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dimension, is performed once the PI team has confidence that it represents the true performance of the process. The team then can better define objectives and the technical approach. In this example, the process could benefit from both improved centering (i.e. perhaps a slight tooling modification) and reduced variability. Evaluating Process

Improvement Alternatives: Over the course of a Six Sigma project, the team analyzes historical and trial data to determine the key input fac- tors affecting the process output. Te intent is to focus on a small number of these factors (the critical few versus the trivial many) and optimize their values to achieve the target performance for the process output. Critical decisions will be made based on samples taken from the process during plant trials. It is easy to see how these deci-

The Six Sigma DMAIC Approach

Define Measure Analyze Improve Control

different aspects of your operation. By definition, Lean Manufactur-

sions could be adversely affected if the samples were contaminated by varia- tion from unknown special causes. If both the baseline and trial data are corrupted, any attempt at draw- ing valid conclusions will be futile. Imagine running an extensive, and perhaps expensive, designed experi- ment only to discover the process was unstable during the trial period and the experiment’s results are suspect.

Lean and Six Sigma One tool to promote process

stability is the effective use of Lean Manufacturing principles. Te rela- tive importance and roles of Lean and Six Sigma efforts is one of the more misunderstood concepts in the industry. Companies debate whether they should implement Lean or Six Sigma, and which will be better for the bottom line. View them not as competing methodologies, but com- plementary approaches that address

ing focuses on identifying and, if possible, eliminating non-value- added steps, i.e. waste. It classifies waste in several well known catego- ries, such as unnecessary motion, inventory, waiting time, etc. From a Six Sigma perspective, these sources of waste also are potential sources of variability in the process out- put. Often, eliminating them will improve process stability. In essence, minimizing the non-

value-added steps focuses the process on its core functions, which even- tually will be the object of process improvement efforts such as Six Sigma. To put it another way, Six Sigma projects are rarely initiated to improve a non-value-added step. Two Lean concepts are of par- ticular significance: • Implementation of Standard Work

• Elimination of in-process waiting time.

ONLINE RESOURCE Read more about lean manufacturing and six sigma at 36 | MODERN CASTING April 2013

Te definition of

Standard Work imposes constraints on the process in the sense that job roles and responsibilities are formally established. It discourages process tweaking and estab- lishes process boundaries that promote stability. Te Standard Work, as initially defined, is not considered permanent. It

is a starting point to help establish the baseline process. As process improvement efforts progress, the Standard Work will have to be revisited and, perhaps, modified significantly.

Eliminating in-process wait- ing time is especially relevant in metalcasting operations, particu- larly high-volume processes that emphasize repeatability. Very few processes in such an operation tol- erate interruptions well, so main- taining a balanced material flow becomes important to the stability of the output quality. Equipment reliability is another critical factor. For these and other reasons, it

is generally beneficial to undertake a Lean Manufacturing program before committing to Six Sigma projects. Lean is easily understood and can be launched with modest levels of training for management, engineers, and operators. Much of it is relatively easy to implement and it provides a quick return on investment. In addition, construct- ing value stream maps can greatly help in prioritizing where to focus more involved process improvement efforts. Undertaking Six Sigma is an

important decision, often with sig- nificant commitment of resources and focused on a problem impor- tant to achieving larger organi- zational objectives. It is therefore critical to ensure the information and process data used for the project are reliable and meaning- ful. You can avoid frustration and wasted effort by ensuring your processes are ready before you launch a Six Sigma project.

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