Creating internal pipe threads offers many challenges. One challenge is the fact that when the tap reverses to exit, the tap must back up over the raised cut chips still in the lands and flutes that it just created. Space for those stored chips is either sufficient or insufficient. If not sufficient, the risk is chipping the tap threads, stripping them off altogether or in extreme cases, catastrophic failure of the tap.
In an effort to create more storage space for cut chips, an interrupted thread pipe tap was created. In this design, every other tap thread (behind the chamfer) is totally ground away in a separate operation.
The missing every other thread obviously creates extra chip space. On adjacent lands, the thread in the same space on the land is not ground away, but rather the adjacent one, to insure correct rows of threads are left to cut. Thus, interrupted thread taps always contain an odd number of lands, five lands is typical.
Interrupted thread taps also reduce friction, allow more space for lubricants and lessen the appearance of “stop marks” typical of pipe threads. One possible negative aspect is that removing every other thread does increase the chip load on the remaining teeth.