Theoretical Datum for a Complex-Contoured Surface... 2

[NutZach]

Good morning folks!

Long-time reader... first-time poster... ASME GDTP-2009 Senior...

Here's the question: Y14.5-2009 figure 4.3, bottom row... We see an example of how a "complex" datum feature MIGHT (in this one particular example from that figure) be able to produce a combination of theoretical datums consisting of an axis, point, and center plane... That's all well and good for a tapered / elongated symmetric feature like the one we see in that image, but... Suppose I've called out as a primary datum feature an irregular lofted complex-contoured surface? A surface that isn't tapered and therefore doesn't converge to any particular point... a surface that doesn't have a median plane... a surface that doesn't have any logical placement for a theoretical axis... I'm talking about a complex-contoured lofted surface.

What is the theoretical "datum" for this lofted surface? Is it just an infinite number of datum points? Does it even exist at all? Why wouldn't we just refer to that datum as a "mathematically-defined surface"? Or perhaps we should just skip this consideration altogether and just go straight to what the true geometric counterpart (simulator) looks like? 2009/2018 both define a datum to be "a theoretically exact point, axis, line, plane, or combination thereof derived from the theoretical datum feature simulator." It certainly seems like we really ought to be including "mathematically-defined complex-contoured surface" in that list as well.

Thoughts?

 

[3DDave]

Already answered. The theoretical datums defined by the feature are the theoretical datums used to create the feature.

If the problem is the Committee failed to mention that clearly, I agree. The distraction of simpler geometry scaling about that theoretical datum used to create the feature is that these simple cases make the inverse operation simpler.

 

[chez311]

The theoretical datums defined by the feature are the theoretical datums used to create the feature.

I'm not sure if you're responding to Burunduk or myself but could you expand on this a bit?

 

[3DDave]

I was responding to the OP.

Most features of choice in the standard used for datum features are simple generated features. Flat surfaces match single datum planes. Round holes are circular cuts along an existing axis of motion. More complicated rotational items, such as cones, are specified as linear or other transformations of circles based on some distance along an axis, usually from a plane, sometimes a point.

In all these cases reducing the form of the feature dimension that is being moved reduces to the path along the axis or just reproduces the plane.

Complex surfaces are typically generate by creating variable distances to points on the surfaces from three datum planes. If one reduces those dimensions to zero then one goes back to the three datum planes the way that reducing the single radial dimension along a cylinder reduces that surface to an axis.

 

[TheTick]

Been through this AGAIN recently...

It's not heresy to put in features that exist solely for providing measurement datums. Having some easy-to-fix features helps everyone.

Usually these features are discreet enough that it does not matter. Sometimes they need to be removed after.