Tech Notes

Creating Pre-Control Charts in ProFicient™

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

    1. Define the specification limits for the Part/Test combination by selecting the Options>Specification Limits menu item (see Figure 1).

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.

  1. Fill in the Upper, Lower and Target value boxes.
  2. Change Target Cp to 1.0, click Save and Close.

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

Once the specification limits have been identified, a Pre-Control chart can be created. To do this:

  1. 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.
  2. Choose Variable Control Chart in the CHART CATEGORIES combo box.
  3. 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.

Figure 2: Chart Type Window Used for Selecting Pre-control Chart.

Figure 3: Traditional Pre-Control Chart Displaying Green, Yellow and Red Zones.


Display Pre-control Zone Colors

To display Pre-Control zone colors:

    1. From the menu, select Chart>Display Options>Graph. This will display the window shown in Figure 4.
  1. On the Pre-Control tab, click Enable Zone Fill Colors.
  2. 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

Figure 4: Graph Options Dialog Showing Zone Fill Colors Enabled.


Enable Pre-control Alarm Rules

Pre-Control alarm rules are enabled by:

  1. Selecting the Option>Alarm Notification Rules menu item. This will display the window shown in Figure 5.
  2. 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.

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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