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Lean production in the furniture industry: the double D assembly cell.


There are five generally recognized manufacturing system types and the Lean Production system is the newest manufacturing system design. Lean Production is functionally and operationally different from any other manufacturing system. It uses less of everything when compared to the most common functional job shop manufacturing system: less labor, less manufacturing space, less tooling investment, and less design hours to develop a new product. Lean Production normally means holding less than half the regularly needed stock on-hand. In addition, the implementation of Lean Production by a manufacturer results in fewer defects, and therefore, an increase in quality. The fundamentals of this newest manufacturing system are: manufacturing and assembly cells, pull system methodology, 100 percent good quality, on-time delivery every time, respect for people, and maximum utilization of non-depreciable resources, i.e., people and raw materials. Lean Production uses the cellular manufacturing system for one-piece flow. This system is flexible and designed to produce superior quality products, on-time, at the lowest possible cost, and on a continuous basis. This research utilizes the basic manufacturing philosophies and methodologies for the design and implementation of a Lean Production subassembly manufacturing cell, a non-typical double D configuration, in the furniture industry. A systematic detailed case study illustrates and proves the flexibility of lean manufacturing; it is adaptive and cost effective, improves quality, and is ergonomically correct for workers in the furniture manufacture sector.


Manufacturing systems are considered essential by most economists for the creation of wealth. The wealth generated by manufacturing in turn makes a considerable impact upon the standard of living of a society. The ease, quickness, and economy of product manufacture are considered by many in industry and academia to form the basis of modern manufacturing (Mital 1995). Manufacturing is loosely defined as the creation of wealth by adding value to raw materials. However, manufacturing systems are complex, as shown in Figure 1; they require careful and considered planning to maximize profits while minimizing risk.

Lean manufacturing systems

It has been estimated that U.S. manufacturers invest more than $250 billon per year in remodeling and building new facilities. This investment is seen as necessary to keep up with increasing performance requirements and customer demand. Even though facilities planning and design is an area that has been significantly researched, the fierce competition that is taking place in the global market demands a complete reorientation in the following areas: facility design projects, manufacturing system design, workstation design, and work. The newest manufacturing system, Lean Production (LP), is specifically designed to compete in today's highly competitive manufacturing arena.



The LP system is considered "lean" because the manufacturing system uses less of everything compared to the traditional functional production manufacturing system design (called the job shop). LP uses less human effort, less manufacturing space, less tooling, and less engineering time required to develop a new product. With LP, there is less than half of the work-in-process (WIP) inventory on hand and there are fewer defects, while producing a greater variety of goods (Womack et al. 1991).

Following the LP system scheme, each step in the manufacturing process sequence is triggered by a need for more material at the next downstream stage. In this respect, LP cells utilize a "pull" system for production and control, where production only takes place by a need for more material at the next step (Black 1988). On the other hand, traditional manufacturing systems such as the functional job shop use "push" production control systems, where operations are triggered by material and labor availability rather than by customer demands.

With the design and implementation of LP, manufacturing moved from the Second Industrial Revolution, the era of mass production, to the Third Industrial Revolution (Black and Hunter 2003). The newest manufacturing system design features the manufacturing cell (MC) as the primary subsystem. The MC is the basic building block in the LP methodology (Hunter 1992). Figure 2 shows a typical layout of a small traditional manufacturing system.

Next, Figure 3 illustrates the same traditional system transformed into an LP system composed of three MCs. An MC is composed of dissimilar processes grouped together and operated by multifunctional workers who use the cell to process a family of parts. MCs are typically laid out in a "U" shape. This configuration provides the shortest walking distance for the cell workers. This shape also encourages communication and conserves floor space. Material flow is downstream and information flows upstream, i.e., the cell is designed to be used as a "pull" system. The cell incorporates inventory and production control, quality assurance, continuous improvement, and preventative maintenance functions. The MC has economy of scope, i.e, the manufacturing cell has the ability to produce a wide variety of products at low cost (Black 1999). The four fundamental attributes of LP are lowest unit cost, 100 percent good quality, shortest throughput, and flexibility (Black and Hunter 2003).


Manufacturing cell workers, operating machining cells, typically follow the material flow and move from process to process, unloading finished parts, checking parts, and loading parts. In this type of cell, the worker initiates the machine, the machine automatically processes the part, and then turns itself off at the end of the process cycle. The cell machine tools are built in-house or are modified commercial processes. They are called single-cycle automatics. In final assembly, subassembly, and in some labor-intensive machining cells, the worker moves from process to process or workstation to workstation completing each step of the assembly process at each. When the worker completes a cycle, an assembly is completed.

The worker is the most important and valuable manufacturing resource in the LP philosophy. One or more workers operate a manufacturing or assembly cell. Each worker should be multifunctional, i.e., each worker should be able to set up and operate each of the cell's processes or carry out all manual work. This is the goal of the cell and, in reality; this multifunctionality must be reached in time. Quality measure is an important aspect of the LP system. In the LP system, each worker is a quality inspector. The worker should have all necessary tools and equipment to ensure their production is 100 percent good quality.

LP is a manufacturing philosophy determined to maximize the two most valuable non-depreciable resources of every manufacturing organization: people and materials. Manned cells are important because even after automation and mechanization, manual work is still necessary mainly because humans are highly flexible and creative. Automation and mechanization have limits (Schonberger 1986). The normal goal of the LP cell designer is job enlargement not job simplification. Job enlargement, adding additional work tasks to the worker, lends itself to beneficial ergonomic effects because the added time required to do more work allows the human body more time to heal micro-injuries. This type of injury is the precursor to longterm chronic work-related musculoskeletal disorders.

The functional job shop

The traditional designed manufacturing system, the functional job shop (Fig. 2), has been used since the beginning of the First Industrial Revolution. In that era, it was necessary for machine tools and processes to be grouped according to type because of the overhead power system used then. Because of this, different departments emerged, such as the lathe department, the milling department, and so forth; all depend upon an overhead power shaft to supply the correct operating speed for a given process.

Routing sheets are used to guide the work through the archaic system. Normal operation of functional jobs dictates one worker per process. Functional job shops are normally capable of producing many varieties of products because of the high skill level of employees and the versatile processes found in job shops. However, most job shops build by the push methodology, which in turn requires extra inventory to be carried by the system. These very common manufacturing systems are batch and queue operations driven by labor/machine availability and schedules.

Lean production system design

With intense international competition, the success of a manufacturing company depends on the design of its manufacturing system. Black (1988) provides a definition of a manufacturing system as, "a complex arrangement of physical elements characterized by measurable parameters." Manufacturing system physical elements include: processes, tooling, material-handling equipment, and most important, the factory workers. Different systems will result in different behavior and outcomes as tracked by selected measures. One of the most critical measurable parameters for evaluating manufacturing performance is time. By systematically reducing the manufacturing interval and its variation, companies can achieve a minimal level of success. However, reduction of manufacturing time via the adoption of LP philosophies and methodologies will lead the company to world-class competitor status.

An LP system design is composed of manufacturing and assembly cells linked together with a physical production and inventory control pull system. Simply moving machines and personnel into a typical cell configuration will not have a positive impact upon cycle time, WIP, quality assurance, or preventative maintenance. These vital subsystems must be integrated into the manufacturing area. The LP design is readily integrated, meaning that the critical control functions, such as quality control, production control, inventory control, and process control, are subsystems. Thus, these subsystems become an integral part of the holistic manufacturing system. Integration is accomplished through proper system design or reengineering.

The LP strategy involves a manufacturing systems design that permits rapid deployment of new products and product design changes in addition to rapid production of existing products, so it is the winner when it comes to time-based competition. In essence, the system design accounts for both the needs of the user and the buyer of the systems products. Thus the design of the manufacturing system is critical to long-term success of a manufacturer. The purpose of designing a manufacturing system is to create a physical entity that minimizes the expenditure of non-depreciable resources, such as materials and labor, while satisfying the specified functional requirements of the system design.

The implementation of LP is an evolutionary project that restructures the factory floor. The redesign of the manufacturing system must satisfy the needs of external customers (those who buy the products) as well as the internal customers of the company (the users of the system). As for the external customer, the system must produce attractive, low-cost, and superior-quality products. These high-quality products must be manufactured in a timely manner in order for the factory to be competitive. To attain internal customer satisfaction, the manufacturing system must be designed with the following functional requirements: safety, flexibility, controllability, unique process technology, waste free. Of these, the most important characteristic is flexibility, i.e., the ability to adapt rapidly to changes in customer demand and product design changes. The LP system has been developed to accomplish these functional requirements.

Cellular manufacturing

The basis building block of the LP system is the manufacturing cell or assembly cell. The LP system is composed of manufacturing and subassembly cells linked by a pull kanban system that holds and physically controls in-process inventory between cells. In manufacturing cells, processes are grouped according to the sequence of manufacture needed to make a family of parts or products. This is Group Technology and is usually directed at product families. A cell usually includes all the processes needed for a complete part or assembly. The cell functions like a flow line manufacturing system in that parts flow, by single piece, from process to process (or workstation to workstation). Small, strictly controlled batches of parts move between cells.

Manufacturing and assembly cells are designed for flexibility (Schonberger 1982). In order to add to this flexibility. LP cells are typically arranged in a U-shape so that workers can walk the shortest distance from process to process, loading and unloading parts. Thus cell workers are standing, walking, and multifunctional. In addition, they are empowered to make decisions about the processing functions; for instance, they have line-stop authority in case their cell has a quality problem.

All manufacturing systems require inventory. The material between cells is WIP inventory, while material inside the manufacturing cell is stock-on-hand (SOH) (Monden 1983). Whether it is WIP or SOH, inventory is the lifeblood of the system. In the LP system, inventory is held in kanban links between the cells, carefully controlled and minimized. These links serve as carefully controlled inventory buffers. Downstream cells and final assembly lines are protected against problems in the upstream cells by the inventory in the kanban links held between cells.

The requirements of the manufacturing system dictate the cycle time for the cell. The system cycle time, called takt time, is the production drumbeat for the system. All the cells in the plant are designed to produce parts as needed, when needed, by the downstream processes, and assembly lines. In the LP factory, manufacturing cells produce components for the subassembly and final assembly lines. The cell is designed to provide parts at exactly the rate the subassembly cell requires the parts and no faster. Piece parts are fabricated at the rate needed by the cells in accordance with the system takt time.

At each process or workstation, the operator performs various manual tasks such as unloading, inspecting, deburring, or loading parts into a process. The time to perform setups is short because the processes are dedicated to a family of parts that flow through the cell (Suzaki 1987). The cycle time reflects the time needed for the worker(s) to complete a walking loop through the cell or cells and to perform all the manual operations at each machine. Therefore, adding or deleting workers can quickly alter cycle time. The LP design relaxes the line-balancing problem, common to flow lines and transfer lines, while greatly enhancing flexibility. There is no need to balance the process time. It is only necessary that no process time be greater than the required cycle time.

Clearly, if product demand is higher than cell capacity, system needs are not being met. One solution is to clone the cell to double the capacity. Proven manufacturing capacity is, therefore, quickly doubled, and so is flexibility. When the cell is replicated, capacity is actually more than doubled. Suppose the cell is manufacturing two parts, A and B, and demand reaches the point where the cell must be replicated. Now one cell can make all "As" and the other cell can make all "Bs." Setup between A and B is eliminated as each cell becomes dedicated to one part. However, if demand should decrease, then one of the cells could go back to making both A and B. Excess capacity would then be available to add new parts to the cells (Monden 1983). Of course duplicate cells making the same product violates the fundamental concept of one serial flow for each part. Duplicate cells will add variation to the components produced by these two cells.

Education and training issues

The systematic conversion of a factory to LP must include education and training of the organization's most valuable resource: its people. We believe that this education should occur on several fronts. First, top management and staff need an overview of LP philosophies and methodologies. This will include an overview seminar on LP. It is absolutely necessary for the top manager to be proactive for this system's change and be a vocal advocate for the change that will occur. Next, several detailed training sessions for engineering and factory floor supervisors should occur. These intense sessions should include ample time for discussion. Third, the workforce, including foremen, should be given an introduction seminar on LP methods, including a general discussion on how LP will affect the workers. In every case, it must be emphasized that LP adoption will make the workers' jobs easier, safer, and more rewarding. Last, a cell design and implementation team should be formed that includes at least one worker. This is important to keeping the workforce informally informed of LP progress.



The case study

The Franklin Corporation, a furniture manufacturer, has been building recliner chairs and other upholstered furniture since 1970. The headquarters and main manufacturing facilities are located in Houston, Mississippi. The company manufactures a large variety of mechanical upholstered furniture while employing approximately 1,200 people. Franklin has vertical depth in its manufacturing operations, processing much of its own raw materials. For instance, Franklin does most all processing such as processing frame components through a cutting edge dimension mill; computer numerical controlled (CNC) cutting of foam, fabric, and leather; sewing; sawing and finishing wood frame materials; CNC routing of plywood components; stamping metallic recliner components; through the subassembly to final assembly and packing of the enormous variety and styles of recliner chairs, recliner sofas, sleeper sofas, loveseats, stationary leather sofas and chairs, and other products. Franklin management decided that in order for the company to not only survive intense foreign and domestic competition but thrive, that they would have to adopt a new way of doing business: LP philosophies and methodologies. The basic building blocks of LP are the manufacturing and assembly cells. This research describes the LP conversion of a flow line to a subassembly cell.

The old flow line

The hardware assembly flow line (Fig. 4) was decided upon by Franklin management and engineering as a good candidate for LP conversion. This particular flow line assembled four sizes of hardware subassemblies for recliners and supports six final assembly lines. The subassembly flow line consisted of two sections (Fig. 4) and was a true flow line with one worker per workstation. The upper portion of the flow line was a straight table with four workstations for the initial assembly of the primary wood and hardware frame. Four workers manned this area and supplied a large queue at the end of the assembly bench with the major subassembly frame. Here the hardware frames remained until passed to the lower circular flow line portion.

On the circular portion of the flow line (Fig. 4), roller tables had assembly fixtures mounted on top and were used to build the subassembly. There the newly assembled frames were passed to the flow line circle and mounted on a movable fixture table. The fixture table supported this frame and the remainder of the hardware assembly components. The movable assembly fixture table, with the freshly mounted primary wood hardware frame, was rolled from workstation to workstation around the interior of the flow line circle. The assembly table was guided by a metal circular perimeter. As the fixture table progressed around the circle, various components were added to the assembly by each workstation worker. The table fixtures rolled on a rough concrete floor and were difficult to push from flow line workstation to workstation by the five flow line circle workers.

The old hardware assembly flow line required a total of 11 workers and produced one completed unit approximately every 30 seconds. The straight (upstream) table portion of the flow line required four workers, while the lower circular area utilized five workers. The flow line also employed two people who had various tasks; they were primarily material handlers. These two workers load and unload assembly fixtures and keep the assembly areas supplied with components. The production rate for the flow line yielded roughly 860 hardware assemblies per 9-hour shift.

Double D cell

The design of the double D hardware assembly cell (Fig. 5) is considerably different than the former circular flow line. The initial straight flow line portion remains the same but the lower circular area was completely redesigned. This area was replaced by two rectangular or double D shaped cell areas. The reason for deciding upon two small Ds rather than one large circular or rectangular shaped cell was because of the shorter distance the workers would have to walk in order to produce the maximum daily requirement of hardware units. In addition, the roller conveyor for a circular configuration was much more expensive than straight sections that form the DD configuration. The double Ds also give an additional element of flexibility to the cell. For example, one D may be utilized for one particular size hardware subassembly and the second D for another size hardware unit. One of the most important functions of the DD configuration is its ability to throttle the cell's output extremely accurately and almost effortlessly. The supervisor can simply add or take out workers to match the desired output. Thus, each D may be manned according to the production requirements for a particular shift. The straight configured flow line supplies both Ds with the basic hardware component upon which the subassembly is built. The two Ds were laid out back to back and the left side will have material flow in a counterclockwise direction while the right side will have the material flow in the clockwise direction (Fig. 5). The assembly fixtures are no longer mounted to roller tables but instead travel around the assembly cell on 30-inch-wide gravity roller conveyors. The roller conveyor provides an almost effortless method of moving the assembly fixtures to the cell workstations.

The DD cell operates in a typical assembly cell manner. The workers start the assembly process by taking an empty fixture and adding a cam & base component. Sometimes the material handler will place the cam & base to the fixture for the cell worker. Then the worker will systematically build the complete hardware assembly by moving the fixture to the first workstation then on to the second. At each workstation, various components are added to the cam & base. This process continues from workstation one through five. After the assembly process at station five is completed, the fixture is rolled to the exit area and a material handler removes the completed hardware unit. The completed hardware assembly is removed from the assembly fixture and placed on a rack. The empty fixture is pushed toward the cell load area and is taken by a cell worker. The worker or material handler then loads a cam & base, thus initiating a new assembly cycle.

It is planned that a kanban inventory and production control system will be implemented to regulate and reduce hardware assembly queues by utilizing these subassembly racks. Presently, there is a large queue of inventory between the hardware flow line and the final assembly areas. A kanban subsystem will drive the inventory queue to the lowest workable level. Reduced WIP inventory will reduce carrying costs, thus add to the bottom line and profitability. Kanban will drive continuous improvement efforts; therefore, increasing quality while improving productivity.

The former circular flow line was producing at the desired rate and was capable of maintaining subassembly requirements by the six downstream final assembly lines. However, the flow line did not take advantage of the benefits of LP. The circular flow line conversion to an LP assembly cell provides several important benefits (Table 1).

A productivity increase was expected after the conversion of the older flow line to a modern LP assembly cell (Table 2). Table 2 reflects the initial results, which are based upon a 430-minute (25,800 sec.) shift.

The assembly cell supervisor can accurately adjust the cell's output by simply adjusting the number of workers in the cell. The average, and in some cases estimated, output columns in Table 2 illustrate the flexibility of the LP methodologies. The cell's output can easily be throttled up or down to meet the needs of the manufacturing system, simply by adding or removing workers. The output columns of Table 2 provide a range of from 226 hardware units for only 1 worker working in 1 of the double D cell areas to a fully manned situation where there would be 10 workers in the 2 double D cell areas producing 2,469 units. However, once either one of the double D cell areas is manned by five workers the assembly cell then becomes a flow line, it would no longer retain the flexibility of a cell and would require line balancing. Line balancing is a moot point as long as there are more cell workstations than there are cell workers.

The system daily volume demand for hardware units is roughly 860 per shift. This output is produced by the circular flow line with 11 workers. It is estimated that the assembly cell will produce 956 units per shift with slightly less than 10 workers. The 100 or so extra units that could be produced by the cell will not actually be manufactured because the supervisor will remove a worker (or part of a worker) to adjust the cell output to the required 860 units. In addition, it is estimated that one material handler will be removed from the cell. Therefore, it is estimated that the cell will produce the required number of hardware units with slightly less than nine workers. This will provide a productivity savings equivalent to slightly more than two workers' wages and benefits.


The subassembly cell provides various ergonomic benefits for the cell workers. Among these are rubber mats to provide a safe, shock-absorbing, surface for the standing and walking cell workers. The cell operates by having cell workers move from workstation to workstation moving the hardware assembly fixtures along roller conveyors. The mats will provide shock absorbance to reduce worker fatigue as they walk around the cell.

The workers are standing and walking. Numerous physiological studies have proved walking to be beneficial to workers. The walking benefits include reduced/eliminated risk of venous pooling (thus reducing the risk of deep-vein thrombosis), increased bone strength, reduced cholesterol and blood vessel plaque, and healthier hearts (Mital 1995).

The components and tools required to build a complete assembly will be provided in an ergonomically correct manner. The free movement afforded by assembly fixtures operating on gravity roller conveyor reduces the possibility of back and shoulder work-related muscular skeletal disorders (WMSDs).

Repetitive motion injury risk is reduced dramatically by cellular manufacturing's job enlargement methodology. Under this scheme, workers have more tasks to carry out on each cycle around the cell, thus giving micro-injuries time to heal. With less work, the same micro-injuries would occur in shorter intervals; thus not allowing sufficient time to heal. Thus, with insufficient time to heal, this condition leads to repetitive motion disorders such as carpal tunnel syndrome and other WMSDs.

Cell results

The double D cell has to this point met its expected results (Table 2). The average daily output is 950 units and is in line with our earlier estimated cell output of 956 units per 9-hour day. This yields approximately an 11 percent productivity increase in output. In addition to yield increases, the double D cell is also enjoying a direct labor reduction of two workers. The old circular system employed 11 workers including two material handlers. The double D cell requires only nine workers including one material handler. This is a real productivity increase where the subassembly cell is producing more product with fewer workers.


The reengineering of the Franklin subassembly flow line into a manufacturing cell signaled the beginning of the factory-wide conversion to LP. The cell consists of an unorthodox design consisting of two double D sections supplied by a traditional straight-line flow line. The design allows easy throttling of the cell output rate by simply adding or removing workers. The cell has benefited the company by providing real productivity gains amounting to 18 percent.

The DD design provides several important ergonomic and physiological benefits including a shorter walking distance for cell workers, easy communication between workers, minimum WIP, and reduced floor space.

LP implementation requires a systems-level change for the factory, i.e., a change that will impact every segment of the company, from accounting to shipping. LP system implementation is not turning a leaf but rather it is growing a new tree.
Table 1. -- Cellular assembly benefits.

Cellular assembly benefits

Productivity increase
Less labor required
Improved quality
No line balancing
Improved ergonomics for workers
Continuous process improvement

Table 2. -- Double D assembly cell (five stations each D element).

Personnel Est. output Cycle Personnel two Est. output Effective
 one D one D time D elements two D cycle time
 (pcs./shift) (sec.) (pcs./shift) (sec.)

 1 226 114.0 2 453 57.0
 2 478 54.0 4 956 27.0
 3 733 35.3 6 1,462 17.7
 4 981 26.3 8 1,962 13.2
 5 1,234 20.9 10 2,469 10.5

[c]Forest Products Society 2004.

Forest Prod. J. 54(4):32-38.

Literature cited

Black, J.T. 1988. The design of manufacturing cells. In: Proc. of Manufacturing Inter. '88. pp. 143-157. Auburn Univ., Auburn, AL.

___________. 1999. Black's blueprint for lean manufacturing. Unpublished manuscript. Industrial and Systems Engineering Dept., Auburn Univ., Auburn, AL.

___________ and S.L. Hunter. 2003. Lean manufacturing systems and cell design. Soc. of Manufacturing Engineers, Dearborn, MI. 336 pp.

Hunter, S.L. 1992. The design and implementation of a manned remanufacturing cell: A case study. Proc. of Am. Production & Inventory Control Society (APICS) Aerospace & Defense Symposium. APICS, Alexandria, VA. p. 177.

Mital, A. 1995. The role of ergo in designing for manufacturability and humans in general in advanced manufacturing technology: Preparing the American workforce for global competition beyond the year 2000. Inter. J. of Indus. Ergonomics 15(2):129-135.

Monden, Y. 1983. Toyota Production System. Industrial Engineering and Management Press, IIE, Norcross, GA. 247 pp.

Schonberger, R.J. 1982. Japanese Manufacturing Techniques: Nine Hidden Lessons in Simplicity. The Free Press, New York. 260 pp.

___________. 1986. World Class Manufacturing: The Lessons of Simplicity Applied. The Free Press, New York. 253 pp.

Suzaki, K. 1987. The New Manufacturing Challenge. The Free Press, New York. 255 pp.

Womack, J.P., D.T. Jones, and D. Roos. 1991. The Machine that Changed the World: The Story of Lean Production. First Harper Perennial Publishers, New York. 323 pp.

Steve L. Hunter

Steven Bullard*

Philip H. Steele*

The authors are, respectively, Associate Professor/Manufacturing Systems Engineer, Forest Products Dept., Forest & Wildlife Research Center, Mississippi State Univ. (MSU), Mississippi State, MS 39762-9820; Professor and Director, The Institute for Furniture Manufacture and Management, Forest & Wildlife Res. Center, MSU; Professor, Forest & Wildlife Res. Center (MSU). The authors would like to acknowledge the financial support for this study provided by Research Work Unit NE-4803, Economics of Eastern Forest Use, Forestry Sci. Lab, USDA Forest Serv., Princeton, WV, under Cooperative Agreement 010-CA-1124234-021. This paper was received for publication in October 2002. Article No. 9558.

*Forest Products Society Member
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Author:Hunter, Steve L.; Bullard, Steven; Steele, Philip H.
Publication:Forest Products Journal
Geographic Code:1USA
Date:Apr 1, 2004
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