leftoverture
Involved In Discussions
Hi Folks,
I wanted to start a discussion to see what others are thinking about the "number of distinct categories" or NDC calculation in MSA studies. The calculation is, per my interpretation of the MSA manual (see page 46), essentially an effort to improve on the old 10:1 rule (where gage discrimination should be at least 1/10 of the specification range).
So, AIAG seems to suggest that making gage discrimination 1/10 of process variation is an improvement over the old 1/10 of spec range rule. I suppose in some cases this may be true, but since the objective of most manufacturers is to reduce process variation, I would suggest that any calculation that carries the risk of penalizing your measuring system because the process variation is low is probably, to some degree, errant.
And this brings me to the subject of NDC. As a reminder, the formula for NDC is 1.41(PV/GRR). I'm sure many of you have experienced what I have, a measurement system with excellent discrimination that exceeds the old 10:1 rule but yields a low NDC anyway. And the reason the NDC is low is that the process variation is low.
I would further guess many of you, like me, have found that your actual R&R raw data has yielded 7,8,9,10 or even more different outcomes yet the NDC came out low anyway. Again, this is most likely because the process variation is very low. I have conversed with other quality professionals who have been as frustrated as I have been by such NDC results.
Some of those folks have taken to inducing variation into their studies, ie: machining parts to increase the variation in the sample set to achieve a better NDC to satisfy their customers. While that does work in many cases, what's the point? To satisfy the customer or to validate a measuring system?
In the old days (I've been at this a while) we used to count (yes, count) the number of distinct outcomes in our study and if the count exceeded 5 we were pretty happy (assuming we met the old 10:1 rule first). And usually we could further validate our measuring system by looking at the X-bar and R charts (bad measurement systems reveal themselves pretty quickly).
I recently had a case study where I had a rather unique measurement for which only one device was available, and the process capability was something in excess of Cpk=5.00, yet the NDC was low because the process did not vary much. And because the person evaluating the study was probably trained to view NDC as an absolute rather than an indicator, they still wanted it improved. (How much money you got to spend?)
So, I would put forth that the real objective of MSA is to have an adequate measurement system; the MSA manual does, in fact, refer to NDC as an indicator not an absolute. So what is really needed is better understanding of what constitutes an adequate measurement system. (Because anything more than adequate could be wasted money.)
So, care to share your thoughts, opinions, experiences with NDC?
By the way, for those of you keeping score, in the AIAG MSA manual, 4th edition, the index lists NDC references as: page 47 (correct); page 80 (blank page); pages 125 & 131 (NDC not specifically mentioned); page 216 (correct); page 218 (not mentioned); and page 227 (Index cover page). Anyone working in Quality Control over there at the AIAG?
I wanted to start a discussion to see what others are thinking about the "number of distinct categories" or NDC calculation in MSA studies. The calculation is, per my interpretation of the MSA manual (see page 46), essentially an effort to improve on the old 10:1 rule (where gage discrimination should be at least 1/10 of the specification range).
So, AIAG seems to suggest that making gage discrimination 1/10 of process variation is an improvement over the old 1/10 of spec range rule. I suppose in some cases this may be true, but since the objective of most manufacturers is to reduce process variation, I would suggest that any calculation that carries the risk of penalizing your measuring system because the process variation is low is probably, to some degree, errant.
And this brings me to the subject of NDC. As a reminder, the formula for NDC is 1.41(PV/GRR). I'm sure many of you have experienced what I have, a measurement system with excellent discrimination that exceeds the old 10:1 rule but yields a low NDC anyway. And the reason the NDC is low is that the process variation is low.
I would further guess many of you, like me, have found that your actual R&R raw data has yielded 7,8,9,10 or even more different outcomes yet the NDC came out low anyway. Again, this is most likely because the process variation is very low. I have conversed with other quality professionals who have been as frustrated as I have been by such NDC results.
Some of those folks have taken to inducing variation into their studies, ie: machining parts to increase the variation in the sample set to achieve a better NDC to satisfy their customers. While that does work in many cases, what's the point? To satisfy the customer or to validate a measuring system?
In the old days (I've been at this a while) we used to count (yes, count) the number of distinct outcomes in our study and if the count exceeded 5 we were pretty happy (assuming we met the old 10:1 rule first). And usually we could further validate our measuring system by looking at the X-bar and R charts (bad measurement systems reveal themselves pretty quickly).
I recently had a case study where I had a rather unique measurement for which only one device was available, and the process capability was something in excess of Cpk=5.00, yet the NDC was low because the process did not vary much. And because the person evaluating the study was probably trained to view NDC as an absolute rather than an indicator, they still wanted it improved. (How much money you got to spend?)
So, I would put forth that the real objective of MSA is to have an adequate measurement system; the MSA manual does, in fact, refer to NDC as an indicator not an absolute. So what is really needed is better understanding of what constitutes an adequate measurement system. (Because anything more than adequate could be wasted money.)
So, care to share your thoughts, opinions, experiences with NDC?
By the way, for those of you keeping score, in the AIAG MSA manual, 4th edition, the index lists NDC references as: page 47 (correct); page 80 (blank page); pages 125 & 131 (NDC not specifically mentioned); page 216 (correct); page 218 (not mentioned); and page 227 (Index cover page). Anyone working in Quality Control over there at the AIAG?
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