Process Control Tolerance

As the name implies, Process Control Tolerance (PCT) is a form of controlling manufacturing processes by monitoring production and automatically reporting conditions that require correction of the process before non-compliance issues arise. Traditionally a feature attribute has a single tolerance value that, when exceeded, was reported only as non-conforming. By the time a non-conformance had been identified, the manufacturing process may have produced parts that may now require expensive repairs.

figure 1, PCT Tolerance Zones

Using PCT allows the inspector to establish different but simultaneous tolerance zones for position and size of features. For example the size of a bore might be specified as 1.000 +/- .001. Under PCT controls the controls values might look like:

Nominal ^{[Zone A] / [Zone B] / [Zone C] / [Zone D]}
-or-
1.000 ^{[+/-.0007] / [+/-.0008] / [+/-.0009] / [+/-.001]}

As shown in figure 1, We break down the tolerance band into zones of control. Each zone is identified by Geomet as Zone A / Zone B / Zone C and Zone D.

The actions required for each tolerance zone is defined by your companies quality procedure. The zone description below are suggestions only.

Zone A: This portion of the tolerance zone is considered in compliance and requires no action. The default value for this zone is 70% of the allowable tolerance, but may be changed to any desirable percent of tolerance.

Zone B: Is the portion of the tolerance zone that exceeds the limit established by Zone A. It is monitored as a first response to provide corrective action to bring the production process back under the trigger limit. Production is still within tolerance limits. The default limit of this zone is 70% - 80% of the allowable tolerance zone.

Zone C: This portion of the tolerance zone requires immediate action to correct a potential production problem. The production is still within the tolerance limit, but beyond a trigger zone that requires immediate attention. The default limit for this value is 80% - 90% of the allowable tolerance zone.

Zone D: This level equals 90% of the allowable tolerance zone and identifies the production as non-conforming that requires a material review.

Zone	Suggested Action
A B C D	No action required. Notify Manufacturing and monitor. Notify Manufacturing and review process. Notify Management and adjust process.

You access the PCT dialog when you apply standard tolerance from within Geomet. In figure 2, the toleranced feature is a standard ID XY Circle. The tolerance box will have buttons labeled as next to the Actual value. This is the button used to start the PCT dialog.

figure 2, Tolerance Dialog	figure 3, PCT Dialog

Each zone can be edited with a custom value or the <Use Default> button can be selected to use the predefined percentages for each zone, see System Options. There exists a validation to exit this dialog, where the zones must be A < B < C <= D to prevent zone overlapping.

If you require a tighter control, select <Use 3 Level> which will compress the allowable tolerance limit by eliminating Zone C and defining it as the non-conformance limit. This will have the same effect as removing the first notification level when the trigger value has been exceeded.

When a feature is being reported, the Dev. Tol. column will indicate which zone the results fall within. When a value is in Zone B you will see a number representing the amount beyond the trigger value. If in Zone C then the value will be tagged with a symbol: <. If the value is in Zone D, then the symbol is: << and special care should be taken to segregate the production.

In the footer of the report, there will be an assigned classification showing a large letter corresponding to the maximum zone reached. There will also be a summary line listing the total count of all attributes that have PCT applied. The summary line will report: Total Inspected count followed by a count for each zone.

PCT Summary Report

The printing of the summary line can be toggled between printing and not printed by accessing the Report Config tab in System Options, see System Options for a complete description.

To print, place a check next to Print Report Summary.

Related Procedures:

System Options→Tolerance Codes, Tolerance, Out-of-Tolerance Flash Message, System Options→Report Config