In this Question Line video, Jason answers the following question, tackling a common source of confusion in GD&T—the difference between circularity and runout:

I work in a wheel workshop and regularly measure wheel treads. I am currently trying to fully understand how roundness works according to GD&T and how it should be evaluated in practice.

The person submitting this question explains that he is determining roundness by rotating an axially fixed wheel and using a dial indicator to measure the total variation. He wants to know whether this method correctly measures circularity according to GD&T standards. 

Jason explains why this approach is actually measuring circular runout, not circularity (commonly referred to as roundness). Though circular runout and circularity are related concepts, they are not the same: one is measured relative to a datum axis, while the other is independent of any datum feature. 

Using animations, Jason walks through the differences between the two and shows how each tolerance is correctly evaluated.

https://youtu.be/0Ck50Q3v2iw

Do you have a GD&T question for us? Submit it at the link below:  

https://www.gdandtbasics.com/share-your-gdt-questions/       

If your question is selected, we will notify you when our video answer is posted to our YouTube channel.       

* If you are a current student, reach out to us directly and we will get you into contact with your instructor. Active students’ questions receive a direct response within 24-48 hours.       

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