When using functional gaging (ASME Y14.43) designed for verification based on virtual condition (absolute policy), the use of zero tolerance on the workpiece is the preferred combination. The range for the zero tolerance part (right side of the graphic) is equal to the virtual condition and will allow acceptance of all parts that meet the hole size and location requirements. 260, regardless of its location, should be rejected. In the example seen in figure 2, for the nonzero toleranced part (left side of graphic), the range of acceptance falls off at the MMC hole size, which is far short of the virtual condition. Manufacturing may not warm up to this idea right away, but when they achieve greater tolerance with this method, they should come around and may even request it. Remember, all the tolerance comes from the bonus, and with no bonus you get no tolerance at all. Caution: Do not apply regardless of feature size with zero tolerance. Virtual condition remains unchanged for both methods. This little change allows acceptance of the full range of parts that will assemble and meet the hole size and location tolerance. (See the example in figure 1.)Īs you can see, the change to zero tolerance isn't difficult. 270 diameter, and the location tolerance is zero at MMC. In other words, we have modified the MMC hole size to equal the MMC fastener diameter. Now we have a value of zero that the MMC hole minus the MMC fastener must satisfy: H(.250) - F(.250) =T(0). We're going to modify the formula slightly to read T(0) = H - F. 250) aren't allowed regardless of their location. The problem with this method is that holes less than. Both H and F will be the MMC value of the respective features. 270 (all figures in inches) and a fastener diameter of. We can see this demonstrated by the formula shown in the ASME Y14.5 (GD&T) standards, where tolerance = hole - fastener, or T = H - F.Īs an example, let's assume a hole diameter of. When using geometric dimensioning and tolerancing (GD&T) and talking about position tolerances, the greatest allowable variation actually comes from the application of zero tolerance on a maximum material condition (MMC) basis. With tolerances, one might think that a large value in a feature-control frame is better than a small value. We've been led to believe that more is better-more speed, more filled seats per flight, more miles per gallon, more inspection tolerance and so on.
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