Datum Targets in MBD 1

[Mech1595]

All,

Had an interesting one related to Datum Targets come through recently, and was hoping you could provide some insight.
Below is a simplified example of the component. The customer has Datum A defined with (3) Datum Target Points: A1, A2, A3.
They also have Parallelism to [A] specified on the yellow surface.

Here's where I'm lost:
Their design intent is for Datum Plane A to be parallel to the yellow surface, but I don't understand how it could be with this info. I looked through Y14.5-2009 and found Fig 4-47 as an example, but they have the Datum A targets offset with the Basic [20]. This component is defined with MBD, with no basic offset like Fig. 4-47, just WCS coordinates for the Datum Targets.
It's also located way out in vehicle position, and not aligned to the WCS in a way that any of the planes (XY, XZ, YZ) would match intended Datum Plane A orientation.
Is this valid? If so, what is the mechanism driving Datum A to be oriented as they suggest? Would Datum A not just be coincident with the (3) Datum Target Points?

 

[Burunduk]

So which of these cases can represent OP's primary datum feature A as specified by the 3 datum target points?

 

[3DDave]

Why would I pick any of them and, more important, why are you insistent that I do?

Is there no cardboard where you are?

 

[Burunduk]

I know how the part can have different positions on the 3 pins fixture,
The interesting question is, how would you classify this?

 

[3DDave]

I classify it as a failure.

 

[Burunduk]

The conclusion might be that it is unclear how many degrees of freedom are constrained by datum feature A as defined by the 3 datum target points.

Except for the Z translation (as shown on the CSYS here), which is independently unconstrained, movement in all other translational and all rotational directions is available but not independently.

 

[3DDave]

The three points in the original post lie on a plane. Because they are on a plane a circle in that plane can be constructed that passes through all three points. The center of that circle with that plane define an axis. The part can be rotated about that axis by rocking the part to maintain contact at the three points. That is unconstrained coupled rotation about Y and X shown in the 14 Sep 24 10:58 diagram.

From the side view, construct surface normals at the location where the datum target points lie on the part surface, normal to the respective surface. They will intersect at a point a finite distance from each surface. This is the instantaneous center of rotation. This allows unconstrained rotation about the Z axis shown in the 14 Sep 24 10:58 diagram.

That totals 3 unconstrained degrees of freedom. Since all the Cartesian coordinates for points elsewhere on the part change relative to the datum target points when the part is rotated about the X, Y, and Z axes, as allowed by these 3 datum target points, it's not clear that one could say the locations of the surfaces are fully constrained either.

The freeedom in the 14 Sep 24 10:58 diagram Z direction is immaterial as that does not affect suitability for determining parallelism or angularity which is the original problem to solve.