Cutting Tools Chicago aka General Cutting Tools along with Greenfield Tap and Die explains the differences between H limits and D limits. H limits are a sequence of numbers in .0005″ increments larger than the minimum limit of the part, starting with H1. An H1 limit is one .0005″ increment larger than the minimum limit or GO gage, an H2 is two .0005″ increments (or .001) larger, an H3 is three .0005″ increments (or .0015″), and so on. The recommended H limit for a particular size should be approximately 40% of the thread tolerance larger than the minimum limit of the thread.
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. First, D limits are slightly larger than H limits. Second, the tap manufacturing tolerances are different. 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 3/4” tap.