Interpreting GD&T-Decorated CAD Models

Continuing with our analysis of the Greatest Design Tool ever, our objective in this article is to take a detailed look at the structure and contents of feature control frames to demonstrate that they represent “encoded” information which may only be “decoded,” not “interpreted.” With their structure defined, we will then demonstrate how to “decode” feature control frames, and in later articles how to “encode” them. In effect, this article will demonstrate that GD&T is a tool with which to research, refine, and ultimately encode the function of each feature of a part, with the objective of guaranteeing functionality and assemblability, as well as decodability for conversion into reliable manufacturing and inspection processes.

Because of the intricacies of imperfect geometry, and the resulting intricacies of GD&T, far too many CAD models are unfortunately “decorated,” rather than “encoded” with it, thereby increasing the potential for nonfunctional designs. Furthermore, like any work of fine art, these decorations must then be “interpreted,” but now at the peril of misleading the machine shop, and confusing the inspection department.

In keeping with the opinion of many a metrologist—that machine parts are merely evidence of violence committed against innocent material—the GD&T found on machine part drawings is therefore often:

• Evidence of an unparalleled condition of maximum confusion in design
• Representative of a composite set of virtually immaterial conditions for manufacturing
• Nothing more than a derived median line of BS (Bodacious Snafus) for inspection

To demonstrate that GD&T is in fact code, however, and if properly implemented can be smoothly and meaningfully decoded, let’s first dissect the encircled feature control frame in Figure 1, and then decode it:

Figure 1.

Referring to the enlarged feature control frame in Figure 2., the first compartment is dedicated to identifying the geometry control tool, the second to defining the shape, size and any modifications of the tolerance zone, and the third—rather than being a collection of datums—is in fact a set of instructions for establishing the datum reference frame, relative to which the tolerance zone is to be oriented and located.

Figure 2.

In greater detail, the tool largely determines the controlled component of the feature under consideration, in the case of position, the bounded portion of the axis of a bore or shaft, as well as the applicable constraints, namely orientation and location. The diameter symbol defines the shape of the tolerance zone, namely cylindrical, and also makes clear that the tolerance value is the diameter, not the radius. The maximum material condition (MMC), tolerance zone size modifier (M), promises Mmm-ore tolerance as the considered feature departs from maximum toward least material condition, and finally the free-state modifier requires the specified conditions to be met in the so-called “free state” of the part, which needs careful specification in an auxiliary note. Next, the collection of datum feature letters and associated maximum material condition—tolerance zone mobility modifiers (M)—specify the steps required to establish the potentially Mmm-obile datum reference frame.

To demonstrate how this is actually communicated by the feature control frame, let’s read and decode it.

Reading: “Position (not true position) within 0.15 mm at MMC in the free state, relative to A, B at MMC and C at MMC.”

Decoding: “With the part in the free state, position requires the bounded axis of the considered feature to lie within a cylindrical tolerance zone of diameter 0.15 mm at MMC, expanding by as much as the total tolerance on the size of the considered feature as it departs from MMC toward LMC, which is oriented and located by basic dimensions relative to a datum reference frame established using datum feature A simulated “rocking,” and datum features B and C simulated Mmm-obily at their Virtual MMC boundaries.”

In review, based on the presence of the tolerance zone size modifier (M), the code tells us that the design team understood the function of the considered bore to be clearance for a mating shaft. Furthermore, based on the presence of the tolerance zone mobility modifiers (M) associated with datum features B and C, they also understood that its position tolerance zone might enjoy some residual mobility during assembly, due to the potential for play between datum features B and C and their mating features. The code clearly informs the manufacturing team of these objectives, and encourages them to favor the least material condition for B and C, regardless of the well-understood need to simulate them regardless of their size in manufacturing fixture design. Finally, the code specifies exactly how datum features A, B, and C are to be simulated for functional gauge design and/or coordinate metrology, and suggests that the assembly team not forget to take advantage of the play before locking things down.

This is code. There is no room for interpretation.

In December we’ll cast much more light on the confusing world of material condition modifiers, which this month’s article broke down into the groups “tolerance zone size” and “tolerance zone mobility” modifiers. In later articles we’ll investigate the painful topic of datums, datum features, datum feature simulators and their resulting datum reference frames, as well as the always delightfully confusing and important topics of 1) the “virtual condition,” 2) GD&T tool selection in design, and 3) the effect of GD&T on manufacturing and inspection process management.

Note: Although the referenced feature control frame instructions are reliable in most cases, they’re not so in all cases, and are often not as explicit as we might like.

InsideMetrology was delighted to get feedback on our first article. Thank you.

From our readers:
John Worrad, 1: “Can GD&T be justified outside its machining/metalworking origins?” Response: As you suggest, we generally think of GD&T as serving primarily to communicate design intent unambiguously to manufacturing and inspection. In fact, however, its primary purpose must be to ensure that what we communicate is worth communicating, that it represents a fault-tolerant, functional part which, if manufactured within spec, can be guaranteed to assemble and perform as hopefully understood and intended. Whereas CAD, standing for Computer Aided Design, merely specifies the geometry of a part, GAD™ on the other hand, standing for GD&T Aided Design, enables us to research, refine and encode the function of each of its features to guarantee functionality and assemblability before design release. Surely CAD without GAD is very BAD.

John Worrad, 2: “Can you delve into line profile vs. surface profile and the mathematics involved?” Response: The surface profile tool defines skin-like tolerance zones within which all the points on the controlled surface must lie. The line profile tool, on the other hand, defines ribbon-like tolerance zones, within which all the points in each section must lie. The skin-like zone represents a normal expansion of the surface partially into space and partially into the material. The ribbon-like zones also represent normal expansions, and must twist back and forth in space as the controlled surface twists. In the end, the stack of line profile ribbons creates a surface profile skin with exactly the same effect. Why do we need two tools to accomplish the same thing? We strongly discourage the use of the line profile tool, because it 1) delivers nothing that the surface profile tool doesn’t already deliver, 2) merely complicates the specification of a requirement, and 3) confuses decoding public by appearing to impose a different requirement. In particular, as opposed to a common misunderstanding that, if imposed without constraining datum features, each of the ribbon-like tolerance zones of the line profile tool is independent of all the others, therefore making it possible to impose independent size and form control on each cross-section of a flexible part, this is not the case due to the rules of simultaneous requirements, which require all the ribbons to dance in unison. The only legitimate, rule-based way to control independent sections of flexible extrusion is with the surface profile tool in conjunction with an incremental tolerance zone modifier, which defines a thin, intrinsically free-floating section of a skin-like zone.

Marel Luksic, 1: “I have to disagree ( . . . that learning GD&T is a great deal of trouble or difficult . . .), because I think GD&T is easy to learn and is a very elegant, powerful, and simple way for a designer to specify functional requirements.” Response: Wow! Where are the rest of you? I obviously agree wholeheartedly with the “very elegant, powerful, and simple way for a designer to specify functional requirements,” but it’s only easy to learn, I would say, given 1) a refined ability to create mental images of 3-D geometric entities and relationships, 2) a visceral familiarity with manufacturing, assembly, and operational processes, and 3) access to crystalline definitions of its concepts, tools, rules, and processes, for which there are few resources.

Marel Luksic, 2: “The problem I have encountered is the narrow-minded attitude of many designers. . . who. . . are resistant to using GD&T without even reading the standard or making an effort to understand it. When used properly, GD&T prompts designers to think through the quality implications of specifications.” Response: In both cases, we fully agree. But let us not be too hard on the resistant ones, for they are first of all very busy, second, not adequately encouraged by competent management, and third, may never have been exposed to crystalline definitions of its concepts, tools, rules, and processes, or to sympathetic and capable trainers.

Scott Gooch, 1: “For most metrologists, GD&T is a love/hate relationship: love the rational behind it, but hate the irrational/impractical usage by designers.” Response: No GD&T is better than bad GD&T, but because all metrology not based on GD&T is pure invention on the part of the inspector, GD&T is essential! The ultimate hope is to largely automate the GD&T encoding process, but in the meantime, crystalline definitions of its concepts, tools, rules and processes as well as management imperatives could at least help.

Scott Gooch, 2: “Please define logical application, practical checking methods, and CMM procedures for correctly checking GD&T.” Response: We will dedicate some of our columns to these important concerns, and we appreciate the encouragement.

Please continue to suggest topics you’d like us to address. Our objective is to help wherever we can and learn as we do.

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