Single Minute Exchange of Die

History

The concept arose in the late 1950s and early 1960s, when Shigeo Shingo, chief engineer of Toyota, was contemplating Toyota's inability to construct vehicles in maximally efficient economic lots.

The problem was that land costs in Japan are very high, and Toyota could not afford the space to store economic lots of its vehicles. The result was that its costs were higher than other producers, because it had to produce vehicles in uneconomic lots.

The "economic lot size" is a well-known manufacturing concept. Historically, the overhead costs of retooling a factory were minimized by calculating the number of items that the factory should construct before changing to another model. The calculation of lot size trades off the interest costs of the extra inventory against the cost of the change-over. The optimum lot size occurs when the interest costs of the lot size of items equals the value lost when the production line is shut down. The problem was that the economic lot size calculation for Toyota included high overhead costs to pay for the land to store the vehicles.

Engineer Shingo realized that both interest and lost production are waste. They should be minimized. Engineer Shingo could do nothing about the interest rate, but he had total control of the factory processes. If the change-over costs could be reduced, then the economic lot size could be reduced, directly reducing expenses.

Over a period of several years, Toyota reworked factory fixtures and vehicle components to maximize their common parts, minimize and standardize assembly tools and steps, and utilize common tooling. Wherever the tooling could not be common, steps were taken to make the tooling quick to change.

The most difficult tooling to change were the dies on the large transfer-stamping machines that produce vehicle bodies. The dies must be changed for each model. They weigh many tons, and must be assembled to the stamping machines with tolerances of less than a millimeter.

When engineers examined the change-over, they discovered that the established procedure was to stop the line, let down the dies by an overhead crane, assemble the dies to the machine by human eyesight, and then adjust their position with crowbars while making individual test stampings. The process took from twelve hours to three days.

The first improvement was to place precision measurement devices on the transfer stamping machines, and record the necessary measurements for each model's die. This immediately cut the change-over to a mere hour and a half.

Observations led to further improvements: Scheduling the die changes in sequence as a new model moved through the factory, dedicating tools to the die-change process, so all needed tools were nearby, and scheduling use of the overhead cranes, so that the new die would be waiting as the old die was removed.

Using these processes, Toyota engineers cut the change-over time to less than one minute per die, and thereby reduced the economic lot size below one vehicle.

The success of this program led directly to just-in-time manufacturing, a further generalization of this concept.

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