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Common Thread Terms

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

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

Back of taper: A slight taper on 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 and relieved cutting teeth at the front end of the threaded section. Common types of chamfer are: Taper, 8 to 10 threads long; Plug, 3 to 5 threads and Bottoming, 1.5 threads.

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

Cutting face: The leading side of the land.

Flute: The longitudinal channels 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 normal to the axis.

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

Interrupted thread: Alternate teeth are removed in the thread helix on a tap having an odd number of flutes.

Land: One of the threaded sections 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 head.

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

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

Pitch: The distance from a point on one thread to a corresponding point on the next thread, measured parallel to the axis.

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. Also, see back taper.

Chamfer Relief: The gradual decreasing 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 of flank of thread: The surface of the thread which connects the crest to the root.

Shank: The portion of the tap by which it is held and driven.

Spiral point: An oblique cutting edge ground into the lands to provide a shear cutting action on the first few threads.

Square: The squared end of the tap shank.

Thread: The helical formed tooth of the tap which produces the thread in a tapped 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 which 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.