Production Flow Analysis

      PRODUCTION  FLOW ANALYSIS

This is an approach to part family identification and machine cell formation that was pioneered by J. Burbidge [6][R]. Production flow analysis (PFA) is a method for identifying part families and associated machine groupings that uses the information contained 011 production route sheets rather than on part drawings. Work parts with identical or similar routings are classified into part families. These families can then be used to form logical machine cells in a group technology layout. Since PFA uses manufacturing data rather than design data to identify part families, it can overcome two possible anomalies that can occur in parts classification and coding. First, parts whose basic geometries are quite different may nevertheless require similar or even identical process routings. Second, parts whose geornetries are quite similar may nevertheless require process routings that are quite different.

The procedure in production flow analysis must begin by defining the scope of the study, which means deciding on the population of parts to be analyzed. Should all of the parts in the shop be Included in the study, or should a representative sample be selected for analysis! Once this decision is made, then the procedure in PFA consists of the following steps:

    Data collection. The minimum data needed in the analysis are the part number and operation sequence, which is contained in shop documents called route sheets or operation sheets or some similar name. Each operation is usually associated with a particular machine, so determining the operation sequence also determines the machine sequence. Additional data. such as lot size, time standards, and annual demand might be useful for designing machine cells of the required production capacity.

    Soriation of process routings. In this step, the parts are arranged into groups according to the similarity of their process routings. To facilitate this step, all operations or machines included in the shop are reduced to code numbers, such as those shown in Table 15.3. For each part, the operation codes are listed in the order in which they are performed. A sortation procedure is then used to arrange parts into "packs," which are groups of parts with identical routings. Some packs may contain only one part number, indicating the uniqueness of the processing of that part. Other packs will contain many parts, and these will constitute a part family.

    PFA chart. The processes used for each pack are then displayed in a PFA chart, a simplified example of which is illustrated in Table 15.4.¹ The chart is a tabulation of the process or machine code numbers for all of the part packs. In recent GT literature [30], the PFA chart has been referred to as part-machine incidence matrix. In this matrix, the entries have a value x_ii=1 or 0: a value of x_ij = 1 indicates that the corresponding part i requires processing on machine j, and X'I = 0 indicates that no processing of component i is accomplished on machine j. For clarity of presenting the matrix, the D's are often indicated as blank (empty) entries, as in our table.

    Cluster analysis. From the pattern of data in the PFA chart. related groupings are identified and rearranged into a new pattern that brings together packs with similar machine sequences. One possible rearrangement of the original PFA chart is shown in Table 15.5, where different machine groupings are indicated within blocks. The blocks might be considered as possible machine cells. It is often the case (but not in Table 15.5) that some packs do not fit into logical groupings. These parts might be analyzed to see if a revised process sequence can be developed that fits into one of the groups. If not, these parts must continue to be fabricated through a conventional process layout. In Section 15.6.1, we examine a systematic technique called rank order clustering that can be used to perform the cluster analysis.

TABLE 15.3 Possible Code Numbers indicating Operations and/or Machines for Sortation in Production Flow Analysis (Highly Simplified)

The weakness of production flow analysis is that the data used in the technique are derived from existing production route sheets. In all likelihood, these route sheets have been prepared by different process planners, and the routings may contain operations that are non optimal, illogical, or unnecessary. Consequently, the final machine groupings obtained in the analysis may be suboptimal. Notwithstanding this weakness, PFA has the virtue of requiring less time than a complete parts classification and coding procedure. This virtue is attractive to many firms wishing to introduce group technology into their plant operations.