TAP NOMENCLATURE

Allowance: The minimum clearance or maximum interference that is intended between mating parts.

Angle of thread: The angle included between the flanks of a thread measured in an axial plane.

Back taper: A slight taper on the threaded portion of the tap making the pitch diameter near the shank smaller than that at the chamfer.

Basic: The theoretical or nominal standard size from which all variations are made.

Chamfer: The tapered cutting teeth at the front end of the tap

Taper tap : 7 to 10 thread lead
Plug tap : 3 to 5 thread lead
Modified bottom : 2-1/2 - 3 thread lead
Bottoming tap : 1-1/2 thread lead

Crest: The top surface joining the two sides or flanks of a thread /

Cutting face: The leading side of the land

Flute: The longitudinal grooves formed on a tap to create cutting edges on the thread profile

Heel: The following side of the land

Height of thread: In profile, distance between crest and bottom section of thread measured to the axis

Hook face: A concave cutting face of the land. This may be varied for different materials and conditions

Interrupted threads: Skip-tooth, where alternate teeth are removed in the thread helix on a tap having an odd number of flutes

Land: Threaded section between the flutes of a tap

Lead of thread: The distance a screw thread advances axially in one turn

Major diameter: The largest diameter of the screw or nut on a straight screw thread

Minor diameter: The smallest diameter of the screw or nut on a straight screw thread

Neck: The reduced diameter, on some taps, between the threaded portion and the shank

Pitch: For metric threads the pitch is the distance from one thread point to the next thread point. For inch sizes measure the count of thread points per inch.

Pitch diameter: On a straight screw thread, the diameter of an imaginary cylinder where the width of the thread and the width of the space between threads is equal

Point diameter: The diameter at the leading end of the chamfered portion

Radial: The straight face of a land, the plane of which passes through the axis of the tap

Rake: The angle of the cutting face of the land in relation to an axial plane intersecting the cutting face at the major diameter.

Relief: The removal of metal behind the cutting edge to provide clearance between the part being threaded and a portion of the threaded land. (see back taper)

CHAMFER RELIEF: The gradual decrease in land height from cutting edge to heel on the chamfered portion of the tap land to provide radial clearance for the cutting edge.

CON-ECCENTRIC RELIEF: Radial relief in the thread form starting back of a concentric margin

ECCENTRIC THREAD RELIEF: Radial relief in the thread form starting at the cutting edge and continuing to the heel

Root: The bottom surface joining the flanks of two adjacent threads /

Side or flank thread: The surface of the thread that connects the crest to the root

Shank: The portion of the tap by which it is held by a tap handle, tap wrench or collet

Spiral point: An oblique cutting edge ground into the lands to provide a shear cutting action on the first few threads, forcing the chips ahead of the tap

Square: The squared end of the tap shank by which the tap is driven

Thread: The helical formed portion of the tap which produces the pitch in a pre-existing hole

Thread lead angle: The angle made by the helix of the thread at the pitch diameter, with a plane perpendicular to the axis

Threads per inch: The number of threads in one inch of length

Thread:
SINGLE: A thread in which lead is equal to pitch
DOUBLE: A thread in which lead is equal to twice the pitch
TRIPLE: A thread in which lead is equal to triple the pitch