Posts by gdtsemin_admin

Words Are Important

Posted by on May 26, 2012 in GD&T questions

Ah, yes.  I remember as a child being told that words are important!  And that is certainly true in the world of geometric dimensioning and tolerancing.  So many people think that GD&T is just a matter of learning the symbols, and it’s true that that is a key part of understanding the language.  But behind the symbols are many rules, acronyms, and definitions that can make a great difference if they are not fully understood. One of the most significant examples is the confusion about the term concentricity. To a casual beginner, the word “concentric” sounds like a simple idea: two or more circles that share a common center.  But in the world of GD&T, concentricity has a very specific meaning that is more specific than what you’ll find in Webster’s Dictionary!   FYI — the same confusion applies to the symmetry symbol. (For more about concentricity, see this blog entry from a couple of years ago.) Here are a few other miscellaneous terms to be careful with: Datum — Technically, a datum is a perfect plane, axis, or point (or combination of these).  So when talking about the actual surface of a part, we shouldn’t call it “datum A,” because that surface may be imperfect: slightly concave, convex, etc.  The...

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Another New Symbol in GD&T

Posted by on Dec 21, 2011 in 2009 ASME standard

If you’ve been keeping track of the new GD&T standard, then you’re probably aware of most of the bigger changes. (Yes, I know that 2009 doesn’t sound “new,” but most people still call it the new standard since it takes a while for companies to switch to a new dimensioning standard.) The new item I want to show you is pretty easy.  It is called the “all over” symbol, and it is very similar to “all around,” which may be familiar to you.  Both of these symbols will be found with feature control frames that use profile of a line or profile of a surface.   Here’s an example of the “all around” symbol, which has been in use for many years: The “all around” symbol is the small circle on the elbow of the leader line for the GD&T feature control frame.  This means that there is a profile zone imposed around the entire perimeter of the part, but only in the left-hand view. It doesn’t cover the two large faces of the part (this is why the 30 mm dimension still has a ± tolerance on it).  Here is the same “all around” profile zone shown in yellow:   OK, but now let’s look at the new...

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How Literally Should We Take the GD&T Standard?

Posted by on Aug 30, 2011 in GD&T questions

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...

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Is There Always a Right vs. Wrong Way in GD&T?

Posted by on Mar 30, 2011 in GD&T Tips

Sorry that it’s been a while since my last blog post!  That simply means that our training schedule has been busy.  Though a good portion of the U.S. economy is still sluggish, I have definitely seen an uptick in the number of requests for GD&T training.  So remember, if you have a group of 4 or more people that need the basics or a refresher in GD&T, don’t hesitate to drop us a line or call for a customized price quote for a group class. Today I should begin by answering the question posed in the title of this blog entry:  No — there isn’t always one right way to tolerance something.  Recall that GD&T is a language, and like any language there may be more than one way of accomplishing something. People often criticize prints for “bad” or “misapplied” GD&T.  And there are certainly many examples of that.  But many times what people are calling “bad GD&T” is simply a different way of doing something! That being said, however, I’d like to present the most common GD&T mistakes which are definite no-nos. Here are my “Top 5” of the most common GD&T errors: —Failure to show a diameter symbol in the feature control frame (if a cylindrical zone is desired...

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The Importance of Blueprint Reading

Posted by on Jan 5, 2011 in Uncategorized

This website and blog naturally focus on GD&T, but it’s a good time to discuss the importance of simple print-reading skills as a prerequisite to learning GD&T.  As I travel around teaching classes on GD&T, you’d be surprised how many people don’t fully understand some of the simple rules of drafting, view layouts, and notation on drawings. First, note that there can be different terms for this skill; the title of this blog entry mentions “blueprint” reading, but nobody uses actual “blue” prints anymore.  (This name was given because at one time they really were blue, due to the chemical process used in producing these drawings; see here for more on the history of this.)    I suppose a more proper term today would be an “engineering drawing” but if you want to call them blueprints still, hey, go ahead. If GD&T is to make sense, then the object being toleranced must certainly be understood first.  Most drawings use “orthographic” projection, which is simply a fancy name referring to the straight-on, flat view of a part from a particular angle.  Think of a cube: each of the six sides can be flattened out to display six orthographic views.  Depending on the part, there may be fewer or more than six orthographic...

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