Using the results from GR&R (Gage R&R) Analysis - One sided dimension

S

swarbur4

#1
An interesting discusion came uip the other day when I was checking a PPAP / PSW submission.

For a SC (Significant Characteristic) the GR&R result was 10%. In production this dimension is inspected 100%. It is a one sided diamension. 0.1mm. The control plan and work instructions for the inspection descibed the actual spec for aceptance or rejection. example =<0.1 ok >0.1 NG.

The discussion was based on should the GR&R result be reflected.

example - spec 0.1mm max. GR&R 10%. Therfore for inspection purposes the acceptance criteria should be 0.1mm - (10% of 0.1) = 0.09. This would then ensure no parts outside of the spec be accepted.

What do you think? - has anybody used the GR&R result this way before.

Cheers
 
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Miner

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#2
Re: Using the results from GR&R Analysis - one sided dimension

A couple of questions:

  1. Are you as the customer free to establish policy, or are you accountable to your customer for the methodology used?
  2. Is the gage used for inspection or process control? If it used for process control, you will be using the process variation, so the tolerance is irrelevant.
Assuming you are free to establish policy and the gage is used for inspection, lets review this situation. The standard approach for calculating % Tolerance assumes a bilateral tolerance. When you have a maximum tolerance, there are several unofficial approaches that have been proposed in this forum:

  1. If zero is a boundary (i.e., results < 0 are not possible), zero can be treated as a lower spec limit to get an idea of the % Tolerance. This may or may not provide a relevant indicator. In the case of a measure such as flatness, it would appear to be relevant. In other cases, it may not.
  2. It can be argued that % Tolerance is irrelevant in this situation and should not be calculated. However, how can you then assess the suitability of the gage?
  3. You may consider Donald Wheeler's "An Honest Gauge Study" approach. You could use the Probable Error and/or the upper watershed limit to assess the suitability. See my blog entry Intro to MSA of Continuous Data – Part 7: R&R using Wheeler’s Honest Gage Study on this method.
This is a gray area in MSA. It has not been addressed by AIAG, Donald Wheeler nor by any article that I have reviewed. I recommend using the approach that economically works for you.
 
S

swarbur4

#3
Re: Using the results from GR&R Analysis - one sided dimension

Miner,

I am both a customer and suppier (Tier1). My main concern at this point is to ensure parts delivered to us are within spec. The gage is used for inspection - currently 100% due to poor capability results. The measurement is for parallelism so the spec is 0 + 0.1mm. Therefore the gauge R&R Tol method should be applicible for this.

The main question - irrespective of the type of diamension / Tol 2 sided or not.

Should the GR&R result be reflected in the final inspection (quality inspection) to ensure only OK parts are accepted. Has anyone actually done this before
 

Miner

Forum Moderator
Staff member
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#4
Re: Using the results from GR&R Analysis - one sided dimension

Since parallelism has a natural boundary of zero, using zero as a LSL would be a legitimate way to create a tolerance range to calculate % Tolerance.

Since you are also concerned about shipping product that is out-of-spec. I recommend that you read the article about "How to Establish Manufacturing Specifications" by Donald Wheeler referenced in my blog link above. The Excel file attached to the blog entry will calculate the manufacturing specs as described in the article.

These manufacturing specs are what are commonly called guard bands. They are statistically tightened spec limits that reduce the probability of shipping product that may be out of specification due to measurement error.
 

bobdoering

Stop X-bar/R Madness!!
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#5
Re: Using the results from GR&R Analysis - One sided dimension

This question is one of the reasons why I like the concept of ndc (or the Wheeler equivalent for those so inclined). Determining the "statistically significant resolution" of a gage in its measurement range is a key question as to its value. It does not matter what occurs in the tolerance zone, just that there is enough "real" resolution to detect where you are within the tolerance zone.
 
R

rbriggs5145

#6
Re: Using the results from GR&R Analysis - One sided dimension

I have found it depends on the customer. I have worked with 2 customers that required the gage R & R to be taken out of the tolerance in order to ensure the parts going out the door were in specification. In this instance your tolerance would be reduced to .09 instead of .1. In other cases, we have not used bonus as part of the calculation in the gage and therefore they considered that extra to cover gage error. Hope this helps!
 

bobdoering

Stop X-bar/R Madness!!
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#7
Re: Using the results from GR&R Analysis - One sided dimension

...that required the gage R & R to be taken out of the tolerance in order to ensure the parts going out the door were in specification.
That is particularly understandable if you are not capable, but really it is only part of the total variation that needs to be considered. It works best if you are 100% inspecting. If not, you need to add sampling error, too, which goes back to your process distribution.
 
S

swarbur4

#8
Re: Using the results from GR&R Analysis - one sided dimension

Miner,

Could you please attach the excel sheet to this entry as my work firewall stops me accessing your blog.

Cheers
 
S

swarbur4

#10
Re: Using the results from GR&R Analysis - One sided dimension

Miner

Thanks for the worksheet, I have read some of Wheelers publications now, and am starting to understand his concepts and logic behind his ideas. I have 1 question in the worksheet it refers to a value Pa - could you please explian what Pa is.

Thanks
 
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