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Process performance is the voice of the process. In order to evaluate their validity, the RUMBA method from the Six Sigma field is used, where R stands for Reasonable, U for Understandable, M for Measurable, B for Believable and A for Achievable, respectively. USL (Upper Specification Limit) is a value above which the process performance is unacceptable, while LSL (Lower Specification Limit) is a value below which the process performance is unacceptable. Due to natural variations within the process, specifications usually are a range with upper and lower bounds. Specification limits are the numerical expressions of the customer requirements. Every process should be capable of fulfilling the customer’s requirements, which must be quantified to be attainable. Specifications are the voice of the customer.
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The results from this analysis may help industrial engineers identify variation within a process and develop further action plans that lead to better yield, lower variation and less number of defects. This analysis measures how a process performance fits the customer’s requirements, which are translated into specification limits for the interesting characteristics of the product to be manufactured or produced. Process capability analysis represents a significant component of the Measure phase from the DMAIC (Define, Measure, Analysis, Improve, Control) cycle during a Six Sigma project. The fact that the process is running off-center (about 1σ below its target) is reflected in the markedly different values for C p, C pk, C pm, and C pkm.Image by Lenny Kuhne available at Unsplash Process Capability Analysis If the upper and lower specification limits of the process are USL and LSL, the target process mean is T, the estimated mean of the process is μ ^ When this is the case, the process capability index is meaningless. If errors of the latter kinds occur, the process is not in a state of statistical control. Errors not addressed include operator errors, or play in the lathe's mechanisms resulting in a wrong or unpredictable tool position. These are variations that naturally occur without a specific cause. The index pertains to statistical (natural) variations only. The process capability index is a measure for how likely it is that a produced axle satisfies this requirement. For instance, the requirement could be that axles need to be between 19.9 and 20.2 mm. As no axle can be made to exactly 20 mm, the designer specifies the maximum admissible deviations (called tolerances or specification limits). 4 Relationship to measures of process falloutĪ company produces axles with nominal diameter 20 mm on a lathe.
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