Threading Terminology Tap and Screw Thread Terminology
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.
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 Taper : A slight taper on the threaded portion of the tap, making the pitch diameter near the shank smaller than that at the chamfer.
Point Diameter : The diameter at the leading end of the chamfered portion.
Rake Angle : 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 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.
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), semi (or modified) bottom (2.5 to 3 threads), and bottoming (1-1/2 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.
Root : The bottom surface joining the flanks of two adjacent threads.
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 Thread : Alternate teeth are removed in the thread helix on a tap having an odd number of flutes.
Side or Flank Thread : The surface of the thread which connects the crest to the root.
Shank : The portion of the tap by which it is held.
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 by which the tap is driven.
Land : Threaded sections between the flutes of a tap.
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.
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 : The distance from a point on one thread to a corresponding point on the next thread, measured parallel to the axis of rotation.
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.
Threading Technical
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