In today’s installment I want to talk about the language of GD&T and perhaps a little about basic geometry and how shapes fuel our thoughts. Any language, to achieve communication, must somehow capture a physical method, accomplishable by our bodies, to change a thought or imagined image, into a physical presentation of some kind.

Put in simpler terms, we have a thought and we find a way to present that thought outside our minds. The process is complicated because for any kind of system that performs this transformation of an intangible “thought” to a tangible communication there is a need for a system that can be digested and understood by a wider audience. That system needs a sophisticated population of tools that by their uniqueness can each evoke a volume of thoughts and images and by their amalgamation can assemble libraries of schemes and constructs. GD&T gives our engineering / manufacturing world that diversity of capability. Let’s go back to grade school, back to when we learned about how our world contained things like mathematics and geometry and scientifically logical conclusions. We learned that ideas like points and lines and planes weren’t just thoughts but actual pieces of a puzzle that when assembled became a warehouse full of ideas and images in which we could store our newly empowered imaginations. Triangles and cubes and solids of various shapes allowed us to see our world in a new light. Mathematics and geometry gave our vision a new depth. With this education came the ability to see and perhaps ‘understand’ some of the components of our observable world.

A world filled with geometry

An example of what I mean is a cylinder. A cylinder is a simple looking object that by virtue of its useful uniqueness can be found in virtually every corner of our lives. Broomsticks, coffee cups, hotdogs, ball point pens and tootsie Rolls all comprise a diverse array of things that we can see without even needing to turn our heads. The simple cylinder is actually not so simple though. If we take a brief look at its geometry, its mathematical structure, we can see that it is composed of a number of geometric principals, all with some fairly complicated interrelationships. A cylinder, in the world of ideas, is made up of an assembly of lines, arranged in parallel and at a common distance from an axis, (another geometric principal,) all arranged to be congruent with circular elements, perpendicular to that axis at any point along its length. This solid can occupy as much or as little space as can be imagined. Now we have taken this diversion of our attention to illustrate our discussion on the needs of language. This simple cylinder, which is not so simple, presents a demand requirement on our ability of giving communicable definition to shape, mathematical logic, size, volume and the perhaps an infinite number of things that may cause one cylinder to differ from another.   Our cylinder needs to exist both in the perfect world of ideas and then find its way into the real world of our manufacturing plants and shops where nothing happens without agreed upon tolerances that govern form, fit and function.   And so we have our cylinder, shown here with a simple callout controlling cylindrical form, and with that one symbol we exert straightness, roundness and coaxiality on its surface elements. The envelope principle of perfect form at MMC is enforced as it enters the real world of tolerances… GD&T speaks a unique language that only a specially trained group of people can appreciate. With a simple symbol by the name of cylindricity, accompanied by a system of applied value using an arrangement of tools like characteristic symbols, modifying symbols, feature control frames, tolerance zones and the many ways to mathematically define and translate these bits and pieces into statements and paragraphs of geometric definition we can communicate a sufficient wealth of data to replace untold pages of written text about our broomstick or coffee cup with a few carefully arranged numbers, symbols and organized compartments. Our language can consolidate time and energy into a form that speaks to the uniquely educated inhabitants of our increasingly diversified manufacturing world and it accomplishes the task with remarkable specificity. Each GD&T engineering statement can be crafted to define any assemblage of shape and function. At the same time it describes this intricate physical relativity that same statement can define to any finite degree a scheme of tolerance that will make the assemblage accomplishable in some real world situation. GD&T is the ultimate tool for describing mankind’s present reality and future.

The alphabet of GD&T

So let’s take a look at the alphabet of our GD&T language. At the heart of GD&T we have the jewels of the system, which are the 14 characteristic symbols. Most of us are familiar with and understand these to be the geometric controls of the system. They include form controls like circularity and flatness,. These symbols define shape, irrespective of relationship to other features. A circle and plane have geometric definition. Others are symbols such as parallelism and angularity, , which are used to control the relationship of parts and features, also mathematically definable. The 14 characteristic symbols have 5 definable types, which are form and orientation, such as the ones above and position, , which allows the specific placement of features with respect to other parts and features. There are runout controls, . These controls define the size and or extent of allowable surface movement of planes and cylinders to axes of rotation, which define a kind of dynamic relationship of features. The last type of characteristic control is known as profile, . Profile is a flexible shape and relationship control. Our modern design facilities are powered by computers configured to aid designers with the super-fast and intricate capabilities of three dimensional computer modeling. Imagine designing and drawing a jumbo jet’s fuselage, detailed on a desktop screen smaller than the windshield of a subcompact automobile, in complete detail. It can be stored in an instant and recalled in a keystroke. The shape, as intricate as it needs to be to carry a million pounds through the air at near mach speed is stored in a computer data file. These profile controls use the definitions saved in those files and give engineers a virtually infinite choice of definable shapes to employ.

Characteristic controls

So as we see, at the heart of the system are the 14 controls, straightness, flatness, circularity, cylindricity, which are the form controls. There are the orientation controls, which are parallelism, perpendicularity and angularity. We have position controls: position, concentricity and symmetry and runout controls, both circular and total. Last we have profile controls. They are profile of a line and profile of a surface. As I mentioned many of us are at least somewhat familiar with these tolerances. However, those that need a brush up or a more in depth familiarization of these things can easily find ample material to study on their browser of choice. This installment has, I hope, given us the opportunity to see the need for a method of communication, written on the backdrop of our system fundamentals, that speaks an abbreviated but fully intricate language of engineering definition. Next time we’ll look at how we use the tools like our characteristic symbols to write sentences that craft geometry into form and establish relationships that accomplish mechanical function.  Readers that may want to look ahead can review terms like datums and constraint and the concepts that guide their use. We’ll be looking at feature control frames and how they’re used to make engineering statements using the tools we’re studying. We’ll look at various kinds of symbols and how they’re used to make brief notes in place of longer and often tiresome text notes. We’ll look at the numerous ways drawings have used and defined styles of tolerance over the years to learn how we can now be more specific and accurate. After we learn the bits and pieces, and after we learn how those pieces assemble to form our language, we’ll begin to look at tolerance zones. Tolerance zones comprise the glue that holds our system together. Tolerance zones are not only the embodiment of values for our system, they’re the arrangement of those values and how they control shape and relationships. I hope this material brings into focus some of the things our readers told us were fuzzy in their minds. GD&T is complicated but it is elegant in its intricacies once a person begins to arrange its pieces into an organized structure. We encourage comments and questions and will continue our efforts to fill these spaces with meaningful information. We’ll talk to you next time.

Having received feedback about our GD&T blog, I am writing material responding to requests to go over the basics. I can appreciate a reader’s desire for basics from the standpoint of people who learned the convention over the course of their job’s daily routines of making parts and seeing what works and sometimes what doesn’t.

Many people use GD&T but have little or no formal training in it. For those people, over the next few months, we will go through some basics from the ground up to fill in the possible gaps that some of you may have.  Subjects covered will be:

1. What is GD&T and how does it apply to engineering drawings and the world of manufacturing?
2. Geometry: describing the world of shape and the arrangement of parts and features.
3. Drawing fundamentals.
4. The syntax of the language of GD&T.
   • Datums
   • Basic dimensions
   • Feature control frames
   • Characteristic symbols
   • Modifiers
   • Tolerance zones

This list should get us started. We’ll see how much interest it generates and expand into the use of the characteristic symbols and tolerance zones if more interest is there.

So let’s talk about what GD&T is.

It’s a language, much like Spanish or English. It uses symbols, like our English symbolic alphabet, to represent an object, function or process. The need for a symbolic method of conveying ideas and function on engineering drawings can be easily understood if a person tries to imagine how they might describe a carburetor in words sufficient to allow a machinist to make one out of a block of aluminum. Pictures and symbols are worth a thousand words, and many more, so our engineering drawings give us access to tremendous caches of information in incredibly abbreviated forms, abbreviated but precise and detailed.

GD&T is many things.

It is a language that improves on the problems created by old coordinate and baseline tolerancing methods. It gives engineers the ability to precisely define geometry and the relationship of geometric features to a degree of complexity that was not possible before it was developed. It accomplishes this task with very few words and gives easy, precise control to both the geometric requirements of features and to the preciseness of the tolerances that will satisfy those requirements.  GD&T gives engineers the ability to precisely say what they need and what will satisfy that need in the same sentence.

Now we’re going to look at some fundamental rules.

GD&T is a language but it needs to layout some fundamentals in order to assure that the playing field of the drawing space is approached the same from all sides. By that we need to understand that a drawing of a rectangle will be appreciated or understood in the same way by everyone who sees it. For instance, when we see a rectangle, you know, a 4 sided geometric figure with two sides measuring one length and two more sides measuring something other than the first two, we recognize it simply as a rectangle, right? Like the shape of a business card. We can picture the rectangle. But to do that we have already assumed some things. Our drawing shows four lines in the shape of our business card and we assume the inside corners of the rectangle are 90 degrees and that the lines meet at the outside corners, you know, like a business card. But if we weren’t allowed to assume that the lines meet at 90 degrees or that the lines actually did meet when we see them touching on a drawing then we would be in trouble. In GD&T drawing fundamentals cover these contingencies. Lines shown to meet at 90 degrees are said to be at 90 degrees without something that says otherwise. Lines shown parallel are said to be parallel. Lines that converge at a point on the drawing, like the 4 corners of our rectangle, are said to be at zero relationship to each other. These fundamentals are the underpinnings of our drawing system so that everyone that may need to understand the drawing will understand it in the same way. So to begin with, engineering drawings must make a complete definition of the items on it and all the items must have a proper tolerance. Those definitions and tolerances must have only one way to interpret them and having said that those things only apply at the drawing level they are on. This text only describes these fundamentals briefly and there is more here to embrace but this is a start. Next time we will begin the task of learning the components of our language and how its syntax, or the arrangement of its words, assembles to form meaningful statements. Our reader’s comments are always appreciated and welcome.  

In this blog, we’ll discuss the tendency for many engineers to produce partially defined drawings (PDDs) that contain elements of GD&T but leave off the applicable basic dimensions. Not only that, but they fail to note most of the X-Y dimensions that define the geometry.

CAD models don’t have tolerances and they don’t have notes

PDDs are becoming all too common in our shops and workplaces thanks to the prevalence of CAD-based engineering. Virtually everyone in the industry uses CAD (computer-aided design) to initiate, translate, or program the nearly infinite array of hardware we all produce. Computer models are now a way of life in our shops…but CAD models don’t have tolerances and they don’t have notes. Instead, of course, they communicate through 3D geometry.

The use of CAD has become so prevalent that the natural tendencies of the competitive marketplace are taking hold. Putting all that data in the CAD files would lead any designer or engineer to feel that using PDDs is acceptable and delivers the throughput they are being driven to achieve. But this happens at the expense of the downline manufacturers who rely on thorough drawing information. Our world of drawings still relies on coordinate or baseline dimensioning techniques. GD&T is becoming more prevalent, and that’s good, all because of one key word: “unambiguous.” As engineering definitions become more unambiguous the process of producing parts that make automobiles roll and airplanes fly becomes less prone to quarrels and disagreements. But “quarrelsome” is how I would all too often describe the necessary exchange of information between designers and builders, because the current practice of part design and drawing interpretation creates disputes. “Quarrelsome” describes rejected parts and late schedules. “Quarrelsome” explains the need for GD&T. Humans communicate in words, while drawings are best when they have few words—a stated goal of GD&T. The ASME Y14.5 standard states that “dimensioning and tolerancing shall be complete so there is full understanding of the characteristics of each feature. Values may be expressed in an engineering drawing or in a CAD product dataset.” The fundamental rules that define engineering drawing practice are established and thorough. Anyone that needs to understand a feature on a drawing must have all the necessary definitions and tolerances.

When is it okay to leave information off drawings?

But the tendency toward PDDs and limited definition drawings is problematic. When is it okay to leave information off drawings? It’s happening—typically, when designers take these shortcuts it may work fine for them and even the company buying the products, but it leaves the Supply Chain out in the cold. The typical scenario is that definitional information is omitted but a note is added: Undimensioned features refer to 3D dataset,” followed by instructions to apply a profile control of perhaps .020” to those values. At a minimum, that kind of guidance is annoying to someone whose job it is to scour the model to see what’s hidden in hard to find places.   Recently I quoted a drawing that had 13 feature control frames for a simple mounting plate. Along with a few hardware installation dimensions there were only two X-Y dimensions and no basic dimensions to go with those 13 frames. The part plate had more than 50 penetrations and the note that described how to find the excluded dimensions said “all of the dimensions taken from the 3D file are considered basic and shall be controlled by the tolerance indicated on the drawing.” That’s a PDD in the truest sense of the phrase, and it’s not uncommon to see something like this—even within the aerospace industry. Again, there are fundamental and detailed rules in place to define engineering drawing practices. But problems arise with ad hoc PDDs that happen to reflect someone’s personal view of what is sensible and sufficient or what a drafter thinks is compliant with ASME standards. If this is our approach as an industry, “quarrelsome” will continue to describe interactions between designers and builders, and this will drive out any hope for productive and efficient collaboration.       

The GD&T (Geometric Dimensioning & Tolerancing) protocol is rapidly becoming the standard for communicating engineering tolerances and allowable variation for product parts. It’s an elegant but not a simple system. Close to 50% of the GD&T drawings coming across my desk use the system incorrectly in whole or in part.

This is not meant to be a critical or accusatory statement. It’s simply an observation, as well as an indication that all of us in the machining and related industries need to take the time to learn the details of GD&T as tolerances get tighter, machining gets more precise, and competition gets fiercer. Machining breakthroughs and software innovations can be overwhelming and it seems like we spend as much time learning techniques and processes as we do delivering for our customers. We all support numerous vertical markets around the country and the world. That’s why it’s more important than ever that we rely on a robust, common convention like GD&T for designing and modeling.