datum plane of inclined datum feature in '09 std

[bxbzq]

I'm looking at fig. 4-7. In the "means this" graph, shouldn't the "third datum plane" be third coordinate plane? The third datum plane or datum plane C should still align with the datum feature simulator of datum feature C. Any thoughts?

 

[Belanger]

The picture is correct. Recall that a datum is a theoretically perfect plane, and datums are always mutually perpendicular to each other. You're thinking of the "simulated datum plane." Check out Fig. 4-10 for the terms; the difference is that 4-7 tosses an angle in there.

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

http://www.gdtseminars.com

 

[bxbzq]

The 3 mutually perpendicular planes of DRF are coordinate planes in the first place, not necessarily datum planes. In many cases coordinate planes coincide with datum planes. In fig. 4-7 this is not the case. Coordinate planes are always mutually perpendicular to each other. Datum planes are not. Fig. 4-10 shows datum plane A derived from high points of datum feature A directly.

 

[Belanger]

Well OK, any randomly chosen datum planes may not be mutually perp to each other, but the datum planes of a "datum reference frame" are always mutually perpendicular to each other (paragraph 4.1), and that was the situation in Fig. 4-7.

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

http://www.gdtseminars.com

 

[DeanD3W]

This is currently a point of confusion in my opinion. "Planes of the datum reference frame" (the "coordinate planes" in bxbzq's perfectly reasonable terms) are always perpendicular to each other, but "datum planes" are something different... Datum planes should be found on, or at the center of, a datum feature simulator. For a simple rectangular block with three planar datum features, the datum planes and planes of the datum reference frame are theoretically aligned with each other. Since parts so simple are rather rare, this alignment of datum planes and planes of the datum reference frame could be considered a special case.

For a part with three spherical features as primary, secondary, and tertiary datum features, there would be three datum center points, located at the center of three datum feature simulators, and once a datum reference frame was constrained to those points, there would then be three planes of a datum reference frame... No "datum planes" exist for this example.

The standard doesn't make the terminology I describe above clear. I'm describing what I think is the best terminology, and also what I hope the next version of Y14.5 will clarify.

Dean

http://www.d3w-engineering.com