MSA vs. Measurement Capability Analysis - What are the differences?

P

potdar

#21
Re: Mr

I would like to contribute a few commonplace "measuring systems" as steven has introduced.

- a surface plate and a height gauge

- a vee block, a master and a dial

in such cases we do a detailed study before reaching a conclusion on the acceptabilty of the system. If not acceptable, we go deeper componentwise to find out what exactly is causing how much trouble.
 
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I

igsram233

#22
Dear Atul,

Thanks for the clarifications.

We were recently audited by Robert Bosch, and the auditor told us to measure Cg & CgK first for finding the capability of the measuring instrument and then go ahead with doing the measurement of the parts.

I am yet to get an answer for the same.

But since I am not a statistics student I am not able to understand your points. Could you please explain by way of example using an excel sheet?


Sriram
 

Jim Wynne

Staff member
Admin
#23
I don't think we're disagreeing here, potdar, just having a little trouble with the language.:D Perhaps my only quibble is this:

So far as I understand, MSA is necessarily done by eliminating effects of process variation. The calculation methods enable that. There is no relation to "what one hopes to accomplish".
And again, perhaps I'm misunderstanding you. The results of GR&R may be used to help determine how a particular gage is to be used (i.e., in accordance with what one hopes to accomplish). Quoting the AIAG MSA manual, Third Edition, page 13:
For product control, variability of the measurement system must be small compared to the specification limits. Assess the system to the feature tolerance.

For process control, the variability of the measurement system ought to demonstrate effective resolution and be small compared to manufacturing process variation. Assess them measurement system to the 6-sigma process variation and/or Total Variation from the MSA study.
(Emphasis in the original)
 
P

potdar

#24
I don't think we're disagreeing here, potdar, just having a little trouble with the language.:D
Yes Jim,

The issues are settled. A gauge that may not be fit for process control may be perfectly OK for incoming inspection of the same spec.:agree1:

I think I shall take some efforts on polishing my presentation.:)
 
A

Atul Khandekar

#25
Dear Atul,

Thanks for the clarifications.

We were recently audited by Robert Bosch, and the auditor told us to measure Cg & CgK first for finding the capability of the measuring instrument and then go ahead with doing the measurement of the parts.

I am yet to get an answer for the same.

But since I am not a statistics student I am not able to understand your points. Could you please explain by way of example using an excel sheet?


Sriram
Hi Sriram,
Sorry, I do not have any Excel sheets for this. However, I do know that Bosch has defined procedures, flowcharts and also worksheets for calculating measurement system capability. You can get these documents directly from them. The formulas I gave above were from my very old notes from my interaction with them at MICO.
 
#26
There also is some confusion regarding the definition of "process variation". Is it "variation due to the process", or "variation in process parameter measurements obtained during analysis"?

The second case is commonly understood as "process variation" and includes variation due to all aspects involved in the process and inspection.

I am still thinking on what's going on here,,,Potdar/Jim if you can explain this difference in terminology used above for the GRR...I know in the GRR calculation Process variation (TV)= sqrt((part var.)^2 +(RR)^2)) .... So I think as Potdar said earlier, the process var. is included in the %GRR calculation (%GRR=RR/TV)...So why don't the rules <10%,etc. apply here?:confused: Any examples to cite here?

This may sound dumb, but any suggestions for this question??
 
P

potdar

#27
I am still thinking on what's going on here,,,Potdar/Jim if you can explain this difference in terminology used above for the GRR...I know in the GRR calculation Process variation (TV)= sqrt((part var.)^2 +(RR)^2)) .... So I think as Potdar said earlier, the process var. is included in the %GRR calculation (%GRR=RR/TV)...So why don't the rules <10%,etc. apply here?:confused: Any examples to cite here?

This may sound dumb, but any suggestions for this question??
If you are talking about the age old thumb rule of 10:1 resolution, it speaks of least count.

GRR talks of measurement error. As you have rightly put, TV consists of PV and RR. SPC techniques are based on tracking the TV and containing it for process control. For the TV to sufficiently accurately reflect PV, the RR component should be limited to a small fraction of TV. Thats why GRR.

Hope I got you right and you get me right.:D
 
#28
Yes, I got you but a a question from your reply:

What is the difference between 10:1 rule, P/T ratio and %GRR calculated over total tolerance???

The AIAG manual specifies some criteria for %GRR (over tolerance) like <10%, 10-30%, & >30% for gage selection which says about the discrimination of the gage, where <10% is considered ok.

10:1 rule says a similar thing about the discrimination of the gage to be atleast 1/10 of the total tolerance.

P/T (Precision/Tolerance) also talks about the same.

To me all means the same.

Hope I am not missing anything important here.

Thanks in advance.
 
T

Themistocle

#29
Hello everybody,

my question concerns p191 of MSA 3rd edition where the link between Cp and %GRR is given.

When analysing the process capability of a supplier, is it acceptable to back calculate the Cpact of the process from the Cpobs and %GRR determined during the MSA and then present the results as the "actual " process of the supplier to our customers?

If yes, shall we use the %GRR to process or the %GRR to tolerance determined on Incomming Inspection measurment data?

Thans a lot for your help

regards
 

Miner

Forum Moderator
Staff member
Admin
#30
You could definitely provide this information supplementally to the customer, but I do not recommend portraying it as what the customer would normally receive from the typical supplier. Be specific about what was done and how.

In the calculation, you do not use either %GRR. You use the standard deviation. StDevActual = SQRT[(StDevObs)^2 - (StDevMeas)^2]. Then use the StDevActual in your calculation of Cp/Cpk.
 
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