The determination of process capability requires a predictable pattern of statistically stable behavior (most frequently a bell-shaped curve). Where the chance causes of variation are compared to the engineering specifications. A capable process is a process whose spread on the bell-shaped curve is narrower than the tolerance range or specification limits. USL is the upper specification limit and LSL is the lower specification limit. It is often necessary to compare the process variation with the engineering or specification tolerances to judge the suitability of the process. Process capability analysis addresses this issue.
A process capability study includes three steps:
Planning for data collection.
Plotting and analyzing the results.
The objective of process quality control is to establish a state of control over the manufacturing process and then maintain that state of control through time. Actions that change or adjust the process are frequently the result of some form of capability study. When the natural process limits are compared with the specification range, any of the following possible courses of action may result:
Do nothing: If the process limits fall well within the specification limits, no action may be required.
Change the specifications: The specification limits may be unrealistic. In some cases, specifications may be set tighter than necessary.
Discuss the situation with the final customer to see if the specifications may be relaxed or modiﬁed. Center the process: When the process spread is approximately the same as the speciﬁcation spread, an adjustment to the centering of the process may bring the bulk of the product within speciﬁcations.
Reduce variability: This is often the most difficult option to achieve.
It may be possible to partition the variation (stream-to-stream, withinthe piece, batch-to-batch, etc.) and work on the largest offender first. For a complicated process, an experimental design may be used to identify the leading source of variation. Accept the losses. In some cases, management must be content with a high loss rate (at least temporarily). Some centering and reduction in variation may be possible, but the principal emphasis is on handling the scrap and rework efficiently.
Other capability applications:
Providing a basis for setting up a variables control chart
Evaluating new equipment
Reviewing tolerances based on the inherent variability of a process
Assigning more capable equipment to tougher jobs
Performing routine process performance audits
Determining the effects of adjustments during processing
The identification of characteristics to be measured in a process capability study should meet the following requirements. The characteristic should be indicative of a key factor in the quality of the product or process. It should be possible to adjust the value of the characteristic. The operating conditions that affect the measured characteristic should be deﬁned and controlled. If a part has fourteen different dimensions, process capability would not normally be performed for all of these dimensions. Selecting one, or possibly two, key dimensions provides a more manageable method of evaluating the process capability. For example in the case of a machined part, the overall length or the diameter of a hole might be the critical dimension. The characteristic selected may also be determined by the history of the part and the parameter that has been the most difficult to control or has created problems in the next higher level of assembly.
The process specifications or tolerances are determined either by:
The Organization’s Engineering department
The process capability study is used to demonstrate that the process is centered within the specification limits and that the process
variation predicts the process is capable of producing parts within the tolerance requirements.
When the process capability study indicates the process is not capable, the information is used to evaluate and improve the process in order to meet the tolerance requirements. There may be situations where the specifications or tolerances are set too tight in relation to the achievable process capability.
In these circumstances, the specification must be re-evaluated.
If the specification cannot be opened, then the action plan is to perform 100% inspection of the process, unless inspection testing is destructive.