The Pros and Cons of Assembly Cells
Workcells come in a variety of shapes and sizes, and they're often touted as one of the main elements of lean manufacturing. In fact, many people believe that they can implement lean principles simply by installing a few cells on their plant floor.
Many engineers mistake cell optimization for lean production, which is a business system that stresses continuous flow, standardized work, visual management and waste reduction. However, contrary to popular belief, assembly cells are not a mandatory component of lean manufacturing. While they may be closely related, cells don't always fit with a lean manufacturing strategy.
Sometimes, cells aren't even the best approach to take. For instance, cells are often incompatible with low-volume, high-mix production.
Manufacturing engineers must weigh the advantages and disadvantages of cellular assembly. Product variety, mix, design volatility and lot sizes are all important considerations.
A workcell is a group of workstations, machines or equipment arranged so that parts can be assembled progressively from one station to another without having to wait for a batch to be completed or requiring additional handling between operations. Cells may be dedicated to a process, a subcomponent or an entire product. One of the main purposes of a cell is to achieve and maintain efficient continuous flow.
Cellular configurations can be in several forms, but equipment within the cell is normally arranged in close proximity to compress time and space. A cell is typically configured for speed and minimal material handling, and can reap substantial benefits in cost saving, time compression and inventory reduction.
To address flexible manufacturing requirements, assembly cells should be capable of easy and quick reconfiguration. Cells must be mobile to adapt to product changeovers and personnel fluctuations.
"Many workstations are now equipped with casters so they can be easily moved and reconfigured," says Karl Wojcikiewicz, product manager at Bosch Rexroth Corp. (Buchanan, MI). "Ideally, any conversion should take no more than 10 minutes." That flexibility requires quick-disconnects for electricity and compressed air.
Benefits of cellular assembly include shorter lead times, higher productivity, decreased throughput time, increased flexibility, improved quality and increased output. In addition, communication is usually enhanced, because operators work closer to each other. Assemblers can see each process-what is coming and how fast-and one person can perform multiple operations. Also, multiple cells can easily produce multiple product designs simultaneously, making the assembly line more flexible.
Changeovers are easier in a cell and, with better communication between workers, cross-training is simpler. Communication also yields better quality. A cell allows parts to be presented to operators from outside the work area, allowing the workflow to continue uninterrupted. In addition, visual control of work in process (WIP) is easier.
Because of all those benefits, cellular manufacturing is more popular now than ever. "Assembly cells continue to be a great way to run a manufacturing operation," says John O'Kelly, vice president of sales and marketing at Production Basics (Watertown, MA). "They promote teamwork and cross-training among operators, reducing turnover and training costs. That leads to better control over product costs, quality and inventory, which [falls] in line with lean manufacturing."
In an assembly cell, employees are held more accountable for their output and quality of the work. "Managers can pinpoint exceptionally productive cells and identify the weaker cells," O'Kelly points out.
Assembly cells typically require greater operator involvement, because they are a key element in the success of the workstation operation. "Managers are interacting more with [operators] on a project level, working with them as a team," says O'Kelly.
But, contrary to popular belief, shape does not make a cell. Flow makes a cell. Many people incorrectly think that all cells have to be U-shaped. While that is the most popular shape, cells also come in other configurations. For instance, C- and T-shaped cells are often used. Neck-shaped cells, such as X or + configurations, allow four different subassemblies to come together.
"Shape depends on the size and complexity of the assembly," explains Rick Harris, president of Harris Lean Systems Inc. (Murrels Inlet, SC). He says the width of the cell is more important than the shape. "A classic mistake is making the cell too big," Harris points out. "No cell should be wider than 5 feet-just enough room for two individuals to pass through."
A U-shape is commonly used when configuring cells because it minimizes walking distance and allows different combinations of work tasks for operators. Harris says this is an important consideration in lean production, because the number of operators in a cell will change with fluctuations in demand. A U shape also facilitates performance of the first and last steps in the assembly process by the same operator, which is helpful in maintaining work pace and smooth flow.
"Many manufacturers don't know how to design cells correctly," warns Quarterman Lee, president of Strategos Inc. (Kansas City, MO). "All too often, we see workcells where some elements, such as layout, are carefully designed and others come about by happenstance." Lee says U-shaped cells typically work best "with small products, when there's not a lot of material coming into the cell."
The size of the product being assembled also affects cell layout. For instance, automotive parts typically are larger than medical devices, so handling distances and parts replenishment inside the cell must be taken into consideration.
Another misconception is that it's always necessary to use separate cells to assemble separate products. However, cells are not necessarily more flexible than a department-function layout, and can actually offer less flexibility depending on product variety and volumes. Although cells usually result in improved use of floor space, because of reductions in WIP and warehouse inventory, they sometimes actually use more floor space than equivalent functional layouts.
"I've seen some good examples of mixed-model cells," notes Harris. For instance, he says a large electrical equipment manufacturer successfully runs multiple product families through the same cell.
Cells Aren't for Everyone
Despite numerous advantages, workcells are not always the best solution. In fact, some assembly applications aren't conducive to cells.
"High mixes of low volume products can make cells impractical," says Ray Gottsleben, sales and marketing manager at Arlink Workstation Systems (Burlington, ON). "In our business, which focuses largely on electronic and light mechanical assembly, we still see a preference for in-line progressive assembly approaches and department-function layouts."
According to Gottsleben, applications that involve high-cost capital equipment also may not be practical for assembly cells. "Equipment utilization rates are generally lower in cells, and if capital costs are high, this can be a detriment," he explains. "Also, custom products, high precision work or work that has variable cycle times, due to the need to calibrate or tune, don't work well in cells."
When deciding whether or not to use cells, manufacturing engineers must consider factors such as assembly processes and the product being produced. "If a part has a short build time with many components, a cell may not be more productive than a progressive assembly line," notes Bob Simmons, national sales manager at Pro-Line (Haverhill, MA).
"In manufacturing facilities where only one function is being performed, or a whole product is not completed, assembly cells don't make sense," adds Production Basics' O'Kelly. "Assembly cells are best implemented where several tasks are required to complete one unit or product. Also, when mobility and the change in process is not an essential component of the business, assembly cells are not necessary."
Assembly cells make sense in certain situations, but they don't work in all plants. In fact, some manufacturers have implemented cells, but then reverted back to traditional, linear assembly lines.
"Some electronic manufacturers are converting from cells to linear workstations," says Eric Dotson, general manager of GWS Inc. (Kennesaw, GA). "They believe there's more waste within the cell that they can squeeze out. There's a different philosophy in different plants."
For example, with linear flow, five operators may be standing or sitting along an assembly line, with each individual progressively performing one task. In a cellular assembly line, one operator may be performing three assembly steps. "They're really doing the same amount of work, but just doing it differently," says Dotson.
Many engineers fail to look at the fine details of operator activity in a cellular environment. They assume that implementing cells will automatically create an effective production team. However, there are often many important staffing issues that need to be addressed.
"Not including the operator is the biggest mistake engineers make when implementing cells," says O'Kelly. "Operators know how far they can reach, the best places to keep supplies, proximity to coworkers, and the tools they need to get the job done. The operator must accept the assembly cell concept for it to be successful, so [engineers should] spend time promoting and educating their employees, along with encouraging feedback."
With cells, there are more tasks to learn, more skills to develop, more responsibility and more accountability. "Typically, cycle times are shorter and frequencies are higher in a cell," says Chris McIntyre, president of Ergonomics at Work Inc. (Waterloo, ON).
It can be hard for operators to adapt to cells. Because more teamwork is required and there is more dependence on others for personal success, personality conflicts can arise. Transition from individual incentives and rewards to team incentives and rewards can lead to problems.
Some engineers underestimate the training needs for workers who must now become adept at many tasks, often requiring new skills.
"With cells, there can be some resistance from operators," says Harris. "[Unlike traditional in-line assembly methods], there is no rest period after every part is built. We're asking them to work all the time, running multiple tasks, rather than just one task.
"However, with assembly cells, operators often see themselves as being more valuable," adds Harris. "Because it challenges their thinking, boredom is often eliminated."
Job rotation is quite common in a cellular manufacturing environment. "But, it's not always the right thing to do," warns McIntyre. "It can create problems, unless the jobs are quite varied." For instance, he says rotating operators through cells may mean that the ergonomic set up is not matched to the individual, but is a compromise for the group. And, if a cell has two or more people in it, McIntyre says there can be personality conflicts.