I hope everyone has had a great summer. Here’s a topic that will be helpful even to seasoned experts in GD&T, and it kind of follows the previous blog entry…
Often, when discussing the finer points of GD&T with others, we end up going to the official standard (or standards) that pertain to dimensioning and tolerancing in order to seek guidance. But if you’ve been in the real world, you know that a technical document can’t capture every possible scenario.
So we naturally look for the example or description in the standard that is closest to our real-world situation. However, we have to make a decision whether we can make the leap of logic to say that a proposed design is still within the “spirit” of the standard. This can sometimes be a sticky point!
An aside: The two major standards when it comes to GD&T are ASME Y14.5 and also the ISO series of standards (ISO has an umbrella of several standards that embrace GD&T, not just one book). The predominant standard in North America — and the one I’m most familiar with — is ASME. In some ways the two standards have different philosophies about the depth of coverage: in some areas ASME tries to nail down every option, and in other areas ISO takes the harder line.
There are those who would say that we must make our designs conform to the exact letter of the law, and any practice which is not described in the standard (or an obvious modification of one given in the standard) is not to be used. But others espouse more leeway and say that the concepts given in the standard can be extended to many other areas that might have been unforeseen by the standard writers.
I put myself into the latter camp. The key is to look carefully at the wording of the standard: if the words “shall” or “must” are used, then the door is pretty much closed to bending that principle. But if no prohibition is given — or better yet, if the verb used is “may” — then there are probably other ways to practice the given idea and still be in conformity to the standard.
One concept that can serve as an example is the “tangent plane” modifier. When introduced in the 1994 ASME standard, this modifier was shown for use on the three orientation symbols (when applied to surfaces). Eventually, someone was bound to ask whether the tangent plane idea could be used with profile of a surface. The standard never said this cannot be done, and so my vote would be that it’s OK. Others said, nope, it wasn’t in the standard, so tangent plane wasn’t to be used on profile of a surface. In the 2009 standard, the notion of extending “tangent plane” to profile of a surface is now clearly allowed; although no specific example is given, they added a footnote saying that the concept of tangent plane is equally applicable to “other geometric characteristic symbols where the feature is related to a datum(s)” (see page 103 of the current edition of Y14.5).
Bottom line: GD&T is a language. And like any language, there are certain rules that must always be followed. However, there are many ways to patch together different parts of that language and still say something clear. We shouldn’t be too legalistic and limit ourselves to designs that are only identical to the examples given in the standard. Obviously, this is where training and knowledge of the fundamentals of GD&T are necessary in order to know when the envelope is being pushed to far!