Dimensions with Datum Targets
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.
GD&T Instructor Wins SAE Award
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.
GD&T Training — On-Site vs. Public Courses
When investigating training in the topic of geometric tolerancing, one thing you’ll probably notice is the various options for the format of a class. The two major types are public classes, sometimes referred to as “open enrollment,” and on-site training. There are advantages to each. A public class is ideal for individuals seeking to learn GD&T. Some companies may only have one or two people that need training, and sending them to a public class is a great way to deepen their knowledge (and it’s nice to get a couple of days out of the office). An on-site class is dedicated to a company or a group within that company. This has two distinct advantages:
- First, because an on-site is targeted to one group, the instructor is able to customize the presentation. I always invite participants to bring sample drawings to class to help generate discussion and get them to think more about the correct application of GD&T to their actual parts.
- The second advantage is cost. The on-site package is usually less expensive as compared to individual tuitions at a public course, although there is a threshold number (usually around 5 or 6 people) needed to make the savings apparent.
If you are considering any type of GD&T training, keep these options in mind. Most of the training I conduct is on-site seminars. But we do offer several public courses each year. The public class is three days, and the cost is $995 per person. The schedule for 2008:
- March 31-April 2 Boston area
- June 9-11 Los Angeles area
- July 7-9 Detroit area
- October 20-22 Austin, Texas area
Please contact us about enrollment information for these classes or to schedule your own on-site seminar.
What Is Resultant Condition?
If you are familiar with GD&T, you’ve probably heard of “virtual condition.” It is a number that represents a worst-case combination of a feature’s size along with its geometric tolerance. For instance, suppose we have the following example of a hole, with a size tolerance of 6 ± 0.2 mm:
The worst case for assembly purposes would be the smallest hole that is also out of position: 5.8 – 0.3 = 5.5. This virtual condition represents the “usable” area that the mating pin must fit within:
Now, virtual condition is usually easy to identify with — think of assembling pins and holes together. But sometimes I may be concerned about the outside boundary, created by the largest hole that is also off center:
This outer boundary, called resultant condition, is calculated as: LMC + stated geo tol + any bonus, or: 6.2 + 0.3 + 0.4 = 6.9 mm. This represents the area where any portion of the hole’s edge may possibly fall.
This resultant condition is not of concern when dealing with assembly of holes and pins! But suppose we are simply punching a drainage hole in the bottom of a drip pan. Nothing will be assembling through this hole, but my concern now is that the hole’s edge not be too close to the edge of the sheet metal.
So be aware of both virtual and resultant condition formulas, for internal and external features.
It’s Just Semantics…
There’s more to learning GD&T than just knowing the symbols. There is also a unique vocabulary — sometimes GD&T terms mesh with everyday usage, but sometimes a word can have a very specific meaning in geometric tolerancing.
Here are some examples:
- The symbol circularity is sometimes called roundness. The two terms are synonymous, but the ASME standard uses circularity.
- The most common GD&T symbol is position. People sometimes call it “true position,” but the ASME standard simply calls the symbol “position.”
The tricky ones, however, are concentricity and symmetry:
- While the words concentric and symmetric are sometimes used in a generic sense, the GD&T symbols for these are quite specific, and are often discouraged. (See below for an earlier blog entry on concentricity.) Also adding to the confusion is the fact that the ISO standard, which most other countries follow, defines these two GD&T symbols in the more generic sense.
- Runout and profile are sometimes used as general terms. But there is no GD&T symbol for “runout” or “profile.” Instead, those are categories which are each divided into more specific symbols; a print will show circular runout or total runout, along with profile of a line or profile of a surface.
- Also, there may be slight differences in pronunciation. For instance, datums can be pronounced with a long or a short “a” sound. I catch myself using both!
Finally, if you write about GD&T in a word processing program, you’ll find that the spell-checkers don’t recognize many of these terms! As I write this, the computer has flagged “tolerancing,” “runout,” and “datums” (I guess it thinks the plural of “datum” is always “data.”)
Admittedly, this is minor stuff, but it’s good to be aware of these things!
The Mysterious Plus/Plus Tolerance
Here’s an interesting tolerance question: Have you ever seen a plus/plus tolerance? (or minus/minus?) The ASME standard does not mention this, so a purist would say that it is illegal (see paragraphs 2.2 and 2.3 of the Y14.5 standard). But I’ve seen examples such as a hole dimensioned with:
The real problem is that, with a casual glance, you might not realize that a part made at the “nominal” size of 1.25 is a bad part! (The size limits are 1.252 – 1.255.) So the obvious question is: Why did the engineer (or designer) choose this odd method of expressing a tolerance?
The most likely answer is because there is some desired fit with a mating feature. Depending on the assembly, they may want a press fit, transition fit, or clearance fit. So if the example given above is a hole, then a pin in the mating part may be dimensioned with a minus/minus tolerance. In this way we can say that the pin and hole each have a nominal size of 1.250 inches, but the respective tolerances will ensure that there is always a little looseness (clearance fit).
In the metric system, an entire method of coding different limits and fits has been developed. A nominal size would be given, along with a letter and number. For example, 16D8 corresponds to a hole with a nominal size of 16 mm, but with actual limits of 16.050 to 16.077 mm (I had to look this up in a table!). The “16D8” is usually not understood in American design usage, so if a design is being converted from another country to an American program, the engineer usually translates it into an odd-looking “plus/plus” tolerance:
(Similarly, external features use a lowercase letter, so a corresponding hole might be 16d8, which corresponds to 15.923 to 15.950 mm.)
For more information on the code letters and numbers for this ISO usage, consult a Machinery’s Handbook or the ANSI standard for “Preferred Metric Limits and Fits,” ANSI B4.2-1978.
What’s the Best Tolerance Stack Method?
When it comes to calculating tolerance stacks, there are many different methods. Some people simply take blank paper, then make a quick sketch and scribble out some numbers. While that may work for a very simple stack, it’s obviously not a very methodical approach! A better way is to use Excel or even special software for tolerance stacks to calculate an answer.
Even when using Excel, there are two schools of thought for performing a stack:
- Some folks prefer to use a stack method that separates everything into an absolute maximum or minimum dimension, and then create two columns to stack the max and min results.
- Others insist on translating everything to a nice symmetric plus/minus tolerance, centered around a nominal.
Both methods will work — it often comes down to how you were taught to perform stacks. Here’s my two cents’ worth: I prefer the max/min method, especially when GD&T is involved. Because geometric tolerancing is based on limits (think about MMC and bonus tolerance) it is usually easier to plug those into a spreadsheet.
For more information check out the Tolerance Stacks course that we offer. Most of the class is devoted to this max/min method, but we also teach the plus/minus method as an alternative. We also have an Excel template available on our “free downloads” page.
Getting Certified as a “GD&T Professional”
For those of you who actively use GD&T on a daily basis, you should be aware of the certification process for GD&T Professionals.
The American Society of Mechanical Engineers (ASME) has established a credential for GD&T proficiency, called GDTP, which stands for Geometric Dimensioning and Tolerancing Professional.
It is a testing process that measures your ability to interpret and apply tolerances correctly.
There are actually two certification levels: the Technologist Level, which measures your ability to read GD&T; and the Senior Level, which tests not only interpretation, but also the application of GD&T to a design. One does not necessarily need to become a Technologist first; some people go right for the Senior Level, although they are required to document at least five years of experience in GD&T (and they have a more difficult test!).
If you decide to pursue either level, be aware that the questions on the test can be taken from any section of the Y14.5 standard. Each chapter has a “weight” that determines the number of questions from the chapter that appear on the test. This means that there is more than just GD&T; you need to be familiar with the nuances of traditional tolerancing as well as the many definitions contained in that standard. Some questions representative of the Technologist Level:
Sample question #1:
A dimension “not to scale” is symbolized by:
a. placing the number in parentheses
b. placing a line under the number
c. including NTS after the number
d. italicizing the number
Sample question #2:
If a datum feature of size is applied RFS, a datum displacement is:
a. not allowed
b. allowed at MMC
c. allowed at LMC
d. allowed at the resultant condition
Sample question #3:
Which of the following datum feature symbols is incorrect?
The GDTP process is not for everybody, but if it’s something that might interest you, contact us for some guidance or you may visit ASME’s website for more detail:
http://www.asme.org/Codes/CertifAccred/Personnel/Y145_Geometric_Dimensioning.cfm
What’s the Deal with Concentricity?
Here’s a question I received at the beginning of a recent GD&T training session. Often I’ll ask participants if there is anything in particular that they are looking to get out of the training. One gentleman didn’t hesitate:
“I’m here to learn why we shouldn’t use the concentricity symbol. No one has been able to tell me why I’m not supposed to use that symbol! Our drawings show a runout tolerance instead, but I’ve never heard a good answer why.”
First, what this person was told by others is indeed true: the concentricity symbol is often discouraged, and another choice such as position or circular runout is usually better. To answer the question, we must analyze how the terms are defined.
- A dictionary definition describes concentric as “having a common axis or center.” That is what we often require for parts such as a camshaft or guide pin holes, right? But the problem is in how we find that axis or center. There are different ways to derive an axis.
- The definition of concentricity in the GD&T standard (ASME Y14.5M-1994) is given in paragraph 5.12. “Concentricity is that condition where the median points of all diametrically opposed elements of a figure of revolution are congruent with the axis (or center point) of a datum feature.”
Now, place yourself in the shoes of an inspector. To check concentricity as defined in GD&T, we must perform a surface analysis at each cross section (or as many cross-sections as practical) to derive a series of median points. The result is an imaginary cloud of points that must all lie within the given tolerance zone. This obviously can be quite time consuming! If the tolerance symbol were simply changed to circular runout, the tolerance zone would be applied around the circumference of the part, which is something that can be physically gaged without getting involved in the tedious math required by concentricity. (The other alternative of position derives an axis from the mating envelope of the part, not each cross section.)
Concentricity is a legitimate choice for some applications where the main concern is equal mass distribution around the center. But for most mechanical parts, you should avoid concentricity. In fact, the automotive companies have explicitly instructed their designers and suppliers to avoid this symbol, along with symmetry, which involves the same concept for planar features.
Because of all this, GD&T folks sometimes use the more ambiguous term “coaxial” in discussion. This term conveys the general idea, without using the specific word concentric. But on a print, be very careful which symbol you choose!
Comments? Ideas for a future blog entry? Let me know!
Welcome to the GD&T Blog!
In this blog we’ll feature some common areas of discussion in GD&T, ranging from the technical nuances of the GD&T symbols to how to convince others that geometric tolerancing does not make a part more expensive to manufacture. You’ll also find out information about our upcoming seminars and updates on revisions to the Y14.5 standard.
Your comments on this are welcome, as well as suggestions for future blog entries. Do you have a sticky GD&T question? Or a real-world example of confusing callouts? Please send me an email at belanger@gdtseminars.com.
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