# GD&T Tips

## 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...

## 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...

## 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...

## A Little-Known Trick in the ASME Standard

Posted by on Oct 12, 2009 in GD&T Tips

While 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...

## When a Supplier Doesn’t Understand GD&T

Posted by on Sep 21, 2009 in GD&T Tips

Several 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...