In the geometric tolerancing system, basic dimensions are used to override general tolerances (sometimes called title block tolerances). But let’s investigate these general tolerances a little more closely. A sample tolerance block is shown below, as taken from a drawing using metric (millimeters). First, notice that the tolerance allowed depends on the number of digits used after the decimal. This is common practice; at other times the tolerances may be divided based on the size of the dimension (1 to 10 mm, then from 10 to 50 mm, etc.). In our example, a separate tolerance is given for angles. Some companies are trying to move away from these title block tolerances. It may be because they want to define everything with GD&T or other direct methods. While that might be OK to some extent, I would be hesitant to eliminate the entire idea of general tolerances, for one specific reason: the angular tolerance. Recall the old drafting rule that 90 degree angles are implied; they do not need to be dimensioned. But if the general tolerance block is removed, these 90 degree angles — unless they have GD&T applied — will have no tolerance!So in your efforts to improve drawings and streamline your designs, don’t go overboard. Title block...
Learn MoreMany of you may know that GD&T has been around for a long time (see an earlier blog entry about the history of this system). And like anything else that’s been around for a while, things sometimes change.The current American national standard for GD&T is maintained by the American Society of Mechanical Engineers (standard number Y14.5M-1994). Prior to 1994, the previous edition was dated from 1982 (and 1973 before that). And each time, there were a few things that were changed in the rules and symbols of GD&T. So, if you look at the history of the standard, you might guess that it’s about time for another revision — and you would be correct! The Y14.5 committee has been hard at work for the past couple of years, and the next edition is in the review phase. (One sneak preview for you GD&T geeks: a new modifier to indicate unilateral profile tolerances.) It is projected that it will be released for general use in the first part of 2009. As a side note, this underscores why it is important to always mention the specific tolerancing standard in the general notes of your prints, or even as part of your company’s standard title block. If this is not mentioned, then...
Learn MoreHere’s another common question that comes up in a GD&T class: Suppose we are applying a position tolerance to a hole. It would typically have a diameter symbol in front of the number, as in the first example below. But what if we omit the diameter symbol in front of the number, as in the second example? Would the shape of the tolerance zone still be assumed as cylindrical? Answer: No, it’s not cylindrical. Without a diameter symbol, a tolerance zone defaults to two parallel planes (unless the BOUNDARY concept is invoked). So the second drawing above is ambiguous; the zone will be two parallel planes, but we don’t know the direction of those two planes. Two solutions: First, if the intent is to control the position in all directions, you must add a diameter symbol as in the first example above. Second, if we really intended two parallel planes, we must graphically indicate the direction of those planes: Now, it is clear that we are controlling position in the left/right direction. Of course, that means that there is no position control in the up/down direction. So let’s take it one more step: This example creates two sets of parallel planes — one in the vertical direction and...
Learn MoreColleagues and friends often ask me what the busiest time of the year is for a technical training company. After teaching GD&T full time for nearly 15 years, I can honestly say that there is no specific cycle that shows up regularly. Of course, training is rarely scheduled at the very end of the year (Christmas holiday, and a busy time in general). But sometimes November and the first part of December are very busy, because a company or department may have to use up budgeted training dollars before the end of the year. And you might think that the summer months are not popular for training, but they usually are, as long as it is scheduled far enough in advance. (I soon discovered that those companies in areas where hunting is popular usually avoid scheduling seminars during those times in the fall when many employees may be perched in a tree!) This is not to say that every month is busy — like any industry, we feel ups and downs. But from year to year, those ups and downs are rarely in a repeatable pattern. It’s a funny thing; obviously the economy drives much of it. But a slow time in the economy can sometimes be an ideal...
Learn MoreInstead of discussing some technical point this time, let’s take a brief look back at the history of GD&T. Some people may think that GD&T is “just the latest fad” (I actually heard someone refer to it that way) and therefore they are implying that it’s not worth learning, since it may soon go away. But the facts show that GD&T has been around for a long time (50+ years), it applies tolerances in a logical and standardized manner, and it saves money — all reasons why it’s not going to fade into the sunset. Supposedly, the story goes that a guy named Stanley Parker came up with the first GD&T concept having to do with position (or “true position” if you prefer). The time was World War II, and the location was Great Britain. As you might imagine, during wartime deadlines are critical, and Mr. Parker ran into a situation where some torpedo parts inspected according to traditional tolerances were rejected. But it turns out that they were actually functional parts, and those parts were sent on their merry way even though they didn’t seem to be to print. He traced the discrepancy to the fact that traditional X-Y tolerances result in a square tolerance zone, but that...
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