Page 76 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Unit 4: Quality Control

The mean for the ranges gives the center line for the R chart. Using standard statistical practice, Notes
the Upper Control Limit (UCL) and the Lower Control Limit (LCL), are calculated at 3 times the
standard deviation. Based on this rationale, the UCL and LCL have been shown in rows 19 and
20 in Table 4.2.

Table 4.2: The R Chart Table

The data can now be converted into a graphical presentation using the Chart Wizard in Excel. As
previously in the case of the µ chart, the ‘line type’ chart type is used and values for the 4 series,
i.e., means of ranges, UCL, CL and LCL incorporated. The y-axis should represent the variable
measurement and the x-axis represents the sequence of samples. The control chart that results is
shown as Figure 4.4.

Figure 4.4: R Chart for Steel Shaft

0.03
UCL

0.02

0.01
CL

0

1 2 3 4 5 6 7 8 9 10
-0.01
LCL
-0.02

It can be seen that all the points are within the UCL and the LCL. Since the process variability is
in control, the process is in control. These charts are stable and can now be used as
benchmarks.

This means that, given the stated levels, these control limits can be adopted for use in
online statistical process control. The X bar chart can now constructed. However, control charts
require to be updated continuously. Old data should be discarded and replaced by new data.
This is necessary, in spite of the fact that the data may show that both the R and the X bar chart
exhibit control.

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