Runout callout with a datum to itself

[sendithard]

I'm not sure I'll ever understand this one... how can you measure a runout from yourself? Since runout doesn't capture the highs like position does...wouldn't runout to itself equal zero for that perscribed datum requirement?
I understand the red circled callout, but I don't understand the blue circled callout.

How do you measure the same surface that is used to capture the surface used to measure against? ie....datum D

Or is this callout saying capture the plane C then find a single point of touch on the diamter D and let that assume the high point...then rotate the part and see how the total indicator moves

 

[3DDave]

Think of it like a collet closing on the feature - if the feature is irregular the collet can still clamp down, but that doesn't mean there aren't gaps - this Feature Control Frame limits the size of those gaps.

 

[sendithard]

So can you use this type callout on other GDT callouts other than runout? I don't remember other gdt geo callouts referencing themselves in other chapters.

 

[3DDave]

First, do you understand why it works in this instance?

Second, perhaps there's not too much in the way of cases where it would matter. Perhaps lack of imagination is why there aren't other examples.

 

[Burunduk]

I don't remember other gdt geo callouts referencing themselves in other chapters.

Did you pay close attention to the figures in section 3?
See figure 3-20, which shows a callout for profile of a surface all-over with reference to datum features A, B. Datum features A and B are included in the all-over control just like all the other features of whatever part that was defined that way. If, for example, A and B are planar datum features, the datum planes will be established based on the high points, but the low points of the same features can't get outside the all-over tolerance zone (half of the specified tolerance will be available for the datum features).

 

[pmarc]

So can you use this type callout on other GDT callouts other than runout?

From the Y14.5 theory point of view, this type callouts can be used on other GD&T tolerances as well. Since you seem to be studying the 2009 version of the standard, we can, for example, imagine a simple pin controlled with a concentricity tolerance relative to a primary datum axis established from the itself. This callout would control location of all median points of the feature with respect to its own axis and would be fine because the controlled elements (i.e., median points) are different elemements than the element used as a datum (i.e., axis of the UAME of the pin).

If, however, the concentricity callout was changed to position, then this wouldn't work anymore since it would basically become a control of the UAME axis relative to the same UAME axis. It would be typical datum self-reference scenario.