3. Group Technology / Cellular Manufacturing1
As early as in the 1920ies it was observed, that using product-oriented departments to manufacture standardized products in machine companies lead to reduced transportation. This can be considered the start of Group Technology (GT). Parts are classified and parts with similar features are manufactured together with standardized processes. As a consequence, small "focused factories" are being created as independent operating units within large facilities. More generally, Group Technology can be considered a theory of management based on the principle that "similar things should be done similarly". In our context, "things" include product design, process planning, fabrication, assembly, and production control. However, in a more general sense GT may be applied to all activities, including administrative functions. The principle of group technology is to divide the manufacturing facility into small groups or cells of machines. The term cellular manufacturing is often used in this regard. Each of these cells is dedicated to a specified family or set of part types. Typically, a cell is a small group of machines (as a rule of thumb not more than five). An example would be a machining center with inspection and monitoring devices, tool and Part Storage, a robot for part handling, and the associated control hardware. The idea of GT can also be used to build larger groups, such as for instance, a department, possibly composed of several automated cells or several manned machines of various types. As mentioned in Chapter 1 (see also Figure 1.5) pure item flow lines are possible, if volumes are very large. If volumes are very small, and parts are very different, a functional layout (job shop) is usually appropriate. In the intermediate case of medium-variety, medium-volume environments, group configuration is most appropriate. GT can produce considerable improvements where it is appropriate and the basic idea can be utilized in all manufacturing environments: • To the manufacturing engineer GT can be viewed as a role model to obtain the advantages of flow line systems in environments previously ruled by job shop layouts. The idea is to form groups and to aim at a product-type layout within each group (for a family of parts). Whenever possible, new parts are designed to be compatible with the processes and tooling of an existing part family. This way, production experience is quickly obtained, and standard process plans and tooling can be developed for this restricted part set. To the design engineer the idea of GT can mean to standardize products and process plans. If a new part should be designed, first retrieve the design for a similar, existing part. Maybe, the need for the new part is eliminated if an existing part will suffice. If a new part is actually needed, the new plan can be developed quickly by relying on decisions and documentation previously made for similar parts. Hence, the resulting plan will match current manufacturing procedures and document preparation time is reduced. The design engineer is freed to concentrate on optimal design.
This chapter is based on Chapter 6 of Askin & Standridge (1993). It is recommended to read this chapter parallel to the course notes.
In this GT context a typical approach would be the use of composite Part families. Consider e.g. the parts family shown in Figure 3.1. The parameter values for the features of this single part family have the same allowable ranges. Each part in the family requires the same set of machines and tools; in our example: turning/lathing (Drehbank), internal drilling (Bohrmaschine), face milling (Planfräsen), etc. Raw material should be reasonably consistent (e.g. plastic and metallic parts require different manufacturing operations and should not be in the same family). Fixtures can be designed that are capable of supporting all the actual realizations of the composite parts within...
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