Please clarify the Rule of 10 to 1 - AND - What is the ndc number?

G

Geese

Can someone please clarify the Rule of 10 to 1, what it is exactly, what I use it for, and so on? An example would help. The book goes on and on about it but doesn't tell me what it is. I searched through the forums but did not see a previous mention of it. :read:

Also, what is the meaning of the ndc number? What range are they looking for to be acceptable? And what is meant by gage discrimination? The book is just too eloquent for me to understand the concept I suppose or I'm too ignorant. :bonk:
 

Ron Rompen

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The "10:1 rule" is a guideline in metrology. Broadly speaking, your measuring instrument chosen should be accurate (not just discriminate) to 1/10th of the tolerance.

In other words, if you have a feature with a tolerance of 0.010", your measuring instrument should be accurate to no less than 0.001".


Unfortunately, I have no idea what 'ndc' is, out of context. What book are you using? There may be some of us here who have the same one, or by the same author, and can extrapolate the meaning from that information.
 

Caster

An Early Cover
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Are you referring to the QS 9000 Measurement Systems Assessment book?

I just went through about a month of reading, re reading, searching the Cove, Googling and general agony on this.

I built myself some crib notes during this process. If you are interested, I will be happy to post them. It may take a few days, life is hectic right now, and I need to tidy the notes a bit before posting.
 
G

Geese

Thats great example on the 10:1 rule. Thankyou.

As for the crib notes, I'd be happy to get what I can. The ndc I'm referring to comes from the MSA manual 3rd edition. It is calculated in an excel spreadsheet after a GRR study is done with 3 appraisers and 10 parts. I've heard mention in some of the other threads that it shows how great your discrimination is by calculating the number of categories you get when performing a GRR. If I could make the connection between categories and what that means with the actual data, I'd be in hog heaven.
 
G

Geese

I finally understand the importance of the 10:1 Rule of Thumb. When completing a GR&R test, measuring a tolerance of .001 with a gage accurate to .0001 gives enough discrimination that the UCL will be useful and not cause problems with numbers falling out of control.

If you make the mistake of measuring a tolerance of .001 with a gage accurate to .001, most of your measurements will fall outside your UCL limit since the discrimination is so low. When you go to correct this, your UCL will fall even lower, possibly pushing more measured values outside the new calculated UCL.
 

Caster

An Early Cover
Trusted Information Resource
Measure twice, cut once

Looks like you have it.

We ran up against this same thing with our CMM machine. Our first MSA studies showed just awful results.

We were reporting measurements to the same number of siginifcant figures as the tolerance. Pages 43 to 46 of the MSA book clearly show this as a mistake.

Another thing to watch for is to select parts for the study that span 6 sigma (or Tolerance if 6 sigma is not known). We initally chose 10 parts in a row from a production run. There was very little part to part variation.

Once we fixed up these problems, we got decent MSA results.

My big lesson learned...slow down and read and understand before you do a lot of testing...the MSA book is actually pretty good. The Cove fills in the holes.
 

Marc

Fully vaccinated are you?
Leader
Caster said:
Are you referring to the QS 9000 Measurement Systems Assessment book? ...... If you are interested, I will be happy to post them.
Please do if you get the time. We'll all appreciate it!
 
D

Daniel Negrea

I have a question for you for a situation that I've encountered.
I have to check a dimension that has a tolerance of +/- 0.1 mm (let's say 10 +/- 0.1 mm). I use a vernier for checking this dimension (the resolution is two decimal places, so it satisfy the 10 to 1 rule). Checking this dimension, is now 9.86 mm a dimension acceptable or not? If you round this one to one decimal place it will be 9.9 mm. Or, is 10.12 mm acceptable (same idea, rounding it it would be 10.1 mm)?

Thanks for your help.
 

Jim Wynne

Leader
Admin
The limit is 0.1mm, so the question you have to ask yourself is "Is 0.14mm greater than 0.1mm?" Even if you round it, the actual value hasn't changed, nor has the fact that the limit has been exceeded.
 
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