SPC/SQC Analysis issue with One sided Specification

[former_member699641]

In the SPC SQC analysis , if only one specification Limit is defined and i want the SPC analysis to ignore the other specification limit, ex: USL. How to do that?

It looks like a bug, that If we dont set the specification limit, the value defaults to zero while getting processed in the action block.

I tried setting nullnumber also but it doesnt work.

It takes the value as zero and generates the statistics results accordingly(CPK,PPK) . further it also generates alarms(False) based on this default value.

Ex: If LSL=96 and USL : NA then the SPC action blocks sets the value as 0 for USL and in the results if the datapoints are found to be say 98  it sets a alarm for its greater than 0 (USL),  whereas it should have ignored it?

Further the CPK value generated is also found to be invalid because it considers the USL as 0.

(Atleast for the CPK value we can have work around to switch between CPL or CPU based on the one sided tolerance limit)

But can we disable the false alarm ?

Need help on the same.

-Karthik

[salvatore_castro]

That's because the SPC calculations require the upper and lower Spec limits to be defined as per the standard definition which we have also posted here in the help documentation:

SPC Algorithms - SAP Manufacturing Integration and Intelligence - SAP Library

How would your propose to ignore a spec limit given the calculation of CPK requiring both an upper and lower limit?

Sam

[former_member699641]

Salvatore,

In the SPC Algorithm definition you had referred, we could see the following sentence just below the CPK definition where it explains about one -sided tolerance.

"A process is capble only when the Cpk value is 1.33 or greater. For a one-sided tolerance, the Cpk is equivalent to either Cpu or Cpl depending on which tolerance limit is specified.".

Here the Specification limit is ignored indirectly by choosing either CPU or CPL depending of the side.

In this case, if i have only USL spec. limit no LSL, then i will choose CPU instead of CPK .

But for  Alarms i could not see such option , the action block considers the missing spec as  0 and gives false list of violations.

This is were we need help.

-Karthik