When tapping in NC-controlled drilling and milling machines you can use either synchronized tapping or conventional tapping.
Synchronized tapping cycle
Synchronized tapping means that the spindle rotation speed and pitch are synchronized to the axial movement of the machine. The tapping cycle always starts with and inoperative spindle. The advantage of synchronized tapping is the accurate depth of the thread. The disadvantage is that the tapping cycle may be to slow. Fig 2 shows that when trying to synchronize the spindle and the machines axial movement, the retardation and acceleration will be limited. The tapping cycle will as a result of this be slow. The process follows graph B in Fig 1 at the risk of incomplete thread profile. In case of modern machines with a very accurate synchronizing, rigid tapping is possible to use (the tap has no axial float). Normally when using synchronized tapping the tap must have an axial float to avoid big axial forces that gives incomplete thread profiles.
Conventional tapping cycle
Conventional tapping cycle means that the spindle rotation and machine axial movement must be programmed separately. The tapping cycle starts with full spindle rotation. The advantage is a faster tapping cycle, and the tapping process can follow graph A in Fig 1, which means that you will get a better thread finish. To obtain a full profile when tapping conventional, a tapping device will ball bearing axial floating must be used. Fig 3 shows the tapping process at conventional tapping. As you can see from the picture, the tapping process can follow graph A in Fig 1 which means that you will get a better thread finish.