This reference defines the standard terminology for taps and screw threads used in manufacturing and machining. It covers tap types (bottoming, plug, taper), thread geometry (pitch diameter, major diameter, minor diameter), cutting features (chamfer, flute, spiral point), and thread characteristics (lead, pitch, angle of thread). The definitions apply to both hand taps and machine taps in all thread standards.

What is a bottoming tap?

Bottoming Tap: A tap having a chamfer length of 1-2 threads.

What is a plug tap?

Plug Tap: A tap with 3 to 5 chamfered threads.

What is a taper tap?

Taper Tap: A tap having a chamfer length of 7 to 10 threads.

What is the chamfer on a tap?

Chamfer: The tapering of the threads at the front end of each land of a chaser, tap or die by cutting away and relieving the crest of the first few teeth to distribute the cutting action over several teeth.

Chamfer Angle: The angle formed between the chamfer and the axis of the tap or die by cutting away the relieving the crest of the first few teeth to distribute the cutting action over several teeth.

Thread geometry terms

Crest: The surface of the thread which joins the flanks of the thread and it’s farthest from the cylinder or cone from whi8ch the thread projects.

Flank: The part of a helical thread surface which connects the crest and the root and which is theoretically a straight line in an axial plane section.

Angle of Thread: The angle included between a flank of the thread measured in an axial plane.

Half Angle of Thread: The angle included between a flank of the thread and the normal 90 degree to the axis, measured in an axial plane.

What is pitch diameter?

Pitch Diameter: The diameter of an imaginary cylinder or cone, at a given point on the axis, of such a diameter and location of its axis that its surface would pass through the thread in such a manner as to make the thread ridge and the thread groove equal and, therefore, is located equidistant between the sharp major and minor cylinders or cones of a given thread form. On a theoretically perfect thread, these widths are equal to one half of the basic pitch (measured parallel to the axis).

What are major diameter and minor diameter?

Major Diameter: The diameter of the major cylinder or cone, at a given position on the axis that bounds the roots an external thread of the crests of an internal thread. The largest diameter of a straight screw thread.

Minor Diameter: The diameter of the minor cylinder or cone, at a given position on the axis that bounds the roots an external thread of the crests of an internal thread.

What is pitch and lead of thread?

Pitch: The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis. The pitch in inches = 1 / (No. of threads per inch).

Lead of Thread: The distance a screw thread advances axially in one complete turn. On a single start tap the lead and pitch are identical. In a multiple start tap the lead is the multiple of the pitch. On a single-thread screw the lead and pitch are identical. On a double thread the lead is 2X pitch, on a triple lead is 3X pitch, etc.

Tap cutting features

Flute: The longitudinal channel formed in a tap to create cutting edges on the thread profile and to provide chip spaces and cutting fluid passage.

Land: One of the threaded sections between the flutes of a tap.

Spiral Point: The angular fluting in the cutting face of the land at the chamfered end. It is 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 great enough to direct the chips ahead of the tap. The tap may or may not have longitudinal lines.

Square: Four driving flats parallel to the axis on a tap shank forming a square or square with round corners.

Hook angles

Hook, Chordal: A concave face having an angle of inclination specified between a chord passing through the root and crest of a thread form at the cutting face, and a radical line through the crest at the cutting edge.

Hook, Tangential: A concave face having an angle of inclination specified between a line tangent to the hook surface at the cutting edge and a radial line to the same point.

Hook Angle: The angle of inclination of a concave face usually specifies either as CHORDAL HOOK or TANFENTIAL HOOK.

What is an interrupted thread tap?

Interrupted Thread Tap: A tap having an odd number of lands with alternative teeth in the thread helix removed. In some cases alternative teeth are removed only for a portion of the thread length.

What is hand of threads?

Hand of Threads: A thread when viewed axially, winds in a clockwise and receding direction for LEFT-HAND THREADS and counter clockwise and receding direction for RIGHT-HAND THREADS.

Tolerance and allowance

Tolerance: It is generally recognized that, in mass production, it is impossible to reproduce in exact detail the theoretically perfect product as laid out on the drawing board. The allowed slight variation between theoretically perfect products is called the TOLERANCE.

Allowance: An intentional difference in correlated dimensions of mating parts. It is the minimum clearance or maximum interference between such parts.