GD&T Workshop: A Top Down View

January 11, 2012
What is geometric dimensioning and tolerancing (GD&T)?

An Example of the GD&T Challenge

The drawings shown here represent three different attempts to specify permissible limits of imperfection for a vacuum flange which includes a pattern of threaded bolt holes which must be mutually tightly located but only loosely constrained relative to the center of the periphery of the flange, and of which one planar surface must be extremely flat to ensure an airtight seal with a mating O-ring, and the other reasonably parallel, but only loosely located relative to the first. In addition, the flat edge of the flange must be tightly perpendicular to the vacuum face and tightly located relative to the bolt hole pattern in order to accurately orient and locate a mating part. We’ll analyze the drawings in our next column. Source: Multi Metrics

So what is GD&T anyway? For some people it’s a grim, depressing and troublesome burden, whose importance they question deeply. For others it’s grand, delightful and terribly important, and the only reliable basis for specifying machine part geometry.

As must be well known, in order to define things which we want other people to manufacture for us-in particular machine parts-there are two essential requirements. First, we must define their fundamental geometry. Secondly, because no physical object can be perfect, we must specify just how imperfect they may be and still function.

In addition to GAD there is the classical alternative for specifying permissible limits of imperfection, namely classical dimensioning and tolerancing (CD&T), but it consists solely of the “±” tool and is just as bad as bad GAD, because it depends entirely on tribal understandings, is beholden to no rules and is therefore just as prone to mislead.

The Origins of GAD

As Stanley Parker, the alleged inventor of GD&T, discovered early one frosty morning in 1940, CD&T is essentially useless. After his team had spent a whole day rejecting a run of the last critical parts for a shipment of torpedoes by Her Majesty’s Torpedo Manufactory in Alexandria, Scotland, Parker set them carefully aside in order to return the next day to do some additional checks before initiating a new manufacturing run. However, when he arrived, he discovered they were all missing.

As it turned out, anxious to make the shipment on time, the night assembly crew was delighted to find the missing parts neatly stacked in the inspection department, and in the wee hours installed the parts, proved they were perfectly functional, and released the shipment. Upon learning this, rather than recalling the shipment and firing the assembly manager for criminal behavior-a highly justifiable response under the circumstances-Parker congratulated him on his pluck and set to work to understand how he, Parker, could have rejected a set of perfectly functional parts.

In the process he tore CD&T apart in an effort to discover possible weaknesses, and since weaknesses were all that he found, he set to inventing a more reliable set of tools, namely the gift of GD&T which not only links permissible limits of imperfection to function, but specifies them so precisely that others can do precisely what they specify.

Today we are the proud inheritors of Parker’s symbolic language-now referred to as GD&T-which represents a set of tools with which to not only communicate design intent unambiguously to manufacturing and inspection, but, much more importantly, with which to ensure that what we communicate is worth communicating, namely represents functional, assemblable parts.

GD&T is a highly sophisticated encodable and decodable symbolic language for managing the risks involved in machine part design, manufacturing, inspection and assembly. It serves first to force the designer to research the functions of each feature of a part, because if not properly understood, said functions can never be properly encoded.

Secondly, it serves to bring to light possible shortcomings in the geometry of certain features of a part to perform their required functions. As a result, it encourages refining the geometries of those features to improve their ability to perform their functions and as a result leads to the creation of more fault tolerant designs.

Thirdly, GD&T provides the means to unambiguously specify preliminary limits of imperfection to enable mathematically reliable tolerance stack-up analysis, which in turn enables refinement of said limits to guarantee assembly and operation, all prior to drawing release.

It is only with GD&T that tolerance stack-up analysis becomes a scientific endeavor; it is only with GD&T that we can unambiguously specify manufacturing objectives; and it is only with GD&T that metrology becomes a fully automatable, scientifically reliable process.

The lack of functional GD&T is demonstrably responsible for bumpy new product ramp up cycles, unnecessarily costly manufacturing processes and unavoidably questionable inspection results, all based on tribal understandings, however, companies are so accustomed to experiencing these problems that they cannot imagine a small investment in GD&T could significantly reduce them. But it can, and in fact, GD&T is not only a gift, but a necessity for an organization which cares about getting it right the first time and which cares about the bottom line.

Path to Successful GAD

Unfortunately, however, grasping GD&T, and making it work, turns out to be somewhat troublesome. This is because its symbolic language needs to be just as rich and complex as the real world of what can go wrong. In order to master GD&T and enable it to produce the bottom line benefits of which it is capable, we must start with absolutely crystalline sets of concepts, tools and rules, which the ASME Y14.5 and ISO 1101 standards attempt to provide, but sometimes with limited success.

Next, with these in hand, we need well-organized, precisely stated processes for encoding and decoding GD&T-something not addressed in the above standards.

Finally, we also need well-documented sets of best practices in order to successfully turn what appears to be a jungle into a veritable English garden.

With these foundations in hand, the final need is for intelligent, software based automation of the encoding and decoding processes in the CAD, CAM and CAI (computer aided inspection) worlds. Although in its early stages, automation has begun to be addressed in CAI and CAD, but still has a substantial distance to go. Maybe it’s time to start demanding it from suppliers of these products, if you haven’t already.

Editor’s note: We encourage readers to submit questions, and we will attempt to provide succinct answers to as many as possible.