From before the Industrial Revolution until the present day, manufacturers have shared common goals: producing a certain number of parts, in a certain amount of time, at a certain cost. Manufacturing processes evolved from craft-made single-item methods to mass production lines and output of increasingly greater numbers of identical parts: a high-volume/low product mix (HVLM) scenario. Most recently, digital technology in programming, machine tool controls and workpiece handling systems are facilitating a manufacturing environment known as Industry 4.0 that enables cost-efficient manufacture of highly diverse parts in small batches: high-mix/low-volume (HMLV) production.In the era of Industry 4.0 it is fashionable to highlight the newest production techniques and digitalization technologies. However, maximum productivity and cost-efficiency still are based on a foundation of operational excellence. In the present economic environment, manufacturers generally consider speed to be a key indicator of operational excellence. A drawing comes into a facility and eventually a completed workpiece leaves the plant; manufacturers want the time between the two events to be as short as possible. Efforts to boost speed typically focus on strategies such as e.g. lean manufacturing or Six Sigma.
However, those strategies generally relate to HVLM production and are not always effective when applied in HMLV scenarios. An important contributor to streamlined HMLV output is the Group Technology approach, in which classifying and coding parts into machinable families enable a shop to achieve the highest level of operational excellence.
Group Technology is a manufacturing organizational strategy in which parts with certain similarities such as geometry, material, manufacturing process or quality standards are classified into groups or families and manufactured under a common production method. Operations are planned for the part family rather than individual workpieces.
Very often when production is organized to handle part families, the arrangement is described as cellular manufacturing. Cellular manufacturing came to prominence in the 1980s, roughly when the era of HMLV production began. Manufacturers recognized that batch sizes were shrinking while the variety of workpieces and new workpiece materials were growing. Shops were confronted with a high diversity of different workpieces, produced in comparatively small batches. Time spent preparing for production rose exponentially, and manufacturers sought to control it.
The creation of part families in Group Technology is based on part codification and classification. Each part is assigned a code consisting of letters or figures or combinations thereof, and each individual letter or figure represents a certain feature of the workpiece or a production technique that is required to produce the workpiece. In Figure 1 the 6th digit in the part code represents workpiece dimensions, the 7th digit the raw material, the 8th digit the original shape of the workpiece material, and the 9th digit the level of quality required. Digits 3 through 5 describe the operations required to machine the part.
The part codes are used to plan production and make price quotes by referring to an imaginary or non-existent part called a complex workpiece, as shown on the second line of Figure 2. Complex in this case does not mean difficult; it describes a generic workpiece that illustrates all the features that a company is able to create, such as high- and low-accuracy holes, deep and shallow pockets, side milled features, etc. The parts on the first line of the figure represent workpieces that can be produced with operations selected from those described on the complex workpiece in the second line. Summing up the costs of producing the required features produces a representative total cost and simplifies estimation of pricing. It is not necessary to analyze the costs on an individual part-by-part basis.
Production planners and estimators work with a drawing of a workpiece and develop a price quote by matching features on the workpiece with those on the complex workpiece and also determine other production elements such as the machine tool required, whether coolant will be needed, etc. In addition, executing the Group Technology technique with the help of a sophisticated CAM system further reduces pre-machining engineering time requirements. Additional benefits include improved communication between departments in a facility as they all work from the same complex workpiece model.
The Group Technology approach initially was based on experience as personnel developing it interviewed process engineers, programmers and planners to gather information regarding the cost of various production operations. Although the development occurred in the 1980s, compiling individual experiences and data and organizing them into a system was a process that resembles today´s initiatives in artificial intelligence.
In some cases, Group Technology prompts reorganization of the shop floor. The left side of Figure 3 shows the circuitous path parts take through a shop that is organized in a traditional layout based on machine functions including turning, milling and grinding. However, when workpieces are grouped and processed as families in a cellular layout, shown at the right in the figure, machine tools can be arranged to streamline manufacturing flow and minimise part movement within the shop. Each different workpiece family is machined in the most efficient way without unnecessary transport within the shop. Significant reduction of the time required to produce the parts is the result.