Datum Target Dimensions

[aniiben]

Why Y14.5-2009 allows datum target dimensions to be either basic or toleranced (±), instead of restricting it to basic only? Are there any advantages of toleranced dimensions on datum target ?
Also, there are no examples of toleranced dimensions, unless I am missing the applicable figure.




4.24.7 Datum Target Dimensions
The location and size, where applicable, of datum targets are defined with either basic or toleranced dimensions.

 

[Sem_D220]

An assumption: perhaps if you use datum targets to establish a datum where the datum feature is a feature of size (probably an irregular one), you could use plus minus toleranced dimensions between the datum targets to control the size of the feature, and to allow the use of a datum feature simulator which is adjustable in size.

 

[CheckerHater]

Go no further than next sentence: "If defined with basic dimensions, established tooling or gaging tolerances apply"

Your datum target can be established by fixture ("datum simulator") or it could be feature of the part itself. To distinguish between the two, toleranced dimensions refer to the part itself, basic dimensions refer to fixture.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

Your datum target can be established by fixture ("datum simulator") or it could be feature of the part itself. To distinguish between the two, toleranced dimensions refer to the part itself, basic dimensions refer to fixture.

CH,
I do not want to stir a debate, but are you sure that this statement is correct:
“Your datum target can be established by fixture ("datum simulator") or it could be feature of the part itself. To distinguish between the two, toleranced dimensions refer to the part itself, basic dimensions refer to fixture.” Not sure if the “feature of the part itself” is the right wording, but it could be….. still learning the terminology.

As far as I know, the datum reference frame is established from the datum feature simulators and not the datum features.

For what is worth figure 4-53 (2009) shows datum target B with ± toleranced dimension (B1, B2, B3 and Ø170±0.1)

 

[CheckerHater]

@greenimi:
Fig. 4-53 tells you that datum targets are established when jaws of the chuck spaced [3X 120deg] (fixture) are touching diameter 170+/-0.1 (part).
Just like I said, toleranced dimensions refer to the part itself, basic dimensions refer to fixture.
Where is the contradiction?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Sem_D220]

CH,
I think the confusing part was when you said that a datum target can be established either from a fixture or from a feature of the part itself. The truth is that it is the fixture which always to simulates datums, including those datums which are defined by datum targets. The fixture simulates the datums by contacting the features of the part at the datum targets points/lines/areas.

 

[CheckerHater]

The confusion starts with the fact that datum / datum simulator / datum feature / datum target are not the same.

Datum target is the area of contact between datum feature and datum simulator. It doesn't belong to any of them.

DIA 26 feature on this drawing belongs to datum simulator (fixture) There is no feature on the part with DIA 26:

Dimension 76 +/-2 on this drawing belongs to datum feature (part) there is no indication how datum simulator looks like:

So once again, datum target belongs to neither datum feature or datum simulator.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Sem_D220]

CH, I agree. You emphasized the distction distinction between the terms very well.
All toleranced dimensions in the examples brought up in this thread so far define datum features, not datum targets.

For me this makes aniiben's question totally valid: considering the paragraph he quoted, in what cases can toleranced dimensions specify "the location and size" of datum targets?