Gage R&R with NDC=1

#11
Sorry missed that part about within part variation. By that do you mean the part flexing/deforming between measurements or a error in where the measurement points are taken from.

The part is a rubber part, so it is viscoelastic in nature and we are using a an optical vision system in order to not have any deformation due to other forms of measurments (calipers). The vision system has very good resolution but does not have automatic detection, so operators manually take measurement points from features they were trained on. This involves lining up a crosshair/lines to the feature as best as visually possible by the eye.
 
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Miner

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#12
Sorry missed that part about within part variation. By that do you mean the part flexing/deforming between measurements or a error in where the measurement points are taken from.

The part is a rubber part, so it is viscoelastic in nature and we are using a an optical vision system in order to not have any deformation due to other forms of measurments (calipers). The vision system has very good resolution but does not have automatic detection, so operators manually take measurement points from features they were trained on. This involves lining up a crosshair/lines to the feature as best as visually possible by the eye.
Flex/deformation may be part of that, but I was speaking about variation in form.

For example, take the cross-sectional diameter of a single O-ring. The cross-sectional diameter is not perfectly circular, so results will vary depending on the angle at which the diameter is taken. Also, the mold pin that forms the inner diameter of the O-ring will not be perfectly concentric with the mold portion forming the outer diameter, so the cross-sectional diameter will vary depending on where the section is measured.

However, you may also be dealing with the uncertainty in lining up to the features. What kind of features are used?
 
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Welshwizard

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#13
@Welshwizard You are correct, and I am a fan of Dr. Wheeler's EMP approach as well as BevD's Youden plots. I tend to stay focused on the specific questions asked by the OP for two reasons. One, most of the time they are required by their customers to use the AIAG approach, and Two, I am afraid to further confuse them by adding another method unless I get an indication that they are open to a different approach.
@Miner, indeed, point taken, its frustrating when people could be spending lots of time and therefore money chasing shadows. You're correct about the different approach, the problem is if you don't keep calling it out and heping to inform the customer, sowing a seed we won't advance. In the aerospace industry in the UK that I was primarily involved in we actually had a choice because we saw the issues early on, I understand that this is not the case in many other industries.

Thanks
 
#14
Flex/deformation may be part of that, but I was speaking about variation in form.

For example, take the cross-sectional diameter of a single O-ring. The cross-sectional diameter is not perfectly circular, so results will vary depending on the angle at which the diameter is taken. Also, the mold pin that forms the inner diameter of the O-ring will not be perfectly concentric with the mold portion forming the outer diameter, so the cross-sectional diameter will vary depending on where the section is measured.

However, you may also be dealing with the uncertainty in lining up to the features. What kind of features are used?
The features for this measurement are to top of the piece which is flat to a small corner feature at the bottom of it. All operators were trained together and shown identical I plan on taking a deeper look on how operator 3 is taking measurements compared to the others.
 
#15
Your measurement process can detect down to 0.020 mm half of the time ( Probable Error) and your tolerance width is 0.8 mm therefore you have ample leeway to detect against spec. The third decimal place is pure noise based upon this study, your measurement process can detect an increment between 0.004 and 0.04 and your recorded increments are 0.001 mm therefore you could drop a decimal place.

Despite the warnings of a lack of consistency (breaching the control limits for operator 2) there is no sign of any detectible systematic differences of repeatability between the operators. There is a detectible difference in part averages (reproducibility) and although small it wouldn't be difficult to understand why operator 3 averages are lower (measurement technique??) than the others and claim a small benefit against the AIAG method.



Cheers
Thanks for the response Welshwizard.

Reading this again, has left me a little confused. You said the measurement process can detect down to 0.020 but then mention it detects increments between 0.004 and 0.04. Could you explain where you are getting these values from and the difference between the 0.020 and 0.04.

And yes I agree. Taking a look at operators 3 measurement technique compared to the others is planned. Also thanks for linking that paper reference. Very informative.
 

Miner

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#16
The features for this measurement are to top of the piece which is flat to a small corner feature at the bottom of it. All operators were trained together and shown identical I plan on taking a deeper look on how operator 3 is taking measurements compared to the others.
This sounds like it might be a combination of handling/placement of a flexible part (possible stretch/compression), alignment and edge detection.
 

Miner

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#17
Reading this again, has left me a little confused. You said the measurement process can detect down to 0.020 but then mention it detects increments between 0.004 and 0.04. Could you explain where you are getting these values from and the difference between the 0.020 and 0.04.
Welshwizard is referring to some concepts unique to Dr. Wheeler's EMP. I cover this in my blog if you are interested in further details.
 

Ninja

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#18
Is it possible to stand the part on edge, and measure the optical profile without touching it?

If feasible, you can make a holding jig that operators can use.
 

Welshwizard

Involved In Discussions
#19
Thanks for the response Welshwizard.

Reading this again, has left me a little confused. You said the measurement process can detect down to 0.020 but then mention it detects increments between 0.004 and 0.04. Could you explain where you are getting these values from and the difference between the 0.020 and 0.04.

And yes I agree. Taking a look at operators 3 measurement technique compared to the others is planned. Also thanks for linking that paper reference. Very informative.
Hello Breadandwater, you are very welcome, it sounds like you are on the right track.

Miner has quoted an excellent resource to help with the use of the Probable Error (PE) and the general EMP approach.

To answer your question generally:

You will see and realise when you read into this some more that the PE places an upper bound on the precision of a measurement, there is therefore no point in attempting to interpret any value more precisely than +/- 1 probable error.

Bearing the above in mind we can see that the PE is the demonstrable resolution of the measurement process. We can use this property to check on whether the recorded increments for the measurements are usefull or just pure noise.

The effective increment is defined by the PE when it exceeds the smallest increment, the increment dominates when its larger than the PE. We can go on to prove that the increments being recorded are justified when in the range of 0.2 to 2 PE. So, when the increment is less than 0.2 PE we can drop a decimal place, when its more than 2 PE we woud be justified in adding a decimal place.

For your data the PE was 0.020 so your recorded increment should be in the range of 0.004 (0.2 PE) and 0.04 (2 PE). As your increments were 0.001 mm the last digit is less than 0.004 therefore its pure noise and it can justifiably be dropped therefore making your increments 0.01 and in the range of 0.2 to 2 PE.

Some people find justification very usefull, I used to work for a National Standards Institute and saw one application were the manufacturer of a very expensive device claimed a resolution of 7 decimal places but it was clear than upon testing the process could only "see" 4. Very often a manufacturer will state the resolution based upon how many digits the display has the ability to record, you now have a tool to validate this, there are other benefits for using PE which are beyond this offering.

The attached is a good explanation of PE to be added to Miners Blog and EMP III. A lot of reading material is on Dr Wheelers website www.spcpress.com.

Thanks for your interest and good luck.
 

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Matt's Quality Handle

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#20
If feasible, you can make a holding jig that operators can use.
This. I would add and cost effective.

In responding to the main thread, Miner is dead on (as usual), on asking what you're doing with this information. I tend to simplify it like this:

NDC/% Variation answers the question "Is my measurement system good enough to use to adjust my process (within control limits)?"
% Tolerance answers the question "Is my measurement system good enough to tell a good part from a bad part?"

However Big 3 SQE's aren't always as enlightened as you, Miner, and me. :)

:2cents:
 
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