Posts by gdtsemin_admin

Sample Senior-Level Questions for ASME Certification in GD&T

Posted by on Sep 6, 2010 in Certification in GD&T

    Several months ago I wrote about the requirements for becoming certified as a “GD&T Professional.”   (View that blog entry here.)    This time, I would like to present some advanced questions. There are two levels of certification for GD&T Professionals: Technologist and Senior. You do not need to be certified at the Technologist level in order to take the Senior level exam.  But you would need to be sure that you are very familiar with all the tricky nuances of GD&T!  (Plus, you must submit a letter of verification that you have been regularly involved with GD&T for at least five years.) So the following are a few questions that might be representative of the more difficult stuff.  Answers are at the end, along with the appropriate paragraph number from the 1994 edition of Y14.5…    Sample question #1: The derived median plane of a feature can best be described as: a. a theoretically perfect plane through the center of a part’s actual mating envelope b. a plane that coincides with the centerplane of the true geometric counterpart c. an imperfect plane passing through center points of all line segments bounded by the feature d. an imperfect plane representing the average height of a surface plate used...

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Tangent Plane Modifier

Posted by on Aug 20, 2010 in GD&T Tips

In my opinion, one of the most underutilized tools in the GD&T toolbox is the tangent plane modifier. It was introduced in the 1994 ASME standard, yet some people still think of it as a new concept.  Shown in the example below, the tangent plane modifier (circled T) can save money by only controlling the high points of a surface, rather than every point: To understand the drawing above, first realize what parallelism controls if no T modifier is given.  Regular parallelism requires that every point on the top of the part be within a tolerance zone of 0.2 mm. This means that regular parallelism inherently controls flatness to the same specification. But there might be times when a designer does not need to control flatness. Perhaps another mating part will contact the top of our part, and we only care about the angle at which the mating part sits.  In that case, we don’t need the surface to be flat within 0.2, since our mating part will only feel the high points anyhow. In that case, the tangent plane modifier makes sense.  It does not give us a “bonus tolerance” as the MMC modifier does with features of size, but it does have the advantage of being more...

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Position with Only One Datum?

Posted by on May 18, 2010 in GD&T Tips

This time around, I’d like to present another “pet peeve” of mine, at least in the world of GD&T.  It involves using the position symbol when the only quality being controlled is perpendicularity. This is very common — it stems from some subconscious notion that if GD&T is going to be used on a hole, it’s got to be the “true position” symbol.  NO! Consider the following example. There is a position tolerance applied to the large hole on the left, and the datum being referenced is A.        But let’s go to the standard and examine how the geometric control called “position” should be used: ASME Y14.5-2009 (and prior editions)  state that position’s main job is to control location — meaning that it involves a distance — and perpendicularity often comes along as part of that position control. Since the large hole given above is already distanced from the edges by plus/minus dimensions, the geometric tolerancing has nothing to do with location. The only relationship that the large hole has with datum A is one of orientation. Therefore, an orientation symbol must be used: Notice the perpendicularity symbol. This is the correct way to identify this hole, since the hole itself now becomes the datum feature...

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Position and Profile for Everything?

Posted by on Mar 2, 2010 in GD&T questions

Occasionally users of GD&T suggest that everything be simplified by just boiling all 14 symbols down to just two or three.  (What, you didn’t know there were 14 symbols? Click here for a handy chart!) There is some logic to what these people are saying — namely, that many GD&T symbols overlap others, and position and profile can be used in such a way as to cover the others.  But as you might guess, there are pros and cons to this. First, realize that position always controls two qualities: location and orientation. Location is obvious, but don’t forget orientation — because position extends all the way through the depth of a feature, it will control any tilt or angling of that feature. Profile of a surface, if used with datum references and basic dimensions tying it back to those datums, can control all four required qualities: location, orientation, size, and form (shape). Since it covers ALL of these, it can be argued that the other GD&T symbols could be ignored and simply use this one symbol (well, two if you count profile of a line). But there are two problems with this minimalist philosophy: For one thing, it may sometimes be necessary to really only control a particular aspect, such as...

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Non-Uniform Profile Tolerancing

Posted by on Feb 8, 2010 in 2009 ASME standard

Another new feature that was introduced in the 2009 standard (ASME Y14.5-2009) is the option of creating a “non-uniform” tolerance zone for either of the two profile symbols. Recall that the profile symbols normally specify a uniform boundary or bandwidth that is centered around the “true” or perfect profile. This true profile is first established by basic dimensions on the drawing or by referencing the CAD model, which is the perfect design.  Here’s a traditional profile callout:   where the tolerance zone looks like:         Notice that the 2 mm zone follows the exact contour of the intended design — this is how profile tolerances have always been understood, and will continue to be understood if no other indication is made.  But the latest version of the Y14.5 standard allows a non-uniform zone, where the feature control frame simply says “non-uniform,” but it is then required that the zone be described in detail on the drawing or by referencing a note or other detailed information. An example: Notice that each side of the tolerance zone has a different radius; the surface of the actual manufactured part can now deviate anywhere within these two curved planes. There may be various reasons why the designer wishes to do...

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