The GD&T Blog

Nobody has to tell you that training is a valuable tool to help improve your skills and that of an entire group. But so often, a desire to implement training is thwarted by roadblocks. These roadblocks come in different forms. Here are a few, and ways to avoid or work around them:

• “We don’t have time.”  This is the most common roadblock. There are always hot projects that can’t wait, especially in the world of engineering and design. But if your company considers training valuable, they should help you make time for it. To minimize the time away from your usual job duties, ask the trainer if the schedule can be broken apart. For our GD&T classes, I am willing to teach a few half-days that are spaced apart. Simply ask for this option, or see if the trainer offers a  condensed version of the training.

• “We don’t have anyone on staff that can teach that.”  Well, that’s when you get on the Web and look around for consultants! I specialize in GD&T; other trainers have other specialties. Don’t be afraid to look outside your company for help; if you are unsure about a consultant’s qualifications, ask questions. How long have they been doing this? Do they hold certifications related to the topic? (A GD&T trainer should be ASME certified; preferably at the Senior Level.)

• “Training is all theory; we need help using it in the real world.” Again, my suggestion would be to ask the trainer about this. While technical training has a theoretical component, the instructor should be able to tie the concepts in with actual designs or real-world situations. I always make sure to learn a little about the company where I will be training. If they make plastic parts, some of my presentation will be different from one at a company that makes machined engine components. Also, ask if the trainer is willing to incorporate your actual prints into the training.

• “Our employees can go online and get the training on their own.”  This might not seem like a roadblock, but there are two potential problems with this. First, are they really going to sit down and do this? There are advantages to online training for someone that is disciplined enough to go through an entire course online. But in reality, roadblock #1 usually creeps in here, and the training never gets done. A second issue is that online training is usually for individuals. A live training class with an instructor allows the entire group to be present, hear the same message, and bounce ideas off of one another. (I love the classes where we have design engineers, manufacturing engineers, and CMM inspectors all together! They all leave the class with a greater understanding of their different viewpoints and how they must work together.)

• The worst roadblock I ever encountered was an HR coordinator who told me that they didn’t need GD&T training because “the engineers should have learned that in college.” I don’t have a good answer for that one! I suppose the engineering manager should take the bull by the horns and make arrangements for the class out of his own department budget, circumventing the HR person. At any rate, don’t let roadblocks stop your company from pursuing technical training. 

So often in using GD&T, people are worried about using it correctly. And this can certainly be a valid question – geometric tolerancing consists of symbols and rules for how to use them. But we also need to remember that GD&T is a language for communicating design requirements. And like any other language, there may be several ways to say the same thing.

When asked about the proper use of GD&T on a sample drawing, I usually classify individual callouts in one of three ways: 1. A-OK;  2. Illegal;  3. Legal, but doesn’t add any value.

Example of #1:

While the use of datum D as a pattern may seem confusing, this datum usage is perfectly fine.

Example of #2:

This is illegal because flatness cannot reference a datum. (Seems obvious, but I’ve seen this on actual drawings!)

Example of #3:

This one is a little harder to assess. The feature control frame itself is legal, but it’s actually redundant with the 0.2 provided by the height tolerance. There is no way that the top surface could exceed 0.2 anyway, due to the plus/minus.

 So keep in mind that GD&T is more than just learning the symbols; it also involves many rules and the interplay of those rules can sometimes be confusing!

In the geometric tolerancing system, basic dimensions are used to override general tolerances (sometimes called title block tolerances). But let’s investigate these general tolerances a little more closely. A sample tolerance block is shown below, as taken from a drawing using metric (millimeters). First, notice that the tolerance allowed depends on the number of digits used after the decimal. This is common practice; at other times the tolerances may be divided based on the size of the dimension (1 to 10 mm, then from 10 to 50 mm, etc.). In our example, a separate tolerance is given for angles.

Some companies are trying to move away from these title block tolerances. It may be because they want to define everything with GD&T or other direct methods. While that might be OK to some extent, I would be hesitant to eliminate the entire idea of general tolerances, for one specific reason: the angular tolerance. Recall the old drafting rule that 90 degree angles are implied; they do not need to be dimensioned. But if the general tolerance block is removed, these 90 degree angles — unless they have GD&T applied — will have no tolerance!

So in your efforts to improve drawings and streamline your designs, don’t go overboard. Title block tolerances are just fine, as long as you don’t get too lazy and let everything fall back on those numbers.

Many of you may know that GD&T has been around for a long time (see an earlier blog entry about the history of this system). And like anything else that’s been around for a while, things sometimes change.

The current American national standard for GD&T is maintained by the American Society of Mechanical Engineers (standard number Y14.5M-1994). Prior to 1994, the previous edition was dated from 1982 (and 1973 before that). And each time, there were a few things that were changed in the rules and symbols of GD&T.

So, if you look at the history of the standard, you might guess that it’s about time for another revision — and you would be correct! The Y14.5 committee has been hard at work for the past couple of years, and the next edition is in the review phase.  (One sneak preview for you GD&T geeks: a new modifier to indicate unilateral profile tolerances.) It is projected that it will be released for general use in the first part of 2009.

As a side note, this underscores why it is important to always mention the specific tolerancing standard in the general notes of your prints, or even as part of your company’s standard title block. If this is not mentioned, then there may be confusion about the meaning of some callouts!

Here’s another common question that comes up in a GD&T class:  Suppose we are applying a position tolerance to a hole. It would typically have a diameter symbol in front of the number, as in the first example below.  But what if we omit the diameter symbol in front of the number, as in the second example? Would the shape of the tolerance zone still be assumed as cylindrical?

Answer: No, it’s not cylindrical. Without a diameter symbol, a tolerance zone defaults to two parallel planes (unless the BOUNDARY concept is invoked). So the second drawing above is ambiguous; the zone will be two parallel planes, but we don’t know the direction of those two planes.

Two solutions:  First, if the intent is to control the position in all directions, you must add a diameter symbol as in the first example above. Second, if we really intended two parallel planes, we must graphically indicate the direction of those planes:

 Now, it is clear that we are controlling position in the left/right direction. Of course, that means that there is no position control in the up/down direction. So let’s take it one more step:

 This example creates two sets of parallel planes — one in the vertical direction and one in the horizontal direction. The result is a square tolerance zone; thus, it’s very similar to using the traditional “coordinate” or plus/minus tolerancing method. But here we still have the advantage of clearly identifying the datum references, and we also have the MMC modifier to gain bonus tolerance, something that the coordinate method cannot do.

Colleagues and friends often ask me what the busiest time of the year is for a technical training company. After teaching GD&T full time for nearly 15 years, I can honestly say that there is no specific cycle that shows up regularly.  Of course, training is rarely scheduled at the very end of the year (Christmas holiday, and a busy time in general). But sometimes November and the first part of December are very busy, because a company or department may have to use up budgeted training dollars before the end of the year. And you might think that the summer months are not popular for training, but they usually are, as long as it is scheduled far enough in advance. (I soon discovered that those companies in areas where hunting is popular usually avoid scheduling seminars during those times in the fall when many employees may be perched in a tree!)

This is not to say that every month is busy — like any industry, we feel ups and downs. But from year to year, those ups and downs are rarely in a repeatable pattern. It’s a funny thing; obviously the economy drives much of it. But a slow time in the economy can sometimes be an ideal time to invest in employees’ technical skills: there will always be a need for knowledge in geometric dimensioning and tolerancing. Usually, our calendar gets pretty full at least one or two months in advance. But sometimes there is a window where our instructor is free, and with only a week or two of notice, we can schedule something. So if your company is in need of GD&T training, don’t think there is a bad time to schedule it!

Instead of discussing some technical point this time, let’s take a brief look back at the history of GD&T. Some people may think that GD&T is “just the latest fad” (I actually heard someone refer to it that way) and therefore they are implying that it’s not worth learning, since it may soon go away. But the facts show that GD&T has been around for a long time (50+ years), it applies tolerances in a logical and standardized manner, and it saves money — all reasons why it’s not going to fade into the sunset. 

Supposedly, the story goes that a guy named Stanley Parker came up with the first GD&T concept having to do with position (or “true position” if you prefer). The time was World War II, and the location was Great Britain. As you might imagine, during wartime deadlines are critical, and Mr. Parker ran into a situation where some torpedo parts inspected according to traditional tolerances were rejected. But it turns out that they were actually functional parts, and those parts were sent on their merry way even though they didn’t seem to be to print. 

He traced the discrepancy to the fact that traditional X-Y tolerances result in a square tolerance zone, but that parts outside the square may actually be good, so long as they are within a circle that encompasses the square’s corners:  

See the logic? If the four corners of the square zone were functional — as the X-Y method clearly allows — then in most cases a circular area would be just as functional. And think how many parts may have been needlessly rejected! (Of course, if your process is capable, you should not really have any parts out near the edge, but that’s a different discussion.) 

From there, GD&T has grown dramatically. Over time, Mr. Parker’s idea of “position” grew to include other concepts such as flatness, parallelism, runout, profile, and many more. And though GD&T became standardized by the military in the 1950s, it gradually became more popular among commercial industries, and has been used by many companies for well over thirty years. So don’t think of geometric tolerancing as a fad; think of it as the way we should have always done things! 

• When it comes to learning geometric tolerancing, some people opt for buying a book to read, or nowadays they can sign up for an online self-study course. The advantages to this are twofold: the ability to study at your own pace, and it is usually less expensive for an individual than a full training seminar.
• Traditional training with an instructor in the same room is sometimes more expensive (not always, depending on the number of students) but the huge advantage here is the ability to ask questions and have the instructor relate the concepts to your specific needs. If people from different areas attend the class together (design, CMM inspection, manufacturing), then they can also learn from each other and be in sync when GD&T issues come up in the future.
• I have dabbled in online training in the past, but we have not pursued that option in depth because we have found that the best service to our clients is to work with them directly to help digest and retain the material. Numerous times people have told me, “I have tried learning about bonus tolerance in the past, but the way you explained it made sense instantly!” While teaching a GD&T concept, I can get feedback — someone asking a question, or just body language — that helps me see if a different way to illustrate something is needed. (For one client, I sometimes teach via television to several of their sites across North America. It’s still not as good as being in the same room; but at least they can ask questions via the audio hookup.)
• Finally, in addition to teaching the concepts of GD&T, I always leave time at the end of class (sometimes the entire last day of class) for hands-on discussion of a group’s drawings. This consultation session is usually very lively, and lets participants apply the theory to reality. This is something an online course cannot do.
• So be sure to weigh the pros and cons when investigating training options. Online courses will work great for many people, but don’t forget the traditional live seminar, especially if a company wants to have several people learn GD&T.

Another question came in recently, having to do with basic dimensions and their use with datum targets. If you are up to speed on GD&T, you should know that a basic dimension is any number enclosed in a box: 

The purpose of this is to show a theoretical dimension, without any tolerance. (Even a general title block tolerance does not apply!) Instead, the feature that is being dimensioned will have some GD&T that provides the actual tolerance for manufacturing.

Now here’s the question: datum targets are usually located using basic dimensions, but there is no GD&T to provide a tolerance:

 Is this legal? What governs the accuracy of where those datum targets are?

Yes, it is legal, and here is the key: Geometric tolerances are applied to features of a part. A datum target is an imaginary point, line, or area that is simply used for fixturing or gage setup. It’s not the responsibility of a product drawing to worry about the tolerancing of a gage or fixture!

The ASME Y14.5 standard says it this way in paragraph 4.6.2: “The location and size, where applicable, of datum targets are defined with either basic or toleranced dimensions. If defined with basic dimensions, established tooling or gaging tolerances apply.”

Thus, most GD&T people use basic dimensions to locate datum targets. Of course, there is the question of where these “established tooling or gaging tolerances” come from, but that is something for a design group to discuss, and perhaps even reference in a note on the drawing.

SAE International (the Society of Automotive Engineers) has recently announced its winners for this year’s awards, and among them is John-Paul Belanger, from Geometric Learning Systems. Mr. Belanger is receiving the Forest R. McFarland Award for outstanding contributions toward the work of the SAE Engineering Meetings Board in the planning, development, and dissemination of technical information through technical meetings, conferences, and professional development programs. This is in recognition of his years of involvement in GD&T training for SAE to their network of members and clients.

“I am proud of my membership in SAE, and am happy to be able to work with them as an instructor in GD&T. They serve as a valuable resource for the automotive industry, and I am grateful for this recognition.”

John-Paul has been involved in training for GD&T and Tolerance Stacks for over fifteen years. He is a principal for Geometric Learning Systems, a consulting firm specializing in GD&T training.