NDC (Number of Distinct Categories) - Long Gage R&R Study in MSA 3rd Edition

Bev D

Heretical Statistician
Staff member
Super Moderator
#61
What's really sad is that the quality profession continues to promote practices that are invalid. We have ceased thinking. The games that are played to get an 'approval' divert effort from true quality improvement. :confused:
 
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bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#62
What's really sad is that the quality profession continues to promote practices that are invalid. We have ceased thinking. The games that are played to get an 'approval' divert effort from true quality improvement. :confused:
I have begun to appreciate how Deming felt.
 

mjoakin

Involved In Discussions
#63
Have a question, what if the NDC is weirdly high?
I have seen NDC of 630 and 780, what does this mean?
If a NDC 5 or higher is acceptable, a really high means what?
Too much equipment for what you are measuring perhaps?
 

Miner

Forum Moderator
Staff member
Admin
#64
I would have to see the data to be certain, but my initial reaction is the same as yours, that you have more gage than you need.
 

Miner

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Staff member
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#67
Even correcting the typo to 6.44, doesn't help. there is a serious repeatability issue, and it is not caused by inadequate resolution.

Can you provide additional information on what is actually being tested? With a destructive test, you might be seeing the inherent variability of the material being tested.
 

bobdoering

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#69
We were pull test the 24awg wire using push pull gage
This type of test has a variety of repeatability problems. Trying to utilize Gage R&R on physical testing is a nightmare. I will give you some tips.

At you measuring ultimate, catastrophic failure? If so, put your pen down and walk away. It will never repeat. You may have to use "surrogate" measures to verify the tester. If it chart records, use yield instead of ultimate failure. It is far more repeatable in materials.

One approach I used was to use trilene fishing line at various test strengths as a surrogate material. It provides - by design - specific variation that your tester should be able to prove it can tell the difference between. You might not have those kinds of samples with your own material. I was working in a small range of pull strengths, so I could easily find line in my range. But, if you can use it in your tester (and you will need to learn the "trilene knot" to keep it from slipping), then input historical variation for your PV, it may work fine.

If your wire is stranded, try using different number of strands to determine of the tester can see the difference.

I hurts my brain when people want to rubber stamp Gage R&R on physical testing, but that rubber stamping is chronic, especially in automotive.
 
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Miner

Forum Moderator
Staff member
Admin
#70
We were pull test the 24awg wire using push pull gage
Depending on the make and model of the gage, this may be a portion of the repeatability issue. However, a large portion will likely be due to the variation in tensile strength of the material, variation in the cross-sectional area of the wire and surface flaws, such as nicks.

After reading Bob's post, I recalled a white paper from Instron that addresses these issues nicely and offers an alternate approach to the R&R.
 
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