Atlas Copco PowerMACS 4000, User Manual

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SPC and Statistics
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Calculation of the process capabilities Cp and Cpk:
 
 
i
i
sl su
LTL UTL
Cam
s sl
LTL X
s su
X UTL
Cpk
s sl su
LTL UTL
Cp
table see c m i
c
S
s
or
table see d m i
d
R
s
k
d d
d
R


*
*
,
*
min
*
) : ( .. 1 ;
) : ( .. 1 ;
864 , 0
645 , 1
' '
'
4
4
'
2
2
'
5 * 5
* 5











 




 
 
  

where

for a normal distribution su = sl = 3

d5 = 2.326 for subgroup size = 5

k = no of subgroups to calculate
Cam is only calculated for subgroup size = 5, and requires range to be collected. Cam is not applicable to
standard deviation.
An error event is generated if the value of Cam, Cp or Cpk is out of limit.
When SPC is wanted for a station, not for its individual bolts, the calculations are made on those variables
that happen to be set up for data collection on that station.
Station, Program, Step and Variable are still defined, so only variables matching those criteria are used.
It is not possible to specify alarm limits or control limits for SPC calculated on a station.The weighing
together of values for the separate bolts are done for each subgroup according to the following rules: