Measurement System Artificially Creates a Skewed Distribution

C

clarked3

#1
Hi All,

I have a question...

I received in a new component for the 1st time and "Goods In" did their inspection. On a batch of 3500, 75 samples were inspected. A dimension was not out of spec but failed to meet the inspection criteria according to ASQ/ANSI Z1.9 2003. Spec is 0.65-0.75 mm, values were recorded at 0.66-0.75. The reason for the failure was because the distribution was positively skewed and when I ran all the stats it turned out 1.9% of the batch could be above the upper spec limit, and therefore failed.

The thing is the supplier performed his inspection on a smaller sampler number (n=15) but his results were all within 0.68-0.70.

I haven't got their individual values and I imagine our inspection isn't up to scratch but my main question is as follows:

Can a poor measurement system introduce a skewness to the distribution?

I'm getting the supplier to measure a representative n=75 sample and I'm expecting it to be normal distribution as the validation up to now has shown it to be a normally distributed process.

So can this happen and is there anything glaringly obvious which could be the cause?

The inspection is on a tube wall thickness with a digital caliper. I agree it is not the most accurate technique, caliper flats and a curved surface but the supplier uses the same.

Thanks

Dan
 
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Miner

Forum Moderator
Staff member
Admin
#2
I have not seen a measurement device that yields skewed data, but measurement methods often do so. For example many GDT methods that take the absolute value or use polar coordinates result in skewed distributions.

In your case, I would look toward anomalies with the part itself such as burrs or similar anomaly in the measured area. Is this tubing DOM (drawn over mandrel) or is it rolled and welded? If the latter, wall thickness at the weld may differ from the rest of the part skewing the data.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
teh other thing to consider is that the distribution of parts within the lot may simply be skewed. the measurement system is fine but he parts are skewed.

the smaller sample doen by your supplier mightsimply not picked up those 'rarer' parts in the tail...Have you perfomed a methdo comparison MSA bewteen your self and your supplier?

when you say you ran the statistics were you using the Normal distribution or a different distributional model?

If you tell us what hte characteristic is and how you are measuring (and maybe even how it is produced - e.g. turned, millled, drilled, formed, etc.) tha twill help us provide better advice.

also posting your data and your supplier's results are helpful...
 
C

clarked3

#4
Hi All,

Its a polypropylene extrusion moulded tube.

AS for the stats...Because ASQ/ANSI Z1.9 is only applicable for normal distributions I first checked that the data was normal. It wasn't.
I checked the skewness and kurtosis, 0.48 and -0.60, respectively.
I ran a goodness of fit test. 3-Parameter Weibull was the only distribution >0.010, (Shape 1.90035, Scale 0.04518, Thresh 0.65697)
I then did a probability plot and worked out the largest wall thickness I could expect would be 0.80 mm

Measuring with digital calipers. Taking an average of 3 points.

Thanks
 

Miner

Forum Moderator
Staff member
Admin
#5
Several possibilities that come to mind are:
  • Variation in the alignment of the caliper anvils with the axis of the tubing
  • Anomolies such as burrs left when the tube is cut
  • Variation in thickness around the circumference of the tube due to the tooling
  • Variation down the length of the tubing due to fluctuations in the extrusion
The first has to due with the measurement method, the second with sample prep and the remainder with part variation.
 
C

clarked3

#7
The tubes are only 230mm long so bagged in lots of 1000.

I imagine it is a true representation and the number the supplier measured was not sufficient to show the skewness.

I am planning a 75 gauge R&R across the supplier and us and we'll see what that throws up!

I was just concerned that we had inadvertantly skewed the data and didn't know if that was even possible. By the sounds of it, it doesn't sound that easy or common to do!

Thanks for everyone's input!
 

Ninja

Looking for Reality
Staff member
Super Moderator
#8
Spec is 0.65-0.75 mm,

Its a polypropylene extrusion moulded tube.
The inspection is on a tube wall thickness with a digital caliper.
Dan
It should be a very revealing GRR...please share what you can of the results.

Wall thickness of < 1mm,
on a round surface (tube)
with calipers...
I will be quite impressed if it holds GRR < 10 on the spec range of 0.1mm with calipers.

Do you have another measurement method to check out at the same time, like a CMM?
 
C

clarked3

#9
No we don't have a CMM available. We queried the use of the digital calipers but the supplier was confident.

I imagine the GRR will highlight some re-training on our end and whether the distribution observed in true.

I'll update when I have more results!
 
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