The Pros and Cons of Cells
Many assemblers believe they can implement lean principles simply by installing a few cells on the plant floor. However, contrary to popular belief, assembly cells are not a mandatory component of lean manufacturing. In fact, there are times when cells can even be counterproductive. For instance, cells are often incompatible with low-volume, high-mix production. It takes hard work and heads-up planning to assure the workcell concept will succeed.
A workcell is a group of workstations or machines arranged so that parts can be assembled progressively from one station to another without additional handling or having to wait for a batch to be completed. 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.
Cell configurations can take several forms, but equipment within the cell is normally arranged in close proximity to compress time and space. To address flexible manufacturing requirements, assembly cells should be mobile to adapt to product changeovers and personnel fluctuations. Many workstations, for example, are equipped with casters so they can be easily moved and reconfigured. They may also come with quick-disconnects for electricity and compressed air.
The benefits of cellular assembly include shorter lead times, higher productivity, decreased throughput time, increased flexibility, improved quality, less unwanted movement and increased output. Because operators work in close proximity to each other, communications are also enhanced. Assemblers can see each process-what is coming and how fast-and one person can perform multiple operations. 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. 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.
Contrary to popular belief, shape does not make a cell. Flow makes a cell. Many assemblers incorrectly think that all cells have to be U-shaped. But 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 an X configuration, allow four different subassemblies to come together.
The U-shape is commonly used when configuring cells because it minimizes walking distance and allows different combinations of work tasks for operators-an important consideration in lean production. 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.
Cells Aren't for Everyone
Text:Despite their many advantages, workcells are not always the best solution. In fact, some assembly applications aren't conducive to cells.
Again, high mixes of low volume products can make cells impractical. For this reason, electronics and light mechanical equipment assemblers may prefer in-line progressive assembly approaches and department-function layouts. The same may be true when assembling custom products, or when doing high-precision work or work that has variable cycle times due to the need to calibrate or tune.
Ultimately, assembly cells are best implemented where several tasks are required to complete a single product, especially in an environment where mobility and process change is a factor. Be aware that there are a number of cases in which manufacturers have implemented cells, and then reverted back to traditional, linear assembly lines.
In addition, it's important to be sure and determine how exactly work will be performed within a cell. Many engineers fail to look at the fine details of operator activity in a cellular environment, which can cause problems down the road. 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.
First among these, is including input from production workers when implementing cells. These individuals know how far they can reach, the best places to keep supplies and the tools they need to get the job done. Production workers must accept the assembly cell concept for it to be successful, so feedback is essential.
Bear in mind that 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.