Page 217 - The Handbook for Quality Management a Complete Guide to Operational Excellence

204 P r o c e s s C o n t r o l Q u a n t i f y i n g P r o c e s s Va r i a t i o n 205

been achieved, the capability is the level created by the common causes of
process variation. The formulas for estimating the process standard devi-
ation are:
R
Range chart method: σ =
X
d
2
S
Sigma chart method: σ = c
X
4
The values d and c are constants from the table in Appendix 1.
2 4

Process Capability Indexes
Only now can the process be compared with engineering requirements.
One way of doing this is by calculating capability indices. Table 9.11 shows
the calculations for several capability indexes, when the distribution of
the raw observations is approximated by a normal distribution. SPC
software is usually used to calculate these values, especially when non-
normality is suspected.

engineering tolerance
C = 6ˆ σ
P
where engineering tolerance = upper specification limit – lower specification
limit

6ˆ σ
C = 100 × engineering tolerance
R
where engineering tolerance = upper specification limit – lower specification
limit
engineering tolerance
C =
M
8ˆ σ
where engineering tolerance = upper specification limit – lower specification
limit
Z = Minimum { Z Z }
,
MIN L U
Z
C = MIN
PK 3
C
C pm = p 2
1 + (µ − T)
ˆ σ 2

Table 9.11 Process Capability Indexes

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