Meaning
Statistical boundaries used to determine if a manufacturing process is in a state of control by identifying variation from assignable causes. These shewhart control limits define the range of expected variation.
Variance Analysis
Data points falling outside these bounds indicate the presence of an assignable cause that requires investigation. The shewhart control limits distinguish between the natural noise of the system and a shift in performance. By ignoring the noise and acting only on the signals, operators can maintain a stable process without over-adjusting the machinery.
Subgroup Selection
Accuracy of the model depends on the grouping of the data into rational sets. Shewhart control limits are most effective when the samples within a group are taken under nearly identical conditions. This structure allows the chart to detect changes that happen between the groups, such as a tool wearing out or a new batch of raw material being introduced.
Consistency within the group ensures that the calculated limits reflect the true capability of the equipment rather than temporary external influences.
Process Capability
Comparison between the control bounds and the customer specifications determines if the process can produce acceptable parts. A process is considered capable only when the shewhart control limits fit well within the specification window.
