Greenfield along with Cutting Tools Chicago aka General Cutting Tools is illustrating some tech tips for use in your shop. Four basic design features are incorporated into the design of all cutting tools, including taps. They are cutting face (hook), relief, base material, and surface treatment or coating. This tip discusses hook and relief. The cutting face, or hook, is that portion of the tap flute between the major and minor diameter of the thread that cuts or shears the workpiece material. The entry angle of the cutting face into the workpiece material is measured in degrees, from positive to negative, from a perpendicular reference line through the axis of the tool. Positive hooks are used for soft materials such as aluminum, mild steel and stainless that produce continuous (stringy) chips. Due to the positive angle, the cutting faces are fragile and may chip easily. Negative hooks are used for materials that produce broken or powder chips, or those that have been hardened. This type of cutting face is much stronger and is less prone to chipping.
The relief of a tap may be found on the chamfer’s major diameter (required for tapping), or in the threads, in the form of radial clearance or back taper. Radial or thread relief is the thinning of the tooth from the cutting face to the heel to relieve cutting pressures and friction across the land of the tap. Thread relief is applied for materials that are tough, hard, or have high elastic memory (shrinks or squeezes the tap creating friction). Back taper is the reduction of the major, pitch, and minor diameters from the first thread at the front of the tap to the last thread near the shank. While thread relief is applied for specific applications, back taper is applied to all taps.