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. ADie is a tool used in manufacturing. However SMED's utility of is not limited to manufacturing (see value stream mapping).

The method can be extended to reduction of �Cycle Time� for any activity like �Overhauling� of equipment, �Installation� of manufacturing set-up, equipment, Projects like power plant installation & commissioning etc.

 

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 changeover takes a long time then the lost production due to changeovers drives up the cost of the actual production itself. This can be seen from the table below where the changeover 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 changeover included. The Ratio is the percentage increase in effective operating time caused by the changeover. SMED is the key to manufacturing flexibility.

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 changeover 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 changeover 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 changeover time. Wherever the tooling could not be common, steps were taken to make the tooling quick to change.

 

Shigeo Shingo, who created the SMED approach, claims that in his data from between 1975 and 1985 that average set-up 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 set-ups

�                                Simplified housekeeping from fewer tools and better organization

�                                Lower expense of set-ups

�                                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.

1.      Separate internal from external setup operations

2.      Convert internal to external set-up

3.      Standardize function, not shape

4.      Use functional clamps or eliminate fasteners altogether

5.      Use intermediate jigs

6.      Adopt parallel operations (see image below)

7.      Eliminate adjustments

8.      Mechanization

NB External set-up 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 set-up actions are performed while the machine is still running,

B)       Separate external and internal set-up actions, ensure that the parts all function and implement efficient ways of transporting the die and other parts,

C)      Convert internal set-up actions to external,

D)      Improve all set-up actions.

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Formal method

There are seven basic steps to reducing changeover using the SMED system:

1.      OBSERVE the current methodology (A)

2.      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.

3.      Convert (where possible) Internal activities into External ones (C) (pre-heating of tools is a good example of this).

4.      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.

5.      Streamline the External activities, so that they are of a similar scale to the Internal ones (D).

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.

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%.

 

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.