GD&T Workshop: Datum Reference Frames
As we know, GD&T is a symbolic language with which to specify permissible limits of imperfection in manufactured parts in order to guarantee their assembly and operation. And yes, the language is just a bit complex, but hopefully only as complex as it needs to be to deal effectively with the complexity of the imperfect real world of machine parts. For example if we are dealing with an unfortunately slightly tapered, slightly bent, slightly tilted and slightly offset shaft, which is supposed to fit into a slightly, similarly distorted mating bore, we need concepts, tools and rules which can impose functional limits of imperfection to guarantee their assembly. That’s what GD&T is all about.
But the less clearly we deal with the concepts, tools and rules of GD&T, the more complex and confusing the subject becomes, making it necessary to “interpret” poorly framed verbal statements and badly stuffed Feature Control Frames, instead of being able to “decode” clear statements and properly “encoded” Feature Control Frames. It is therefore clear that crystalline concepts, tools and rules are essential to make GD&T work, and are the only way to turn the all too common jungle of GD&T into a well laid out, formal garden.
The World of DatumsThe many concepts associated with the word “datum” represent a particularly important part of this jungle-oops, garden-and our purpose in this and the following workshop is to define them so clearly that we can use them with confidence and communicate them clearly.
In the order in which we’ll deal with them, these concepts are:
Composite Datum Features
Datum Feature & Datum Target Simulators
Datum Reference Frames
Datum Reference Frame Components & Labels
Datum Reference Frame Mobility
In each case we will provide the Y14.5 definition, followed by certain refinements for clarification.
By way of preparation, we show a graphic overview of these many concepts in Figure 1. The functions of the bore in the lower right hand corner of the CAD defined part are encoded with the encircled feature control frame, which references three datum features. Datum Feature A is a planar surface, B is a bore, and C is a slot. The associated Datum Feature Simulators, A, B and C are shown in blue in the lower left hand corner of the figure, along with their associated Datums A, B and C, in red, along with the Datum Reference Frame they define, also shown in blue, which is also shown transferred to the actual part in the lower right hand corner of the figure.
Note: Although the Regardless of Feature Size modifier (S) is no longer required by the Y14.5M 1994 Standard and forbidden by the 2009 Standard, its explicit use is still highly recommended in order to 1) manage the “encoding” process intelligently, 2) speed the decoding process, and 3) prove that the design engineer had considered the (M) and (L) alternatives and selected the (S) intentionally.
1. Datum Features
Y14.5 2009 Definition (§1.3.16 p.3): “A feature that is identified with either a datum feature symbol or a datum target symbol.”
What are Datum Features for? They serve to constrain the rotational and translational degrees of freedom of a part relative to the mating features of a mating part, manufacturing fixture or gage during an assembly process.
Note: As shown in Figure 1, datum features are identified using datum feature labels which consist of 1) certain authorized capital letters of the alphabet and 2) the datum feature symbol, a hollow or filled triangle which can be attached to the surface of the feature, to a leader or to an extension line, or hung from the bottom of a feature control frame. There are no “datum labels,” and the CAD defined features identified by datum feature labels are in fact “datum features,” not “datums.” Furthermore, contrary to a widely held misconception, the letters listed in the last compartment of a feature control frame also represent “datum features,” not “datums.” Simple proof of this is the fact that these letters are often associated with a material condition modifier (S), (M) or (L) (we like to refer to them as “Tolerance Zone Mobility” modifiers to clearly represent their effects; see Workshop Article #4), and only Datum Features can perform their duties “Regardless of Feature Size” or at their “Maximum” or “Least Material Boundaries,” because the points, axes and planes representing datums have neither size nor material.
2. Composite (Multiple) Datum Features
Y14.5 2009 Definition (§4.12 p.65): “Where more than one datum feature is used to establish a datum feature simulator for a single datum, the appropriate datum feature reference letters and associated modifiers, separated by a dash are entered into one compartment of the feature control frame.”
What is a Composite Datum Feature? A collection of two or more features of a part that serve as a group to constrain certain degrees of freedom during an assembly process. A Composite Datum Feature may be represented by a single Datum Feature label referencing all the associated features (e.g. in the case of a bore pattern), or by a list of the labels of the individually identified, participating Datum Features, separated by dashes in any order.
What are Composite Datum Features for? Composite Datum Features serve the same purpose as single Datum Features.
3. Datum Targets
Y14.5 2009 Definition (§4.24 p.83): “The designated points, lines or areas that are used in establishing a datum.”
What are datum targets? Specially labeled portions of a datum feature in the form of points, lines or limited areas that are oriented and located by Basic dimensions relative to the Datum Reference Frames they help define.
What are datum targets for? Datum targets serve to constrain the rotational and translational degrees of freedom of a part relative to the equally limited portions of a mating datum feature, or relative to their associated datum target simulators in a manufacturing fixture or a gage.
Note: As shown in Figure 2, datum targets are identified using datum target labels. These consist of a circle with a horizontal bar, the lower half of which contains the name of the associated datum feature together with a potentially incrementing number, and the upper half of which must contain the diameter of the spherical or cylindrical simulator for a point or line datum target, and the diameter or side length of an area datum target. One caveat: Datum targets should only be used when they represent the actual mating portions of mating datum features or the actual mating features of functional gages and manufacturing fixtures.
In our next workshop we’ll complete the definitions of the major concepts having to do with defining and establishing Datum Reference Frames, and in the following workshop, we’ll tackle the Datum Reference Fame establishment process itself and the rules that manage it.