Introduction to Fastener Threads 221

12-UN series

A set of medium-pitch Unified threads that have 12 threads per inch. 12-UN series threads are used in place of UNF threads for diameters over 1 inch.

16-UN series

A set of fine-pitch Unified threads that have 16 threads per inch. 16-UN series threads are used in place of UNF threads for diameters over 1 inch.

8-UN series

A set of coarse-pitch Unified threads that have 8 threads per inch. 8-UN series threads are used in place of UNC threads for diameters over 1 inch.

acme threads

A thread with a symmetrical trapezoid-like profile. Acme threads have flat crests and 29° included angles for power and motion transfer.

adhesive bonding

The joining of two or more materials using adhesive substances such as pastes, gels, glues, and tape. Adhesive bonding can be used to join similar and dissimilar materials.

allowance

The difference in dimension or space between parts. Allowance on threaded parts is a specific amount of clearance or interference between threads.

asymmetrical

Having features on either side of a point, line, or plane that differ in size, shape, or position. Asymmetrical flanks are a characteristic of buttress threads.

blind threaded hole

An internally threaded hole that does not go completely through a material or object. Blind threaded holes are typically used with screws.

bolt

A cylindrical threaded fastener with a head that usually mates with a nut. Bolts typically have blunt ends.

buttress threads

A thread with an asymmetrical profile that has one straight and one angled flank. Buttress threads have flat crests and roots.

callout

The listed designations for a specific thread. The callout for Unified threads and metric threads include different information relating to nominal size, pitch, fit, and type of thread, among others.

chamfered

Angled or beveled along an edge. Chamfered ends allow parts to fit together easily and protect against damage.

chips

An unwanted piece of metal that is removed from a workpiece. Chips are formed when a tool cuts or grinds metal.

class of fit

A designation that indicates how loose or tight a Unified thread fits. Class of fit uses numbers to designate tightness, with higher numbers being the tightest.

clearance

The amount of space or distance between two mating parts. Clearance is greater between threads with looser fits.

cold forming

A process that compresses or shapes metal into a finished product at low or room temperatures. Cold forming improves the strength of the metal.

crest

The top or peak of a thread. The crest is formed by the meeting of two flanks away from the body of the fastener.

cross threading

A condition in which the internal and external threads of a fastener assembly are not properly aligned. Cross threading can cause threads to become damaged or locked together.

cutting tools

A device made of hard, tough material with one or more sharp edges used to create chips and remove material. Cutting tools are either single-point or multi-point tools.

cylinder

A solid, three-dimensional shape with circular ends and straight sides. Cylinders form the basic shape of threaded fasteners.

depth

The distance between the root and the crest of a thread. Depth is also known as thread height.

diameter

The distance from one edge of a circle or circular object to the opposite edge, as measured through the center. Diameter describes the size of round and cylindrical objects, such as fasteners, workpieces, and tools.

double-start thread

A thread consisting of two parallel ridges wrapped around the cylinder. A double-start thread gives a fastener a lead that equals two times its pitch.

engine blocks

A casing that holds various moving parts in an automotive engine and provides channels for fluids to flow. Engine blocks on larger engines often use 8-UN series threads.

external threads

Threads formed on the outside of a cylinder, such as on bolts and screws. External threads are also called male threads.

fastener

A device that holds objects together or locates them in relation to one another. Fasteners can be threaded or non-threaded.

fatigue resistant

Able to withstand load conditions that normally cause failure. Fatigue-resistant threads are manufactured by thread rolling.

female threads

Threads formed on the inside of a cylinder or hole, such as in nuts. Female threads are also called internal threads.

flank

An angled side of a thread that connects the root to the crest. Flanks occur in pairs.

grinding wheel

A disc made of a bonded abrasive used to remove material from a workpiece surface. Grinding wheels are typically mounted in a spindle that allows them to rotate.

head

The end of a fastener that provides a feature for gripping and turning and allows it to bear loads. Head shape and style varies depending on the type of fastener.

helical

Winding in the shape of a spiral or coil. Helical ridges called threads appear on screws and other fasteners.

helix

A spiral structure with a repeating pattern. The helix of a thread can be right-handed or left-handed.

hexagonal

Having six sides. Hexagonal fasteners like nuts often accommodate a driving tool.

included angle

The angle between two defined features. Included angles are created by two adjacent flanks on a thread.

inside diameter

ID. The interior surface of a spherical or cylindrical object. Threads on the inside diameter are called internal or female threads.

interference

An amount of overlap between two mating parts. Interference between threads requires significant force for assembly or disassembly.

interference fit

A tight fit designed so that the smallest permissible shaft is larger than the largest permissible hole. Force is required to assemble parts with an interference fit.

internal threads

Threads formed on the inside of a cylinder or hole, such as in nuts. Internal threads are also called female threads.

International Organization for Standardization

ISO. An organization based in Switzerland that develops and publishes standards for its international membership base. The International Organization for Standardization developed the metric thread standard.

ISO metric thread standard

A system for cataloging and defining threads based on millimeters. The ISO metric thread standard was developed in Europe by the International Organization for Standardization (ISO), and is also known as the metric thread standard.

lead

The distance a threaded part advances in one revolution. Lead is equal to the pitch multiplied by the number of starts.

leadscrews

A long, threaded bar that transfers power between machine tool components. Leadscrews often have multiple-start threads.

left-hand thread

A thread that is driven by rotating it counterclockwise. Left-hand threads are rare.

major diameter

The largest diameter of a thread. Major diameter is measured from crest to crest on an external thread and from root to root on an internal thread.

male threads

Threads formed on the outside of a cylinder, such as on bolts and screws. Male threads are also called external threads.

metric designation

The letter "M" that identifies all metric threads. The metric designation appears at the beginning of all metric thread specifications and distinguishes metric threads from other types of threads.

metric thread standard

A system for cataloging and defining threads based on millimeters. The metric thread standard was developed in Europe by the International Organization for Standardization (ISO), and is also known as the ISO metric thread standard.

minor diameter

The smallest diameter of a thread. Minor diameter is measured from root to root on an external thread and from crest to crest on an internal thread.

nominal diameter

The overall diameter of the fastener, which may be different from its actual, measurable size. Nominal diameter is used to indicate a fastener's nominal size in metric and Unified thread specifications.

nominal size

The size by which something is known, which may be different from its actual, measurable size. Nominal size of fasteners is given as the nominal diameter in metric and Unified thread designations.

nut

A fastener with a threaded hole that mates with a bolt. Nuts are often hexagon-shaped to accommodate a driving tool.

outside diameter

OD. The exterior surface of a spherical or cylindrical object. Threads on the outside diameter are called external or male threads.

parallel

Extending in the same direction and equally distance from one another at all points. The number of parallel threads on a fastener is equal to the number of starts.

perpendicular

Intersecting at a right, or 90° angle. Perpendicular flanks are found on square threads.

pitch

The distance between corresponding points on consecutive threads. Pitch can be determined by measuring the distance between two adjacent crests or roots.

pitch diameter

The diameter of an imaginary cylinder that passes through the thread form in such a way that it intersects the thread groove and the thread ridge equally. Pitch diameter is larger than the minor diameter but smaller than the major diameter and marks the theoretical point where threads of a fastener and threads of a hole meet.

pressure vessels

A closed container whose contents are held at a pressure different from the pressure of the air outside the vessel. Pressure vessels often use 8-UN threads.

radius

A curved or rounded profile at the junction of two surfaces. Radius surfaces at the root are a characteristic of V-shaped threads.

right-hand thread

A thread that is driven by rotating it clockwise. Right-hand threads are found on most threaded fasteners.

root

The groove at the base of a thread. The root is formed by the meeting of two flanks on the body of the fastener.

screw

A cylindrical threaded fastener that either fits into a threaded hole or forms threads in a material. Screws may have blunt or pointed ends.

set screws

A type of screw that is fully threaded from end to end with a rounded or cone-shaped end and a driving recess or external drive. Set screws are typically used to fit into a machining recess and lock a component onto a shaft.

single-start thread

A thread consisting of one ridge wrapped around the cylinder. A single-start thread gives a fastener a lead that equals its pitch.

spindle

A rotating component of a machine tool. Spindles can hold and rotate grinding wheels.

square threads

A thread with a symmetrical box-like profile. Square threads have straight flanks that are perpendicular to the crest and root.

start

The number of individual parallel threads that wrap around a fastener. The start affects the lead of a threaded fastener.

stress area

The cross-sectional area of a threaded fastener that is subjected to a tensile force. Stress area is the weakest, and usually smallest, cross section of the fastener, and therefore determines the fastener’s strength.

studs

A cylindrical fastener that has external threads at both ends. Studs do not have heads.

symmetrical

Having features on either side of a point, line, or plane that are identical in size, shape, and position. Symmetrical flanks are a characteristic of V-shaped threads.

thread

A raised, helical ridge around the interior or exterior of a cylindrical object or hole. Threads help fasteners grip material and hold components together.

thread cutting

A method of producing threads by using cutting tools to remove material from a solid blank or workpiece. Thread cutting produces weaker threads than thread rolling and is often used to manufacture internal threads.

thread form

The unique shape or profile of a thread. Thread forms vary depending on the shape of their crests, roots, and flanks.

thread height

The distance between the root and the crest of a thread. Thread height is also known as depth.

thread nicking

A condition in which the crests of external threads are damaged, usually by rough handling. Thread nicking can cause problems with assembly.

thread rolling

A method of producing threads by forcing hard dies into a solid blank or workpiece. Thread rolling is an example of cold forming and produces stronger threads than thread cutting.

thread series

A designation that indicates the coarseness or fineness of Unified threads. Thread series is usually either Unified coarse (UNC) or Unified fine (UNF).

Thread standards

A policy and guideline established to ensure thread compatibility and interchangeability. Thread standards include metric and Unified thread standards.

threaded fastener

A fastener that has threads to hold objects together or grip material. Threaded fasteners include screws, nuts, and bolts.

thread-rolling dies

Thread-forming components with grooves and ridges that shape metal into threads when extreme pressure is applied. Thread-rolling dies produce strong and fatigue-resistant threads.

threads per inch

TPI. The number of threads in one linear inch. Threads per inch depends on the pitch and is used in Unified thread specifications.

through hole

An internal hole that completely passes through an object or material. Through holes are often used with bolts and nuts.

tolerance

The unwanted but acceptable deviation from a desired dimension. Tolerance determines how much an object can vary from the desired dimension and still meet specifications.

tolerance class

A number and letter combination that indicates a thread’s degree of fit and whether it is an internal or external thread. Tolerance class identifies the degree of fit with a number and the tolerance position and type of thread with a letter.

TPI

Threads per inch. The number of threads in one linear inch. TPI depends on the pitch and is used in Unified thread specifications.

trapezoid

A four-sided shape that resembles a triangle with a flattened top. Trapezoid-shaped threads are called acme threads.

UNC

Unified coarse. A Unified thread series that identifies threads with larger pitches and thread depths. UNC threads are easier to assemble and disassemble than Unified fine (UNF) threads.

UNF

Unified fine. A Unified thread series that identifies threads with smaller pitches and thread depths. UNF threads have a greater stress area than Unified coarse (UNC) threads.

Unified coarse

UNC. A Unified thread series that identifies threads with larger pitches and thread depths. Unified coarse threads are easier to assemble and disassemble than Unified fine (UNF) threads.

Unified fine

UNF. A Unified thread series that identifies threads with smaller pitches and thread depths. Unified fine threads have a greater stress area than Unified coarse (UNC) threads.

Unified thread standard

UN. A system for cataloging and defining threads based on inches. The Unified thread standard was developed by the United States, Canada, and the United Kingdom.

V-shaped threads

A thread with a symmetrical peaked profile. V-shaped threads generally have 60° included angles.

welding

The joining of materials, usually metals, through the use of heat, pressure, and/or chemicals. Welding fuses two materials together permanently.