The GD&T Blog

Three reasons for on-site GD&T training — Why a hands-on, tailored approach is better for your team…

 1 — An on-site GD&T class can be customized to suit the needs of your group. If they are immersed in GD&T regularly, we can spend less time on the introductory material and jump into the issues that are most relevant to the team. However, the basics cannot be completely avoided, because we need to make sure that everyone is on the same page for the essentials.  But overall, there is more “bang for the buck” when the class is custom-fitted to the GD&T topics that are applicable to your products.

2 —  Another advantage to having an on-site class is that your actual prints can be incorporated into the discussion. At a “public” GD&T class we can’t do that because everyone comes from different companies; there are time limitations, and sometimes confidentiality issues of displaying a print for the entire group to see.  But a special on-site class for your team makes the GD&T relate directly to the “real world” because the instructor can help you sort out real tolerancing issues on the spot. A video or web-based course can’t do that.

3 — Contrary to what you might think, the third benefit for a custom on-site GD&T class is a lower cost when compared to a public class — and the class can be scheduled based on your convenience. We price our seminars based on the course length and any travel costs, not the number of participants. So instead of having 15 people go across town (or out of town!) to a seminar at $900 per person, we can do the class in your building for about half of that total. And a flexible schedule can be arranged with the instructor, sometimes even spreading it out if consecutive days are not possible.

While an on-site class is not always ideal (perhaps there are only one or two employees that need the training), the benefits can be noticeable because the theory and the application are integrated together!  Call us or use the “Contact Us” button above to learn more about hosting an on-site GD&T seminar.

When I teach a GD&T class, I have to presume that eveyone is “green” about the topic. Even if some folks have been using GD&T regularly, I find it best to start from the beginning. This ensures that everyone is on the same page, and it sets the stage for presenting the various GD&T topics that will be examined in the class.

However, in order to effectively learn the GD&T system, there is some prerequisite knowledge. Before signing up for a GD&T class, make sure you are comfortable with basic blueprint reading, such as how to interpret the various views on a print (top, front, side, sections, etc.). You should also be familiar with plus/minus tolerancing (including unilateral plus or minus) and common drafting practices.

Here’s a simple example:

Common drafting practice tells us to assume that the corners in the right-hand view are 90 degrees, and we also assume that the inside and outside diameters are to be made on the same center line.

But that raises two questions:  What is the tolerance on the 90 degree corners?  And what is the tolerance on the possible offset between the diameters?

According to the general tolerance given for the print, the corners can deviate anywhere from 89-91 degrees. So we’re OK there. But this print does not provide any tolerance for the “coaxiality” of the two diameters!  The general tolerance cannot help us here, so technically this print is ambiguous.

Of course, we could go on to discuss adding a geometric tolerance such as concentricity or position for the diameters, but the point here is to get you to think about basic drafting rules before you proceed to the GD&T.

One last prerequisite needed before attending a GD&T class: a willingness to learn the material, and in some cases unlearn preconceived notions! I’ve had numerous people come to a seminar as a refresher, but at the end they realize that they had a poor or erroneous understanding of some of the symbols. That’s not a criticism of them; it’s a compliment to their honestly and ability to evaluate technical concepts in a new light. We all learn something new each day — even us GD&T experts!

For engineers who regularly perform tolerance stacks, handling regular dimensions is pretty straightforward. And even when GD&T is involved, there is usually not much difficulty, until one encounters the MMC modifier. How can the effect of this “M” symbol be accounted for?

First, a primer on what the effect of the MMC modifier is. Suppose the example shown below is applied to a pin:

The “M” symbol tells us that the given position tolerance of 0.5 applies if the pin is made at its maximum size of 11.8 mm. But if the pin is made at any size less than that, then the position tolerance gets a corresponding “bonus” tolerance. Thus, each part that is made gets its own customized geometric tolerance.  (Example: a pin made at 11.5 gets a position tolerance of 0.8, and a pin made at the smallest size of 11.2 gets a position tolerance of 1.1 mm.) The advantage to this system is that some parts that are made will get more positional tolerance, while still ensuring that those pins will assemble with the mating parts.

With two-column tolerance stack calculations, then, we must be careful.  Without the “M” symbol, we would simply add another line item in our stack to account for the 0.5 mm position error (which is actually 0.25 in each column). With the “M” symbol, that line should still be added, but then another line should also be included for the bonus portion. Then the key is to realize that sometimes the bonus amount is applied to both columns, and other times it only occurs in one column. Here is an example where the bonus is in both columns, because the pin’s size is not a factor; the stack is directed to the axis of the pin:

If the stack were leading to the edge of the pin, then the size of the pin gets factored into the stack, and one column would have zero and the other would get the 0.3 bonus.

The complete methodology for this spreadsheet method is covered in our 2-day class on Tolerance Stacks. We teach this approach for the bonus tolerance, “shift tolerance,” as well as the correct handling of all geometric tolerancing symbols.

When it comes to GD&T training, I am often asked which units of measurement are preferred. The answer: It doesn’t really matter! The GD&T system works the same using inches or millimeters; the only thing that changes is the number.

The technical standard ASME Y14.5M-1994 uses SI units (millimeters). Paragraph 1.1.2 phrases it this way: “The International System of Units (SI) is featured in this standard because SI units are expected to supersede United States (U.S.) customary units specified on engineering drawings. Customary units could equally well have been used without prejudice to the principles established.”

This may be humorous to those companies that have always used inches and continue to do so. (Weren’t we all told back in the late 1970s that everything would soon be metric?) The millimeter is widely used by countries besides the United States, and within the U.S. many industries have made the complete changeover to metric (including the automotive industry). But other industries, such as the aircraft industry, continue to use inches, as do smaller suppliers and machine shops.

Obviously, dimensions and tolerances given in one system can be easily converted to the other, but several things need to be addressed when doing this. First, keep in mind that rounding error may occur. Second, there are different customs to follow when displaying millimeters on a drawing than for inches. One custom is that a number less than one millimeter is to be preceded by a zero (such as 0.5 mm) but a number less than one inch should not (such as .500 in). Other minor differences are spelled out in the Y14.5 standard in paragraphs 1.6 and 2.3.

Finally, I should mention that our training is available in either system of units. Our GD&T seminars are based on the ASME standard, so our training manual was originally developed using millimeters. But we recently finished converting the manual to inches. So if your company engages us to do GD&T training, the same class can be taught using either system! 

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!