Threads Terms

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Glossary of Screw Threads Gaging Terms

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GLOSSARY OF SCREW THREADS GAGING TERMS:

Definitions of Screw Threads. The following definitions are based on American National Standard ANSI B1.7M-1984, "Nomenclature, Definitions, and Letter Symbols for Screw Threads," and refer to both straight and taper threads.

A
ACTUAL SIZE:	An actual size is a measured size.
ALLOWANCE:	An allowance is the prescribed difference between the design (maximum material) size and the basic size. It is numerically equal to the absolute value of the ISO term fundamental deviation.
AXIS OF THREAD:	Thread axis is coincident with the axis of its pitch cylinder or cone.

B
BASIC PROFILE OF THREAD:	The basic profile of a thread is the cyclical outline, in an axial plane, of the permanently established boundary between the provinces of the external and internal threads. All deviations are with respect to this boundary.
BASIC SIZE:	The basic size is that size from which the limits of size are derived by the application of allowances and tolerances.
BILATERAL TOLERANCE:	This is a tolerance in which variation is permitted in both directions from the specified dimension.
BLACK CREST THREAD:	This is a thread whose crest displays an unfinished cast, rolled, or forged surface.
BLUNT START THREAD:	"Blunt start" designates the removal of the incomplete thread at the starting 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. It was formerly known as a Higbee cut.

C
CHAMFER:	This is a conical surface at the starting end of a thread.
CLASS OF THREAD:	The class of a thread is an alphanumerical designation to indicate the standard grade of tolerance and allowance specified for a thread.
CLEARANCE FIT:	This is a fit having limits of size so prescribed that a clearance always results when mating parts are assembled at their maximum material condition.
COMPLETE THREAD:	The complete thread is that thread whose profile lies within the size limits. (See also Effective Thread and Length of Complete Thread.) Note: Formerly in pipe thread terminology this was referred to as "the perfect thread" but that term is no longer considered desirable.
CREST:	This is that surface of a thread which joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects.
CREST TRUNCATION:	This is the radial distance between the sharp crest (crest apex) and the cylinder or cone that would bound the crest.

D
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 SIZE:	This is the basic size with allowance applied, from which the limits of size are derived by the application of a tolerance. If there is no allowance, the design size is the same as the basic size.
DEVIATION:	Deviation is a variation from an established dimension, position, standard, or value. In ISO usage, it is the algebraic difference between a size (actual, maximum, or minimum) and the corresponding basic size. The term deviation does not necessarily indicate an error. (See also Error.)
DEVIATION, FUNDAMENTAL (ISO TERM):	For standard threads, the fundamental deviation is the upper or lower deviation closer to the basic size. It is the upper deviation es for an external thread and the lower deviation El for an internal thread. (See also Allowance and Tolerance Position.)
DEVIATION, LOWER (ISO TERM):	The algebraic difference between the minimum limit of size and the basic size. It is designated El for internal and ei for external thread diameters.
DEVIATION, UPPER (ISO TERM):	The algebraic difference between the maximum limit of size and the basic size. It is designated ES for internal and es for external thread diameters.
DIMENSION:	This is a numerical value expressed in appropriate units of measure and indicated on drawings along with lines, symbols, and notes to define the geometrical characteristic of an object.

E
EFFECTIVE SIZE:	See Pitch Diameter, Functional 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.
ERROR:	This is the algebraic difference between an observed or measured value beyond tolerance limits, and the specified value.
EXTERNAL THREAD:	This is a thread on a cylindrical or conical external surface.

F
FIT:	Fit is the relationship resulting from the designed difference, before assembly, between the sizes of two mating parts which are to be assembled.
FLANK:	The flank 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 DIAMETRAL DISPLACEMENT:	In a boundary profile defined system, flank diametral displacement is twice the radial distance between the straight thread flank segments of the maximum and minimum boundary profiles. The value of flank diametral displacement is equal to pitch diameter tolerance in a pitch line reference thread system.

H
HEIGHT OF THREAD:	The height (or depth) of thread is the distance, measured radially, between the major and minor cylinders or cones, respectively.
HELIX ANGLE:	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. (See also page 1717 for diagram.)
HIGBEE CUT:	See Blunt Start Thread.

I
IMPERFECT THREAD:	See Incomplete Thread.
INCLUDED ANGLE:	See Thread Angle.
INCOMPLETE THREAD:	This is a threaded profile having either crests or roots or both, 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.
INTERFERENCE FIT:	This is a fit having limits of size so prescribed that an interference always results when mating parts are assembled.
INTERNAL THREAD:	This is a thread on a cylindrical or conical internal surface.

L
LEAD:	Lead is the axial distance between two consecutive points of intersection of a helix by a line parallel to the axis of the cylinder on which it lies, i.e., the axial movement of a threaded part rotates one turn in its mating thread.
LEAD ANGLE:	On a straight thread, the lead angle is the angle made by the helix of the thread at the pitch line with a plane perpendicular to the axis. On a taper thread, the lead angle at a given axial position is the angle made by the conical spiral of the thread with the perpendicular to the axis at the pitch line.
LEAD THREAD:	This is that portion of the incomplete thread that is fully formed at the root but not fully formed at the crest which occurs at the entering end of either an external or internal thread.
LEFT-HAND THREAD:	A thread is a left-hand thread if, when viewed axially, it winds in a counterclockwise and receding direction. Left-hand threads are designated LH.
LENGTH OF COMPLETE THREAD:	This is the axial length of a thread section having full form at both crest and root but also including a maximum of two pitches at the start of the thread which may have a chamfer or incomplete crests.
LENGTH OF THREAD ENGAGEMENT:	The length of thread engagement of two mating threads is the axial distance over which the two threads, each having full form at both crest and root, are designed to contact. (See also Length of Complete Thread.)
LIMITS OF SIZE:	These are the applicable maximum and minimum sizes.

M
MAJOR CLEARANCE:	This is the radial distance between the root of the internal thread and the crest of the external thread of the coaxially assembled designed forms of mating threads.
MAJOR CONE:	This is the imaginary cone that would bound the crests of an external taper thread or the roots of an internal taper thread.
MAJOR CYLINDER:	This is the imaginary cylinder that would bound the crests of an external straight thread or the roots of an internal straight thread.
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. (See also Major Cylinder and Major Cone.)
MAXIMUM MATERIAL CONDITION (MMC):	This is the condition where a feature of size contains the maximum amount of material within the stated limits of size. For example, minimum internal thread size or maximum external thread size.
MINIMUM MATERIAL CONDITION (LEAST MATERIAL CONDITION (LMC):	This is the condition where a feature of size contains the least amount of material within the stated limits of size. For example, maximum internal thread size or minimum external thread size.
MINOR CLEARANCE:	This is the radial distance between the crest of the internal thread and the root of the external thread of the coaxially assembled design forms of mating threads.
MINOR CONE:	This is the imaginary cone that would bound the roots of an external taper thread or the crests of an internal taper thread
MINOR CYLINDER:	This is the imaginary cylinder that would bound the roots of an external straight thread or the crests of an internal straight thread.
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. (See also Minor Cylinder and Minor Cone.)
MULTIPLE-START THREAD:	This is a thread in which the lead is an integral multiple, other than one, of the pitch.

N
NOMINAL SIZE:	This is the designation used for the purpose of general identification.

P
PARALLEL THREAD:	See Screw Thread.
PARTIAL THREAD:	See Vanish Thread.
PITCH:	The pitch of a thread having uniform spacing is the distance measured parallel with its axis between corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis. Pitch is equal to the lead divided by the number of thread starts.
PITCH CONE:	The pitch cone is an imaginary cone of such apex angle and location of its vertex and axis that its surface would pass through a taper thread in such a manner as to make the widths of the thread ridge and the thread groove equal. It is, therefore, located equidistantly between the sharp major and minor cones of a given thread form. On a theoretically perfect taper thread, these widths are equal to one-half the basic pitch. (See also Axis of Thread and Pitch Diameter.)
PITCH CYLINDER:	The pitch cylinder is an imaginary cylinder of such diameter and location of its axis that its surface would pass through a straight thread in such a manner as to make the widths of the thread ridge and groove equal. It is, therefor, located equidistantly between the sharp major and minor cylinders of a given thread form. On a theoretically perfect thread these widths are equal to one-half the basic pitch. (See also Axis of Thread and Pitch Diameter.)
PITCH DIAMETER:	On a straight thread the pitch diameter is the diameter of the pitch cylinder. 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. Note: When the crest of a thread is truncated beyond the pitch line, the pitch diameter and pitch cylinder or pitch cone would be based on a theoretical extension of the thread flanks.
PITCH LINE:	This is the generator of the cylinder or cone specified in Pitch Cylinder and Pitch Cone.

R
RIGHT-HAND THREAD:	A thread is a right-hand thread if, when viewed axially, it winds in a clockwise and receding direction. A thread is considered to be right-hand unless specifically indicated otherwise.
ROOT:	A root is that surface of the thread which joins the flanks of adjacent thread forms and is immediately adjacent to the cylinder or cone from which the thread projects.
ROOT TRUNCATION:	This is the radial distance between the sharp root (root apex) and the cylinder or cone that would bound the root.
RUNOUT:	As applied to screw threads, unless otherwise specified, runout refers to circular runout of major and minor cylinders with respect to the pitch cylinder. Circular runout, in accordance with ANSI Y14.5M, controls cumulative variations of circularity and coaxiality. Runout includes variations due to eccentricity and out-of-roundness. The amount of runout is usually expressed in terms of full indicator movement (FIM).

S
SCREW THREAD:	A screw thread is a continuous and projecting helical ridge usually of uniform section on a cylindrical or conical surface.
SHARP CREST (CREST APEX):	This is the apex formed by the intersection of the flanks of a thread when extended, if necessary, beyond the crest.
SHARP ROOT (ROOT APEX):	This is the apex formed by the intersection of the adjacent flanks of adjacent threads when extended, if necessary, beyond the root.
STANDOFF:	This is the axial distance between specified reference points on external and internal taper thread members or gages, when assembled with a specified torque or under other specified conditions.
STRAIGHT THREAD:	A straight thread is a screw thread projecting from a cylindrical surface.

T
TAPER THREAD:	A taper thread is a screw thread projecting from a conical surface.
TENSILE STRESS AREA:	The tensile stress area is an arbitrarily selected area for computing the tensile strength of an externally threaded fastener so that the fastener strength is consistent with the basic material strength of the fastener. It is typically defined as a function of pitch diameter and/or minor diameter to calculate a circular cross section of the fastener correcting for the notch and helix effects of the threads.
THREAD:	A thread is a portion of a screw thread encompassed by on pitch. On a single-start thread it is equal to one turn. (See also Threads per Inch and Turns per Inch.)
THREAD RUNOUT:	See Vanish Thread.
THREAD SERIES:	Thread Series are groups of diameter/pitch combinations distinguished from each other by the number of threads per inch applied to specific diameters.
THREAD SHEAR AREA:	The thread shear area is the total ridge cross-sectional area intersected by a specified cylinder with diameter and length equal to the mating thread engagement. Usually the cylinder diameter for external thread shearing is the minor diameter of the internal thread and for internal thread shearing it is the major diameter of the external thread.
UNIFIED SCREW THREADS
THREADS PER INCH:	The number of threads per inch is the reciprocal of the axial pitch in inches.
TOLERANCE:	The total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.
TOLERANCE CLASS (METRIC):	The tolerance class (metric) is the combination of a tolerance position with a tolerance grade. It specifies the allowance (fundamental deviation), pitch diameter tolerance (flank diametral displacement), and the crest diameter tolerance.
TOLERANCE GRADE (METRIC):	The tolerance grade (metric) is a numerical symbol that designates the tolerances of crest diameters and pitch diameters applied to the design profiles.
TOLERANCE LIMIT:	This is the variation, positive or negative, by which a size is permitted to depart from the design size.
TOLERANCE POSITION (METRIC):	The tolerance position (metric) is a letter symbol that designates the position of the tolerance zone in relation to the basic size. This position provides the allowance (fundamental deviation).
TOTAL THREAD:	This includes the complete and all of the incomplete thread, thus including the vanish thread and the lead thread.
TRANSITION FIT:	This is a fit having limits of size so prescribed that either a clearance or an interference may result when mating parts are assembled.
TURNS PER INCH:	The number of turns per inch is the reciprocal of the lead in inches.

U
UNILATERAL TOLERANCE:	A tolerance in which variation is permitted in one direction from the specified dimension.

V
VANISH THREAD (PARTIAL THREAD, WASHOUT THREAD, OR THREAD RUNOUT):	This is that portion of the incomplete thread which is not fully formed at the root or at crest and root. It is produced by the chamfer at the starting end of the thread forming tool.
VIRTUAL DIAMETER:	See Pitch Diameter, Functional Diameter.

W
WASHOUT THREAD:	See Vanish Thread.

UNIFIED SCREW THREADS
American Standard for Unified Screw Threads. - American Standard B1.1-1949 was the first American standard to cover those Unified Thread Series agreed upon by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. These Unified threads are now the basic American standard for fastening types of screw threads. In relation to previous American practice, Unified threads have substantially the same thread form and are mechanically interchangeable with the former American National threads of the same diameter and pitch. The principal differences between the two systems lie in (1) application of allowances; (2) variation of tolerances with size; (3) difference in amount of pitch diameter tolerance on external and internal threads; and (4) differences in thread designation. In the Unified system an allowance is provided on both the Classes 1A and 2A external threads whereas in the American National system only the Class 1 external thread has an allowance. Also, in the Unified system the pitch diameter tolerance of an internal thread is 30 per cent greater than that of the external thread, whereas they are equal in the American National system.
REVISED STANDARD	The revised screw thread standard ANSI B1.1-1982 is much the same as that of ANSI B1.1-1974. The definition of screw thread acceptability criteria in reference to B1.1-1960 has been deleted. Acceptability criteria are described in ANSI B1.3M-1986, Screw Thread Gaging Systems for Dimensional Acceptability, Inch or Metric Screw Threads (UN, UNR, UNJ, M, and MJ). Where the letters U, A or B do not appear in the thread designations, the threads conform to the outdated American National screw threads.
ADVANTAGES OF UNIFIED THREADS	The Unified standard is designed to correct certain production difficulties resulting from the former standard. Often, under the old system, the tolerances of the product were practically absorbed by the combined tool and gage tolerances, leaving little for a working tolerance in manufacture. Somewhat greater tolerances are now provided for nut threads. As contrasted with the old "classes of fit" 1, 2, and 3, for each of which the pitch diameter tolerance on the external and internal threads were equal, the Classes 1B, 2B, and 3B (internal) threads in the new standard have, respectively, a 30 per cent larger pitch diameter tolerance than the 1A, 2A, and 3A (external) threads. Relatively more tolerance is provided for fine threads than for coarse threads of the same pitch. In cases where previous tolerances were more liberal than required, they were reduced.
THREAD FORM	The Design Profiles for Unified screw threads, shown on page 1481, define the maximum material condition for external and internal threads with no allowance and are derived from the Basic Profile, shown on page 1474.
UN EXTERNAL SCREW THREADS	A flat root contour is specified, but it is necessary to provide for some threading tool crest wear, hence a rounded root contour cleared beyond the 0.2p flat width of the Basic Profile is optional.
UNR EXTERNAL SCREW THREADS	In order to reduce the rate of threading tool crest wear and to improve fatigue strength of a flat root thread, the Design Profile of the UNR thread has a smooth, continuous, non-reversing contour with a radius of curvature not less than 0.108p at any point and blends tangentially into the flanks and any straight segment. At the maximum material condition, the point of tangency is specified to be at a distance not less than 0.625H (where H is the height of a sharp V-thread) below the basic major diameter.
UN AND UNR EXTERNAL SCREW THREADS	The Design Profiles of both UN and UNR external screw threads have flat crests. However, in practice, product threads are produced with partially or completely rounded crests. A rounded crest tangent at 0.125p flat is shown as an option on page 1481.
UN INTERNAL SCREW THREAD	In practice it is necessary to provide for some threading tool crest wear, therefor the root of the Design Profile is rounded and cleared beyond the 0.125p flat width of the Basic Profile. There is no internal UNR screw thread.
THREAD SERIES	Thread series are groups of diameter-pitch combinations distinguished from each other by the number of threads per inch applied to a specific diameter. The various diameter-pitch combinations of eleven standard series are shown on Table 2. The limits of size of threads in the eleven standard series together with certain selected combinations of diameter and pitch, as well as the symbols for designating the various threads, are given in Table 4.
COARSE-THREAD SERIES	This series, UNC/UNRC, is the one most commonly used in the bulk production of bolts, screws, nuts and other general engineering applications. It is also used for threading into lower tensile strength materials such as cast iron, mild steel and softer materials (bronze, brass, aluminum, magnesium and plastics) to obtain the optimum resistance to stripping of the internal thread. It is applicable for rapid assembly or disassembly, or if corrosion or slight damage is possible.
FINE-THREAD SERIES	This series, UNF/UNRF, is suitable for the production of bolts, screws, and nuts and for other applications where the Coarse series is not applicable. External threads of this series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member (see p. 1278). It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.
EXTRA-FINE-THREAD SERIES	This series, UNEF/UNREF, is applicable where even finer pitches of threads are desirable, as for short lengths of engagement and for thin-walled tubes, nuts, ferrules, or couplings. It is generally applicable under the conditions stated above for the fine threads.

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Last modified: April 22, 2003