Single Minute Exchange of Die
From Wikipedia, the free encyclopedia
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. It is also often referred to as Quick Changeover. It is a concept that says all changeovers (and startups) can and should take less than 10 minutes ... hence the phrase Single Minute. Closely associated is an advanced concept of One-Touch Exchange of Die, (OTED), which says changeovers can and should take less than 100 seconds.
[edit] 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.
[edit] Formal Method
There are seven basic steps [1] to reducing changeover using the SMED system:
1. OBSERVE the current methodology
2. Separate the INTERNAL and EXTERNAL activities. Internal activities are those that can only be performed when the process is stopped, while External activities can be done while the last batch is being produced, or once the next batch has started. For example, go and get the required tools for the job BEFORE the machine stops.
3. Convert (where possible) Internal activities into External ones (pre-heating of tools is a good example of this).
4. Streamline the remaining internal activities, by simplifying them. Focus on fixings - Shigeo Shingo rightly observed that it's only the last turn of a bolt that tightens it - the rest is just movement.
5. Streamline the External activities, so that they are of a similar scale to the Internal ones.
6. Document the new procedure, and actions that are yet to be completed.
7. Do it all again: For each iteration of the above process, a 45% improvement in set-up times should be expected, so it may take several iterations to cross the ten minute line.
The SMED concept is credited to Shigeo Shingo, one of the main contributors to the consolidation of the Toyota Production System, along with Taiichi Ohno.
[edit] External links
Advisors Inc.
