Continuous Flow – Cellular Manufacturing
Flow is the result of removing wastes from processes. Cellular thinking is the opposite of Batch and Queue thinking. The purest form of manufacturing or information transfer is One-Piece-Flow. Not all manufacturing or Information transfer can be converted to One-Piece-Flow, however that state is the goal, and it helps us attack issues that would otherwise be disguised, hidden or just accepted as “the nature” of the thing.
As in all things in Lean, standardization is key to the Cellular manufacturing process. We follow an eight step process to convert to Cellular Flow. It is important to group like products and processes to assure compatibility. Although flexibility and mix-model capability is important, too much variation can negatively impact the performance of the Cell.
When wastes are removed what you have left are the Value Added operations or steps. We focus on the Value Added steps and even work them to improve how they are accomplished. The machine and manual operations are optimized individually. We now know the “Cycle Time” for the operation. These are documented.
Now that the individual steps are optimized, they are analyzed and compared to the target throughput of the line or department. What is that target? TAKT time. Takt is a German phrase that roughly means “Beat” or “Pulse”. It is used in manufacturing to refer to a demand RATE. How fast (or slow) should we be producing so that ultimately customer demands are achieved? Understanding TAKT time and comparing to Cycle Time allow us to properly staff the Cell.
We know how long it will take (Cycle Time), we know how long we need it to take (TAKT Time), we staff accordingly. But now we need to know the proper work distribution to assure balance within the cell between operators. When balance is achieved and no operator is burdened with an amount of work that exceeds TAKT time we should be able to achieve customer demand.
In reality there are many other obstacles to Cellular Flow or Cellular Manufacturing. First, in order for the promise of balance and achieving TAKT time to be realized, the performance of the 6 elements of process must be stable and predictable. People, Methods, Materials, Machines, Environment, and Information must demonstrate performance that will be repeatable in the cellular environment. Batch and Queue manufacturing has some benefits.
If a supplying process goes down for any reason, the next consuming process has lots of inventory, “buffer”, to work with still and is “insulated” from the downtime (for a little while anyway) while the supplying process is repaired.
In Cellular manufacturing, there is little to no “buffer” to guard against downtime or other interruptions to product or information flow. This means the processes are critically linked to each other and require high performance from each other. This is why before introducing Cellular structure the 6 elements are reviewed and process capability is understood.
Other challenges to Cellular Flow are human nature and sometimes culture. Often times there is resistance to sharing workload. In some cases there are physical barriers to flow that need to be eliminated.
The appropriate hand offs from one person to the next needs to be regulated within the cell. Since perfect balance and perfect performance are not always achieved, very small buffers between operators can help to minimize those effects.
There can be external influences that negatively impact the Cell’s performance and those need to be addresses as well.
Assuring the Cell performs to design is the effort of follow up and owned by the operators and their supervision. Goals and metrics must be established to track throughput and performance goals as well as Quality targets. These metrics guide the organization to making further improvements and coaching operators when needed.
Lastly, achieving the Cellular state, whether traditional manufacturing or in the administrative environment, is beneficial and impactful. A traditional Kaizen could develop and deliver a Cell in a three-day event.