Tertiary datum on Drawing

[ashivu123]

I have attached drawing which contains GD & T callout. Two datum are used in the drawing to constrain part, however only 5 degrees of freedom is constrained part may still rotate. Since part is free to rotate it will not have any impact on GD&T callouts?

 

[CheckerHater]

Ask yourself: will it become different part if rotated?

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

 

[KENAT]

Specifically, if other features are rotated about datum B does it become different?

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[drawoh]

CheckerHater and KENAT,

Actually, no! There is no problem rotating the part. This ceases to be true if you add any features not concentric with the hole.

--
JHG

 

[drawoh]

Still, I would apply the third, clocking datum.

--
JHG

 

[KENAT]

drawoh - CH & I were trying to get the OP to come to his own conclusion by giving him advice on what to consider rather than just giving him the specific answer for this case.

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[Belanger]

Since the profile tolerance goes all around, there really is no option for a tertiary datum. The rotation is actually controlled by the profile tolerance itself.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

The rotation is actually controlled by the profile tolerance itself

Rotation implies some sort of an angle. Which angle in this case is controlled by Profile tolerance?

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

 

[CheckerHater]

Still, I would apply the third, clocking datum.

What for? Implied 90 deg. angles and Sim. requirements are keeping the pieces together.

See ASME 14.5-2009 Fig. 4-39

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

 

[Belanger]

Rotation implies some sort of an angle. Which angle in this case is controlled by Profile tolerance?

All four corner angles!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

I am OK with corners. OP's drawing is actually pretty tight. The only thing you can rotate around datum B is the drawing view itself.

Which brings interesting afterthought: there is no such thing like implied horizontal (or vertical) line. We take for granted that certain edges of the part are (arbitrarily) made parallel to the edges of drawing sheet, but it is merely a matter of convenience.

What if you are trying to make a sketch on a piece of paper that is not rectangular at all? Could anyone reference the rule that makes the view on the right of enclosed picture "illegal"? No, seriously.

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