Cellular Manufacturing

      CELLULAR  MANUFACTURING

Whether part families have been determined by visual inspection. parts classification and coding, or production flow analysis, there is advantage in producing those parts using group technology machine cells rather than a traditional process type machine layout. When the machines are grouped. the term cellular manufacturing is used to describe this work organization. Cellular manufacturing is an application of group technology in which dissimilar machines or processes have been aggregated into cells, each of which is dedicated to the production of a part or product family or a limited group of families. The typical objectives in cellular manufacturing are similar to those of group technology:

   To shorten manufacturing lead times, by reducing setup, workpart handling, waiting times, and batch sizes

   To reduce work-in-process inventory. Smaller batch sizes and shorter lead times reduce work-in-process.

   To improve  quality. This is accomplished by allowing each cell to specialize in producing a smaller number of different parts. This reduces process variations.

   To simplify production scheduling. The similarity among parts in the family reduces the complexity of production scheduling. Instead of scheduling parts through a sequence of machines in a process-type shop layout, the parts are simply scheduled though the cell.

   To reduce setup times. This is accomplished by using group tooling (cutting tools, jigs, and fixtures) that have been designed to process the part family, rather than part tooling, which is designed for an individual part. This reduces the number of individual tools required as well as the time to change tooling between parts.

Additional reasons for implementing cellular manufacturing are given in Table 15.7. In this section, we consider several aspects of cellular manufacturing and the design of machine ceils.

           Composite   Part  Concept

Part families are defined by the fact that their members have similar design and/or manufacturing features. The composite part concept takes this part family definition to its logical conclusion. It conceives of a hypothetical part, a composite part for a given family, which includes all of the design and manufacturing attributes of the family. In general. an individual part in the family will have some of the features that characterize the family but not all of them. The composite part possesses all of the features.

There is always a correlation between part design features and the production operations required 10 generate those features. Round holes are made by drilling, cylindrical shapes are made by turning, flat surfaces by milling, and so on. A production cell designed for the part family would include those machines required to make the composite part. Such a cell would be capable of producing any member of the family, simply by omitting those operations corresponding to features not possessed by the particular part. The cell would also be designed to allow for size variations within the family as well as feature variations.

To illustrate, consider the composite part in Figure 15.1O(a).1l represents a family of rotational parts with features defined in Figure 15.1O(b). Associated with each feature is a certain machining operation as summarized in Table 15.6. A machine cell to produce this

part family would be designed with the capability to accomplish all seven operations required to produce the composite pan (the last column in the table). To produce a specific member of the family, operations would be included to fabricate the required features of the part. For parts without all seven features, unnecessary operations would simply be omitted. Machines, fixtures, and tools would be organized for efficient flow of work-parts through the cell,

In practice, the number of design and manufacturing attributes is greater than seven, and allowances must be made for variations in overall size and shape of the parts in the family. Nevertheless, the composite part concept is useful for visualizing the machine cell design problem.

           Machine Cell Design

Design of the machine cell is critical in cellular manufacturing. The cell design determines to a great degree the performance of the cell. In this subsection, we discuss types of machine cells, cell layouts, and the key machine concept.

Types of Machine Cells and Layouts. GT manufacturing cells can be classified according to the number of machines and the degree to which the material flow is mechanized between machines. In our classification scheme for manufacturing systems (Section 13.2), all GT cells are classified as type X in terms of part or product variety (Section 13.2.4, Table 13.3). Here we identify four common GT cell configurations (with system type identified in parenthesis from Section 13.2):

    single machine  cell (type  I M)

    group  machine  cell with manual  handling  (type n M generally, type III M less common)

    group machine cell with semi-integrated handling (type II M generally, type III M less common)

    flexible manufacturing cell or flexible manufacturing system (type IT A generally, type III A less common)

As its name indicates, the single machine cell consists of one machine plus supporting fixtures and tooling. This type of cell can be applied to work-parts whose attributes allow them to be made on one basic type of process, such as turning or milling. For example, the composite part of Figure 15.10 could be produced on a conventional turret lathe, with the possible exception of the cylindrical grinding operation (step 4)

The group machine cell with manual handling is an arrangement of more than one machine used collectively to produce one or more part families. There is no provision for mechanized parts movement between the machines in the cell. Instead, the human operators who run the cell perform the material handling function. The cell is often organized into a U shaped layout, as shown in Figure 15.11. This layout is considered appropriate when there is variation in the work flow among the parts made in the cell. It also allows the multifunctional workers in the cell to move easily between machines [29].

The group machine cell with manual handling is sometimes achieved in a conventional process type layout without rearranging the equipment. This is done simply by assigning certain machines to be included in the machine group and restricting their work to specified part families. This allows many of the benefits of cellular manufacturing to be achieved without the expense of rearranging equipment in the shop. Obviously, the material handling benefits of OT are minimized with this organization.

The group machine cell with semi integrated handling uses a mechanized handling system, such as a conveyor, to move parts between machines in the cell. The flexible manufacturing system (FMS) combines a fully integrated material handling system with automated processing stations. The FMS is the most highly automated of the group technology machine cells. The following chapter is devoted to this form of automation, and we defer discussion till then.

A variety of layouts are used in GT cells, The U-shape, as in Figure 15.11, is a popular configuration in cellular manufacturing. Other GT layouts include inline, loop, and rectangular, shown in Figure 15.12 for the case of semi integrated handling.

Determining the most appropriate cell layout depends on the routings of parts produced in the cell. Four types of part movement can be distinguished in a mixed model part production system. They are illustrated in Figure 15.13 and are defined as follows, where the forward direction of work flow is defined as being from left to right in the figure:(l) repeat operation, in which a consecutive operation is carried out on the same machine, so that the part does not actually move; (2) in-sequence move, in which the part moves from the current machine to an immediate neighbor in the forward direction; (3) bypassing move, in which the part moves forward from the current machine to another machine that is two or more machines ahead; and (4) backtracking move, in which the part moves from the current machine in the backward direction to another machine.

When the application consists exclusively of in sequence moves, then an inline layout is appropriate. A V-shaped layout also works well here and has the advantage of closer interaction among the workers in the cell. When the application includes repeated operations, then multiple stations (machines) are often required. For cells requiring bypassing moves, the U-shape layout is appropriate. When backtracking moves are needed, a loop or rectangular layout is appropriate to accommodate recirculation of parts within the cell. Additional factors that must be accounted for in the cell design include:

   Quantity of work to be done by the cell. This includes the number of parts per year and the processing (or assembly) time per part at each station. These factors determine the workload that must be accomplished by the cell and therefore the number of machines that must be included, as well as total operating cost of the cell and the investment that can be justified .

   Part size, shape, weight, and other physical attributes. These factors determine the size and type of material handling and processing equipment that must be used.

Key Machine Concept. In some respects, a GT machine cell operates like a man. ual assembly line (Chapter 17), and it is desirable to spread the workload evenly among the machines in the cell as much as possible. On the other hand, there is typically a certain rnachine in a cell (or perhaps more than one machine in a large cell) that is more expensive to operate than the other machines or that performs certain critical operations in the plant This machine is referred to as the key machine. It is important that the utilization of this key machine be high. even if it means that the other machines in the cell have relatively low utilization. The other machines are referred to as supporting machines, and they should be organized in the cell to keep the key machine busy. In a sense, the cell is designed so that the key machine becomes the bottleneck in the system.

The key machine concept is sometimes used to plan the GT machine cell. The approach is to decide what parts should be processed through the key machine and then determine what supporting machines are required to complete the processing of those parts.

There are generally two measures of utilization that are of interest in a GT cell: the utilization of the key machine and the utilization of the overall cell. The utilization of the key machine can be measured using the usual definition (Section 2.4.3). The utilization of each of the other machines can also be evaluated similarly. The cell utilization is obtained by taking a simple arithmetic average of all the machines in the cell. One of the exercise problems at the end of the chapter serves to illustrate the key machine concept and the determination of utilization