Cellular manufacturing

Cellular manufacturing, a model for workplace design, is an integral part of just-in-time manufacturing and lean manufacturing, with additional applications in administrative processes. The cell, or work cell, has roots in group technology, which seeks to align process flows by families of component parts or, sometimes, families of target customers (A primary reference for this discussion on cellular manufacturing comes from a 770-page book by Hyer and Wemmerlöv devoted to the topic.^[1])

The cellular mode of process organization is in contrast to the functional, or “job-shop,” form, which “groups similar equipment” (e.g. lathes, mills, drills etc.) “into functionally specialized units in order to manufacturing a variety of dissimilar parts that may follow highly variable routings. . . . A cell, on the other hand, groups dissimilar equipment in order to produce similar parts using identical or closely related routings.”^[2] The cellular mode may be developed in the form of networks of “linked cells” in which manufacturing cells feed parts to subassembly cells, which feed subassemblies to final assembly cells.^[3] Some machines in a cell may not be “used to their capacity, which is a concern if machines are expensive, and a reason to avoid expensive machines when possible.” Among the many benefits of cells, “the overall effect is to (1) greatly shorten lead times of material through production and (2) provide excellent visibility and immediate feedback among the operations in the cell.”^[4]

History

Cellular manufacturing is derivative of principles of group technology, which were proposed by Flanders in 1925^[5] and adopted in Russia by Mitrofanov in 1933 (whose book^[6] was translated into English in 1959). Burbidge actively promoted group technology in the 1970s.^[7] "Apparently, Japanese firms began implementing cellular manufacturing sometime in the 1970s," and in the 1980s cells migrated to the United States as an element of just-in-time (JIT) production.^[8]

One of the first English-language books to discuss cellular manufacturing, that of Hall in 1983, referred to a cell as a “U-line,” for the common, or ideal, U-shaped configuration of a cell^[9]—ideal because that shape puts all cell processes and operatives into a cluster, affording high visibility and contact. By 1990 cells had come to be treated as foundation practices in JIT manufacturing, so much so that Harmon and Peterson, in their book, Reinventing the Factory, included a section entitled, “Cell: Fundamental Factory of the Future”^[10] Cellular manufacturing was carried forward in the 1990s, when just-in-time was renamed lean manufacturing.^[11] Finally, when JIT/lean became widely attractive in the service sector, cellular concepts found their way into that realm; for example, Hyer and Wemmerlöv’s final chapter is devoted to office cells^[12]

Cell Design

“A cell is a small organizational unit . . . designed to exploit similarities in how you process information, make products, and serve customers. Manufacturing cells [closely locate] people and equipment required for processing families of like products. [Prior to cellularization, parts] may have traveled miles to visit all the equipment and labor needed for their fabrication. . . . After reorganization, families of similar parts are produced together within the physical confines of cells that house most or all of the required resources, . . . facilitating the rapid flow and efficient processing of material and information. . . . Furthermore, cell operators can be cross-trained in several machines, engage in job rotation, and assume responsibilities for tasks [that] previously belonged to supervisors and support staff [including] activities such as planning and scheduling, quality control, trouble-shooting, parts ordering, interfacing with customers and suppliers, and record-keeping.”^[13]

The short travel distances within cells serve to quicken the flows. Moreover, the compactness of a cell minimizes space that might allow build-ups of inventory between cell stations. To formalize that advantage, cells often have designed-in rules or physical devices that limit the amount of inventory between stations. Such a rule is known, in JIT/lean parlance, as kanban (from the Japanese), which establishes a maximum number of units allowable between a providing and a using work station. (Discussion and illustrations of cells in combinations with kanban are found in^[14]) The simplest form, kanban squares, are marked areas on floors or tables between work stations. The rule, applied to the producing station: “If all squares are full, stop. If not, fill them up.”^[15]]

An office cell applies the same ideas: clusters of broadly trained cell-team members that, in concert, quickly handle all of the processing for a family of services or customers.^[16]

A virtual cell is a variation in which all cell resources are not brought together in a physical space. In a virtual cell, as in the standard model, team members and their equipment are dedicated to a family of products or services. Although people and equipment are physically dispersed, as in a job shop, their narrow product focus aims for and achieves quick throughput, with all its advantages, just as if the equipment were moved into a cellular cluster.^[17] Lacking the visibility of physical cells, virtual cells may employ the discipline of kanban rules in order to tightly link the flows from process to process.

A simple but rather complete description of cell implementation comes from a 1985 booklet of 96 pages by Kone Corp. in Finland, producer of elevators, escalators, and the like. Excerpts follow: "The first step involved creating cells in the assembly, electrical and chemical testing departments. In April 1984 six cells, identified by different colors, were established. . . . All devices manufactured in cells are identified by the cell's color, and all feed-back from quality control is directed straight to the workers of the cell concerned. . . . The second step, in summer, 1984, was to "cellularize" manufacture of the analyzer subassemblies [that are] needed in the analyzer cells, and to test them if necessary. Production of the five sub-assembly cells consists exclusively of certain analyzer sub-units. The parts and materials are located in the cells. . . . Material control between the cells is based on the pull system and actual demand. In the analyzer cells there is a buffer consisting of two pieces for each (roughly 25 different) sub-unit. When one piece is taken into assembly, a new one is ordered from the corresponding unit-cell. The order is made [using] a magnetic [kanban] button, which identifies the ordering cell (by color), unit (by code), and order date. . . . When the manufacturing cell has completed the order, the unit is taken with the [kanban] button to its place on the ordering cell shelf. Orders from the unit cells to the sub-cells are based on the same principle. The only difference is that the buffer size is six sub-units. This [procedure] was implemented in August, 1984.^[18]

Benefits and costs

Cellular manufacturing brings scattered processes together to form short, focused paths in concentrated physical space. So constructed, by logic a cell reduces flow time, flow distance, floor space, inventory, handling, scheduling transactions, and scrap and rework (the latter because of quick discovery of nonconformities). Moreover, cells lead to simplified, higher validity costing, since the costs of producing items are contained within the cell rather scattered in distance and the passage of reporting time.^[19][20]

Case studies in just-in-time and lean manufacturing are replete with impressive quantitative measures along those lines. For example, BAE Systems, Platform Solutions (Fort Wayne, Ind.), producing aircraft engine monitors and controls, implemented cells for 80 percent of production, reducing customer lead time 90 percent, work-in-process inventory 70 percent, space for one product family from 6,000 square feet to 1,200 square feet, while increasing product reliability 300 percent, multi-skilling the union-shop work force, and being designated an Industry Week Best Plant for the year 2000^[21] By five years later, rework and scrap had been cut 50 percent, new product introduction cycles 60 percent, and transactions 90 percent, while also increasing inventory turns three-fold and service turn times 30 percent, and being awarded a Shingo Prize for the year 2005.^[22]

It appears to be difficult to isolate how much of those benefits accrue from cellular organization itself; among many case studies researched for this article few include attempts at isolating the benefits. One exception is the contention, at Steward, Inc. (Chattanooga, Tenn.), producing nickel zinc ferrite parts for electromagnetic interference suppression. According to case study authors, cells resulted in reductions of cycle time from 14 to 2 days, work-in-process inventories by 80 percent, finished inventories by 60 percent, lateness by 96 percent, and space by 56 percent.^[23]

Another cellular case study includes quantitative estimates of the extent to which cells contributed to overall benefits. At Hughes Ground Systems Group (Fullerton, Calif.), producing circuit cards for defense equipment, the first cell, which began as a pilot project with 15 volunteers, was launched in 1987. One month later a second cell began, and by 1992 all production employees, numbering about 150, had been integrated into seven cells. Prior to cells, circuit card cycle time, from kit release to shipment to the customer, had been 38 weeks. After the cells had taken over the full production sequence (mechanical assembly, wave solder, thermal cycle, and conformal coat), cycle time had fallen to 30.5 weeks, of which production manager John Reiss attributed 20 weeks to use of a “WIP chart system” by the cell teams and the other 10.5 weeks to the cellular organization itself. Later, when it seemed that the cells were overly large and cumbersome, cell sizes were shrunk by two-thirds, resulting in “micro cells” that cut cycle time by another 1.5 weeks. Finally, by adopting certain other improvements, cycle times had decreased to four weeks. Other improvements included reducing work-in-process inventory from 6 or 7 days to one day and percent defective from 0.04 to 0.01^[24] Switching from a functional (job-shop) layout to cells often costs has a minus net cost, inasmuch as the cell reduces costs of transport, work-in-process and finished inventory, transactions, and rework.^[25] When large, heavy, expensive pieces of equipment (sometimes called “monuments” in lean lingo) must be moved, however, the initial costs can be high to the point where cells are not feasible.^[26]

See also

• Cross-training (business)

References

Further reading

• Black, J. T. (1991). The Design of the Factory with a Future, New York, NY: McGraw-Hill, Inc., 1991.
• Black, J. T. (2000). 'Lean Manufacturing Implementation', in Paul M. Swamidass (ed.), Innovations in competitive manufacturing, Boston, Mass.; London: Kluwer Academic, 177-86.
• Burbidge, J.L. (1978), The Principles of Production Control, MacDonald and Evans, England, ISBN 0-7121-1676-1.
• Brandon, John. (1996). Cellular Manufacturing: Integrating Technology and Management, Somerset, England: Research Studies Press LTD.
• Feld, William M., (2001). Lean Manufacturing: tools, techniques, and how to use them, Boca Raton, FL; Alexandria, VA: St. Lucie Press; Apics.
• Irani, Shahrukh. (1999). Handbook of Cellular Manufacturing Systems, New York, NY: John Wiley & Sons, Inc., 1999.
• Kannan, V.R. 1996. A virtual cellular manufacturing approach to batch production. Decision Sciences. 27 (3), 519-539.
• McLean, C.R., H.M. Bloom, and T.H. Hopp. 1982. The virtual manufacturing cell. Proceedings of the Fourth IFAC/IFIP Conference on Information Control Problems in Manufacturing Technology. Gaithersburg, Md. (October).
• Singh, Nanua and Divakar Rajamani. (1996). Cellular Manufacturing Systems Design, Planning and Control, London, UK: Chapman & Hall.
• Swamdimass, Paul M. and Darlow, Neil R. (2000). 'Manufacturing Strategy', in Paul M. Swamidass (ed.), Innovations in competitive manufacturing, Boston, Mass.; London: Kluwer Academic, 17-24.