Lean Six Sigma For Dummies by Morgan John & Brenig-Jones Martin

Author:Morgan, John & Brenig-Jones, Martin [Morgan, John]
Language: eng
Format: epub
Publisher: Wiley
Published: 2012-03-11T22:00:00+00:00

Figure 7-10: The driving needs improvement.

In Figure 7-10, the driving needs some improvement. Here, the Cpk value is less than the Cp value. If Cpk is less than 1.0, it doesn’t meet the CTQ. In process terms, you need to shift the mean by improving this threshold process. Reducing the variation also makes the ‘fit’ of the car in the arch that little bit easier.

To fully assess a process you need both the Cp and the Cpk values. You also need to be in control of the process for these indices to be meaningful. The Cpk value is never greater than the Cp value. When the process mean is running on the ‘nominal’ – the mid-point of the customer specification – Cpk and Cp are the same value. This involves good management of the process, or continuing our car analogy, some careful driving.

Figure 7-11 shows the formula for calculating the Cp and Cpk values for the X moving R chart (see the ‘Choosing the right control chart’ section earlier in this chapter). The difference between the control limits, the UCL and the LCL, on your control chart covers six standard deviations. The difference between the upper and lower customer specification (USL–LSL) is usually referred to as the tolerance. The USL and LSL represent the range of the customer’s CTQ. For example, they may want a product delivered within five days, but describe their requirement as an upper specification of five days and a lower specification of one day. Three days would be the nominal value of this specification, the mid-point, and the tolerance, the difference between the specification limits, would be four days.

Dividing the tolerance by the distance between your control limits (six standard deviations), you have the Cp value – the theoretical number of times you can fit the car within the arch. Your Cp value must be at least 1.0 if you are to meet the CTQ.

Figure 7-11 describes how the formula for working out your Cpk depends on the position of the mean on your control chart.

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