How Do You Select a Tap for CNC Internal Threading?

A tap is a common tool for producing internal threads. Its basic form is an externally threaded tool with axial flutes.

The threaded portion consists of a chamfered cutting section and a full-form sizing section. The chamfer has a taper angle so that the stock is removed gradually. The sizing section has complete thread teeth and performs finishing, sizing, and guidance. The shank connects to the machine-spindle taper through a holder and a standard taper shank.

Tapping is a form-cutting process. Internal-thread quality is affected by the tap’s thread form, pitch, flute geometry, chamfer type, material, the workpiece material, and the tap holder.

Tap Chamfer Types

According to chamfer length, taps can be classified as bottoming, plug, and taper taps. Chamfer length affects the programmed depth required in CNC machining.

Chamfer length is commonly expressed in the number of threads. A taper tap normally has 8 to 10 chamfered threads, a plug tap 3 to 5, and a bottoming tap 1 to 1.5. Typical chamfer angles are 4° to 5° for a taper tap, 8° to 13° for a plug tap, and 25° to 35° for a bottoming tap.

Blind holes normally require a bottoming tap. Through holes usually use a plug tap and only rarely a taper tap. In general, the longer the chamfer, the more extra drilled depth is required below the complete thread.

Rigid and Floating Taps

Taps can also be classified by holder connection as rigid taps or floating, tension-compression taps.

A floating tap holder gives the tap behavior similar to the feel required in manual tapping. It allows limited axial compression and extension, and its adjustable torque changes the tapping tension.

A rigid tap requires a CNC control with synchronized operation. During tapping, thread lead and spindle speed must remain synchronized:

Feed rate = thread lead × spindle speed

Unless the CNC supports synchronized rigid tapping, a floating tap should be selected, although floating tap holders are more expensive.

When tapping with a floating holder, reducing the programmed feed rate by about 5% can improve the result. If the commanded Z-axis feed is slightly lower than the axial speed imposed by the helical thread, the helix pulls the tap downward after the first few teeth enter the hole. This helps protect the floating tap because most tapping holders extend more freely than they compress.

Gang Taps

CNC machines sometimes use a gang tap whose working section is equivalent to two or three taps arranged in series. The sections successively perform roughing and finishing. This construction is suitable for high-strength or high-hardness materials and for large, high-accuracy threads.