![]() ![]() ![]() These tolerances have been observed through many years of experience. The tolerance for each manufacturing process is usually figured as the range incorporating three standard deviations from the mean. This variation usually follows a normal distribution. There are many sources of variation, such as stock size or blank thickness, tool wear, process, temperature, or the machine operator. Tolerances are due to variation in the manufacturing process. Obviously, reworking or scrapping parts adds to the component's cost and reduces the throughput of the assembly line. If it can't be reworked, then the part must be scrapped. If it is bad, the part needs to be reworked before it can be used. If the part is good, it will meet its function and fit appropriately in the assembly. Tolerances allow you, the engineer, to describe what a "good part" is and what a "bad part" is. Unfortunately, when the parts are made, they are never exactly that size - features are a little larger or smaller, surfaces aren't exactly flat, and angles aren't always perpendicular. When you design a part or assembly in CAD, the design is created at a nominal condition. Why do we need to assign tolerances to our design? This should balance all of the aspects of part design: fit, function, form, and fee/cost. ![]() Tolerancing is the process of setting the allowable variation in a part. You use tolerances on a drawing to communicate the definition of what is a usable part and what is an unusable part. Proper selection of tolerances accommodates the reality of manufacturing while still maintaining the part or the assembly's ability to function and to fit with mating parts. If the holes were not at their MMC the GD&T tolerance would be given bonus tolerance, effectively increasing the position tolerance of the holes.Tolerances are necessary due to variations in the manufacturing process. In this case, the maximum material condition for the holes would be at 2.5, or the smallest hole. The boxed symbols can be read “the position of these two holes may vary within a cylindrical tolerance zone of 0.1 relative to datums A, B, and C when the holes are at their maximum material condition”. An example use of the MMC symbol is shown below. The MMC and LMC symbols are, respectively, the letter M or L inside of a circle. In contrast, least material condition (LMC) refers to a feature of size containing the least amount of material, yet remains within its tolerance zone: In GD&T, maximum material condition (MMC) refers to a feature-of-size that contains the greatest amount of material, yet remains within its tolerance zone. ![]()
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