What is the difference between Process Variation and Product Tolerance for GR&R?

H

harryYin - 2010

#1
I confused on the difference when I analysis with the data.
sometimes the result shows big difference between use process variation method and product tolerance method.

Can somebody give a clear explaination on that ? and when is the time to use process variation and when to use product tolerance?:caution:
 
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A

aras97

#2
Tol% is used to evaluate the measurement syetem whether detect a good or bad part part;R&R% is used to evaluate the measurement syetem whether detect a process change.
 
A

alspread

#3
Product tolerance is (obviously) the drawing or design tolerance that must be met in order for the part to function.

Process variation is the amount of variation that the process has proven it produces.

So, if you happen to use a process that is very fussing (extra-capable) for producing a part with a (relatively) large drawing tolerance, you will see a big difference between the gage analysis when you compare its results to the product or the process.

What do you want the gage to do: Be able to detect a good part from a bad part; or Be able to detect changes in the process??

Hopwe this helps.

Good luck
 
I

isotsadmin

#4
Hi,

How about during conducting the MSA, Is it safe to use Range method for GR&R for tolerance? How about for GR&R for process variation, what method is to be used? is it still Range method? I see two formual used for this method and I wonder if both are correct.

By formula:
a. for product tolerance :
GR & R = 6R/d2
%GR &R = 100* [GR&R/ (USL-LSL)]
b. for process variation :
GR & R = R/d2
%GR &R = 100* [GR&R/ (process stdev)];
where process stdev is from the previous tudy.

I have read the MSA manual and it mentioned these two %GR &R to tolerance and %GR & R to process variation but it does not mention what formula to be used. It also mentioned that when an independent estimate of process variation is not available the sample parts must be selected from the process and represent the entire production range. How do I do that for an initial study?
 
N

NumberCruncher

#5
Hi HarryYin

Given the tag, I guess that you are talking about GRR studies. However, there are several statistics which are calculated during these studies.

Equipment variation
Appraiser variation
Part variation
GRR
Total variation

Which of these are you referring to?

As to which you should use:

"Either or both approaches can be taken depending on the intended use of the measurement system and the desires of the customer" (MSA manual, 4th edition, page 123).

Neither approach is the "correct" approach. They simply give you different comparisons. If you compare to process, you get a measure of your ability to detect what your process actually produces.

If you compare to tolerance, you are only looking at your ability to detect if a product is within specification.

You may think that the two statements are equivalent, but in the real world, process capability and tolerance can be very different.

Let's say you have a completely lousy manufacturing system where most of your items are either way too big or way too small. You can still have acceptable product going out of the door.

How?

Easy! Just have 100% inspection with a measurement system that can just about tell the difference between 'in tolerance' and 'out of tolerance'. Then you throw away all the useless parts. You may have a scrap rate of 99%, but your customer only gets good parts.

On the other hand, you have a truly awful manufacturing system.

Now carry out a GRR study on this setup. If you compare your barely capable measurement system to the product tolerance (what the customer requires), you will find it is barely capable.

If you now compare your barely capable measurement system to the truly awful manufacturing system, you will find that your measurement system is amazingly good.

Actually, your measurement system is barely capable, you are just comparing it to a production variability that is so great you may as well sort your initial output using a couple of industrial sized seives. ("If it falls through these holes, it's too small, if it doesn't fall through those holes, it's too big" :lol:)

NC
 
N

NumberCruncher

#6
Hi Isotsadmin

It also mentioned that when an independent estimate of process variation is not available the sample parts must be selected from the process and represent the entire production range. How do I do that for an initial study?
This is an engineering decision. If you know your process well enough you may decide that there is no significant hour/day/week/production shift variation so a single 'grab' sample would be ok.

If you suspect that morning production could be different to afternoon production, collect parts from the morning and parts from the afternoon. Similarly for any other time related variability. If you think that parts from shift to shift are different, collect parts from each shift.

If you have no idea what sort of variation exists in your manufacturing system, I would recommend collecting parts from different hours/days/weeks etc.

There is no single value that is the "correct number of parts". It depends on the variability of your parts, which is what you are trying to determine!

A useful rule of thumb is "at least 30 parts, preferably 50 or more parts". With that number of samples, the chance of you selecting 'all good' or 'all bad' is very low, as long as your sample is totally random.

The method I have suggested above is not totally random, its actually a form of stratified sampling. I think it will work better than trying to choose truly random samples simply because it is very difficult to be truly random.

NC
 
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