- Slides: 7
Specification Limits, Tolerances • Specification limit: conformance boundary specified for a characteristic. • Statistical tolerance limits: limits of the interval for which it can be stated with a given level of confidence that it contains at least a specified proportion of the population
Process Specification Conflicts • Type I conflict: the inherent variability of the process is too great or the specification limits are too narrow (Two sided specification limits and less than 6 ). • Type II conflict: one-sided specification limits and less than 3 . • Type III conflict: the specification limits are too wide for acceptable quality.
Actions in the case of Type I Specification Conflict • Action 1 - Changing the process (reduce the spread of the distribution) • Action 2 – try to avoid setting the specification limits tighter than necessary. • Action 3 – setting up an inspection/sorting operation (an interim procedure, not a permanent solution) • Action 4 – adjusting the center
Actions in the Case of Type II Specification Conflict • Action 1 - Attempt to center the process at a value far enough from the specification limit to avoid individual items outside of the limit (use xbar chart) • Action 2 – Changing the specification limit • Action 3 – Setting up an inspection/sorting operation to find and remove or repair the units of product that fall outside the specification limit.
Actions in the Case of Type III Specification Conflict • Action 1 – Conduct more formal programs of process experimentation to determine what range of values cause non-conformities. • Action 2 – Make specification limits more tight. • Action 3 – Establish specification limits on a temporary basis until final values are determined.
The Process Capability Index Cp Cp = (U-L)/6 If Cp = 1, then the natural spread of the process equals the width of specification limits If Cp < 1 a type I specification conflict occurs The most typical Cp = 1. 33 (U-L = 8)