D limits are similar to H limits and follow the same numbering system. However, H limits are used primarily for inch threads, while D limits are used exclusively for metric threads. As is the case with H limits, D limits are a sequence of numbers in increments larger than the minimum limit or GO gage, but the increments are .013mm or .00051. Starting with D1, a D1 is one .013mm increment larger, a D2 is two .013mm (or .026mm) increments larger, and so forth. As with inch threads, the 40% rule is applied based upon the tolerance or class of thread.
There are two primary differences between H limits and D limits. The first is very subtle. As we have discussed, H limit numbers are in .0005 increments, while D limits are in .013mm increments, which are slightly larger than .0005. The second difference is much more dramatic. An H or D limit number defines the maximum size of the tap. A tap manufacturing tolerance is then applied to establish the minimum limit. For H limit taps the tolerance is minus .0005 for all sizes through 1 diameter. Tap tolerances for D limits are larger for all thread sizes. The smallest tolerance is .015mm (.00059) and becomes larger with the diameter. For example, an M3 has a .015mm (or .0006) tolerance, an M6 has a .020mm (or .00079) tolerance, and an M20 tap has a .041mm (or .00161) tolerance. That is over 3 times the amount of tolerance of an equivalent Ύ tap. As you can see from these examples, the tolerance allowable becomes quite large.
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