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QM: Control Charts

Description

A control chart is a graphic utility for statistical process control (SPC). The following control charts can be maintained for inspection characteristics for which measured values are recorded:

• mean value chart where tolerance is taken into account (acceptance chart)

• mean value chart where tolerance is not taken into account (Shewhart chart)

• standard deviation chart

You can combine a mean value chart and a standard deviation chart to form a two-track control chart which monitors the position and dispersion of a process simultaneously.

Planning a control chart

If you want to maintain a control chart for an inspection characteristic, you must first create a sampling procedure with a fixed sample. You must enter the type of control chart in this sampling procedure.
Set the indicator for the SPC characteristic and the sampling procedure in the routing or in the inspection plan. Then assign the sampling procedure to the characteristic and enter an SPC criterion.

Creating and allocating a control chart

During sample calculation for a characteristic with a control chart, the sample is automatically assigned to the most recent, active control chart in accordance with the SPC criterion. If an active control chart does not exist, a new control chart is created. Number assignment is carried out internally.

Calling up a control chart

You can call up the control chart during results recording via a pushbutton. A separate graphics window is opened for every track on the chart. The results of the current inspection and the completed results of other inspections assigned to this control chart are displayed on the chart.
The following functions are available in the graphics window:

• change the status of the control chart from new to active.

• change the status of the control chart from active to completed.

• calculate the control limits for the track

• save changes

• return to results recording without exiting the graphic

• exit the graphic

You require the appropriate authorization to carry out status changes and calculate the control limits (authorization object Q_SPC).

Change system parameters in customizing

The SPC criteria, the control charts and the number ranges are set in the Customizing application. Programming changes or extensions are possible by replacing the function modules entered in this application.

• SPC criteria
  An SPC criterion determines whether separate or common control charts are maintained for different orders, work centers, materials, manufacturers or customers.

• Control charts
  You can specify the requirements for the control indicators of the inspection characteristic and the sampling procedure for each control chart type.
  At a second hierarchy level, you can define the tracks of the control chart. For each track, you can specify which control variable is used and the algorithms in accordance with which the warning and action control limits are calculated. Using setting parameters, you can adapt statistical characteristics to special features of your own company or country.

• Number ranges
  You must create the number ranges used in the control charts.

You can find out more about the settings in the Implementation Guide for control charts.

Changes to the interface

The control charts have resulted in the following changes to the existing functions:

• You can enter a control chart type in the sampling procedures with fixed samples.

• You can enter an SPC criterion in the sample screen of the plan characteristic.

• The pushbutton Control chart has been incorporated in the results recording function.