OK — time to dive into another item that is new in the world of geometric tolerancing. The standard that was released earlier this year expanded the definition of a “feature of size.” This has an impact in that some GD&T symbols can only be applied to “features of size,” most notably position. The 1994 standard defined a feature of size as a single entity: “One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces….” This is just a fancy way of describing things like a hole, pin, a part thickness, or other feature that can be measured directly for size. A traditional “feature of size” The 2009 standard now breaks “features of size” into two categories: regular and irregular features of size. The regular feature of size keeps the definition given above. The irregular feature of size is new: it is defined as “a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope….” Thus, a grouping of objects can now form a “feature of size.” This allows a geometric tolerance to be applied to the group as if it were one feature. In the example below, I can position the imaginary circle with GD&T,...
Learn MoreWhile everyone is ooohing and aaaahing over the new GD&T standard released in April, there is a rule that dates back to 1994 that very few people know about. And although it’s not something that would be used very often, it’s something that might be worth filing in the back of your brain… This little-known rule is the idea that the variable tolerance created by an MMC or LMC modifier — often called “bonus tolerance” — can have a limit placed on it. Most GD&T people are comfortable with the notion of bonus tolerance, but recall that the maximum amount of bonus is completely dependent on the tolerance allowed for size. Here’s an example: The position tolerance of each hole is going to be 0.2 at a minimum, and this happens when the hole is manufactured at 10.8 mm. If you manufacture a hole at 11.4, then the allowable position tolerance is 0.8 (this is the original tolerance of 0.2 plus the bonus of 0.6, which comes from the spread on the hole’s size). Now suppose that the designer wants to allow extra position tolerance with the MMC modifier, but for some reason can’t let it grow to 0.8. (Maybe there is concern about having a hole drift...
Learn MoreSeveral times I have heard that a designer is hesitant to use GD&T because he knows that the manufacturer will not understand it. There are several ways to answer this dilemma: Too bad; the burden is on them to learn it Use traditional tolerancing even though it lacks the benefits of GD&T Use a hybrid approach, and make yourself available for guidance if they have GD&T questions Without sounding callous, the best answer is probably the first option. Even small machine shops go through the process of becoming certified in ISO, so why shouldn’t they be fluent in GD&T? They will be handicapped in their business by not knowing this important tolerancing system. I don’t know if this is true or a tall tale, but one engineer at our seminar said that he once worked at a small manufacturer that, when bidding on a new job, would count the number of GD&T callouts and multiply it by a cost factor. The thinking was that more GD&T callouts meant a more expensive product to produce! Needless to say, that is nonsense. Of course, you may want to gently ease into the GD&T waters if you know that a supplier isn’t comfortable with it. (You might not want to throw a...
Learn MoreOut there in the GD&T world, there is often confusion about parts that have irregular shape. We are told that the theory of GD&T requires datums to be 90º to one another. Sure, that’s great in a textbook where the examples are nice, rectangular, flat plates! But what about those other shapes? It’s actually very easy. The confusion is that people mistake the term “datum” for “datum feature. The standard defines a datum as a theoretically exact point, axis, or plane. But a datum feature is defined as a physical portion of the actual part from which the datum is derived. Think about those two terms, and you’ll see that irregularly shaped parts pose no problem. Even something shaped like a blob or a potato chip has a physical surface. It may require using datum targets, but a theoretical plane can still be constructed from those targets. So again, it’s true that the theoretical datums mentioned in a feature control frame are perpendicular to each other. But those theoretical datums can be derived from any crazy-shaped surface. If you have the new 2009 ASME standard, see pages 81-90 for some neat examples. (If you have the 1994 edition, see pages 54 and 78-79.) Stay tuned...
Learn MoreIf you are a regular user of GD&T, you probably know that the ASME standard was recently revised (for details, see the blog entry below dated March 28, 2009). In today’s column, I’d like to introduce you to one of the changes. The term “maximum material condition” or MMC has been around for a long time. This concept is invoked when the circled M symbol is placed in a feature control frame. Well, a new item for the 2009 standard is something very similar, called “maximum material boundary,” or MMB. Yet it is invoked using the same circled M symbol. The reason this was introduced was to eliminate confusion when the M symbol is modifying a datum that has its own geometric tolerance. Consider the following example: The position tolerance references datum feature B with the M symbol. But here’s the key: think about the size of a gage pin that would be inserted into the center hole (it should be attached to a flat plate that simulates datum A). It would not really be simulating the MMC of 22.0; instead, it would be 21.8 in order to accommodate the perpendicularity tolerance of 0.2. Thus the confusion — people would say “MMC” when discussing the datum, but...
Learn MoreMeet The Expert
Watch John-Paul talk about the benefits of GD&T training courses.