retrieve it from a shelf. The internal organisation of the assembly area dictates these differences in part handling.
The second major factor in assembly productivity is the design of the assembly work, including:
As mentioned above, in-plant logistics involves a series of interconnected processes, including procuring raw materials, coordinating with suppliers, managing inventory, and ensuring that products are delivered to the right place at the right time. This encloses the reception, handling, and storage of inbound raw materials, components, and parts as they enter the plant, as well as their transportation to various production lines and workstations.
There are two areas of in-plant logistics: the process of unloading raw materials (and the respective loading of finished products) and the supply of materials to the single assembly stations.
Unloading (and loading) processes include aspects such as:
The primary goal of assembly station logistics is to meet the needs of production workers by minimising unnecessary walking, material handling, and wasted time. To achieve this, all the materials required for a specific task must be readily available and positioned in a convenient material zone near the worker's station.
When determining the best location for each part in the assembly area, several key factors should be taken into account:
Additionally, to ensure smooth operations, large parts containers should not be arranged in a way that blocks access to one another, as this can hinder the efficient flow of materials and lead to delays, errors, and reduced productivity. Clear, unobstructed access to each container allows for seamless retrieval of parts, promoting efficiency.
To maintain organisation, all parts containers and racks should have clearly marked positions on the floor. Implementing a coordinate system at the assembly station is particularly useful. Each part can be assigned a specific coordinate, which can then be integrated into the production planning module of your ERP system. This system can be used for large and small parts and help automate the delivery process from the warehouse to the assembly point. Automated work orders can be generated, and even AGVs (automated guided vehicles) can handle the entire parts delivery process in the factory (see also: RFID for asset tracking).
What is the assembly process in manufacturing?
The assembly process is the final stage in discrete manufacturing, where multiple distinct components are combined to create a finished product. It often involves tasks like joining parts through various methods (e.g., screwing, welding) and is typically labour-intensive. Assembly ensures that all components come together efficiently and correctly, making it a critical step in producing high-quality goods.
How does the design of an assembly process impact productivity?
The design of the assembly process significantly affects productivity. Proper planning of part placement, workflow sequencing, and ergonomic considerations can minimise time and effort. A well-designed assembly area ensures that frequently used parts are easily accessible, reducing worker strain and optimisation deficiencies. A smart assembly layout can greatly improve production speed and output quality when combined with effective in-plant logistics.
Can automation fully replace human workers in the assembly process?
While automation is advancing, human workers remain crucial in many aspects of the assembly process. Certain tasks, particularly those requiring flexibility, problem-solving, and the ability to handle errors or anomalies, are difficult to automate. Humans provide the adaptability needed in processes with high variability or custom products, making them indispensable in many assembly operations despite ongoing automation efforts.
The assembly process is vital in manufacturing, combining efficiency with human adaptability. Its success depends on careful design, ensuring smooth workflows, ergonomic part placement, and the integration of automation where possible. A well-organised assembly area can reduce wasted time and improve productivity, but effective part supply is equally important. This leads to the broader field of intra-logistics, which focuses on optimising the movement of materials within a facility. By refining intra-logistics, manufacturers can ensure that parts flow seamlessly to the assembly line, further enhancing the efficiency and performance of the assembly process.
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Sources:
(1) Baudin M., Netland T. (2023): Introduction to Manufacturing. New York: Routledge