A funny question arrived in the inbox yesterday, and it reminded me to get on here and post another entry. The questioner wondered if I’m undercutting some of my training business by giving out GD&T tips and explanations via this blog.
Not at all. GD&T is not a secret society! It’s a very useful language and the more people that know it, the better. By explaining some of the symbology here on the blog, we hope to do two things: educate people, especially those that need assistance with a specific topic; and at the same time, generate interest in our complete GD&T seminars (wink). So it is possible to be helpful and advertise at the same time! Here’s today’s GD&T “secret”: the tangent plane modifier:
First, a review of the parallelism symbol in general. The top of this block is to be parallel to datum A within .006 inches (notice that the distance between the top and bottom is a separate, more generous tolerance). Regular parallelism, without the T symbol, would require all points across the top to be within our zone of .006.
But the T symbol changes things — instead of controlling every point across the top surface, now it says that the “tangent plane” formed across the top surface must be parallel within .006. This imaginary tangent plane is formed by the highest points only:
Thus, the tangent plane modifier is loosening the parallelism idea because we ignore the valleys, or low points! (Of course, the low points cannot dip below 4.405 because of the height requirement.) Tangent plane makes sense for a mating surface whose counterpart only feels the high points anyway. The T symbol can be used with parallelism, angularity, or perpendicularity when those are applied to surfaces.
So there’s another GD&T secret out of the bag.
OK — everyone else is writing about the present economy, so I guess we should look at how it affects the training industry. I’d like to resurrect an old blog entry (about a year ago) but tweak it a little to give you all some ideas on how to sell your managers on getting people trained, even when the budgets are tight. When approaching your managers for approval on hosting a GD&T class, one form of resistance that you may get is:
- “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.
Of course, the latest version of this is “We don’t have the money.” First, emphasize that in these tight times, only the cutting-edge engineering groups will survive. And knowledge of GD&T is essential to a company’s survival. Second, ask us (or any trainer that you contract with) about how flexible the pricing is. (Perhaps your company already owns a copy of the Y14.5 standard so we wouldn’t need to provide one; that’s $150 in savings.)
Another common response from management that was detailed in our previous blog entry:
- “Employees can go online and get the training on their own.” There is some truth to this, but there are two potential problems. 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, the training often 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.)
And the last “roadblock” has nothing to do with the present economy, but with a closed-minded manager:
- Once I encountered an HR coordinator who told me that they didn’t need GD&T training because “the engineers should have learned that in college.” In that case 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.
In summary, the need for training will always be there. But don’t be afraid to ask the instructor about creative solutions such as staggered classes or a condensed course to help optimize everyone’s resources!
Let’s go back to the question box —
“Why is profile allowed to be designated as one-sided? Can other GD&T symbols also be one-sided?” First, let’s address a couple of points: there are two profile symbols: profile of a line and profile of a surface. Yes, each of them can have the tolerance amount that is unilateral (one-sided) or bilateral (both sides of the intended profile). Also realize that the profile symbols can be “unequal bilateral” where there is tolerance on both sides of the perfect shape, but more tolerance exists on one side. Some examples:
No other GD&T symbol can use these options; there is no such thing as unilateral flatness or unilateral parallelism. Why is that? Well, when we talk about flatness or parallelism, the feature in question has no curvature. So the surface “is where it is.” Profile tolerances, however, have a curve (usually) and also one or more basic dimensions that describe the radius or other values. The prescribed radius is important, and if the curve dips in or out, it is directly impacting the given radius. With this in mind, I should point out the tolerance zones for some other GD&T symbols can float to one side or the other. For instance, a parallelism tolerance of 0.5 allows a surface to tilt 0.5 mm in either direction, but that is not considered a unilateral tolerance, because it is a flat surface. (If it curves in or out, it doesn’t have to follow a particular radius.)
Whether you our your company need a GD&T refresher or a comprehensive training program, please contact us for information about how we can help everyone in your group get on the same page when it comes to the language of GD&T!
If you’ve spent any time around geometric tolerancing, you have probably heard it touted as the best thing to come along in mechanical design since the drafting board. And most of it’s true — GD&T Â helps us communicate a product’s design in a big way. So it shouldn’t be surprising that most industries have adopted GD&T to some extent in their designs.
Here’s a short list of some of the industries that use GD&T regularly:
- medical devices
- military hardware
- household appliances
- electronic components
- even furniture…
I admit that I have been surprised a few times by working with clients in a sector where I probably wouldn’t have guessed that they use GD&T! It is interesting, as a GD&T consultant and trainer, to work with various groups and help them implement GD&T where others may not have bothered. A few of the more interesting clients that have worked with us manufacture things such as golf clubs, blenders, explosives, gas pump handles, and even municipal water supply mains (you should see the tolerancing on the huge water valves and bulkheads!).
You may have heard that GD&T is great for mass-produced parts (so a blender might need it) but not for low-volume or custom parts, such as a water main connection. But guess what: somebody still has to build the thing, and some form of tolerance is still needed! So kudos to those of you that want to design with GD&T; don’t let the fact that you’re making a non-traditional component stop you from taking advantage of the benefits of geometric tolerancing.
This blog post may seem like splitting hairs to some of you, but it’s a question that came up in my class. And you know the saying: if someone has a question, chances are that others are thinking the same question.
When inspecting parts for runout (and a few other characteristics), you may know that the classical method involves holding a dial indicator to the surface and then watching for the highest and lowest reading. This difference is then compared to the specification allowed by the drawing. Here’s a visual of runout being checked on an end face:
The absolute value of highest to lowest gage reading is often called “TIR,” or “total indicator reading.” In the past it was quite common to specify runout by adding a note to the drawing such as “.040 max. TIR.” (Nowadays, it is more proper to use GD&T to control this, especially because the former method is ambiguous when it comes to identifying the datum.)
Well, somewhere along the line another acronym crept into the vocabulary: “FIM,” which is “full indicator movement.” It essentially means the same thing — the total variation from highest to lowest gage point. But the Y14.5 standard uses FIM exclusively in its explanations for runout. Is there a reason?
Yes, and here’s where the “splitting hairs” part comes in: the way TIR is phrased emphasizes the reading shown on the dial face. But there may be a small error inherent in the dial indicator: after all, it’s made of springs and other mechanisms that might display a number that varies from the actual distance travelled by the gage tip. So the term FIM implies that we want to know the distance that the tip actually moves.
It may be semantics, but the world of technical standards is permeated by legalisms, and this was a small change in terminology to avoid the discussion of inherent error in the reading shown on the dial face. Aren’t you glad someone asked…. 🙂
A question came up recently about how GD&T relates to CAD dimensions: The rules of GD&T say that basic dimensions are required when using a position tolerance. But does that always mean that the print must show these “boxed” dimensions?
Answer: No, it doesn’t. While classical GD&T seems to say that the boxed dimensions are required, there are two alternative ways to meet the rule. First, if most or all of the dimensions on a print are to be basic dimensions, then a general note can be placed on the drawing that states “all dimensions shown are assumed to be basic” or something to that effect. This means that the dimension is still shown, but without the box. Caution: if this is the case, make sure that the title block does not show any default plus/minus tolerances.
The other option is one that is becoming more common: Leave the dimensions off the print, and add a note saying something like “all undimensioned features are dimensioned in CAD, and are basic.” To its extreme, this means that the entire print can leave off all dimensions, requiring the reader to have access to the CAD data if they really want to know the dimension. While there are advantages to this (the designer doesn’t have to cram all the dimensions on the drawing), it also assumes that everyone downstream will have access to the CAD data.
Like anything else, moderation is the key. I say that if you can show the dimensions, put them on there (with the box, for basic dimensions). However, if it’s a complex shape, or if you’re certain that everyone can access CAD, then you could go dimensionless. As always, your comments are welcome!
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 state!) 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 at the top of this page 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 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.
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!