Transcript Process Capability

Process Capability
Process limits (The “Voice of the Process” or
The “Voice of the Data”) - based on natural
(common cause) variation

Tolerance limits (The “Voice of the Customer”)
– customer requirements

Process Capability – A measure of how
“capable” the process is to meet customer
requirements; compares process limits to
tolerance limits

Process Capability
(1)
(3)
specification
specification
common variation
common variation
(2)
(4)
specification
specification
common variation
common variation
Process Capability Index
 X  LTL UTL - X  ,

Cpk = min 
or

3

3



where x-bar is the mean of the process,
sigma is the standard deviation of the
process, UTL is the customer’s upper
tolerance limit (specification), and LTL is
the customer’s lower tolerance limit.
Interpreting the Process Capability
Index
Cpk < 1
Not Capable
Cpk > 1
Capable at 3
Cpk > 1.33
Capable at 4
Cpk > 1.67
Capable at 5
Cpk > 2
Capable at 6
Process Capability Index, Cpk
A process has a mean of 45.5 and a standard
deviation of 0.9. The product has a specification
of 45.0 ± 3.0.
Find the Cpk .
Process Capability Index, Cpk
Example problem solution:
 X  LTL UTL - X 

Cpk = min 
or

3

3



= min { (45.5 – 42.0)/3(0.9) or (48.0-45.5)/3(0.9) }
= min { (3.5/2.7) or (2.5/2.7) }
= min { 1.30 or 0.93 } = 0.93 (Not capable!)
However, by adjusting the mean, the process can
become capable.