Pre-Control charts allow the comparison of individual measurements to specification limits. In this way, Pre-Control charts can be used to alert an operator to the potential for producing defective products. Although Pre-Control charts look similar to standard control charts, they should not be mistaken for the Shewhart-type control charts. Control charts alarm users to statistically significant process changes based on control limits that are calculated from production data. Control limits are normal bounds of variation (±3σ for the statistic or data values being evaluated) that identify significant process changes even if no defective products are being produced.
In contrast to the Shewhart charts, Pre-Control charts are designed to highlight unacceptable deviations from a desired target. They should be used only with processes that are already in a state of statistical control and have a Cpk value greater than 1.0 (ideally in the 2 to 3 range). Creating a Pre-Control chart within ProFicient is a 4-step process, starting with configuring specification limits.
Configure Specification Limit Record
- Define the specification limits for the Part/Test combination by selecting the Options>Specification Limits menu item (see Figure 1).
- Fill in the Upper, Lower and Target value boxes.
- Change Target Cp to 1.0, click Save and Close.
Note: The specification limits will be displayed as the Pre-Control limits only when the Target Cp value is equal to 1.00. If the user wants pre-control limits to be tighter than the specification limits, then the value of Target Cp should be increased to a number larger than 1.00.
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Figure 1: For Traditional Pre-Control Charts, Set Upper, Lower and Target Values and Specify a Target Cp of 1.0.
Select Pre-Control Chart
- Select the Chart>New menu item. After choosing a name for the chart and optionally linking it to an existing Data Entry Configuration, ProFicient will present the user with the Chart Type window shown in Figure 2.
- Choose Variable Control Chart in the CHART CATEGORIES combo box.
- Then, from the GRAPH 1 combo box, choose Pre-Control from the list. When doing this for the first time, make sure the TEMPLATES combo box reads <None>. Make selections from the remaining windows as you would normally when creating any new chart within ProFicient. A Traditional Pre-Control chart is shown in Figure 3.
Once the specification limits have been identified, a Pre-Control chart can be created. To do this:
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Figure 2: Chart Type Window Used for Selecting Pre-control Chart.
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Figure 3: Traditional Pre-Control Chart Displaying Green, Yellow and Red Zones.
Display Pre-control Zone Colors
To display Pre-Control zone colors:
- From the menu, select Chart>Display Options>Graph. This will display the window shown in Figure 4.
- On the Pre-Control tab, click Enable Zone Fill Colors.
- To display the red zone fill color (representing the area outside of specifications), enlarge the graph's scale by optionally filling in the Increase Scale By
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Figure 4: Graph Options Dialog Showing Zone Fill Colors Enabled.
Enable Pre-control Alarm Rules
Pre-Control alarm rules are enabled by:
- Selecting the Option>Alarm Notification Rules menu item. This will display the window shown in Figure 5.
- Pre-Control alarm rules are displayed with a "PC" prefix. To enable the Pre-Control alarm rule of choice, double-click on the PC alarm rule(s). As with all ProFicient alarm rules, the word <enabled> is displayed to the right of the rule, confirming its selection.
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Figure 5: Alarm Notification Rules Window Displaying Pre-Control Alarms.
How ProFicient Calculates Pre-Control Limits
ProFicient Pre-Control charts display limits whose values are based on both specification limits and the Target Cp found in the Specification Limits record. Pre-control limits are calculated using the formulas found in Table 1 below. Note that using a Target Cp value greater than 1.0 results in Pre-control limits that are tighter than specification limits.
Table 1: Formulas for Calculating Control Limits for Default ProFicient Pre-Control Charts.
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