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 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. A die is a tool used in manufacturing. However SMED's utility of is not limited to manufacturing (see value stream mapping).
The concept arose in the late 1950s and early 1960s, when Shigeo Shingo was consulting to a variety of companies including Toyota, and was contemplating their inability to eliminate bottlenecks at car body-moulding presses. The bottlenecks were caused by long tool changeover times which drove up production lot sizes. 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. 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||4.8%|
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, Economic Order Quantity) 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.
Effects of implementation
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:
- Stockless production which drives inventory turnover rates,
- Reduction in footprint of processes with reduced inventory freeing floor space
- Productivity increases or reduced production time
- Increased machine work rates from reduced setup times even if number of changeovers increases
- Elimination of setup errors and elimination of trial runs reduces defect rates
- Improved quality from fully regulated operating conditions in advance
- Increased safety from simpler setups
- Simplified housekeeping from fewer tools and better organization
- Lower expense of setups
- Operator preferred since easier to achieve
- Lower skill requirements since changes are now designed into the process rather than a matter of skilled judgment
- Elimination of unusable stock from model changeovers and demand estimate errors
- Goods are not lost through deterioration
- Ability to mix production gives flexibility and further inventory reductions as well as opening the door to revolutionized production methods (large orders ≠ large production lot sizes)
- New attitudes on controllability of work process amongst staff
Shigeo Shingo recognises eight techniques that should be considered in implementing SMED.
- Separate internal from external setup operations
- Convert internal to external setup
- Standardize function, not shape
- Use functional clamps or eliminate fasteners altogether
- Use intermediate jigs
- Adopt parallel operations (see image below)
- Eliminate adjustments
NB External setup can be done without the line being stopped whereas internal setup requires that the line 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:
- OBSERVE the current methodology (A)
- Separate the INTERNAL and EXTERNAL activities (B). 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.
- Convert (where possible) Internal activities into External ones (C) (pre-heating of tools is a good example of this).
- Streamline the remaining internal activities, by simplifying them (D). Focus on fixings - Shigeo Shingo observed that it's only the last turn of a bolt that tightens it - the rest is just movement.
- Streamline the External activities, so that they are of a similar scale to the Internal ones (D).
- Document the new procedure, and actions that are yet to be completed.
- 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.
This diagram shows four successive runs with learning from each run and improvements applied before the next.
- Run 1 illustrates the original situation.
- Run 2 shows what would happen if more changeovers were included.
- Run 3 shows the impact of the improvements in changeover times that come from doing more of them and building learning into their execution.
- Run 4 shows how these improvements can get you back to the same production time but now with more flexibility in production capacity.
- Run N (not illustrated) would have changeovers that take 1.5 minutes (97% reduction) and whole shift time reduced from 420 minutes to 368 minutes a productivity improvement of 12%.
Key elements to observe
|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%|
- shortages, mistakes, inadequate verification of equipment causing delays and can be avoided by check tables, especially visual ones, and setup on an intermediary jig
- inadequate or incomplete repairs to equipment causing rework and delays
- optimization for least work as opposed to least delay
- unheated molds which require several wasted 'tests' before they will be at the temperature to work
- using slow precise adjustment equipment for the large coarse part of adjustment
- lack of visual lines or benchmarks for part placement on the equipment
- forcing a changeover between different raw materials when a continuous feed, or near equivalent, is possible
- lack of functional standardization, that is standardization of only the parts necessary for setup e.g. all bolts use same size spanner, die grip points are in the same place on all dies
- much operator movement around the equipment during setup
- more attachment points than actually required for the forces to be constrained
- attachment points that take more than one turn to fasten
- any adjustments after initial setup
- any use of experts during setup
- any adjustments of assisting tools such as guides or switches
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.
- Study of Toyota Production System, Shigeo Shingo, 1981, p 70
- Shingo, Shigeo (1985). A Revolution in Manufacturing: The SMED System. Stamford, CT: Productivity Pres. pp. xxii. ISBN 0-915299-03-8.
- Theory of Constraints, Eliyahu Goldratt, North River Press, 1990, p 40
- A revolution in manufacturing: The SMED system, Shigeo Shingo, Productivity Press, 1985, p 113
- A study of the Toyota Production System, Shigeo Shingo, Productivity Press, 1989, p 47
- A revolution in manufacturing: The SMED system, Shigeo Shingo, Productivity Press, 1985, p 27