Industrial Statistics with Minitab

16.2 Diameter_capability_1

The previously mentioned part must be produced for a client that requires that the capability index of the critical diameter meets Cp > 1.33. A first series is produced during two days in order to verify that all of the client's requirements are met.

To carry out the capability study, five parts are taken every half an hour (16 samples each day, 32 samples in total), and the results obtained are stored in file DIAMETER_CAPABILITY_1.MTW. The data are organized in rows, that is, the first row contains the values of the first sample, the second row the values of the second sample, etc.

Is the requirement Cp > 1.33 met?

Use: Stat > Quality Tools > Capability Sixpack > Normal

Since the data are organized by rows, use the option Subgroups across rows of: and introduce C2-C6. Remember that the specifications are 7.5 ± 0.1.

An evident conclusion obtained from the graph of the sample means (Xbar Chart) is that the process becomes decentred towards larger values. The reason for that should be studied; perhaps it is a badly calibrated machine, a wear and tear of the tools, etc. Concerning the range chart, there is nothing to object. The histogram is decentred with respect to the target value; the default values of the scale are changed to show this more clearly.

The required capability index Cp > 1.33 is met in this case (also Pp > 1.33), although it appears ...