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Lean Approaches to Variability

 

The Variability Perspective

Normal CurveMost Lean Manufacturing elements either reduce variability, adapt to it or both. This can be a useful perspective for developing Lean Manufacturing strategy. Here are some examples:

TQM/Six Sigma

Capability analysis, part of SPC, ensures that the inherent variability in a process is small enough to ensure defect-free production. Control charts discover unrecognized sources of variability before they can produce defects.

Cellular Manufacturing

Grouping similar parts (Group Technology) and processing them in workcells reduces the variability from differences in routing. It also reduces the variability associated with transport and batching.

Kanban & Pull Scheduling

Kanban reduces variability from the scheduling system. It smoothes the flow and helps reduce batch sizes.

Team Development

Teams reduce variability in many ways: their lines of communication are shorter and more predictable; their responses to situations more consistent; they enhance the effects of workcells, SMED and other Lean elements. Teams both reduce variability and enhance the ability to cope with it.

Setup Reduction (SMED)

Shigeo Shingo's SMED approach reduces setup cost and enables lot size reduction. This, in turn, reduces the variability from batching. In addition, SMED makes setups not only shorter, it makes them more consistent in terms of time and quality.

Mixed Model Production

Running multiple products on a production line smoothes upstream demand and, hence, reduces upstream variability. Configuring a production line is an example of coping with unavoidable customer demand.

Efficiency, Balance & Variability

Efficiency is the traditional focus of manufacturing managers. However, Little's Law, Theory of Constraints and the relationships shown elsewhere in this series show that, when designing and managing production operations, balance and variability are equally important.

Efficiency-- focuses on accomplishment of individual process steps and completing each step with minimal resource input. Efficiency is well appreciated and most engineering efforts are directed to it.

Balance-- focuses on several or all steps and attempts to balance a resource, i.e., time. Balance is well appreciated in line operations but is often ignored when processes are disconnected. When balance is attempted, it is often based on overall averages rather than balance on each cycle.

Variability-- is the consistency of each step to ensure that each cycle of the process is identical. Variability is often under-appreciated and causes many (perhaps most) manufacturing difficulties.

References

 Sterman, John D., Business Dynamics: Systems Thinking & Modeling for a Complex World, Irwin McGraw-Hill, New York, 2000.

Hopp, Wallace J. and Spearman, Mark L., Factory Physics, Irwin McGraw Hill, New York, 1996.

Forrester, Jay Wright, Industrial Dynamics, Pegasus Communications, (1961)

Stalk, George, Competing Against Time, Free Press, 2003.

Blackburn, Joseph D., Time-Based Competition, Business One Irwin, 1991.

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