Statistical Process Control Example

Your organisation manufactures cylinder liners for the marine and truck engine markets. You wish to understand the levels of process control and capability for the critical characteristics of the liners. One such characteristic is bore diameter. You have a tolerance for this dimension of 33.0mm ± 0.80mm. You have measured 50 subgroups of 5 (each subgroup consisting of consecutively produced parts) in order to create a pair of Shewhart Statistical Process Control charts. The table below shows the averages and ranges for the subgroups.

Subgroup / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10
Average / 33.2 / 32.5 / 33.8 / 33.8 / 33.1 / 32.8 / 33.3 / 34.0 / 34.7 / 33.4
Range / 1.6 / 1.3 / 1.2 / 1.3 / 1.5 / 1.1 / 1.4 / 1.5 / 1.2 / 1.2
Subgroup / 11 / 12 / 13 / 14 / 15 / 16 / 17 / 18 / 19 / 20
Average / 33.2 / 33.7 / 33.0 / 34.1 / 33.2 / 32.9 / 33.2 / 33.5 / 33.0 / 32.8
Range / 1.3 / 1.1 / 1.7 / 1.2 / 1.3 / 1.8 / 1.4 / 1.5 / 1.0 / 1.3

a)  Draw and appropriately annotate an appropriate pair of control charts for the data (NB control limit calculation constants for a subgroup size of 5 are A2 = 0.58, D4 = 2.114). Ensure that all equations used are shown and explained

b)  Briefly discuss your observations of the patterns of the chart.

c)  Discuss the level of capability of the process, clearly indicating your diagnosis of the situation. (2 marks)

d)  Given the situation you have described in c) above, how would you go about improving the process capability?