Single Minutes Exchange of Dies (SMED). A technic that is the core of lean developed by Shigeo Shingo.
But what I learned from my coach was slightly different. He didn’t say that SMED is wrong. It is about how to get there.
It started with “Datum (Point).” Something fundamental in mechanical engineering and geometric dimensioning and tolerancing.
Are you starting an improvement of changeover? Great idea. And when we google this topic, you will run into Shigeo Shingo’s Single Minutes Exchange of Dies (SMED). The SMED is a method of improving a machine's changeover (set up) activity. A changeover starts when a machine stops producing goods and needs to exchange dies, fixtures, or toolings from what is currently being used to the next. It is non-value-adding activity. The shorter time it takes, the better. The shorter the time, we should be able to reduce the economical batch sizes and become flexible to the changes in the market. SMED improvement technics can be used in many other situations. SMED brings nothing but good things to the business.
SMED starts with observing and identifying what work can be done externally and what work must be done internally. Internally means when the machine is stopped for the changeover. A classic example of such improvement is searching & bringing up dies can be done externally. We don’t have to stop the machine to “prepare.” Because it is simple but powerful, SMED applies to many situations. One example we see is the pit stop of the racing car. You do not see the crew searching and bringing tires and other equipment after the race car has stopped at the pit. Everything is prepared before the race car enters the pit. If not, the scene of the crews panicking to search and rushing will be posted as a funny video or not-so-impressive moments of the day on the sports channel.
But my coach always asked something different first for SMED. He asked, “Where (or what) is datum (point)?”
Datum (point) is defined as the following on ISO5459:2011;
one or more situation features of one or more features associated with one or more real integral features selected to define the location or orientation, or both, of a tolerance zone or an ideal feature representing for instance a virtual condition
Note 1 to entry: A datum is a theoretically exact reference; it is defined by a plane, a straight line or a point, or a combination thereof.
Datum should exist for any artificial part. It starts with the design of the part. The Datum of the part and the fixtures should align. But datum is not just engineering knowledge. The datum should be used for daily production. The datum should be referenced first when a part is set to the fixture. Datum should be a part of standardized work. The same thinking applies to changeover. The datum should exist inside the machine and the die so that when they come together, the location, orientation, or both align and are standardized. This assures quality and also time. Instead of “figuring out” where the location or the orientation is, the datum should guide the two into the best condition. [I kept using the word “should” because I have seen so many places where the datum is ignored or destroyed.]
The race car pit stop provides a great example. The race car stops at a position. It sounds simple, but in real life, when I park my car, I have to maneuver my car to stop. You do not see the race car adjusting left-right or back and forth at the pit stop. Yes, the race car driver has far more sophisticated technics than me. But that is not the only reason. The front of the race stops at the location where a crew lifts the vehicle. The datum is the front of the race car. Because this datum is clearly defined, the other crews can wait to conduct their tasks at one position. The tire guy can stay at the tire's location while the gas charger can wait for the nozzle. Each crew’s positions are predefined according to the race car's datum and design. If there were no datum, we should see the teams moving back and forth according to where their race car stopped. In real life, some hang a tennis ball in the garage to guide the car to park at one position so that we can use the space more efficiently.
So why “datum” is vital for SMED?
Because the datum point defines what is internal and external work is. We prepare or conduct external work. But such work is not based on opinion. The race crew should not prepare the tires where they think it is close enough. The specific location of the tires can be defined according to the datum. The external was not done correctly if the tire was not at that location. I often encountered situations where the external work was “poorly” done, while people thought that it was part of internal work. Many internal or external decisions are made randomly. It should not be. It should be scientific. That science starts from the datum.