MSA (% Tolerance: 10 %)

B

Black arrow

#1
Hello,

One question i got from my colleague which also made me unsure what to answer:

If MSA turns out to be acceptable for example (only for inspection), no actions needs to be taken.

% Tolerance= 10 % => Total Gage R&r: 2 microns
Tolerance: Up to 20 microns
___________________________________________________________
The MSA is ok, the measuring equipment can be used to detect parts, whether they are within tolerance or not. So far so good.

The colleague asked me this:

"In practice, the operator measure the part and gets 19.2 microns. We know that the measuring system spreads 2 microns, should we decrease sorting limits from 20 microns to 18 microns? To be sure that the part is not out of sec. limits?"
 
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Jim Wynne

Staff member
Admin
#4
Hello,

One question i got from my colleague which also made me unsure what to answer:

If MSA turns out to be acceptable for example (only for inspection), no actions needs to be taken.

% Tolerance= 10 % => Total Gage R&r: 2 microns
Tolerance: Up to 20 microns
___________________________________________________________
The MSA is ok, the measuring equipment can be used to detect parts, whether they are within tolerance or not. So far so good.

The colleague asked me this:

"In practice, the operator measure the part and gets 19.2 microns. We know that the measuring system spreads 2 microns, should we decrease sorting limits from 20 microns to 18 microns? To be sure that the part is not out of sec. limits?"
You don't know for sure what the uncertainty is if you're using only the R&R output. There are problems with the standard AIAG GR&R that make those kinds of conclusions unreliable. For more information I recommend
An Honest Gauge R&R Study [PDF] from Donald Wheeler to help you understand the limitations and potential pitfalls.

If you assume that you are indeed losing 10% of your tolerance to measurement error, you need to do some guardbanding to account for it. Amost any time a process approaches a specification limit there is risk. There is an economic decision to be made--how many "good" parts can you afford to lose by reducing the tolerance in production? Can the process be moved away from the limit, thereby reducing the risk? There are details and factors other than GR&R results that need to be known and taken into account for these kinds of decisions.
 
Z

zancky

#5
from my undertanding,

MSA make sense if You are using the device for the X-r/X-s chart i.e when you are measuring cp cpk. In such a case you can not be so close to the limit.

If you are deciding on a signle part about compliance ok the limit should be reduced in my opinion in order to avoid type II error
 
B

Black arrow

#7
You don't know for sure what the uncertainty is if you're using only the R&R output. There are problems with the standard AIAG GR&R that make those kinds of conclusions unreliable. For more information I recommend
An Honest Gauge R&R Study [PDF] from Donald Wheeler to help you understand the limitations and potential pitfalls.

If you assume that you are indeed losing 10% of your tolerance to measurement error, you need to do some guardbanding to account for it. Amost any time a process approaches a specification limit there is risk. There is an economic decision to be made--how many "good" parts can you afford to lose by reducing the tolerance in production? Can the process be moved away from the limit, thereby reducing the risk? There are details and factors other than GR&R results that need to be known and taken into account for these kinds of decisions.
As i understand the % Tolerance it should be used when your measurement equipment is used for inspection and i saw in a thread in this forum and it says that "... to determine whether it is in specification or not".

Wether part is in specification or not? Please explain more what does that mean in practice? So an acceptable % Tolerance for example 10 % , means that the measuring eq. is ok for determine if the parts is within spec. or not? I would like to have an example which explains this a little bit better or picture which shows this. The measurement error vs tolerance range.:confused:

Thanks in advance!
 
Last edited by a moderator:

Jim Wynne

Staff member
Admin
#8
As i understand the % Tolerance it should be used when your measurement equipment is used for inspection and i saw in a thread in this forum and it says that "... to determine whether it is in specification or not".
Please explain more what does that mean in practice? So an acceptable % Tolerance for example 10 % , means that the measuring eq. is ok for determine if the parts is within spec. or not? I would like to have an example which explains this a little bit better.

Thanks in advance!
GR&R results categories are not absolute rules. They're guidelines, and some would say that they're not very good guidelines. Rather than just looking at the output numbers (percent of tolerance, percent of variation, e.g.) you need to look at the contributors as well. It's not a good idea to make a decision based on GR&R results alone. MSA is tool use in helping you to make decisions.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#9
Black Arrow: I strongly recommend that you read soem of Donald Wheeler's articles on MSA. it is a complicated topic and I know that if you invest soem time to understand the fundamentals from an expert (and of course you should read Miner's blogs here at the cove) you will accelerate your understanding. it will be faster - and less confusing - for you than a series of 'disjointed' questions.

I am not trying to discourage you from askign questions here - I am merely trying to provide you wiht additional high value references.

In addition to the article that Jim referenced I would direct you to the following:

How to Establish Manufacturing Specifications

Good Data, Bad Data and Process Behavior Charts
 
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