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Title Block Tolerances and GD&T

Posted by on May 13, 2008 in Uncategorized

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...

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History of GD&T

Posted by on Mar 25, 2008 in Uncategorized

Instead 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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GD&T Instructor Wins SAE Award

Posted by on Feb 25, 2008 in Uncategorized

SAE International (the Society of Automotive Engineers) has recently announced its winners for this year’s awards, and among them is John-Paul Belanger, from Geometric Learning Systems. Mr. Belanger is receiving the Forest R. McFarland Award for outstanding contributions toward the work of the SAE Engineering Meetings Board in the planning, development, and dissemination of technical information through technical meetings, conferences, and professional development programs. This is in recognition of his years of involvement in GD&T training for SAE to their network of members and clients. “I am proud of my membership in SAE, and am happy to be able to work with them as an instructor in GD&T. They serve as a valuable resource for the automotive industry, and I am grateful for this recognition.” John-Paul has been involved in training for GD&T and Tolerance Stacks for over fifteen years. He is a principal for Geometric Learning Systems, a consulting firm specializing in GD&T...

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The Mysterious Plus/Plus Tolerance

Posted by on Jan 26, 2008 in Uncategorized

Here’s an interesting tolerance question: Have you ever seen a plus/plus tolerance? (or minus/minus?) The ASME standard does not mention this, so a purist would say that it is illegal (see paragraphs 2.2 and 2.3 of the Y14.5 standard). But I’ve seen examples such as a hole dimensioned with:      The real problem is that, with a casual glance, you might not realize that a part made at the “nominal” size of 1.25 is a bad part! (The size limits are 1.252 – 1.255.) So the obvious question is: Why did the engineer (or designer) choose this odd method of expressing a tolerance?    The most likely answer is because there is some desired fit with a mating feature. Depending on the assembly, they may want a press fit, transition fit, or clearance fit. So if the example given above is a hole, then a pin in the mating part may be dimensioned with a minus/minus tolerance. In this way we can say that the pin and hole each have a nominal size of 1.250 inches, but the respective tolerances will ensure that there is always a little looseness (clearance fit). In the metric system, an entire method of coding different limits and fits has been developed. A nominal size would be...

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