84
Process capability
coefficient (C
pk
)
Is defined as the minimum of:
1.)
s
Mittelwert
Obergrenze
C
pk
3
or
2.)
s
e
Untergrenz
Mittelwert
C
pk
3
This index provides information about the placing as well as
the spreading of torque values.
The lower acceptable C
pk
- value for a screwdriver is 1.33
(corresponds to a calculation of the C
pk
on limits of 4 Sigma
(s)).
C
p
/ C
pk
Limit values
within limit
values
Error
contribution
0.30
s
68.3 >
30%
0.70
2s
95.4 ~
5%
1.00
3s
99.7 3
1.33
4s
99.994 60
ppm
1.63
4.891s
99.9999 1
ppm
1.66
5s
99.99994 0.6
ppm
2.00
6s
99.9999998
The mean value must not deviate from the target value by
more than 3%!!!
A correct statement about the process capability index is
then no longer possible.
Process capability
examination
Prior to using a processing device for the series production it
is possible to check if the requirements are met with sufficient
safety.
It is the objective of the process capability examination to
verify that the
a) processes to be examined is processing with the
recognizable are legality. If legality is not recognizable
then the process has no defined capability. Especially then
it is not permissible to allocate a statistically calculated
capability characteristic.
b) the process is able to produce the parts in the indicated
tolerances. The required documentation for this can only
be provided under point a) found legality.
The verification of the process ability must be done over a
longer time period. Random sampling is done in set intervals
from the running process and quality characteristics are
recorded (measured). The influence of the machines, the
materials and the operator are assumed by the process
ability.
The basis of the analysis is the Gauss distribution curve.
Mean value
Mean value on a certain amount of measuring values.
n
i
n
x
n
x
1
1
