Effective Training Inc. Message Board

I am currently taking the online GD&T course. I am very confused by the circularity callout.

When Circularity is specifically called out, the tolerance given is a radial tolerance. This is explained in the online course and the book.

When I am asked to determine the circularity tolerance based on Rule#1 with no specific circularity callouts, the correct answer is the diameteral tolerance.

I have attached a screenshot of question to further explain my confusion. For this question, I answered 0.4. I do not understand why this is wrong.

Hi Adam,

The answer is 0.7 because it is the largest value less than the size tolerance of 0.8. As you correctly stated in your question, Rule #1 provides an indirect roundness tolerance equal to the diameter tolerance. The size tolerance for the part shown is 0.8. To understand why Rule #1 provides an indirect roundness equal to the size tolerance it is best to look at the example in Alex's book "Fundamentals of Geometric Dimensioning & Tolerancing". On page 90, figure 4-17 shows a pin with a max size of 12.8 and a min size of 12.0 You will see that the rule #1 MMC boundery of 12.8 diameter is circumscribed about the high points of a three lobed pin. The drawing shows that the 12.0 LMC size is the distance across the pin from high point to lowest point opposite. This creates a radial distance from highest point to lowest point of 0.8 equal to the size tolerance. I attached a copy for you.

_________________
Best Regards,
Roy Cross
ASME GDTP Senior Level S-0488
Dimensional Engineering Mentor
Effective Training, Inc.
2118 S. Wayne Rd.
Westland, MI. 48186
royc@etinews.com
http://www.etinews.com