Alex,

WRT…”the method of verifying a size dimension can make a difference between a functional part and a failed product. (e.g. minimum wall thickness, allowable roundness deviation, etc.)”

Alex, Do not dismiss 2 point measurement using roundness as the reason. Diameter (size) is not the same as roundness.

Although roundness and size both play a significant role in the way things fit together, roundness is evaluated independent of size and must therefore be measured in a different manner.

Definition of roundness
A component is described as round if all points of a cross section are equidistant to a common center. Therefore, to measure roundness, rotation of the component is necessary coupled with the ability to measure change in radius.

Vee-block (3 point) method
Rotational datum method
Radial variations output from the gauge as a polar profile or graph from a manual template overlay or computer generated “perfect circle.”

With Respect Always,

Charles Roscoe
Senior Designer ~ Retired

From your assumption section. In the following text, we shall read Y14.5.1 instead of Y15.1.
“Until the Y14.5 and Y15.1 standards adequately cover how to interpret size dimensions, consider one of the following actions:”
My interrogation
Where is the definition of “Size dimension” in ASME Y14.5?
How could we explain the difference between “limits of size” and “size dimension”?
Does “size dimension” is reserved to limit the size of a regular feature of size?
Does size dimension is only express using limits of size?
Does a size dimension of a feature of size could be express as a basic dimension and its variation is express by using a profile tolerance?
Why ASME Y14.5 maintain Rule No 1 as a default? Does an industry survey could be performed which will demonstrate if the default Rule No 1 is functionally required on most regular features of size defined with a limits of size?
If we go back in the industry standard (mil-std-8), rule No 1 was applied only when the only controlled we have is a limit of size.

I am having some trouble understanding how the inspection method can be the difference between design success and failure. A Diameter, for example, must have all points on the surface comply with the controls shown (They must lie within the boundary set by the diameter tolerance, and any additional form tolerance). When I inspect a part, I recognize that using a micrometer to check this diameter can be very risky, allowing a section of surface I did not check wander outside of the tolerance zone. Lastly, as a design engineer, I have always been taught not to place manufacturing or inspection methods on the drawing. I can force a more detailed inspection of critical surfaces by using a 3D geometric tolerance in place of a 2D control (Cylindricity in place of circularity or total runout vs. circular), but I cannot force the QA inspector to measure the part in a particular way. How do you propose I force the inspector to utilize a specific method like the theoretical swept ball?

Thanks Alex for the survey. I believe this would be helpful in pulling information together on the subject.

I think this highlights the issues when the standard runs into practical considerations.

Technically (as per the survey, ignoring the effects of rule 1), per the standard, you need to determine the axis from the unrelated actual mating envelope. You can then sweep the surface, and determine that the distance from this axis of all points is within the limits of size.

With a CMM, this might be relatively easily achieved, if your machine supports it.
Otherwise, you could perhaps set up something clever to lock the part’s axis along the axis of its unrelated actual mating envelope, allow it to rotate on this axis, and then set up an indicator that can sweep along the direction of the axis.

…But how often is this actually going to happen?