Thread Standards and Inspection 151

Acme threads

A thread with a symmetrical trapezoid-like profile. Acme threads have flat crests and 29-degree included angles.

anvils

A fixed element against which an object to be measured is placed. On a thread snap gage, the anvils are segments of threaded cylinders of precise dimensions.

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.

attribute inspection

A quality control method that determines whether a part possesses certain characteristics, or attributes, required for it to be acceptable. In attribute inspection, a part that possesses the required attributes passes inspection, and a part that does not fails.

automated

Run by a preprogrammed mechanical system with little to no human intervention. Automated machines operate more efficiently and precisely than machines that an operator manually controls.

axes

An imaginary straight line that describes the positions of objects in three-dimensional space. Axes refer to locations or movements along linear and rotational paths.

bind

Become difficult to turn or move a threaded component within a mating part. A threaded part should bind on a no-go gage.

bolt

An externally threaded, cylindrical fastener with a head at one end and a blunt tip at the other end. Bolts are assembled with nuts and are designed to fit into non-threaded holes.

buttress threads

A thread with an asymmetrical profile that has one straight and one angled flank. Buttress threads are used when the screw must withstand a great deal of pressure.

chips

An unwanted piece of material that is removed from a workpiece. Chips are a byproduct of machining that can cling to threads.

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.

concave

Curving inward like the interior of a bowl. A full-form segment is a rectangular block with a concave, threaded side.

cone and vee rolls

A pair of cylinders on an indicating thread gage that includes a vee roll that fits to thread roots and a cone roll that fits to thread flanks. Cone and vee rolls measure pitch diameter.

coordinate measuring machines

CMMs. A sophisticated measuring instrument with a flat polished table and a suspended probe that measures parts in three-dimensional space. Coordinate measuring machines can measure using either contact or noncontact methods.

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.

criticality

The level of importance or consequence associated with a specific task or component, particularly in terms of safety, performance, or reliability. Criticality determines how stringent the inspection process must be to ensure that parts meet the necessary standards for their intended application, especially in safety-sensitive or high-performance environments.

dial indicator

A measuring instrument that consists of a circular scale and a needle that moves to point to different measurements on the scale. Dial indicators often appear on variable inspection devices.

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.

drunken threads

A thread defect characterized by a waviness or misalignment of the thread helix, causing it to deviate from its intended path. Drunken threads do not follow a consistent and uniform helical shape.

external threads

A thread formed on the outside of a cylinder, such as on bolts and screws. External threads are found on the outer diameter (OD) of an object and may also be called OD threads.

fastener

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

fittings

A piece of hardware used to connect two or more pipes. Fittings may use pipe threads.

flank

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

full-form segments

A rectangular block on an indicating thread gage that has a concave, threaded side. Full-form segments are used to measure functional diameter.

functional diameter

The smallest or largest pitch diameter that fits with a mating thread. System 21 requires that the largest example of a part's functional diameter fit into its mating part.

gages

An instrument with an established standard size that is used to determine whether a part feature passes or fails inspection. Gages do not determine the degree of variation or provide measurement.

go gage

A tool to verify that a part is within the minimum acceptable dimension. A go gage is used to check if a part can fit into designated hole or around a designated tool.

go/no-go gages

A set of two gages used to determine whether a part feature passes or fails inspection. A go/no-go gage consists of a go gage and a no-go gage.

go/no-go gaging

The use of a gage to determine whether a part feature passes or fails inspection. In go/no-go gaging, no effort is made to determine the exact degree of error.

included angle

The angle between two defined features. On a thread, included angles are created by two adjacent flanks.

indicating thread gages

A variable inspection device that has a dial or digital readout that displays thread measurements. Indicating thread gages show not only pitch diameter and functional diameter, but also variation as the thread is turned along the gage.

inner diameter

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

interchangeable assembly

The ability to assemble parts manufactured separately without any additional fitting or adjustment. Interchangeable assembly ensures that parts can be produced in different locations or times, and still fit together when assembled.

internal threads

A thread formed on the inside of a cylinder or hole, such as in nuts. Internal threads found on the inner diameter (ID) of an object and may also be called ID threads.

International Organization for Standardization

ISO. An organization based in Switzerland that develops and publishes standards for various aspects of manufacturing and industry. The International Organization for Standardization is responsible for standards across a number of different industries.

ISO metric thread standard

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

jacks

A mechanical lifting device used to apply great force or lift heavy loads. Jacks usually use Acme threads.

lathe

A machine tool used to create cylindrical parts. A lathe holds a cylindrical workpiece on one or both ends while a single-point cutting tool is gradually passed along the surface of the rotating part.

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.

leaves

A single section of a screw pitch gage with teeth that match a specific thread pitch. Leaves can lock into place to prevent movement during thread inspection.

machinist's rule

A simple measuring instrument consisting of a long, thin, metal strip with a marked scale of unit divisions. Machinist's rules, also known as metal rules, come in many sizes and forms and can be rigid or flexible.

magnifying glass

A lens held in front of the eye to enlarge an object for the viewer. Magnifying glasses can be used when conducting visual inspection.

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.

master

A threaded part used a basic go gage for threads with minimal tolerance requirements. A master nut, bolt, or screw can be used to test the fit of a mating part.

maximum material limits

The greatest amount of material that an object may have and still pass inspection. System 21 requires that threads function under maximum material limits.

metal rule

A simple measuring instrument consisting of a long, thin, metal strip with a marked scale of unit divisions. Metal rules, also known as machinist's rules, come in many sizes and forms and can be rigid or flexible.

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, also known as the ISO metric thread standard, was developed in Europe by the International Organization for Standardization (ISO).

metric threads

A thread set to a measurement standard based on millimeters. Metric thread standards were established by the International Organization for Standardization (ISO).

micrometer

A precision measuring instrument that contains a calibrated screw mechanism and a spindle that moves along a fixed scale. A micrometer provides highly accurate measurements of small distances.

minimum material limits

The smallest amount of material that an object may have and still pass inspection. System 22 and System 23 require that threads function under minimum material limits.

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.

multi-rib rolls

A pair of threaded cylinders on an indicating thread gage that make full contact with a part's threads except for a small amount of clearance at the crests and roots. Multi-rib rolls measure functional diameter.

no-go gage

A tool to verify that a part does not exceed the maximum allowable dimension. A no-go gage checks that the part cannot fit into a designated hole or around a designated tool.

nominal diameter

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

nominal size

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

nut

An internally threaded fastener consisting of a square or hexagonal block with a hole in the center. Nuts are designed to fit with the external threads of a bolt or screw.

outer diameter

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

parallel

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

pipe threads

A V-shaped thread that wraps around a tapered cylinder. Pipe threads are used on threaded pipes and fittings for plumbing.

pitch

The distance between corresponding points on consecutive threads. Pitch describes the distance from a point on one thread to a matching point on the next thread.

pitch diameter

The diameter of an imaginary cylinder that intersects a threaded object at the point where the width of the threads and the width of the spaces between the threads are the same. Pitch diameter is larger than the minor diameter but smaller than the major diameter.

precision

The degree of consistency and repeatability in measurements or processes. In the context of inspection, precision indicates how closely multiple measurements come to each other, even if they are not necessarily accurate.

precision wires

A thin, hardened steel rod with a very precise diameter. Precision wires are used in the three-wire method of variable thread inspection.

probe

A device that gathers measurement data from a part being inspected. The probe of a coordinate measuring machine uses either contact or noncontact methods to measure a part.

roll thread snap gage

A special type of thread snap gage that has free-turning, threaded rollers instead of anvils. Roll thread snap gages can provide more accurate and repeatable measurements because their rollers reduce friction and wear compared to anvils.

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.

scale

A standard of measurement that is often displayed as a series of lines. The lines on a scale indicate measurement unit divisions.

screw

An externally threaded, cylindrical fastener with a head on one end and a flat or pointed tip on the other end. Screws are designed to either fit into threaded holes or form threads in material.

screw pitch gages

An attribute inspection instrument with several different leaves, each of which has teeth calibrated to a particular thread pitch. A screw pitch gage is used to check the pitch of threaded parts like screws.

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.

software

A set of programs and instructions that control computer hardware functions and operations. Inspection software can be used to automatically measure parts.

specifications

A description of the essential physical and technical properties of a finished part. Specifications outline important information including finished part dimensions and acceptable tolerance ranges.

square threads

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

standards

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

start

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

surface finish

The degree of roughness and variation on the surface of a part after it has been manufactured. Due to irregularities created when machining a part, surface finish cannot be perfectly smooth.

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 and Acme threads.

System 21

A thread tolerance standard that requires threads to be suitable for interchangeable assembly and pass a go/no-go inspection. System 21 deals only with specifications for the functional diameter of a thread.

System 22

A thread tolerance standard that requires threads to meet both minimum and maximum material limits. System 22 includes specifications for both functional diameter and pitch diameter.

System 23

A thread tolerance standard that requires threads to conform with specifications for pitch diameter, functional diameter, major diameter, minor diameter, and surface finish. System 23 is used for safety-critical applications such as aerospace parts.

tangent

A straight line that touches a curve at a single point without crossing it at that point. When using the three-wire method, wires touch the thread flanks at points that are tangent to the pitch diameter of the thread.

thread

A long, spiral ridge around the exterior or interior of a cylindrically shaped object. Threads are used to fasten or provide motion.

thread depth

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

thread forms

The unique shape or profile of a thread. Thread forms include V-shaped threads, Acme threads, square threads, buttress threads, and pipe threads.

thread gages

An attribute inspection instrument used to check the accuracy of threads. Thread gages include ring gages, plug gages, and snap gages.

thread plug gage

A cylinder of heavy metal with threaded ends made to match particular internal threads. Thread plug gages typically have a go end and a no-go end.

thread ring gage

A disk of heavy metal with a central, threaded hole made to match a particular external thread. A thread ring gage is screwed onto the part being inspected to check pitch diameter.

thread series

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

thread snap gage

A C-shaped device that is used to inspect the major diameter and the pitch diameter of external threads that have no free end on which to place a thread ring gage. Thread snap gages are typically made for specific thread sizes and types.

threaded fastener

A device that uses compatible threads to hold two or more objects together. Common threaded fasteners include screws, bolts, and nuts.

threads per inch

TPI. The number of threads in one linear inch. Threads per inch is used in Unified thread specifications.

three-dimensional

3D. Having length, width, and height. Three-dimensional objects occur within three planes of space.

three-wire method

A variable thread inspection technique that uses precision wires and a micrometer to take measurements. The measurements found by the three-wire method can be used to calculate pitch diameter.

tolerance class

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

tolerances

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

TPI

Threads per inch. The number of threads in one linear inch. TPI 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.

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

UTS. 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.

Unified threads

A thread set to a measurement standard based on inches. Unified threads conform to specifications of the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI).

variable inspection

A quality control method that reveals the degree of variation from a given standard or expected measurement. Variable inspection gives the actual measurements of a part feature that can then be compared to its specifications.

vises

A workholding device with one fixed jaw and one movable jaw. Vises use buttress threads.

V-shaped threads

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

wire size chart

A table accompanying a set of precision wires that lists their specific sizes. A wire size chart also notes the corresponding constants used for calculating pitch diameter.