For decades Toyota was doing just fine in applying and improving TPS on the shop floor day in and day out without documenting TPS theory. Workers and managers were constantly learning new methods and variations on old methods through actual practice on the shop floor. Communication was strong in what was a relatively small company, so “best practices” developed within Toyota spread to other Toyota plants and ultimately to suppliers. But as the practices matured within Toyota, it became clear that the task of teaching TPS to the supply base was never ending. So Taiichi Ohno disciple Fujio Cho developed a simple representation—a house.
The “TPS house” diagram has become one of the most recognizable symbols in modern manufacturing. Why a house? Because a house is a structural system. The house is strong only if the roof, the pillars, and the foundation are strong. A weak link weakens the whole system. There are different versions of the house, but the core principles remain the same. It starts with the goals of best quality, lowest cost, and shortest lead time—the roof. There are then two outer pillars—just-in-time, probably the most visible and highly publicized characteristic of TPS, and jidoka, which in essence means never letting a defect pass into the next station and freeing people from machines—automation with a human touch. In the center of the system are people. Finally there are various foundational elements, which include the need for standardized, stable, reliable processes, and also heijunka, which means leveling out the production schedule in both volume and variety. A leveled schedule or heijunka is necessary to keep the system stable and to allow for minimum inventory. Big spikes in the production of certain products to the exclusion of others will create part shortages unless lots of inventory are added into the system.
Each element of the house by itself is critical, but more important is the way the elements reinforce each other. JIT means removing, as much as possible, the inventory used to buffer operations against problems that may arise in production. The ideal of one-piece flow is to make one unit at a time at the rate of customer demand or takt (German word for meter). Using smaller buffers (removing the “safety net”) means that problems like quality defects become immediately visible. This reinforces jidoka, which halts the production process. This means workers must resolve the problems immediately and urgently to resume production. At the foundation of the house is stability. Ironically, the requirement for working with little inventory and stopping production when there is a problem causes instability and a sense of urgency among workers. In mass production, when a machine goes down, there is no sense of urgency: the maintenance department is scheduled to fix it while inventory keeps the operations running. By contrast, in lean production, when an operator shuts down equipment to fix a problem, other operations will soon stop producing, creating a crisis. So there is always a sense of urgency for everyone in production to fix problems together to get the equipment up and running. If the same problem happens repeatedly, management will quickly conclude that this is a critical situation and it may be time to invest in Total Productive Maintenance (TPM), where everyone learns how to clean, inspect, and maintain equipment. A high degree of stability is needed so that the system is not constantly stopped. People are at the center of the house because only through continuous improvement can the operation ever attain this needed stability. People must be trained to see waste and solve problems at the root cause by repeatedly asking why the problem really occurs. Problem solving is at the actual place to see what is really going on (genchi genbutsu).
In some versions of the “house” model, several of the Toyota Way philosophies are added into the foundation, such as “respect for humanity.” While Toyota often presents this house with the goals of cost, quality, and timely delivery, in actuality their plants follow a common practice in Japan of focusing on QCDSM (quality, cost, delivery, safety, and morale) or some variation. Toyota will never sacrifice the safety of their workers for production. And they do not need to, as eliminating waste does not imply creating stressful, unsafe work practices. As Ohno wrote:
Every method available for man-hour reduction to reduce cost must, of course, be pursued vigorously; but we must never forget that safety is the foundation of all our activities. There are times when improvement activities do not proceed in the name of safety. In such instances, return to the starting point and take another look at the purpose of that operation. Never be satisfied with inaction. Question and redefine your purpose to attain progress.