The tap is usually single or group. Small and medium size thread through hole by a single tap into an attack. When the blind hole or large size hole when used with tap tap group, processing more than 2 in order to complete a hole. The group set an isometric and unequal diameter two design. Tap size design, the only cutting cone length different; unequal diameter design of tap, the thread sizes are not the same, only last a has complete tooth profile. Tap the cutting load distribution of different diameter design reasonable, high processing quality, but the manufacturing cost is high. Trapezoidal thread tap is often used with different diameter design.