Does GD&T Make Things Too Complicated?

I write this blog each month and try to focus on how to convey to our readers what GD&T is and how we can better understand it. There’s always a question about how to apply and use a tolerancing system that is supposed to apply to all the mechanical things we design and build. So, I try to explain the system to those readers, like the many people I meet, who need to use and understand it.

What is GD&T?

GD&T is a complex system of symbols and values designed to give engineers and manufacturers the ability to impart an almost infinite degree of geometric definition to parts while controlling those definitions to an almost limitless degree of tolerance.

The Difference Between GD&T and ISO Standard 

I teach the subject to a wide variety of students at local community colleges and area manufacturers, including my employer. Students range from teens to folks in their 60s. So, the approach needs to be flexible while knowing that the world of GD&T has little flexibility. After all, it’s that quality that makes GD&T a world standard. In other parts of the world, it’s called the ISO standard, but the two systems are essentially the same. 

GD&T Adapts to Everything That Can Be Designed

GD&T frustrates a lot of people. I’ve had students ask, “If we’ve been making airplanes fly and ships float for decades then why is a complicated system like GD&T needed?” I hesitate to answer a question like that, but in truth, it’s a good one. GD&T is complicated, and it’s evident on the surface that it makes things complicated and more expensive. However, the reason for this system’s complexity is that it’s adaptable to virtually everything that can be designed. From simple to frightfully complex, it can describe features and parts with as much or as little detail as needed, and when used effectively, it’s more cost-efficient.   

How GD&T Was Used for Planes in WWII

In 1941, during World War II, the Boeing Corporation began rolling 16 B17 bombers off the runway every day in Seattle. Remember those planes with polished aluminum skins and rivets everywhere? Eventually, by the end of the war, those numbers would swell to a total of over 12,000 planes.

As a side note, I have asked Google more than a few times how many rivets were in a B17. I was never given an answer or even provided a guess about how many there were until a few days ago. I was surfing to find that elusive number when I found an entry by Michael Lombardi, who is a Boeing Historian. After some fair amount of labor, Lombardi put the number at 394,484. He didn’t know what the specific model of the plane that number was accurate for, but I think we can agree that’s a lot of rivets. At 16 planes per day, that amounts to almost 6.5 million rivets. All of these are individually installed by hand on a daily basis.    

Boeing’s engineers, mechanics, machinists, and installers accomplished that feat without GD&T. So, why wasn’t that sufficient to avoid the troublesome and costly undertaking known as GD&T? The answer is entwined in the details of what manufacturing was like then and how it has progressed since. 

How Designers Use GD&T To Manufacture World-Class Parts

The men and women who design and make parts for things like automobiles, planes, and military weapons have always been tenacious innovators. If a device or part did not function as intended, they’d make changes until it did. Machinists making parts on machines designed to hold thousandths of an inch learned how to coax those machines to far exceed those tolerances. Form, fit, and function were the result of people on our American (and its allies like Britain and France) design and assembly lines making things fit and function. Work was being done off the page so to speak. 

Is GD&T a Necessary System for Manufacturers?

So, since this is true, why was there a need to create a system like GD&T? To answer that, I need to explain something very basic; so basic it often gets ignored or passed over. Engineering drawings are pictures. These pictures are conceived and arranged to show and describe mechanical parts with sufficient detail such that almost anyone who is trained to understand what 3.207” means or what 304 CRES is can decode the details of that drawing and reliably let the wheels of their manufacturing world produce that part. Drawings provide pictures with a universally accepted set of instructions. 

GD&T in Today’s Manufacturing World

Until our manufacturing worlds accepted and began to rely on the GD&T system to describe the technical specifics of parts, it relied on various dimensioning systems to detail those specifics. Dimensioning systems such as chain, parallel, combined, co-ordinate, and baseline (and there are variants of each) filled in the part-pictures with details of a part’s size and peculiarities. So, was GD&T developed to enable the design of parts that were different than existed before? Looking at this through the eyes of today’s manufacturing world, we can see that some manufactured parts have gotten more complicated and without a doubt the processes that produce them have gotten much more sophisticated, but the parts themselves are not that different. 

GD&T Makes Fabricating Complex Parts Much Easier

I have made and seen some very complex parts and assemblies during my time in manufacturing, but those parts do not surpass the beauty and simplicity of the radial engines that kept planes flying during WWII. Watch an animated model of a running Pratt and Whitney radial engine on Google and be amazed at its sophistication.  

To explain where this is going, I’ll illustrate my words with an example of what we’re discussing. This morning, I received an email with an attached drawing detailing a simple part that a customer submitted an RFQ for (You can see it below).

I have redacted all of the customer’s info, but included all the necessary technical info except for tolerancing, which for this part was simply +/-.005” for 3 place decimals, angles held to +/- 1 degree, and a required 125 rms surface finish. The drawing doesn’t specify any adherence to ASME Y14.5. So, if this is important, then why?

GD&T Reduces Manufacturing Times

Let’s look at this simple part. There are not many parts with fewer details, but because of the possibility of geometric variations, this part is more complicated than it appears. What does that mean? When I look at this part, I immediately try to think of a path to manufacture that takes the least time to machine or a method that consumes the least amount of material in its path to the shipping container, moderated by its required volume. Those are valid concerns, but as a manager, I need to first be concerned with a finished product that works when it arrives on the assembly line. Because of the increasingly more diverse and complicated world we live in, the necessity that our part functions properly is far more important. 

After a quick look at it, I can think of some versions of this part that will meet the print but will likely not function on the assembly line. The things I am talking about are related to the part’s geometry. So, why do I think I see allowable versions of the part that will not function at assembly? Because the drawing allows versions of the part that are out of tolerance by as much as two to three times the tightest basic tolerance on the drawing. So, you say a part made to the drawing’s tolerances might be two or three magnitudes more inaccurate than the tolerance? No, that’s hogwash! This possibility is, however, true. And we will see why that might be.

Why Cubes Need GD&T

Before we dive into why this might be true, let’s remember that if the part’s design engineer did a proper job of designing this part, then the part that will function properly at a given tolerance will not function properly outside that tolerance, or he would have allowed those dimensions in the first place because looser tolerances generally cost less. That’s proper engineering design. The correct tolerance costs the least, but functions properly. 

So, let’s look at our cube part again. The sketch below shows the A, B, C and D angles to be square as the drawing shows. Also, the sides are shown equal, making the part a cube per the geometric definition.

But what if the sides of the cube were not perfectly square, but they were within the tolerance of +/- 1 degree? Let’s say the 90-degree angle (D) in one view was produced at 91 degrees and its corresponding adjacent angle (C) was at 89 degrees. The cube would be a parallelogram (the term rhombus is technically correct) like the one shown below (with opposing angles, both acute and obtuse) being equal and all sides equal and parallel. We could then apply the drawing’s tolerance to any two opposing sides allowing them to be .500” +/- .005”.  

I made this sketch with the angles pictured exaggerated past 91 and 89 degrees, so we could easily recognize the part’s inaccuracies. But let’s agree, for our math’s sake, that the part shown is in angular and linear tolerance. 

As our drawing shows, an inspector could measure the cube, one feature at a time, per the tolerances on the drawing, and both the angles and sides would be in tolerance, but the cube’s affected geometry would be outside the limits of its fit in an assembly (Remember, first and foremost the part needs to function).

How GD&T’s Rules Have Changed

Using the verbal description above, the actual fit-up of the block could be oversize by a considerable amount, as we said before (see the sketch below, which is only 1 of its 3 dimensions.) That same inspector might notice that the cube is leaning to one side, as in our sketch, but can he reject it? There is nothing on the drawing that requires it to stand up straight to any value smaller than +/- 1 degree. So, what would be his justification for the rejection? There would be none.   

From this sketch, we can see that the envelope of our cube exceeds its 3-place tolerance allowance, but the scope of the drawing does not control that actuality.

So, here we are with the question staring us in the face that asks: Whose responsibility is it? Who says we can’t live with simple? I can hear my father-in-law now, “When I was a boy, my father could fix anything on our tractor any day, anyhow.” When it comes to airplanes and fighter jets and weapons that can fire thousands of rounds per minute, the responsibility for safety and reliability falls on our society’s regulatory authorities. Since my father-in-law’s days, those rules have changed.

Metalcraft’s Next Chapter on GD&T

Stay tuned next chapter for some thoughts on that and more close-up examinations of why GD&T, as technical as it is, serves its social and mechanical responsibilities well.

Thanks, Chris Lindenberger

This month, let’s review some definitions and rules. Let’s imagine that we’re going to make a simple part in our machine shop. The foreman returns to the quality lab or machine shop from a morning production meeting holding a customer’s drawing for a small shaft that will be sold to a customer in the aerospace industry. The drawing is for a simple small shaft that is .505” in diameter and 5.670” in length. A look at the drawing shows us that the tolerance for 3 place decimals is +/-.005”

That means that the shaft could be turned at .505” plus as much as .005” and be acceptable, correct? We have wiggle room and should be able to hit that range of values. Well, the answer to this first question is “NO!” We can’t turn the .505” diameter to .510” and be in tolerance! Can anyone tell me why the answer is “no?” 

Because this is a month-to-month blog, whenever we touch on something important about our subject, I will create a quiz for everyone to take. I just asked a question about our discussion part: If we turn it to .505” plus as much as .005” more, will it be acceptable? Then, I said that the answer is “no.” Accordingly, our first quiz will focus on why .505” diameter plus .005” is not acceptable for this part in GD&T-land. The following four statements will comprise quiz #1. Some of you, as readers of this blog, likely use GD&T. With that noted, let’s see how you do. Choose the numbered statement that you think best describes why a measured value is insufficient to approve our part’s OD.

Below is your quiz:

1. In the world of GD&T, size is not everything.
2. We might be living in an apartment in France.
3. How the part will be measured is not identified.
4. The oxen are slow, but the earth is patient.

What answer did you choose on the first quiz? If you selected number 1, 2, 3, or 4, you receive an A! Now, why did you receive an A?

Number 1: In the GD&T world, size is somewhat meaningless without knowledge about its geometry. It’s a round shaft, and it may be .510” everywhere you can put a mic on it. However, what if it’s not perfectly straight? UGH!

Number 2: Why would it matter if we were living in France? Well, if our shaft is made in France, it may no longer fit when it reaches a customer in Seattle, even though it is not above .510” in diameter.

Number 3: The drawing we’re working from describes the part, but not the gauge to measure it with. Consequently, something other than what the job requires could be created.

Number 4: “The oxen are slow, but the earth is patient” is an age-old philosophy about, among other things, the benefits of taking life one step at a time.

Now, let’s test a part for its adherence to GD&T scrutiny. The first thing we’ll learn is that there are a few things we can assume will be required when we enter the world of GD&T. One of the first is called “rule number one.” It states that there is a thing called maximum material condition that applies to all parts.

Rule number one ties size and geometry together when assessing a feature of size. Since we’re just getting started on our no definition approach to using GD&T, and because we’re going to define something, I’ll use our quiz to explain why a part’s size is not everything in GD&T. I’ll also explain why living in France may be pivotal to our discussion and why the drawing did not tell us what gauge or tool to measure it with.

Rule number one is more of a philosophy than a rule, but it is a rule. It is slightly more complicated than what we are saying, but it will serve as a starting point. Essentially, this rule states that features of size must be perfect in form at MMC. Expressed differently, it says that as a feature of size (our shaft, for example) approaches its capacity (in tolerance), it must resemble its perfect form. Let’s remember that GD&T means “geometry stuff.” Since our shaft is a cylinder, the geometry we’re concerned with is cylindricity. If combining the two ideas (size and form), we must ensure that our part is a perfect cylinder when it’s at its northernmost (biggest) measurement. 

If our shaft measures .505” +/-.005”, then its biggest allowable size is .510” in diameter. If we superimpose rule number one on top of size tolerance, we can determine how we did on our quiz.

1. Size is not everything. After all, when our part measures .510”, it must also be a perfect cylinder without any of its molecules extending past a .510” diameter perfect cylinder.
2. In France, a 12.594 mm diameter shaft would not fit in a .5100” diameter bore unless it’s a perfect cylinder. If a Boeing 737 Max’s landing gear was made outside the confines of rule number one, this could cause a catastrophic failure. 
3. Our drawing did not specify that the shaft needed to be measured in a certified, 6-inch-long fixed .510-inch OD ring gauge. However, if we had such an item, we wouldn’t require GD&T because our shaft wouldn’t exceed .5100 inches in diameter or be less than perfectly straight and 5.67 inches in length. 
4. The oxen are slow proverb shows that a close-up study of life can be simultaneously enlightening and enraging. Specifically, there is no way to grasp GD&T without struggling over a few details. With this subject, we must practice patience. 

Let’s try a new approach, called “Fun with GD&T,” to learn and understand this concept. We’ll adopt this approach to avoid a strict fact and rule-filled reading session that leans in the direction of taking a part through the process of assessing whether it meets the real-world requirements of designing, making, or quality checking a part in GD&T-land. 

This method may help some people learn how to apply GD&T without first memorizing a million factoids. You will eventually learn all those factoids, but one at a time could be the superior approach. By the way, brace yourself, there are more than a million factoids.

I was a handle-turning machinist in the ’70s and, even then, we began to see GD&T controls on aerospace drawings. This required us to learn and use some GD&T concepts and we didn’t have Google to conduct research. The internet is an incredible tool to solve learning problems. Hence, it would be good to have your fingers oiled and poised above your keyboards. In addition to reading blogs on the subject, get a GD&T dictionary and refer to it when designing or making parts with GD&T controls. 

Another way to learn about GD&T is by reading this monthly blog, writing down any questions, and submitting them here. Subsequently, we’ll schedule an online appointment or phone call to discuss your inquiry. There is a limit to how many, and how complicated, your questions can be. Please provide us with a little time so we can evaluate your question and answer it thoroughly.

We are currently working to implement this format, so your suggestions will be appreciated.

With that noted, I hope to talk with you soon. Did I mention the response will be from me? 

I usually write blogs about how the symbols and rules that govern GD&T should be translated because it’s a system based on rules and definitions. What does parallelism mean? What rules govern datums used at maximum material boundaries? What exactly differentiates a regular feature of size from an irregular one? These questions are examples of why GD&T could be considered boring and tiring. Nevertheless, rules and definitions help us learn GD&T and without them, there wouldn’t be a usable system. So, to prevent boredom, let’s review how GD&T can be used in everyday life.

GD&T facts and rules are necessary and invaluable when determining the right approach to a particular part and its features. What GD&T rules will you need to follow to handle a particular part? 

Hopefully, we can use a GD&T viewpoint to start with our subject part. After beginning that process, we can rummage through our junk drawer to locate some important details. When getting started in the mechanical parts business, we were taught how to step up to our CAD-CAM system, lathe, drafting board, or surface plate, and whammo!  We began turning, extruding, indicating, or drawing our part. Essentially, we learned how to apply what we learned. 

Now, let’s again concentrate on how to design, machine, or inspect a part. Of course, it will relate to the GD&T-controlled universe. For this exercise, let’s employ a “no definitions approach” to applying GD&T to a part. Explanation text insert.

Let’s further analyze what a “no definitions approach” to applying GD&T means. Think about an astrophysicist, who needs to learn a lexicon of words and terms. This individual undoubtedly maintains a memory system to use that field’s language and aptly apply it. 

An astrophysicist studies scientific phenomenons, mathematics, subjects that explain gravitational relationships of bodies in space, and more. It’s a complex and difficult task to combine all the aforementioned studies into one cohesive and acceptable overview. 

GD&T is just as complex as astronomy. An individual tasked with applying GD&T to their field of engineering/manufacturing must grasp a sizable boatload of facts, relationships, math formulas, concepts of size and geometry, and much more. There are several books available on GD&T that can serve as a reference guide and boost your ongoing education. 

Becoming a skilled machinist, inspector, or CAD designer is a difficult and time-consuming process. However, when adding GD&T to the equation, the process becomes even more demanding and daunting.  

GD&T is an intricate concept that is grueling to fully comprehend. Therefore, if we ever cover something that creates confusion, please submit your question(s) here. 

Stephen Hawking once said, “The enemy of knowledge is not ignorance it is the illusion of knowledge.”

Think about that for a moment. What is the “illusion of knowledge?” Hawking suggests that what impedes a greater understanding is the tendency of people to believe they already possess knowledge of it.

When we apply that to GD&T, we see engineers and designers use GD&T incorrectly because they think they are defining their parts and geometry correctly. In reality, they have simply failed to learn the current definitions of how to apply controls.

Design Mistakes Reflect Poorly on Our Industry

Experts estimate that at least 50% of drawings that employ GD&T use it incorrectly. This is unacceptable. Machine shops must not just strive to improve their manufacturing skills, but pursue excellence in achieving their goals. Anything short of that is a failure.

Consider this in the context of a machine shop. If 50% of the designs—and hence the drawings—that come through the doors are beset with mistakes and problems, then how does that reflect on us as manufacturers?

 

 

Inaccuracies Go Unchecked

GD&T allows us to define precise part geometry. Everything the designer needs to say about a part’s form, fit, and function can be outlined to a nearly infinite degree of detail. So, if the designer includes inaccuracies on the drawings that we then estimate and build on a daily basis, then perhaps our inability to understand the convention sufficiently allows these problems to go unchecked. The “Illusion of knowledge” is winning the battle.

Inaccuracies Go Unchecked, So What?

Parts still get made. Machinists and technicians still work their eight hours a day. We don’t make any less money as a business or as workers when we’re less than excellent. If the parts are inaccurate, but we maintain our operations, then, “so what?”  

The “so what” translates to the countless hours spent accomplishing work on parts that don’t need it.

If the parts we make pass both the shop floor’s interpretations of the call outs and the quality department’s interpretations of the call outs, and both teams made decisions based on a less-than-perfect interpretation of the job requirements, then there is overcompensation going on. 

Vague Tolerance Interpretations Push Back the Goal Post

Remember, meeting any particular tolerance can only be accomplished by being “slightly” better than the call out. But, being exactly in-tolerance is not a real-world possibility. 

A part feature is either better than or less than a given tolerance, so if the exactness of the given tolerances’ interpretation is vague or blurred, the distance to exceed it is harder to reach.

Create a Culture of Questioning

Often, inspectors and customers quote explanations of why a feature was inspected in a certain fashion, only to realize they have no idea why they were doing something for that call out. 

Creating a playing field that questions how we interpret the tolerances that govern our performance will help shops find the most efficient way to ship good parts. After all, that’s how we compete.

Study the Standards. Look for Correctness. Know Your Tolerance Zones. 

Know what you absolutely have to know to meet your drawing tolerances to help machine your parts correctly, and know what you absolutely do not have to do to meet those same tolerances.

The truth is, we as builders and designers only begin to excel on the world stage when we do exactly what we’re supposed to.

The “Illusion of Knowledge” can be the biggest hurdle we face in our pursuit of excellence. Remember, however, excellence is not a goal, it’s a journey, and achievement is not an outcome, it’s a process.

Partner with Metalcraft

Our engineer-minded machinists prioritize staying up-to-date with the latest GD&T standards. We examine part drawings to find the most efficient way to produce your product at the lowest price point. Are you looking for a metal manufacturing partner with a reputation for precise accuracy and tight tolerances?  Contact us for a bid, today.

GD&T is growing more common in our industry, which burdens us with forming a working knowledge of its convention.  How we use and translate GD&T changes over the decades as new governing bodies take oversight and release new revisions. The one constant, however, is the great diversity of opinion on how to employ and translate various complications and refinements of the craft.

Differing Opinions Muddy the Waters

Often, machinists are thoroughly ensconced in making parts that need to fit inside the confines of a language that has black and white definitions but work in an atmosphere of users that likely have varying opinions about how things work. Aerospace engineers in the halls of Lockheed can be found arguing about total runout controls, while a variety of RF or hydraulic engineers follow the way their company translates a certain profile callout and that’s the way it is. 

 GD&T Can Be Miraculous When Employed Correctly

There are a lot of opinions flying around about how GD&T is being used that its preciseness is being watered down. GD&T is a wide and deep practice designed to make the design and fabrication of a miraculous world of mechanical technology as precise as it can be.  Given those pursuits and the achievements inherent in their success, fewer planes will fall out of the air, and technologies will function longer and perform more efficiently when we keep our standards at hand and our minds open to finding the best consensus of opinions.

Partner with Metalcraft

Metalcraft understands the importance of continual GD&T education. Our engineer-minded technicians optimize product designs to find the most efficient and cost-effective manufacturing process. Contact us for a bid, today.