Value of secondary B in the circular runout

[greenimi]

I had a dispute with the author few days ago on about the value added of secondary datum feature B in the circular runout in the unless otherwise specified note.

Do you think B is provided any value as a secondary datum feature in the circular runout?

I would love to hear your opinions.
Thank you very much.

Disclaimer: do not focus on the other mishaps on the embedded drawing nor on the comments shown on the face of the print. Just strictly on the circular runot note.

 

[chez311]

greenimi,

Theoretically the constraint provided by datum feature B of the translational DOF along the axis of datum feature A shouldn't have an impact, however in reality it depends on what exactly datum feature A is meant to be as currently it is (as noted) applied incorrectly to the axis as well as what the assembly condition is. If it is meant to be the stubby diameter (40 -.025/-.064) to the right of the flange face (datum feature B) I think you'll find that the reality of the high diameter to length ratio will cause some issues during measurement if only datum feature A is utilized.

Furthermore it is highly likely that the assembly condition would dictate that the flat flange face datum feature B should be primary - if you note in Y14.5-2009 pretty much all of the examples utilizing a flat face as a datum feature have it as primary for this reason. Take a look at fig 9-7 for what seems to be a similar case with a flat mating face and what looks to be bolt holes.

 

[greenimi]

My point then and now was/is, if in theory B, as a secondary does nothing then in practice someone shouldn't put use it as such in the first place.
We have to understand the theory first before we can have any hope to apply such of said theory in any sort of practical way.

All the examples for runout in 2009 standard show the plannar surface used primary and NOT secondary (as it is in my picture).
I wouldn't have any problem with B being used primary.

 

[chez311]

greenimi,

With such a short length of engagement, I think it would probably be difficult to reliably simulate datum feature A without some amount of contact on datum feature B even if it doesn't change the outcome. I don't know if I would say theres anything inherently wrong with adding it though, even if its extraneous - and especially if it reflects part function/assembly. For example, additional datum feature references can be added to the lower frames of a composite tolerance even if they don't constrain any additional rotational DOF and have no impact on the behavior of the tolerance zone - I wouldn't call this wrong though. A restraint note to restrain the part to B could be added that would no longer make datum feature B extraneous, however that would probably be bad practice as it would fight the primary datum feature A for constraint of DOF - B should probably be primary in this case.

All the examples for runout in 2009 standard show the plannar surface used primary and NOT secondary (as it is in my picture).
I wouldn't have any problem with B being used primary.

Right, I was trying to suggest that although B is shown as secondary in the print you have provided, changing it to primary might better reflect part function and assembly condition as well as mitigate some of the issues with trying to simulate a feature with such a high *diameter/length ratio at RMB.

 

[TheTick]

I was the same ignoramus that put the datum on the centerline,