MSA Manual asks for NDC higher than 5 - NDC (number of distinct categories)

M

Mexicanquality

#1
I know that MSA asks for NDC higher tant 5, but what i still dont understand is what is the meanning of this...I have seen that if we use parts span in all variation tolerance or 6 sigma the NDC increases...but it this a trick to approve the GR&R? what does the NDC gives you in a gage study??

Thanks
 
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bobdoering

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#2
An ndc of 5 gives you 5 statistically significant categories of measurement (or true resolution when considering gage error)- just a little better than a gas gage in a car. It is OK for simple measurement, but inadequate for SPC - in spite of what the chart says on page 47 of the MSA 4th ed. For SPC, the ndc>10 (5 categories on either side of the mean), or you will get "chunky" data and marginally useful results, if that.

The parts you should present to the Gage R&R should represent the variation you expect that gage to see over time for that characteristic with that process. Otherwise, all of the data will be either overstated or understated.
 
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