Definitions of Thread Terms:

Definitions of Thread Terms

ALLOWANCE: The minimum clearance between two mating parts. The variations from the basic size which are prescribed to permit the desired amount of play in a metal-to-metal fit.

BASIC SIZE: The theoretical size (usually the same as the nominal size), from which the design size limits are derived by the application of tolerances and allowances.

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Definitions Continued...

CLEARANCE: The radial distance between an external diameter and an internal diameter.

CONVOLUTED: Removal of the incomplete threads at the end faces of a threaded part.

CREST: The top of the thread form. The major diameter of an external thread, or the minor diameter of an internal thread.

FIT: The term used to designate the tightness or looseness of two mating parts, resulting from a combination of tolerances and allowances applied to the basic size of the parts.

FLANKS: The sides of the thread form or groove, connecting the crest and the root.

FLANK ANGLE: The angle between the flank and a line perpendicular to the axis of the thread. This term is often referred to as half angle or lead angle. Some exceptions to this definition will be encountered, such as tapered thread flank angles measured perpendicular to the taper, and should be noted.

FUNCTIONAL DIAMETER: The actual (measured) pitch diameter of a thread adjusted by the cumulative effects of lead error and angle error, which is always added to external threads and subtracted from internal threads.

INCLUDED ANGLE: Two flank angles that complete a thread form.

LEAD: The distance advanced by a thread when rotated 360 degrees on its mating thread.

LIMITS: The largest and smallest extremes in the size of a dimension.

MAJOR DIAMETER: The largest diameter of a thread form. The root diameter of an internal thread and the crest diameter of an external thread.

MINOR DIAMETER: The smallest diameter of a thread form. The bore or crest diameter of an internal thread and the root diameter of an external thread.

NOMENCLATURE: In relation to a thread, the complete identification, including the nominal size, threads per inch or pitch, thread series, class of fit, and possibly a designation for internal or external.

NOMINAL SIZE: The size used for purposes of identification. Usually the same as the basic size.

PITCH: The distance between corresponding points on adjacent threads. The reciprocal of threads per inch.

PITCH DIAMETER: The theoretical diametrical plane which passes through a thread at the point where the width of the thread tooth and groove are equal.

RELIEF: Removal of the material from a portion of the surface or diameter to avoid contact with the mating part when it is engaged.

REFERENCE GAGE: A master gage generally held to close tolerance limits which is preserved for a periodic comparison with working gages. A truncated seating plug is a reference gage that is used to inspect as well as set an adjustable thread ring.

ROOT: The bottom of the thread form. The major diameter of an external thread and the minor diameter on an internal thread.

THREAD ANGLE: The included angle of the thread form, which has two thread flanks as sides.

TOLERANCE: The amount of variation permitted from the designated dimension, or the difference between the minimum and maximum dimension.

TRUNCATE: To remove the crest of a thread, thus reduce its height, usually done to guarantee non-interference.

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