Pull System, One-Piece Flow, and Single Minute Exchange of Die (SMED) 225

12.4 Single Minute Exchange of Die (SMED)

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 (QCO). Performing faster changeovers is important in manufacturing, or any process, because they make low cost flexible opera-tions possible.

The phrase “single minute” does not mean that all changeovers and startup should take only one minute, but that they should take less than ten minutes (in other words, “single digit minute”). Closely associated is a yet more chal-lenging concept of one-touch exchange of die (OTED), which says change-overs can and should take less than 100 seconds.

The SMED 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 changeover takes a long time, then the lost production due to changeover drives up the cost of the actual production itself. This can be seen from Table 12.1 where the changeover and processing time per unit are held constant while 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 changeover.

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.

The economic lot size (or EOQ) is a well-known, and hugely debated, man-ufacturing 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

Table 12.1

Changeover Time

Changeover Time Lot Size

Process Time Per Item

Operation Time Ratio

8 hr 100 1 min 5.8 min 580%8 hr 1,000 1 min 1.48 min 48%8 hr 10,000 1 min 1.048 min 5%

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226 Lean Manufacturing: Business Bottom-Line Based

overhead per manufactured unit low. According to some sources, opti-mum 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 over-head costs to pay for the land to store the vehicles. Shingo could do noth-ing 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 pos-sible by SMED.

Over a period of several years, Toyota reworked factory fixtures and vehi-cle components to maximize their common parts, minimize and standardize assembly tools and steps, and utilize common tooling. This common parts or tooling reduced changeover time. Wherever the tooling could not be com-mon, steps were taken to make the tooling quick to change.

exaMPle 12.1

The most difficult tooling to change was 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 changeover, 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 process took from twelve hours to three days.

The first improvement was to place precision measurement devices on the trans-fer stamping machines and record the necessary measurements for each model’s die. Installing the die against these measurements rather than by eye immediately cut the changeover 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.Scheduling use of the overhead cranes, so that the new die would be wait-•ing as the old die was removed.

Using SMED processes, Toyota engineers cut the changeover time to less than ten minutes per die, and thereby reduced the economic lot size below one vehicle. The success of this program contributed directly to JIT manufactur-ing that is part of the Toyota Production System. SMED makes load balancing much more achievable by reducing economic lot size and thus stock levels.

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Pull System, One-Piece Flow, and Single Minute Exchange of Die (SMED) 227

12.5 How to Implement Single Minute Exchange of Die (SMED)

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 (see Figure 12.2). 7. Eliminate adjustments. 8. Mechanization

SMED improvement should pass through the following four conceptual stages:

1. Ensure that external setup actions are performed while the machine is still running.

2. Separate external and internal setup actions. 3. Ensure that the parts all function and implement efficient ways of

transporting the die and other parts. 4. Convert internal setup actions to external. 5. Improve all setup actions.

Cabinetassembly

Doorassembly

Finalassembly

Cabinetassembly

Doorassembly

Finalassembly

Cabinetassembly

Doorassembly

Finalassembly

Figure 12.2Adopting parallel operations to a refrigerator assembly.

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228 Lean Manufacturing: Business Bottom-Line Based

12.5.1 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, get the required tools for the job before the machine stops.

3. Convert (where possible) internal activities into external ones (C) (preheating of tools is a good example of this).

4. Streamline the remaining internal activities, by simplifying them (D). Focus on fixings: 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 (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-percent

improvement in setup times should be expected, so it may take sev-eral iterations to cross the ten-minute line.

12.5.2 Key elements to Observe

Look for and record all necessary data:

Shortages, mistakes, inadequate verification of equipment causing •delays, which can be avoided by check tables, especially visual ones, and setup on an intermediary jigInadequate or incomplete repairs to equipment causing rework •and delaysOptimization for least work as opposed to least delay•

Table 12.2

Operation and Proportion of Time

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%

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Pull System, One-Piece Flow, and Single Minute Exchange of Die (SMED) 229

Unheated molds that require several wasted “tests” before they will •be at the temperature to workUsing slow precise adjustment equipment for the large coarse part •of adjustmentLack of visual lines or benchmarks for part placement on the •equipmentForcing a changeover between different raw materials when a con-•tinuous feed, or near equivalent, is possibleLack of functional standardization, that is, standardization of only •the parts necessary for setup, for example, all bolts use same size spanner, die grip points are in the same place on all diesToo much operator movement around the equipment during setup•More attachment points than actually required for the forces to •be constrainedAttachment 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•

12.5.3 effects of implementation

Shigeo Shingo, who created the SMED approach, claims that in his data from between 1975 and 1985 average setup times he has dealt with have reduced to 2.5 percent of the time originally required, a 97-percent 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 capital turnover rates•Reduction in footprint of processes with reduced inventory freeing •floor spaceProductivity increases or reduced production time•

Increased machine work rates from reduced setup times even if •number of changeovers increasesElimination of setup errors and elimination of trial runs reduces •defect ratesImproved quality from fully regulated operating conditions •in advanceIncreased safety from simpler setups•Simplified housekeeping from fewer tools and better organization•Lower expense of setups•

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230 Lean Manufacturing: Business Bottom-Line Based

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 errorsGoods 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 among staff•

12.6 Case Study: Flow Management of an Integrated Circuit Packaging Work-in-Process System

Integrated circuit (IC) manufacturing can generally be divided into five major parts: design, fabrication, chip probe testing, packaging, and final testing. IC packaging transforms wafers into chips. Since the product cycle time is short and the product arrival cannot be planned, IC packaging industry manag-ers heavily rely on shop-floor information to timely handle dynamic shop-floor environments and respond to customer requests. Therefore, improving the competitiveness of a company mostly depends on flexibly incorporating shop-floor information into the production flow and handling manufactur-ing data quickly according to the production flow. A standard IC packaging production flow consists of the following operations:

Wafer grindingWafer sawingDie bondingEpoxy cureWire bondingPost-bonding inspection (PBI)MoldingMarkingDejunking/trimmingSolder platingForming/singulationOpen/short testingLead scan

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