Result of a Flatness MSA on a CMM

[Black arrow]

Hello,

I need some support from you experts concerning a result i recently had from a MSA concerning flatness.

1 operator
20 parts
3 repetitions

One sided tolerance (Flatness)=> Only enter upper tolerance limit 0.05 in minitab.

Measuring equipment: CMM

This CMM is used for both inspection and for cap.studies.

As you can see % Tolerance. % Study Var & ndc are acceptable.

But part 12 & 39 are out of control limit in R-chart.

What conclusions can i take from that? Is this MSA only partly acceptable now?

I have checked the surfaces on part 12 & 39 and can not see anything suspect. So i have no good explanations concerning the deviations for those two results when it becomes to values out of control limit.

Thanks!

 

[Miner]

I recommend carefully examining both parts for the presence of a surface flaw that might have skewed the results. If your CMM stores the individual XYZ data that make up the flatness, see if one of the points has an unusual value for the (I'm guessing) Z value.

 

[Black arrow]

I have checked the plots and i can not see any unusal. In another hand the sample range (max-min) for part 12 is 0.001347 mm, it is quite small value.

 

[Jhintze]

You really should be using more than one operator for the MSA, even with a CMM. Your variation is going to come from fixturing, cleanliness of the part, etc. You may be surprised to learn the amount of variation there is between operators in automated inspection.

That said, I agree with Black Arrow. The range of 0.001347 mm is very small, especially in comparison to 50 mm. I wouldn't worry too much about it.

If your CMM is measuring in inch, the range is a good percentage of your tolerance and should be investigated. Maybe there was burr or other defect on the two parts that the CMM picked up.

 

[RSEGRIGGY]

Your program itself can have a lot of variation, depending on how you write it. Number of hits, probing system, machine speed, alignment, algorithm, etc....

Since it seems these two parts are an issue, I'd look at them a little closer. Are they sitting on an uneven surface? Is the part restrained?

We have a part that sits on a cast face. Sometimes the variations in the cast part would allow the part to rock when probing the machined surface. Could that be the case here?

Could there be some type of transient vibration, electrical surge, or air supply issue?

 

[Black arrow]

Thanks. I will tomorrow do some tests on the parts that are in focus. E.g 50 measurings without taking the part from yhe fixture. Then measure the part 50 times, and taking it away and mount it again between the measurings. Just as a first step...

 

[SasiKumar]

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