Choosing the Size Tap to Maximize Tool Life And Meet the Required Class of Thread

Different classes of thread, frequently called class of fit, determine the tightness of mating internal and external threaded parts. It’s essential the two parts assemble to a degree of tightness for the use of the assembly. Parts that are frequently assembled and disassembled typically are designed with a loose fit, while an assembly subject to extreme vibration will require a much tighter fit.

(Click Image to See Full Size)

The vast majority of manufacturing uses the Unified Thread Series with the designation for the internal thread of 1B, 2B or 3B with counterpart for the external thread of 1A, 2A or 3A. The first two designations are used for quick and easy assembly, while the 3B and 3A tolerances usually indicate accuracy of thread form and lead are important.

Manufacturers of taps produce their tools to specifications for pitch diameter in .0005 increments known as “H” or “L” limits measured in relation to the basic pitch diameter. Only in rare cases do taps cut to the exact size, more often, based on material being tapped and the speed of the spindle, they cut oversize. Class of thread 2B is the most common and has a tolerance about 25% greater than a 3B Class of Thread.  Some confusion occurs because the numbers 1, 2 and 3 are used in both systems, but do not have an exact relationship.

If you know the cutting characteristics of the material from past history, choose the highest “H” limit that will produce a thread meeting the Class of Thread required. More common practice is to choose a middle of the tolerance to insure you don’t cut oversize. Either method is acceptable; however, going with the highest “H” limit tap to produce to specifications will always ensure the maximum tool life.

Regal Cutting Tools offers a variety of H limits in all styles of standard and popular special taps. Contact us today to learn more about our offering and which is right for you!