Tap Terms- Thread angle Pitch

Flank Angle: Angle between the individual flank and the perpendicular to the axis of the
thread, measured in an axial plane. A flank angle of a symmetrical thread is commonly
termed the “half angle of thread.”
Flank—Trailing: The trailing flank of a thread is the one opposite the leading flank.
Flutes: Longitudinal channels formed in a tap to create cutting edges on the thread profile
and to provide chip spaces and cutting fluid passages. On a parallel or straight thread
tap they may be straight, angular or helical; on a taper thread tap they may be straight,
angular or spiral.
Flute-Angular: A flute lying in a plane intersecting the tool axis at an angle.
Flute-Helical: A flute with uniform axial lead and constant helix in a helical path around
the axis of a cylindrical tap.
Flute-Spiral: A flute with uniform axial lead in a spiral path around the axis of a conical
tap.
Flute Lead Angle: Angle at which a helical or spiral cutting edge at a given point makes
with an axial plane through the same point.
Flute-Straight: A flute which forms a cutting edge lying in an axial plane.
Front Taper: A gradual increase in the diameter of the thread form on a tap from the
leading end of the tool toward the back.
Hook Angle: Inclination of a concave cutting face, usually specified either as Chordal
Hook or Tangential Hook.
Hook-Chordal Angle: Angle between the chord passing through the root and crest of a
thread form at the cutting face, and a radial line through the crest at the cutting edge.
Lead Deviation: Deviation from the basic nominal lead.
Progressive Lead Deviation: (1) On a straight thread the deviation from a true helix
where the thread helix advances uniformly. (2) On a taper thread the deviation from a true
spiral where the thread spiral advances uniformly.
Length of Thread: The length of the thread of the tap includes the chamfered threads and
the full threads but does not include an external center. It is indicated by the letter “B” in the
illustrations at the heads of the tables.
Limits: The limits of size are the applicable maximum and minimum sizes.
Major Diameter: On a straight thread the major diameter is that of the major cylinder.
On a taper thread the major diameter at a given position on the thread axis is that of the
major cone at that position.
Minor Diameter: On a straight thread the minor diameter is that of the minor cylinder.
On a taper thread the minor diameter at a given position on the thread axis is that of the
minor cone at that position.
Pitch Diameter (Simple Effective Diameter: On a straight thread, the pitch diameter is
the diameter of the imaginary coaxial cylinder, the surface of which would pass through
the thread profiles at such points as to make the width of the groove equal to one-half the
basic pitch. On a perfect thread this coincidence occurs at the point where the widths of the
thread and groove are equal. On a taper thread, the pitch diameter at a given position on the
thread axis is the diameter of the pitch cone at that position.

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