Tap terminology
d₁
Nominal diameter
The diameter used for the purpose of general identification.
dₘ
Pitch diameter
The diameter measured where the width of the thread is equal to half the pitch.
d₂
Shank diameter
The diameter of the shank, important for the tapping attachment.
d₃
Chamfer diameter
The diameter at the leading end of the chamfer.
d₄
Neck diameter
The diameter of the reduced section between the thread and shank of the tap.
d₅
Core diameter
The diameter of a circle tangent to the bottom of the flutes.
I
Chamfer
The taper on the threads at the front end of the tap made by grinding and relieving the crests of the first few teeth.
a
Square
The square with rounded corners formed by four flats parallel to the tap axis. The square serves to drive the tap.
L
Square length
The length of the flats that form the square.
L₁
Total length
The complete length of the tap from end to end, excluding external centres.
L₂
Thread length
The length of the threaded section of the tap.
L₃
Usable length
The length measured from the front end of the tap to the end of the neck section. This length determines the maximum threadable depth on taps with reinforced shank.
L₄
Chamfer length
The length of the chamfer measured parallel to the tap axis, excluding the chamfer bevel.
L₅
Flute length
The axial length of the flute including the cutter sweep.
Cm
External centre
The pointed end of the tap.
Cf
Internal centre
The countersink in one or both ends of the tap.
P
Pitch
The distance, measured parallel to the tap axis, between two corresponding and successive points on the thread profile.
α
Angle of thread
The angle between the flanks of the thread (measured in an axial plane).
δ
Thread lead angle
The angle made by the spiral of the thread and a plane perpendicular to the tap axis, measured on the pitch diameter line.
β
Chamfer angle
The angle between the chamfer and the tap axis, measured in an axial plane.
γ
Rake angle
The angle between the cutting face of the tap and a radial line passing through the crest of the tooth at the cutting edge.
T
Land width
The chordal width of material between two successive flutes.
S
Flute
The longitudinal channels in a tap which create cutting edges. The flutes provide space for chips and passage for coolant/ lubricant.
Δ1
Pitch diameter relief
The radial reduction of the pitch and/ or major diameter behind the cutting edge of the tap. The relief confers cutting properties and provides clearance between the part being threaded and the tap threads.
Δ
Chamfer relief
The radial reduction of the major diameter on the tap chamfer behind the cutting edge. The chamfer relief confers cutting properties to the tap.
ε
Spiral flute angle
The angle formed by the flutes and the tap axis.