Does a hole create a datum feature point or axis

[sendithard]

Does the 1.350 diameter hole in the upper left create a datum that is a point or an axis?

Am I correct that the 1.350 diam hole location has no positional tolerance b/c it is the origin? Is the positional tolerance then only applied to the counterbore?

Thank you.

 

[chez311]

Does the 1.350 diameter hole in the upper left create a datum that is a point or an axis?

Axis, though the fact that it creates an axis is less important in my mind than the DOF it constrains.

Am I correct that the 1.350 diam hole location has no positional tolerance b/c it is the origin?

It has no position tolerance because most simply it has an orientation tolerance applied.

To expand on that - unless one utilizes Simultaneous Requirements or the feature is part of a pattern, a position tolerance is not applicable because its DRF (simply A in the FCF) has no higher order datum features from which a location constraint can be established. B has only orientation constraint to A. Also the concept of "origin" is relatively arbitrary and not usually very useful in GD&T terms. Again, not to sound like a broken record but I would concentrate on how and where your DOF are constrained than where an origin might be - in some cases its not very clear or obvious where it might be (say, with an asymmetric pattern of holes as a datum feature), and in that case it just matters that your quality team (and CMM software) utilizes a consistent origin for measurements. Its really outside the scope of the drawing to define an origin unless you're creating a customized DRF.

Is the positional tolerance then only applied to the counterbore?

The orientation tolerance is applied to the 1.350 hole, the position tolerance is applied to the 2.500 hole. Thats relatively clear from how the FCFs are attached to the dimensions.

 

[sendithard]

Chez,

Thanks for your time replying. I see more clearly now how you don't apply a positional tolerance to the 1.350 hole due to it only referencing datum A. I am new to GDT and didn't know at first when looking at this that you could separate callouts on perpendicularity and positional like this and they not apply to the entire family. I haven't read the standards, but I guess whatever tolerance callout you place underneath only applies to whatever is directly above.

I guess my next question would be what does a 'point' datum look like. Are there several basiic dimensions that tie it down or some datum feature that ties it down? I've yet to encounter a point datum.

 

[3DDave]

Datum points are determined from spheres.

 

[drawoh]

sendithard,

If you define a hole as a secondary datum, its position is perfect, by your definition.

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JHG