How to Factor Bonus Tolerance into a Tolerance Stack
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 a 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.
Inch vs. Metric
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!
Roadblocks to Effective Training
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.” Â 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.
Are You Using GD&T Correctly?
`So often in using GD&T, people are worried about using it correctly. And this can certainly be a valid concern — 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 several times 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 on the height dimension.
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!
Title Block Tolerances and GD&T
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.
Does Geometric Tolerancing Ever Change?
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!
Does GD&T Require a Diameter Symbol?
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.
What Is the Best Time for GD&T Training?
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!
History of GD&T
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!
Live GD&T Training or “Canned”?
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.
Meet The Expert
Watch John-Paul talk about the benefits of GD&T training courses.