GD&T without concentricity

[Jbeaker]

Please see the simplified drawing, I am omitting other dimensional and geometric tolerances for simplicity.

With the removal of the concentricity geometric tolerance in ASME 2018, I was trying to sort out how to ensure the inner diameters of the lens barrel are co-axial. A couple ways to do it, hole location, runout, and profile I think (controlling form and location).

Am I missing anything? I didn’t opt for a tertiary datum since I don’t really care about rotation. I want to ensure co-alignment between datum B (hole) and the other two holes.

 

[3DDave]

There are cases that concentricity can handle that runout cannot meaningfully be used - for example concentricity can be applied to a hexagonal feature. I suspect one could apply it to a elements of a bolt circle or propeller blades. Note the definition accepts "radially disposed features"

There can be no "better than was asked for" in substituting the control by the total runout characteristic over controlling via the concentricity characteristic for those cases.

 

[mfgenggear]

generally it was used on shafts for ground diameters.
here is my take as a designer.
stick with runout and total runout on shafts
and life is good.

 

[3DDave]

Runout and total runout are the best descriptions of an interaction with a mating part such that the mating surface slides or rolls over the surface controlled by runout or total runout. Typically the mating part is adjustable to conform to the size or location of the controlled surface.

If the mating part is relatively rigid and encompasses the majority of the surface then it will act as the Actual Mating Envelope which is the basis for the position characteristic tolerance for cylindrical features.

It is possible to have a feature that is entirely acceptable via position characteristic tolerance that is rejected by a runout or total runout characteristic tolerance of similar magnitude.

To be fair, it's easier to blame engineering for rejecting parts when they are coerced into using runout or total runout characteristic symbols for cases when there isn't sliding contact and for which position is the more representative characteristic to control.

 

[mfgenggear]

there has been components such as gear shaft, shafts that where ground for bearing journals. that are high rpm application. and required balancing. with very tight balancing
requirements. tight tolerance. if the oarts had to much runout even in tolerance the parts could not be balanced. runout is a great attribute for high speed rpm shafts.