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
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