Reducing Variation through Multiple Readings - MSA 3rd edition Chapter 5d

C

cyberjyothi

#1
Hi friends,

can some body help me !

During Audits I'm facing lot of queries with the external auditors(ISO/TS16949)
,What is the corrective action u h've taken while the GRR is exceeding >30%.
For this i found one alternative, i.e In MSA 3rd edition Chap-V -sec-d under reducing variation through multiple readings.
They given one Ex :% of Tol GRR variation is 25.5%, With a 6 Sigma spread equal to 0.24 ,If we reduce this fig. to atleast 15%,Which would have an equivalent 6 sigma spread of 0.14.

Pl. can anybody explain how the figure(0.24&0.14) arrived .

Thanks,
Jyothiswar:thanx:
 
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Jim Wynne

Staff member
Admin
#2
cyberjyothi said:
Hi friends,

can some body help me !

During Audits I'm facing lot of queries with the external auditors(ISO/TS16949)
,What is the corrective action u h've taken while the GRR is exceeding >30%.
For this i found one alternative, i.e In MSA 3rd edition Chap-V -sec-d under reducing variation through multiple readings.
They given one Ex :% of Tol GRR variation is 25.5%, With a 6 Sigma spread equal to 0.24 ,If we reduce this fig. to atleast 15%,Which would have an equivalent 6 sigma spread of 0.14.

Pl. can anybody explain how the figure(0.24&0.14) arrived .

Thanks,
Jyothiswar:thanx:
Those numbers are just examples, although I'm not sure where the examples came from. On page 175 of the 3rd Edition MSA manual, under Example, it says,

In the XYZ Corporate example, the percentage of tolerance GRR variation is 25.5% with a 6 sigma spread equal to 0.24. The customer wants to reduce this figure to at least 15%, which would have an equivalent 6 sigma spread of 0.14.
The reference to "the XYZ Corporate example" is unclear; I did a brief scan and couldn't find such and example in the manual, but perhaps I missed it. Nonetheless, you can use the information and formulae by just plugging in your own data.
 

Miner

Forum Moderator
Staff member
Admin
#3
There is a simple relationship between the measurement error and the sample size. Take the existing measurement error in StdDev and divide by the square root of the sample size.

For example, if you take a sample size of 4 and use the average as your measurement, you will reduce your measurement error by half.

measurement error / SQRT(4) = mesaurement error / 2
 
M

mos_mos

#4
Re: Reducing variation through multiple readings - MSA 3rd edition Chapter 5 d

If grr to tol value s say 130%, can this method allow me to do a capability study through multiple meas however painful?
 

Miner

Forum Moderator
Staff member
Admin
#5
If you were to take a small number of individual measurements, you could take the average of 16 measurements to reduce the error by 4 (SQRT of 16). However, I am guessing that you do not want to measure 1600 pieces for a capability study.

A better, less painful option, follows:


  1. Perform 100 pieces capability study, calculating mean and standard deviation
  2. Square standard deviation to calculate total observed variance
  3. Find Repeatability and Reproducibility standard deviation from your R&R study
  4. Square standard deviation to calculate total R&R variance
  5. Subtract total R&R variance from total observed variance to calculate product variance
  6. Take square root of product variance to calculate product standard deviation
  7. Use mean from step 1 and standard deviation from step 6 to calculate Cp and Cpk as appropriate.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#6
The other option is to review the resulting data from your gage R&R, and determining the various within- and between- errors and try to eliminate as many of the causes of error as possible - either by improving the technique (such as fixturing, etc.) or at least using another gage that will eliminate the most significant contributors of the error by its design.
 
M

mos_mos

#7
Thanks for the fast reply!!. What assumptions underlie plan B in terms of variation proportion? What do i need to check in order to close the loop? Thanks!!!!
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
I suggest reviewing page 53 through 54 of 3rd ed. MSA and look for those issues during the Gage R&R study. After identifying them, look to reduce them to improve Gage R&R. Reducing the variation will reduce the GRR as % of tolerance, as the tolerance remains the same.
 
M

mos_mos

#9
I understand how you sort the measuring system but in this case it is not easily/timewise etc/cheaply done, and historical we want to bookend. Right now we no choice but to use the existing and modify to improve measurement through repeated meas and also do capability with it.

Hence my reason for asking the previous questions. Thanks for the information very much appreciated.
 
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