1. Higher National Certificate in Engineering
Unit 36 – LO3
Capability

2. Learning Outcome 3.1
LO 3.1:
describe process capability

3. 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.

4. 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.

5. 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

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

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

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

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

10. 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

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

12. 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

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

14. 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

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

16. 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

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