H Limits and Classes of Thread

We discussed selecting the recommended “standard” H Limit for the various classes, 2B, 3B, etc. However, there are situations when larger or smaller H limits would be more appropriate.
Larger H limits: The primary reason for using larger H limits is for plated or heat-treated parts. In both cases the thread must be made larger (and sometimes oversize) to accommodate the reduction in size due to the plating thickness or heat shrinkage.
Selecting an H limit for plated parts is relatively easy. Using the plating thickness, the oversize pre-plate thread dimensions can be calculated. Then, using the “40% rule”, the H limit for the oversize thread can be calculated.
Selecting a larger H limit for heat-treated parts is more difficult and generally requires trial and error due to the fact that different materials and part configurations shrink by varying amounts.

Another reason for larger H limits is part configuration. If the part is very thin walled, the tap may expand the part when cutting, but will close in once the tap is removed. Again, trial and error is required to find the appropriate H limit size.

The final reason for using larger H limits is extremely important but seldom used. A larger tap will allow for more tool wear, producing more parts! When the tapping operation is very consistent, the largest H limit that will provide good gauging with the NOT GO thread plug can be used for greater productivity.

Smaller H Limits: Generally, the only time a smaller H limit is used is to overcome problems with the process that would result in an oversize thread, such as runout, misalignment, or improper feed of the tap. However, this technique is usually used as a temporary “band-aid” fix, until a permanent solution can be employed.
Also, small or undersized H limit taps are occasionally used for parts that require a tight or smooth, slop free fit between the mating threads.