Quick Changeover (SMED)
As lean production is dependent upon small lot sizes, small lot sizes are dependent upon quick changeovers. If set-ups or changeovers are lengthy, it is mathematically impossible to run small lots of parts with low inventory because large in-process inventories must be maintained to feed production during changeovers.
For example, large stamping dies commonly took hours to change until the development of quick die change methods (Single Minute Exchange of Die, or SMED). Long changeovers necessitate cycle times well below the Takt time, and hence a buildup of inventory to supply production while the die is being changed (see Pull Scheduling for a discussion of Takt time).
The editors are familiar with an appliance plant that is a good example of changeover problems. In order to minimize capital investment when the plant was launched, many dies were designed to make multiple parts. Not a bad idea in and of itself, but to accomplish this mission the dies had to be reconfigured in the tool room by adding or removing bolt-on modules.
As a result, changeovers between parts took as long as eight hours. Massive in-process inventories were required to maintain production levels while the dies were reconfigured. The short-term decision to reduce investment through common dies was more than offset by the increase in inventory and tool-room costs! The System Dynamics were clearly not considered when the initial investment decision was made.
Single Minute Exchange of Die (SMED)
SMED was developed by Shigeo Shingo in Japan in the sixties and early seventies at Toyota and other Japanese firms. The impetus was to reduce costly inventories and improve efficiency.
An important foundation to the SMED system is the distinction between changeover work that occurs while the machine is not running, called Internal Setup, and preparatory work that occurs while the machine is running, called External Setup. At the point in time when SMED was developed, almost all changeover work was performed while the machine (press) was down.
The SMED philosophy breaks down into four stages:
A useful comparison is the pit crew of a racing team changing tires.
If the tools and parts are not prepared until the car enters the pit, then the pit stop will be lengthy. If the parts and tools are prepared beforehand (External Setup), the crew's every move is choreographed for efficiency, and special tools and fittings are used to increase speed, then the pit stop will be short.
Changeover Time Reduction Techniques
Specific techniques to reduce changeover time are:
Staged Tooling and Fixtures
If a part or tool must be placed into a fixture prior to an operation, such as machining, add a fixture so that while one part or tool is in the machine, the second can be set up in the fixture.
Operations Conducted in Parallel
Consider the pit crew example again: if the four tires are changed simultaneously by four crew members that pit stop is a lot faster than if the crew moves from tire to tire in sequence.
Tools that are the same size with the same attachment location can be changed much more quickly. Consider how long it would take a pot crew to change tires if the replacement tire had a different lug nut configuration. In the same vein, a common die height allows the shut height of the press to be set to a common dimension.
Standardized attachment points and methods reduce confusion and the number of fasteners - saving time. Standardization also applies to tool storage organization so that tools can be quickly located. See the 5S tutorial of the Toolbox for additional discussion of workstation organization.
Clamps, cams, and u-shaped washers are used for quick attachments. If a bolt can be pre-tightened during external setup, then it only needs one final turn to provide clamping force during internal setup. Another example to consider is the quick-release hub on a bicycle tire, which facilitates rapid changeover by a lever and cam device so that a wrench and the time that goes with it is not required.
Use locating pins, guides, and fixtures to completely eliminate adjustment.
Duplicate tooling can minimize or eliminate the need for changeovers, and the cost can quickly be offset by reduction in inventory and set-up labor costs. Simple pre-set gauges and templates can speed up changeovers without large expense.
Assisted Tool Movement
Large dies or other heavy tools can be moved much faster using dedicated die carts, roller tables, or small conveyors than with forklifts or cranes. Mechanized tool change equipment allows pre-staging of the replacement tool, and may also facilitate changeovers by the base equipment operator without additional indirect labor.
Benefits of reduced changeover time
By incorporating the above methods, massive dies are commonly changed in less than 10 minutes - setups that used to take hours. Benefits of reduced changeover time extend beyond the direct reduction of inventory to include: