ASME Y14.5 Datum terminology question 2

[Burunduk]

In fig. 4-7 of ASME Y14.5-2009,
If needed to explain how establishment of datums and DRF is done, what would you describe as "datum C"? An inclined plane coincident with "Datum feature simulator of datum feature C", or the "Third datum plane" of the datum reference frame?

 

[3DDave]

You mean theoretical datum C? As derived from the theoretical datum simulator C which is derived from the physical datum feature simulator C which is derived from datum feature C? For which it is defined relative to the A|B DRF.

I'm more interested in explaining to people how a part might have multiple theoretical datum feature Cs; [C] is different than in [A|C] is different than in [A|B|C]. They all have the same name and refer to the same datum feature, but result in entirely unique planes.

 

[Belanger]

Datums are mutually perpendicular to each other.

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

 

[Burunduk]

Datums are mutually perpendicular to each other.

That's exactly what I'm confused about.
According to the standard, a datum is "a theoretically exact point, axis, line, plane, or combination thereof derived from the theoretical datum feature simulator".
A datum point can't be perpendicular to anything, it can only have a location relationship with other datums. A datum axis I think can be "inclined" in the same sense that a "Datum feature simulator of datum feature C" is inclined in fig. 4-7. My approach was always that datum plane C in that figure is an "inclined" datum plane derived directly from the datum feature simulator of datum feature C, and the "Third datum plane" of the DRF is derived from datum planes A B and the inclined datum plane C. But I have seen this concept shown otherwise recently - see the attached image from a textbook, explaining the datum reference frame of a part with an inclined datum feature shown at 70° basic to B. There, "datum plane C" which is associated with an inclined datum feature C is a plane of the DRF. So what is correct?

On a similar note, If we consider a primary datum axis, it is associated with 2 theoretical planes of the DRF intersecting at right angles at the axis. Are these two planes the datums or are they derived from the datum (axis)? Fig. 4-3 shows only an axis as the "datum" for a cylindrical datum feature.

 

[axym]

Burunduk,

I think you're on the right track with this. I would agree that in datum C in Fig. 4-7 is an inclined plane coincident with the datum feature simulator of datum feature C.

To be sure we understand Figure 4-7 correctly, we need to understand the distinction between the following things:
-a planar theoretical datum feature simulator
-a datum plane
-a plane of the datum reference frame

However, most of the examples in Y14.5 show cases in which two or more of these planes are coincident. For example, in Fig. 4-2 the planes shown in the Means This are labeled as First Datum Plane, Second Datum Plane and Third Datum Plane. But these planes are also theoretical datum feature simulators and datum planes. So it's easy to lose track of the distinction. There is also some rather loose and inconsistent usage of the term "datum plane" in several figures in Section 4, which adds to the difficulty. Then there are other figures which are fully consistent. Based on my experience, trying to make sense of this section can do serious damage to one's sanity ;^).

If we look at Figure 4-7, the Means This figure is not fully consistent with the text in 4.10.2:

"For parts with inclined datum features as shown in Fig. 4-7, a datum feature simulator plane is oriented at the basic angle of the datum feature. The corresponding plane of the datum reference frame passes through the vertex of the basic angle and is mutually perpendicular to the other two planes."

The text uses the term "plane of the datum reference frame" but the figure labels the vertical plane as the "Third datum plane". This is inconsistent and confusing - it should be labeled "Third plane of the datum reference frame". Here's a rule of thumb - when a datum plane is referred to as "first, second, third", they're really referring to a plane of the datum reference frame.

I would say that Figure 4-6 is a good example of correct and consistent usage of terminology. Figure 4-9 is also good - for feature C, we can see the distinction between the datum feature simulator (expanding cylinder), datum (axis derived from the datum feature simulator) and plane of the datum reference frame.

So what is the full story with Figure 4-7? Here is my take on it:

A: The plane labeled "First datum plane" is all 3 of the following:
-the Datum Feature Simulator of Datum Feature A
-Datum Plane A
-First Plane of the Datum Reference Frame

B: The plane labeled "Second datum plane" is all 3 of the following:
-the Datum Feature Simulator of Datum Feature B
-Datum Plane B
-Second Plane of the Datum Reference Frame

C: The plane labeled "Datum feature simulator of datum feature C" is the following:
-the Datum Feature Simulator of Datum Feature C
-Datum Plane C

The plane labeled "Third datum plane" is the following:
-Third Plane of the Datum Reference Frame

The textbook excerpt appears to conflict with the standard. The vertical plane should not be labeled "Datum plane C". Y14.5 would describe this plane in the text as a "plane of the datum reference frame" and label it in a figure as the "Third datum plane". Datum plane C is the 70 degree tilted plane (in this case it is coincident with the datum feature simulator of datum feature C).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

Evan, thanks a lot for the detailed clarifications. It seems I am not far off in thinking that "datum planes" and "planes of the datum reference frame" are completely different entities (although often coincident with each other).

The standard is kind of indecisive about the distinction between those concepts. It also doesn't help that sometimes the origin of measurement/dimensions is the datum reference frame and sometimes it is the datums. A couple of examples: para 4.7.1: "This theoretical reference frame constitutes the three-plane dimensioning system used for dimensioning and tolerancing". Para. 4.10.3: "This axis serves as the origin for relationships defined by geometric tolerances." I suppose it's not wrong, but If we are establishing a datum reference frame which is often described as a coordinate system to locate/orient the feature that referenced the associated datum features, let it be the origin of measurements and let the datums be just the auxiliary means to establish this coordinate system. It helps when every concept has its own distinct meaning and purpose and things don't get mixed up.