Single Minute Exchange of Die (SMED) is one of the many lean production methods for reducing waste in a manufacturing process. It provides a rapid and efficient way of converting a manufacturing process from running the current product to running the next product. This rapid changeover is key to reducing production lot sizes and thereby improving flow (Mura).
The phrase "single minute" does not mean that all changeovers and startups should take only one minute, but that they should take less than 10 minutes (in other words, "single digit minute"). Closely associated is a yet more difficult concept, One-Touch Exchange of Die, (OTED), which says changeovers can and should take less than 100 seconds.
An increase in effective operating time caused by the change-over. SMED is the key to manufacturing flexibility.
|Changeover time||Lot size||Process time per item||Operation time||Ratio|
|8 hours||100||1 min||5.8 min||480%|
|8 hours||1,000||1 min||1.48 min||48%|
|8 hours||10,000||1 min||1.048 min||5%|
Toyota's additional problem was that land costs in Japan are very high and therefore it was very expensive to store 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" (or EOQ) is a well-known, and heavily debated, manufacturing concept. Historically, the overhead costs of retooling a process were minimized by maximizing the number of items that the process should construct before changing to another model. This makes the change-over overhead per manufactured unit low. According to some sources optimum lot size occurs when the interest costs of storing the lot size of items equals the value lost when the production line is shut down. The difference, for Toyota, was that the economic lot size calculation included high overhead costs to pay for the land to store the vehicles. 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. Indeed the whole debate over EOQ becomes restructured if still relevant. It should also be noted that large lot sizes require higher stock levels to be kept in the rest of the process and these, more hidden costs, are also reduced by the smaller lot sizes made possible by SMED.
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. These common parts or tooling reduced change-over time. Wherever the tooling could not be common, steps were taken to make the tooling quick to change.
Toyota found that the most difficult tools to change were the dies on the large transfer-stamping machines that produce car vehicle bodies. The dies – which must be changed for each model – weigh many tons, and must be assembled in the stamping machines with tolerances of less than a millimeter, otherwise the stamped metal will wrinkle, if not melt, under the intense heat and pressure.
When Toyota engineers examined the change-over, they discovered that the established procedure was to stop the line, let down the dies by an overhead crane, position the dies in the machine by human eyesight, and then adjust their position with crowbars while making individual test stampings. The existing process took from twelve hours to almost three days to complete.
Toyota's first improvement was to place precision measurement devices on the transfer stamping machines, and record the necessary measurements for each model's die. Installing the die against these measurements, rather than by human eyesight, immediately cut the change-over to a mere hour and a half.
Further observations led to further improvements – scheduling the die changes in a standard sequence (as part of FRS) as a new model moved through the factory, dedicating tools to the die-change process so that 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 10 minutes per die, and thereby reduced the economic lot size below one vehicle.
The success of this program contributed directly to just-in-time manufacturing which is part of the Toyota Production System. SMED makes Load balancing much more achievable by reducing economic lot size and thus stock levels.
Shigeo Shingo, who created the SMED approach, claims that in his data from between 1975 and 1985 that average setup times he has dealt with have reduced to 2.5% of the time originally required; a 40 times improvement.
However, the power of SMED is that it has a lot of other effects which come from systematically looking at operations; these include:
Shigeo Shingo recognises eight techniques that should be considered in implementing SMED.
NB External setup can be done without the line being stopped whereas internal needs the line to be stopped.
He suggests that SMED improvement should pass through four conceptual stages:
A. ensure that external setup actions are performed while the machine is still running, B. separate external and internal setup actions, ensure that the parts all function and implement efficient ways of transporting the die and other parts, C. convert internal setup actions to external, D. improve all setup actions.
There are seven basic steps  to reducing changeover using the SMED system:
This diagram shows four successive runs with learning from each run and improvements applied before the next.
|Operation||Proportion of time|
|Preparation, after-process adjustment, and checking of raw materials, blades, dies, jigs, gauges, etc.||30%|
|Mounting and removing blades, etc.||5%|
|Centering, dimensioning and setting of conditions||15%|
|Trial runs and adjustments||50%|
Record all necessary data
Parallel operations using multiple operators By taking the 'actual' operations and making them into a network which contains the dependencies it is possible to optimise task attribution and further optimize setup time. Issues of effective communication between the operators must be managed to ensure safety is assured where potentially noisy or visually obstructive conditions occur.