Tapping into Tapping Terms

edited by Bernard Martin

Getting a good understanding of the definitions of the parts of a tap will help you to better understand the functions of  tap designs.  Special thanks to North American Tool for letting us share their short and simple explanations!

ALLOWANCE
Minimum clearance between two mating parts; the prescribed variations from the basic size.

ANGLE OF THREAD
The angle included between the sides of the thread measured in an axial plane.

AXIS
The imaginary straight line that forms the longitudinal centerline of the tool or threaded part.

BACK TAPER
A gradual decrease in the diameter of the thread form on a tap from the chamfered end of the land towards the back which creates a slight radial relief in the threads.

BASE OF THREAD
The bottom section of the thread; the greatest section between the two adjacent roots.

BASIC SIZE
The theoretical or nominal standard size from which all variations are derived by application of allowances and tolerances.

CHAMFER
The tapering of the threads at the front end of each land of a tap by cutting away and relieving the crest of the first few teeth to distribute the cutting action over several teeth; Taper taps are chamfered 7-10 threads; plug tapsare chamfered 3-5 threads; semi-bottoming (or modified bottoming) taps are chamfered 2-2.5 threads; bottom-ing taps are chamfered 1-2 threads; taper pipe taps are chamfered 2-3.5 threads.

CHAMFER RELIEF
The gradual decrease in land height from cutting edge to heel on the chamfered portion, to provide clearance for the cutting action as the tap advances.

CREST
The top surface joining the two sides or flanks of the thread; the crest of an external thread is at its major diameter, while the crest of an internal thread is at its minor diameter.

CUTTING FACE
The leading side of the land in the direction of cutting rotation on which the chip forms.

FLUTE
The longitudinal channels formed in a tap to create cutting edges on the thread profile, and to provide chip spaces and cutting fluid passages.

HEEL
The edge of the land opposite the cutting edge.

HEIGHT OF THREAD
The distance, measured radially, between the crest and the base of a thread.

HELIX ANGLE
The angle made by the advance of the thread as it wraps around an imaginary cylinder.

HOOK
The undercut on the face of the teeth.

HOOK ANGLE
The inclination of a concave cutting face, usually specified either as Chordal Hook or Tangential Hook.

• Chordal Hook Angle: The angle between the chordpassing through the root and crest of a thread form at the cutting face, and a radial line through the crest at the cutting edge.
• Tangential Hook Angle: The angle between a line tangent to a hook cutting face at the cutting edge and a radial line to the same point.

INTERRUPTED THREAD TAP
A tap having an odd number of lands with alternate teeth along the thread helix removed. In some cases alternate teeth are removed only for a portion of the thread length.

LAND
The part of the tap body which remains after the flutes are cut, and on which the threads are finally ground. The threaded section between the flutes of a tap.

LEAD
The axial distance a tap will advance along its axis in one revolution. On a single start, the lead and the pitch are identical; on a double start, the lead is twice the pitch.

MAJOR DIAMETER
Commonly known as the “outside diameter.” It is the largest diameter of the thread.

MINOR DIAMETER
Commonly known as the “root diameter.” It is the small-est diameter of the thread.

PERCENT OF THREAD
One-half the difference between the basic major diameter and the actual minor diameter of an internal thread, divided by the basic thread height, expressed as a percentage.

PITCH
The distance from any point on a screw thread to a cor-responding point on the next thread, measured parallel to the axis and on the same side of the axis. The pitch equals one divided by the number of threads per inch.

PITCH DIAMETER
On a straight thread, the pitch diameter is the diameter of the imaginary co-axial 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 of the basic pitch. On a perfect thread this 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.

RAKE
The angular relationship of the straight cutting face of a tooth with respect to a radial line through the crest of the tooth at the cutting edge.

• Negative rake means that the crest of the cutting face is angularly behind the balance of the cutting face of the tooth.
• Neutral or Zero rake means that the cutting face is directly on a radial line.
• Positive rake means that the crest of the cutting face is angularly ahead of the balance of the cutting face of the tooth.

RELIEF (or Thread Relief)
The removal of metal from behind the cutting edge to provide clearance and reduce friction between the part being threaded and the threaded land.

ROOT
The bottom surface joining the sides of two adjacent threads, and is identical with or immediately adjacent to the cylinder or cone from which the thread projects.

SPIRAL FLUTE
A flute with uniform axial lead in a spiral path around the axis of a tap.

SPIRAL POINT
The angular fluting in the cutting face of the land at the chamfered end; formed at an angle with respect to the tap axis of opposite hand to that of rotation. Its length is usually greater than the chamfer length and its angle with respect to the tap axis is usually made great enough to direct the chips ahead of the taps cutting action.

STRAIGHT FLUTE
A flute that forms a cutting edge lying in an axial plane.

TOLERANCE
In producing a tap to given specifications, tolerance is:
(a.) the total permissible variation of a size;
(b.) the difference between the limits of size.