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Gage R&R Example

If My Parts Are So Good, Why Is My Gage R&R So Bad?

Almost every week, we get this question from someone trying to conduct a Gage R&R study because their customers are requiring it. They think if their product is good, their Gage R&R should be too.

You need BAD parts to conduct a GOOD Gage R&R Study

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Gage R&R Study Basics

Gage repeatability and reproducibility calculates the total variation (TV) from three factors. It then uses total variation to determine how much variation is attributable to appraisers and equipment (%R&R).

If your parts are identical, there is NO Part Variation and all of the variation falls to the appraisers and equipment.

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How to Set Up a Good Gage R&R Study

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Gage R&R Example

I have dozens of examples, but here's a recent one. The supplier produces parts:

• Target: 43.11
• Tolerance: +/-0.13 (0.26 total)
• USL = 43.11 + .13 = 43.24, LSL = 43.11 - .13 = 42.98

They measured 10 parts with three appraisers. As you can see from the data table below, all parts are only off from the target by a few thousands. NO PART VARIATION.

 

If we run a frequency histogram on this data, you'll see that the capability indices (Cp, Cpk, Pp, Ppk) are excellent:

 

Even though the parts are good, they are too good to test the measurement system. When we put this data into QI Macros Gage R&R template, built in logic tells us the same thing. The calculations in cells A48:B56 contain the warning "NDC < 5".

NDC is the Number of Distinct Categories. It is a measurement of the variation in your sample parts. If your NDC is less than 5, there isn't enough part variation to use the Average and Range Method or the ANOVA method to calculate Gage R&R.

Specification Tolerance Method

The alternative is to use the Specification Tolerance Method. The Spec Tolerance equals (USL-LSL).
A one-sided Specification Tolerance equals 2*(USL-Xbar) or 2*(Xbar-LSL). AIAG MSA V3 pg. 116-117.

In this example, the spec tolerance is equal to 0.26. If we input this value into cell B45, we get results using the spec tolerance method in column C.

You can even use the specification tolerance with the ANOVA method (%R&R = 1.3%).

But, it can still be hard to explain this to your customers.

A better choice is to go back and make sure you have 10 parts that span the tolerance: (42.98-43.24):

42.98, 43.01, 43.04, 43.07, 43.10, 43.13, 43.16, 43.19, 43.22, 43.24

When you do, there's much more part variation and the NDC will change accordingly. Here is an example of Gage R&R results with a good NDC and usable results.

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Here's the Point

Gage R&R doesn't measure the quality of your parts, only the quality of your measurement system. If you want customers to trust your part quality, you have to learn how to properly evaluate measurement variation.

Learn More...

• Another Example Using QI Macros Gage R&R Template for Excel and Ford Test Data

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Why Choose QI Macros Gage R&R Software for Excel?