GD&T Workshop: Establishing Datum Reference Frames

The concepts, tool and rules...continued.

Figure 1 - This shows the Datum Feature Simulators that are defined by the Datum Features referenced in the encircled Feature Control Frame.

Figure 2 - The six possible Datums

In our last workshop we dealt with an initial set of the concepts that impact the definition and establishment of Datum Reference Frames, namely 1) Datum Features, 2) Composite Datum Features and 3) Datum Targets. In this column, we will address the remaining concepts, namely 4) Datum Feature and Datum Target Simulators, 5) Datums, 6) Datum Reference Frames, 7) Datum Reference Frame Components and Labels, and 8) Datum Reference Frame Mobility.

4. Datum Feature Simulators & Datum Target Simulators

Y14.5 2009 Definition: “Theoretical Datum Feature Simulator (§ p.3)”: The theoretically perfect boundary used to establish a datum from a specified datum feature. “Physical Datum Feature Simulator (§ p.3)”: The physical boundary used to establish a simulated datum from a specified datum feature.

Note: The Y14.5 2009 Standard does not define a Datum Target Simulator.


What are Datum Feature Simulators? Datum Feature Simulators are “conceptually” perfectly perfect, or “physically” almost perfect, inverse Datum Features, which represent the mating Datum Features of a part or the staging surfaces of a manufacturing or a gaging fixture.

What are Datum Target Simulators? Point Datum Target Simulators are spheres, Line Datum Target Simulators are straight or bent cylinders, and Area Datum Target Simulators are limited Datum Feature Simulators.

What are Datum Feature and Datum Target Simulators for? Datum Feature/Target Simulators serve as bridges between the imperfect real world of datum features and the perfect imaginary world of datums:

from which we extract datums,

in which we first establish Datum Reference Frames, and

with which we transfer Datum Reference Frames to actual parts.

Figure 1 shows the Datum Feature Simulators which are defined by the Datum Features referenced in the encircled Feature Control Frame. The simulator for planar surface A is an inverted planar surface, that the simulator for bore B is an expanding shaft, and that the simulator for the slot is a tombstone fixed in size at the virtual Maximum Material Boundary of the slot, namely of thickness 16 millimeters. Figure 1 also shows the Datums extracted from their simulators.

5. Datums

Y14.5 2009 Definition (§1.3.13 p.3): “A theoretically exact point, axis, plane or combination thereof derived from the theoretical datum feature simulator.”


What are Datums? The minimum, mutually embedded set of a single, perfect imaginary point, and/or axis, and/or plane, which together fully characterize the orientation and location of a Datum Feature Simulator.

What are they for? Datums serve to constrain the rotational and translational degrees of freedom of a “starter” coordinate system, thereby turning it into a Datum Reference Frame relative to which Basic dimensions serve to orient and locate tolerance zones.

As illustrated in Figure 2, there are six datum types, each shown embedded in its defining Datum Feature Simulator: a point, a line, a plane, a point-on-a-line, a line-in-a-plane and a point-on-a-line-in-a-plane.

6. Datum Reference Frames

Y14.5 2009 Definition (§4.1 p.48): “A set of three mutually perpendicular, intersecting datum planes.”


What are Datum Reference Frames? Datum reference frames are Cartesian coordinate systems that have been partially or wholly constrained by a set of Datums extracted from a set of Datum Feature Simulators, defined by a set of Datum Features referenced in a Feature Control Frame.

What are Datum Reference Frames for? Datum reference frames serve, with the help of basic dimensions, to orient and locate tolerance zones.

Although the fundamental Datum Reference Frame is always a Cartesian coordinate system that consists of three mutually perpendicular axes, three mutually perpendicular base planes and a point representing the origin, a cylindrical or a spherical coordinate system may be derived from it where required. The Datum Reference Frame defined by the Datum Features referenced in the encircled Feature Control Frame in Figure 1 is shown in Figure 1 superimposed on the CAD model, buried in the simulator set, and established in the actual part.

7. Datum Reference Frame Components & Labels

Y14.5 2009 Definition (§4.21 p.79): “ . . . the X,Y and Z axes representing the intersections of a set of three mutually perpendicular datum planes which are identified by their names followed by a list of the datum feature labels used to establish the specified datum reference frame, separated by commas and contained in brackets. . .”


What are Datum Reference Frame Components? The components of a Datum Reference Frame are its X, Y and Z axes, its XY, YZ and ZX base planes, and its XYZ origin. The labels identifying Datum Reference Frame components should always include a list of the Datum Features responsible for establishing the Datum Reference Frame, as follows: X[A,B,C], . . . XY[A,B,C] . . . and XYZ[A,B,C].

What are Datum Reference Frame Component Labels for? Datum reference frame components and their labels can be included in CAD models, and in their associated 2-D representations, in order 1) to significantly accelerate the process of comprehending the Datum Reference Frame defined by the Datum Features listed in a particular Feature Control Frame, 2) to uniquely specify the preferred Datum Reference Frame defined by Datums which can take multiple forms (typical in the case of Composite Datum Features) and 3) to uniquely specify the axes of a Datum Reference Frame established using the Y14.5 2009 Standard (§4.22 p.81) defined “Degrees of Constraint” modifiers [u,v,w,x,y,z], which require them. Furthermore, labeling Datum Reference Frame components in a drawing provides the additional benefit of ensuring that coordinate measuring machine (CMM)-generated reports coming from different laboratories present feature offsets in identically labeled formats.

For examples of datum reference frame components and their labels, see figure 1.

8. Datum Reference Frame Mobility

Y14.5 2009 Definition (§4.11.9 + p.63): “MMB or LMB modifiers applied to the datum feature reference will allow the datum feature to shift/displace from the boundary established by the datum feature simulator in an amount that is equal to the difference between the applicable (unrelated or related) actual mating envelope for MMB, actual minimum material envelope for LMB, or surface of the feature and the datum feature simulator.”


What is Datum Reference Frame Mobility? Datum Reference Frames are Cartesian coordinate systems established using sets of Datums extracted from sets of Datum Feature Simulators to constrain the degrees of rotational and translational degrees of freedom of a “starter” coordinate system. Datum Reference Frames are always initially established in the Datum Feature Simulators from which the Datums are extracted, and transferred to actual parts by “marrying” their Datum Features to their associated simulators. If some of the “marriage ceremonies” are impacted by the Tolerance Zone Mobility (Material Boundary) modifiers (M) or (L), in which certain Datum Features do not fully engage their mating simulators, the result is “residual mobility” in terms of the degrees of freedom the “marriage” would otherwise fully constrain. If the “marriage ceremonies” fail to even partially constrain certain degrees of freedom-in case the specified Datum Features cannot or may not constrain certain degrees of freedom-the resulting Datum Reference Frame will retain total freedom in their regard, namely to, for example, roll or to translate in the X direction.

What function does Datum Reference Frame Mobility Encode? Datum Reference Frame mobility encodes the permission to take advantage of potential play between mating parts during the assembly process.

What is the impact of Datum Reference Frame Mobility? 1) If a Datum Reference Frame is mobile, all the tolerance zones referenced to it become mobile as well, thus able to shift to improve the ability of the controlled features to meet their imposed requirements. 2) The presence of Datum Reference Frame mobility imposes the Rules of Simultaneous Requirements, which requires all features referenced to the same mobile Datum Reference Frame to meet their requirements “simultaneously.”

With all the most critical concept definitions behind us, the next workshop will be dedicated to the Datum Reference Frame establishment process itself, namely the process of constructing Cartesian (as well as cylindrical and spherical) coordinate systems defined by the Datum Features and associated Tolerance Zone Mobility (Material Boundary) modifiers listed in a Feature Control Frame.

How are we doing? Are we helping you? Please let us know.

Bill Tandler is the president and chief technical officer of Multi Metrics Inc. (Menlo Park, CA). He can be reached at or (650) 328-0200.

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