Threaded fasteners explained

Acme thread

A screw thread having a 29° flank angle. Used largely for feed and adjusting screws on machine tools. The American acme thread, while accepted worldwide, is very similar to the European metric standard trapezoidal thread, which has a 30° flank angle.

Addendum

The addendum of an external thread is the radial distance between the major and pitch cylinders or cones. This term applies to those threads having a recognized pitch diameter or pitch cone. You can also define the addendum as the height by which a tooth projects beyond a pitch cylinder, plane, or cone.

Allowance

An intentional clearance between internal or external thread and the design form of the thread when the thread form is on its maximum metal condition. Not all classes of fit have an allowance. For metric threads the allowance is called the fundamental deviation.

Axis of thread

The line, real or imaginary, passing through the centre of an object about which it could rotate; a point of reference.

Axis

The axis of a thread is coincident with the axis of its pitch cylinder or cone.

Back taper

A slight decrease in diameter from point to back in the body of the drill. Back taper can also be defined as a light decrease in diameter, from front to back in the flute length of reamers, or as 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.

Basic form of thread

The permanent reference profile, from which the design forms for both external and internal threads are developed.

Basic thread profile

The theoretical profile of external and internal threads with no manufacturing tolerance applied.

Blunt start (blunt end) thread

“Blunt start” (“blunt end”) designates the removal of the incomplete thread at the end of the thread. This is a feature of threaded parts that are repeatedly assembled by hand, such as hose couplings and thread plug gages, to prevent cutting of hands and crossing of threads, and which was formerly known as a Higbee cut.

Centre gage

A small, flat gage having 60° angles used for grinding and setting the thread cutting tools in a lathe. It may also be used to check the pitch of threads and the points of centre.

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 multiples of the thread pitch from the tap point. Plug taps are chamfered 3-5 times the pitch. Bottoming taps are chamfered 1-2 times the pitch. Specification of chamfer lengths in multiples of pitch avoids variations in the counting of threads on various tap lands.

Chasing threads

Cutting threads in a lathe or screw machine.

Classes of threads

Threads of a given type are distinguished from each other by the amounts of tolerance or tolerance and allowance specified. Various combinations of these tolerances and allowances have been set in tables to form a set of standard classes.

Complete thread

The complete (full form) thread is the cross section of a threaded length having full form at crest and root. (See effective thread, length of complete thread.)

Crest

The surface of the thread that joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects. The crest of an external thread is at its major diameter while the crest of an internal thread is at its minor diameter.

Crest truncation

The crest truncation of a thread is the radial distance between the crest (crest apex) and the cylinder or cone that would bound the crest.

Cumulative pitch

The distance measured parallel to the axis of the thread between corresponding points on any two threads whether or not they are in the same axial plane.

Dedendum (thread)

The dedendum of an external thread is the radial distance between the pitch and minor cylinders or cones, respectively. The dedendum of an internal thread is the radial distance between the major and pitch cylinders or cones, respectively. (This term applies to those threads having a recognized PD or pitch cone.)

Depth of thread engagement

The depth (or height) of thread engagement between two coaxially assembled mating threads is the radial distance by which their thread forms overlap each other.

Design form of thread

The design form of an internal or external thread is the thread form in its maximum metal condition. It is the same as the basic thread profile except that the thread roots are rounded. If either the internal or external thread form exceeds the design form of the thread profile, a potential interference exists.

Design thread form

The design thread form is the maximum material form permitted for the external or internal thread. In practice, unless otherwise specified, the form of root is an indeterminate contour not encroaching on the maximum material form of the mating thread when assembled.

Die (threading)

A tool used to cut external threads.

Die stock

The frame and two handles (bars) which hold the dies (chasers) used for cutting (chasing) external screw threads.

Die threading

A process for producing external threads, usually on small diameter rod, wire, pipe or tubing, using a one-piece or two-piece die with integral cutting edges. Die threading is the counterpart of tapping.

Effective diameter

This is the diameter of an imaginary cylinder coaxial with the thread, which has equal metal and space widths. It is often referred to as pitch diameter. Sometimes referred to as the simple effective diameter to differentiate from the virtual effective diameter.

Effective thread

The effective (or useful) thread includes the complete thread, and those portions of the incomplete thread which are fully formed at the root but not at the crest (in taper pipe threads this includes the so-called black crest threads), thus excluding the vanish thread.

External thread

A thread on a cylindrical or conical external surface.

Feather edge

Same as feather burr except that feather edge can also refer to the ends of a lead-in or lead-out thread, which is a very thin machined ridge. Sometimes called a wire edge or whisker-type burr.

First full thread

The first full thread on the cutting edge back of the chamfer. It is at this point that rake, hook, and thread elements are measured.

Fishtail

A common name for the center- gage. It is used to set thread cutting tools and has scales on it for determining the number of threads per inch.

Flank

The flank, or side, of a thread is either surface connecting the crest with the root. The flank-surface intersection with an axial plane is theoretically a straight line.

Flank angle

The flank angles are the angles between the individual flanks 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-leading

1. The flank of a thread facing toward the chamfered end of a threading tool. 2. The leading flank of a thread is the one which, when the thread is about to be assembled with a mating thread, faces the mating thread.

Flank-trailing

The trailing flank of a thread is the one that is opposite to the leading flank.

Flutes (tap)

The 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.

Following flank

The following (trailing) flank of a thread is the one that is opposite to the leading flank.

Form diameter

The diameter at the point nearest the root from which the flank is required to be straight.

Form of thread

The form of a thread is its profile in an axial plane for a length of one pitch of the complete thread.

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.

Functional (virtual) diameter

The functional diameter (virtual condition per ANSI Y14.5M) of an external or internal thread is the pitch diameter of the enveloping thread of perfect pitch, lead, and flank angles, having full depth of engagement but clear at crests and roots and of a specified length engagement. It may be derived by adding to the pitch diameter in the case of an external thread, or subtracting from the pitch diameter in the case of an internal thread, the cumulative effects of deviations from specified profile, including variations in lead (uniformity of helix) and flank angle over a specified length of engagement. The effects of taper, out-of-roundness, and surface defects may be positive or negative on either external or internal threads. A perfect internal or external GO-thread gage having a pitch diameter equal to that of the specified material limit and having clearance at crest and root is the enveloping thread corresponding to that limit.

Fundamental deviation

An intentional clearance between internal or external thread and the design form of the thread when the thread form is on its maximum metal condition. For metric threads the fundamental deviations are designated by letters--capitals for internal threads and small letters for external threads. Some tolerance classes havea fundamental deviation of zero. For imperial threads the fundamental deviation is called the allowance.

Fundamental triangle height

The fundamental triangle height is normally designated with the letter H. This is the height of the thread when the profile is extended to a sharp vee form. For 60˚ thread forms such as metric and Unified Thread Series, H equals 0.866025 times the thread pitch.

Gage, screw pitch

A gage consisting of a group of thin blades, used for checking the number of screw threads per unit of distance--usually per inch--on a screw, bolt, nut, pipe, or fitting.

Half nut

A lever-operated mechanism that resembles a split nut that can be closed on the lead screw of a lathe when threads are being cut.

Height of fundamental triangle

The height of the fundamental triangle of a thread, that is, the height of a sharp-V thread, is the distance, measured radially, between the sharp major and minor cylinders or cones, respectively.

Height of thread

The height (or depth) of thread is the distance, measured radially between the major and minor cylinders or cones, respectively.

Helix

A path formed as a point advances uniformly around a cylinder, as the thread on a screw or the flutes on a drill.

Helix angle (threading)

On a straight thread, the helix angle is the angle made by the helix of the thread and its relation to the thread axis. On a taper thread, the helix angle at a given axial position is the angle made by the conical spiral of the thread with the axis of the thread. The helix angle is the complement of the lead angle.

Included angle

The included angle of a thread (or angle of thread) is the angle between the flanks of the thread measured in an axial plane.

Incomplete thread

A threaded profile having either crests or roots, or both crests and roots not fully formed, resulting from their intersection with the cylindrical or end surface of the work or the vanish cone. It may occur at either end of the thread.

Internal thread

A thread on a cylindrical or conical internal surface.

Interpolation

A function of control enabling data points to be generated between specific coordinate positions to allow simultaneous movement of two or more axes of motion in a defined geometric pattern. For example. in NC, curved sections can be approximated by a series of straight lines or parabolic segments. Also known as linear interpolation.

Interrupted thread tap

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

Land (tap)

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

Lead

The distance a thread will advance along its axis in one completerevolution.

Left-hand thread

A screw thread that is screwed in by rotating counter clockwise.Length of engagement: The axial distance over which an external thread is in contact with an internal thread.

Minor diameter

The smallest diameter of a screw thread. Also known as the "root diameter."

Multiple-thread screw

A screw made of two or more threads to provide an increased lead with a specified pitch.

Nominal diameter

The diameter equal to the external diameter of the threads.

Overtapping

Tapping of a thread following a plating operation so that the thread tolerances comply within specification, allowing the internal and external threads to assemble. It is normal practice to overtap the internal rather than the external thread.

Pitch

The distance from any point on a thread to the corresponding point on the adjacent thread, measured parallel to the axis. Also applied to spur gears. See diametral pitch.

Pitch diameter

The diameter of a thread at an imaginary point where the width of the groove and the width of the thread are equal.

Pitch line

An imaginary line which passes through threads at such points that the length of the part of the line between adjacent threads is equal to the length of the line within a thread.

Right-hand thread

A screw thread that is screwed in by rotating clockwise. The majority of screw threads are right handed.

Rolled thread

A thread formed by plastically deforming a blank rather than by cutting. Increases both fatigue strength and thread shear strength.

Screw

Threaded fastener with the thread running up to the head of the fastener; has no plain shank.

Screw thread

A ridge of constant section which is manufactured so that a helix is developed on the internal or external surface of a cylinder.

Square threads

A thread having a depth, width, and space between threads that are equal. It’s used on heavy jack screws, vise screws, and other similar items.

Stress area (fasteners)

The effective cross-sectional area of a thread when subjected to a tensile force. It is based upon a diameter which is the mean of the pitch (or effective) and the minor (or root) diameters of the thread.

Symmetrical thread

A symmetrical thread is one which has both flanks of the thread profile inclined are the same angle.

Tap

A tool used to cut threads on the inside of a round hole.

Tapping

The process of cutting screw threads in a round hole with a tap (an internal thread cutting tool).

Thread

A helical projection of uniform section on the internal or external surface of cylinder or cone. Also, the operation of cutting a screw thread.

Thread angle

The angle formed by the two sides of the thread (or their projections) with each other.

Thread axis

A line running lengthwise through the centre of the screw.

Thread crest

The top part of the thread. For external threads, the crest is the region of the thread which is on its outer surface; for internal threads it is the region which forms the inner diameter.

Thread depth

The distance between the crest and the root of a thread.

Thread flank

The thread flanks join the thread roots to the crest.

Thread height

This is the distance between the minor and major diameters of the thread measured radially.

Thread length

Length of the portion of the fastener with threads.

Thread pitch

The distance from a point on one screw thread to a corresponding point on the next thread.

Thread pitch diameter

The diameter of a screw thread measured from the thread pitch line on one side to the thread pitch line on the opposite side.

Thread root

The thread root is the bottom of the thread. On external threads the roots are usually rounded so that fatigue performance is improved. ‘Thread root’ is also used to refer to the bottom surface joining the sides of two adjacent threads.

Thread runout

The portion at the end of a threaded shank which is not cut or rolled to full depth, but which provides a transition between full depth threads and the fastener shank or head.

Tolerance class

A combination of tolerance grade and a fundamental deviation which is given to an internal or external thread. A tolerance class for an internal thread when combined with the tolerance class for an external thread gives the class of fit for the mating threads.

Tolerance grade

The difference between maximum and minimum metal conditions for a tolerance applied to a screw thread. For metric threads the tolerance grade is given a number.

Virtual effective diameter

The effective diameter of a thread, but allowing for errors in pitch and flank angles.