Higher National Certificate in Engineering Unit 36 – LO3 Capability

Learning Outcome 3.1 LO 3.1: describe process capability

How good is your Process? Seeking answers to questions such as… Are we capable of doing the job correctly? Have we done the job correctly? Are we continuing to do the job correctly? Could we do the job more consistently and on target? …provides us with an understanding of the process capability and the sources of non- conforming outputs.

Process Capability A process capability index is a key metric used to predict the ability of the process to meet the customer requirements. As a prediction tool (Note: the process capability index can only be used when the process is in statistical control, as demonstrated on a suitable chart). It is thus best to reference the chart used.

Process Capability Cp = engineering tolerance 6σ The statistical control chart provides the estimate of the process variation and where needed, the process location. Cp (Process Capability Index): historically this was one of the first capability indices developed. The natural tolerance of the process is computed as 6σ using the control chart… Cp = engineering tolerance 6σ

Process Capability Cp = engineering tolerance 6σ i.e. Cp = Upper Spec. Limit – Lower Spec. Limit

Non-Capable Process CP<1 rework +3σ USL Nominal X-bar-bar LSL -3σ scrap

Capable Process USL LSL Nominal X-bar-bar +3σ -3σ CP=1

Capable Process USL LSL Nominal X-bar-bar +3σ -3σ CP>1

Capable Process CP = USL – LSL 6σ CP = 8 = 1.33 6 USL σ +3σ Nominal X-bar-bar 6σ CP = 8 = 1.33 6 -3σ σ LSL

Capability v Control A process may be in control but not capable A process needs to be in-control and capable

Capable Process v Setting Note in each case Cp>1 but as x-bar drifts the scrap or rework can be produced X-bar USL +3σ X-bar X-bar Nominal X-bar-bar X-bar LSL -3σ

The Capability Index Cpk USL LSL Nominal X-bar-bar +3σ -3σ Cp = Cpk = 1

Cp v Cpk Cp = 1; Cpk = 0.5 i.e. Cpk < 1 USL LSL Nominal X-bar-bar +3σ -3σ Cp = 1; Cpk = 0.5 i.e. Cpk < 1 X-bar Cpk = USL – x-bar-bar 3σ y/2

Cp v Cpk Cp = 1; Cpk < 1 USL LSL Nominal X-bar-bar +3σ -3σ X-bar Cpk = x-bar-bar - LSL 3σ

The Minimum Goal for Cp &Cpk USL σ CP = USL – LSL 6σ +3σ Nominal X-bar-bar 6σ CP = 8 = 1.33 = Cpk 6 -3σ σ LSL

Process Capability In the western World studies have shown that process capability Cp on average has a value of 0.6