Profile with 2 planes as the Primary datum

[[To...]]

There is a profile to |D-E|A|.  The drawing calls out 2 planes, one as datum D and the other as datum E.  The plane on the bottom is datum A.  Any suggestions for how to set up D-E as the Primary Datum?  Is this a case of using a Geometry Best Fit alignment of D and E and using the alignment in the profile?

I really don't care about datum A because it doesn't control any degrees of freedom.

[[Ma...]]

Wouldn't be D-E as symmetry?

[[Ch...]]

It depends to the software that are you are planning to use for this inspection task. For ZI the answer would be that you can use a wedge as a datum starting with ZI 2025. For Calypso I have no idea what a good solution would be. 

[[Je...]]

Like Martin told, in Calypso a symmetrie of the (outer tangent) planes should work

Edited December 13, 2024

[[To...]]

Thank you for your feedback.

I don't know if this matters but the 2 planes are not at an equal angle netting an A2 projection angle of 4.25° on the symmetry plane. This symmetry reduces the degrees of freedom from 5 to 4, which now means Datum A can control translation in Z.  The considered feature is a plane (not shown) that is parallel to A.  I don't know if the design intent/function is to locate/orient from D-E or A.

[[Je...]]

This is an interesting characteristic, Tom.  I agree with Martin and the others that a symmetry of the two planes, in ASME speak a "midplane," is the primary datum.  A literal interpretation of the print would be to have D-E constrain everything that it can, which I surmise is two degrees of rotation and one degree of translation. That allows Datum A to control one degree of rotation this is the planar rotation (clocking, skew, etc.), and also one degree of translation which is perpendicular to the axis of the surface of the D-E symmetry until its intersection of Datum A.  The only remaining degree of freedom that is left unconstrained is translation mutually perpendicular to the axis of both Datum D-E and Datum A.

Edited December 13, 2024

[[To...]]

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My bad on the number of degrees of the midplane.

[[Ti...]]

The idea the planes are not equally angled from each other becomes flawed when they are the primary datum. Why? That is because it is a matter of perspective. I throw you into a dark room and give you a part with two planes a primary and a fixture with the same angled two planes. You put the part in the fixture and the midplane sets an X plane immediatly and eventually dertermins Z+.  The only reason you could have to assume they are not equal to each other is by looking at other features on the part. But the other features are a slave to the primary datums. In the below image, the left would be thinking the two planes in the fixture are not a symmetrical angle to each other, but the right fixture the planes are symmetrical. 

 

I bring this up b/c this callout should remove 5 dof. it is identical to a tapered slotted hole/boss which is the only geometric shape that removes all 6 dof, except in this case there lacks the two opposing radii to stop the translation in the fixture. Here you toss the part below in a 105 deg v-block glued to a surface plate and the part can only be pushed to and from the operator. Z is set by the intersection line of the two planes. Similar to a vertex point of a cone primary datum, or again the line created with a tapered slotted hole.  You could customize your datum frames to remove Z from the D-E and insert A if that be the case.

 

 

 

Edited December 13, 2024