TQM CONCEPTS

Single Minute Exchange of Die

Single Minute Exchange of Die (SMED) is one of the many lean production methods for reducing waste in a manufacturing process. It forms one of the key factors behind the success of the Just – in – Time (JIT) concept. It provides a rapid and efficient way of converting a manufacturing process from running the current product to running the next product. It is based on the concept of the Seven Wastes. Reducing time in changing over from one die to another is considered a saving in non-value adding process time. As reducing the change over time also results reduced inventories due to shorter runs, it is also considered a saving in the inventory. It is also often referred to as Quick Changeover (QCO). Closely associated is a yet more challenging concept of One- Touch Exchange of Die, (OTED), which says changeovers can and should take less than 100 seconds.

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 economic lot size is calculated from the ratio of actual production time and the 'change-over' time; which is the time taken to stop production of a product and start production of the same, or another, product. If change-over takes a long time then the lost production due to change-overs drives up the cost of the actual production itself. This can be seen from the table below where the change-over and processing time per unit are held constant whilst the lot size is changed. The Operation time is the unit processing time with the overhead of the change-over included. The Ratio is the percentage increase in effective operating time caused by the change-over.

Due to this there was a tendency to produce in large lots based on the concept of ‘economic lot size’ leading to higher inventories. Toyota's additional problem was that land costs in Japan are very high and therefore it was very expensive 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.

If the change-over costs could be reduced, then the economic lot size could be reduced, directly reducing expenses. 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. This 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.

The most difficult tooling to change were the dies on the large transfer-stamping machines that produce car vehicle bodies. The dies must be changed for each model. They weigh many tons, and must be assembled in 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, position the dies in the machine by human eyesight, and then adjust their position with crowbars while making individual test stamping. 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.

Further observations led to further improvements: Scheduling the die changes in a standard sequence, 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. Note: The Single in Single Minute Exchange of Die meant single digit minutes, so less than 10 minutes, not less than 1 minute.

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.

Implementation

Shigeo Shingo recognizes eight techniques that should be considered in implementing SMED.

1. Separate Internal from External setup operations
2. Convert Internal to External setup
3. Standardize function , not shape
4. Use functional clamps or eliminate fasteners altogether
5. Use intermediate jigs
6. Adopt parallel operations
7. Eliminate adjustments
8. Mechanization

He suggests that SMED improvement should pass through four conceptual stages:

1. ensure that external setup actions are performed while the machine is still running,
2. separate external and internal setup actions, ensure the parts all function and implement efficient ways of transporting the die and other parts,
3. convert internal setup actions to external,
4. improve all setup actions.

Formal method

There are seven basic steps 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.

SMED Methodology

Advantages of using multiple shift changeovers - perhaps to serve several clients or to make several models and allow load balancing.

This diagram shows four successive runs with learning from each run and improvements applied before the next.

How to achieve it?

Separate the internal from the external tasks and then improve their efficiency.

Use multiple operator optimized task attribution to further optimize setup time