Basic Measurement 101

accuracy

The difference between a measurement reading and the true value of the measured dimension. The smaller the difference, the greater the accuracy of the reading.

American National Standards Institute

ANSI. A nonprofit organization that establishes standards and guidelines for several industries and related processes and products. The American National Standards Institute defines measurement practices for standard pin gages and ring gages.

American Society Of Mechanical Engineers

ASME. An organization that publishes technical materials and sets industrial and manufacturing standards. Along with the American National Standards Institute (ANSI), The American Society of Mechanical Engineers provides written standards for gaging devices and other inspection practices.

anvil

A fixed jaw against which an object to be measured is placed. Micrometer anvils may be positioned differently depending on the type of micrometer.

attribute inspection

A type of inspection of that uses a device with an established standard size to determine whether a measurement falls within the acceptable tolerance range. Attribute inspection, also known as gaging, determines whether a part passes or fails inspection.

ball anvil micrometer

A type of micrometer with a spherical-shaped anvil. Ball anvil micrometers, or spherical face micrometers, can more easily measure parts with curved surfaces.

beam

The long bar-like portion of a caliper that an inspector holds while measuring a feature. The caliper beam displays a graduated scale, similar to a steel rule.

blade micrometer

An outside micrometer with narrow tips similar to a blade that can fit into grooves or gaps in an object. Blade micrometers measure the outside diameter of a part at grooves, gaps, and other features along the part.

calibrate

To compare and adjust a device with unknown accuracy to a device with a known, accurate standard. Inspectors calibrate measuring devices to eliminate any variation in the device being checked.

calibration

The comparison and adjustment of a device with unknown accuracy to a device with a known, accurate standard. Calibration eliminates any variation in the device being checked.

calipers

A measuring instrument with a pair of jaws on one end and a long beam containing a marked scale of unit divisions. The jaws on most calipers can measure both internal and external features.

certified pin gages

A pin gage that has been accurately calibrated to a known, standard dimension. Certified pin gages are standardized by organizations such as the American Society of Mechanical Engineers (ASME).

decimal ruler

A steel rule or other type of ruler with a resolution of one one hundredth of an inch, or 0.01 in. (0.254 mm). The increments on a decimal ruler are easily converted to decimal values.

depth micrometer

A micrometer with an anvil that reaches into holes and other internal part features to measure depth. Depth micrometers are one-sided micrometers.

dial

A flat disk with a set of circular numbers or markings and a needle that rotates to display a specific value. Dials are used on dial calipers to increase measurement resolution and accuracy.

dial calipers

A type of caliper that uses main scale graduations on the beam and graduations on a round dial to indicate measurements. Dial calipers are easier to read than slide calipers.

digital calipers

A type of caliper that calculates measurements electronically and shows these measurements on a digital display. Digital calipers are as accurate as vernier calipers and are more precise.

discrimination

The smallest unit of measurement that an instrument is capable of displaying. Discrimination, also known as resolution or least count, affects how accurate an instrument's measurement readings can be.

English system

A system of measurements based on the inch, pound, and degrees Fahrenheit. The English system is primarily used in the United States and England.

gage block

A hardened steel block manufactured with highly accurate dimensions that is used to master and calibrate measuring instruments. Gage blocks are available in a set of standardized lengths.

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.

gaging

A type of inspection of that uses a device with an established standard size to determine whether a measurement falls within the acceptable tolerance range. Gaging, also known as attribute inspection, determines whether a part passes or fails inspection.

go gage

A gage designed to fit only with correctly sized parts. Go gages are often combined with no-go gages to form a go/no-go gage.

go/no-go gage

A set of two gages used to determine whether a part is within tolerance. A go/no-go gage consists of a go gage and a no-go gage.

go/no-go gages

A set of two gages used to determine whether a part is within tolerance. A go/no-go gage consists of a go gage and a no-go gage.

graduated scales

A series of measurement markings or lines that are divided into stages or quantities. Graduated scales are used on variable inspection devices to provide specific measurements.

graduations

A visible line on a measuring device that indicates a unit of measurement within a scale. Graduations on a steel rule may indicate English system units, metric system units, or both.

granite

A dense, hard type of rock that exhibits excellent wear resistance, stability, and flatness. Granite tables and surface plates are used for inspection surfaces and reference planes.

groove micrometer

A type of internal micrometer with a single spindle end that separates from a fixed end as an operator turns the spindle screw. On a groove micrometer, the fixed end and spindle end contact opposing surfaces of a groove or other internal feature to measure its width.

height gage

A measuring device with a column mounted on a base such as a granite surface plate, and a unit with an extending arm that slides up and down along a main scale beam. Height gages may also have a vernier scale, a dial indicator, or a digital display and are used to measure vertical dimensions and other distances.

inside micrometers

A type of micrometer with one or more spindle tips that extend or expand to press against the interior edges of a part. Inside micrometers, or internal micrometers, come in a variety of types and shapes for various measurement purposes.

inspection

The examination of a part during or after its creation to confirm that it adheres to specifications. During inspection, defects may be identified and corrected.

internal micrometers

A type of micrometer with one or more spindle tips that extend or expand to press against the interior edges of a part. Internal micrometers, or inside micrometers, come in a variety of types and shapes for various measurement purposes.

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 establishes inspection standards, rules, and guidelines to help ensure that manufacturing processes and products are fit for their purposes.

jaws

The part of a caliper that contacts a part to measure its dimensions. One jaw on a caliper is fixed, while the other jaw slides up and down the beam.

liquid crystal display

LCD. A light-driven display device made from nematic liquid sealed between two pieces of polarized glass. Some digital height gages and other digital measuring devices have a liquid crystal display screen to show readings electronically.

locking nut

A type of nut that contains a locking element that uses friction to create a tight grip on threads. Locking nuts are used on micrometers to lock the spindle in place after measuring a part to help prevent errors in reading measurements.

machinist's rule

A simple measuring instrument consisting of a long, thin metal strip with a graduated scale. The machinist's rule, or steel rule, comes in many sizes and forms and can be rigid or flexible.

main scale

The primary measurement scale on a measurement instrument. The main scale has graduations to indicate different measurement units, such as inches or millimeters.

master device

A device with a known measurement used to determine if a variable inspection device is providing accurate measurements. Master devices such as standard gage blocks and ring gages are used to calibrate inspection instruments.

master gage blocks

A set of gage blocks used by an organization to calibrate and certify other gage blocks and measuring devices. Master gage blocks regulated by the National Institute of Standards and Technology (NIST) are accurate to within one millionth of an inch.

mastering

A quick check of a measuring device against a known standard reference, such as a gage block, to determine if the device is providing accurate measurements. Mastering should be done regularly for measuring devices.

metric system

A system of measurements based on the meter, kilogram, and degrees Celsius. The metric system is an internationally recognized standard.

micrometer

A measuring instrument with a threaded spindle that slowly advances toward a small anvil. Micrometers are available in numerous types for measuring a variety of dimensions and features.

micrometers

A measuring instrument with a threaded spindle that slowly advances toward a small anvil. Micrometers are available in numerous types for measuring a variety of dimensions and features.

National Institute of Standards and Technology

NIST. NIST. A non-regulatory federal agency within the U.S. Department of Commerce. The National Institute of Standards and Technology manages standards for inspection equipment and maintains master gage blocks for gage block calibration in the United States.

no-go gage

A gage purposely designed to not fit with correctly sized parts. No-go gages are often combined with go gages to form a go/no-go gage.

nominal

A stated measurement that may not correlate exactly with a physical part. Nominal measurements for a part indicate the size specified in the part print, which may differ from the size of the actual part.

outside micrometer

A U-shaped type of micrometer with a threaded spindle that slowly advances toward a small anvil to press against the outer edges of an object. Outside micrometers are the most common type of micrometer.

pin gages

A hardened, cylindrical gage used to inspect the size of small holes. Pin gages are available in sets of different sizes that range from 0.011 in. (0.279 mm) to 1.000 in. (25.400 mm) in increments of 0.001 in. (0.0254 mm).

plug gage

A hardened, cylindrical gage used to inspect the size of a hole. Plug gages are available in standardized diameters.

plug gages

A hardened, cylindrical gage used to inspect the size of a hole. Plug gages are available in standardized diameters.

precision

The degree to which an instrument will repeat the same measurement over a period of time. Precision is also called repeatability because precise instruments will provide the same results under unchanged conditions.

prints

A document containing all the instructions necessary to manufacture a part. A print includes a part drawing, dimensions, and notes that describe important part specifications and the order and type of manufacturing operations.

ratchet stop

A component attached to the handle end of a micrometer that turns the spindle with a small amount of constant pressure to improve measurement accuracy and prevent over-tightening. Ratchet stops make a clicking noise when turned by the operator, which helps the operator determine how far the spindle is rotating.

reference plane

A common surface chosen as a point of reference for measuring the dimensions of tools or manufactured parts. Granite surface plates are commonly used as reference planes for height gages and other tools since their dimensions do not change significantly due to thermal expansion.

resolution

The smallest unit of measurement that an instrument is capable of displaying. Resolution, also known as discrimination or least count, affects how accurate an instrument's measurement readings can be.

ring gage

A hardened, round gage with a hole used to inspect the size of cylindrical parts or features. Ring gages often come in go/no-go pairs.

sensitivity

The smallest amount of force or pressure that a measuring instrument is capable of detecting. High sensitivity can improve the accuracy of measuring instruments but can decrease precision due to the likelihood of operator error.

sleeve

The cylindrical part of the micrometer that contains the main scale. The micrometer sleeve also contains an index line for reading the thimble scale.

slide calipers

A type of caliper that uses a movable jaw and and fixed jaw to indicate measurements on a graduated beam that is similar to a steel rule. Slide calipers may have different shapes for measuring internal and external part features.

snap gage

A U-shaped gage with hardened, adjustable anvils on opposite ends used to inspect the length of part features. Snap gages are typically go/no-go gages.

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.

spherical face micrometer

A type of micrometer with a spherical-shaped anvil. Spherical face micrometers, or ball anvil micrometers, can more easily measure parts with curved surfaces.

spindle

A component on a machine or device that rotates or spins. The spindle on a micrometer rotates to move one or more anvils to contact the surface of a part.

spindle screw

A rotating screw on the handle end of a micrometer that moves the spindle end as the operator turns it. The thread pitch of a spindle screw determines how far the spindle advances with each revolution.

standard dimensions

An agreed-upon standard length, width, or other dimension used as a reference for gaging or variable inspection devices. Standard dimensions for gaging are established the National Institute of Standards and Technology (NIST), the American Society of Mechanical Engineers (ASME), and other organizations.

steel rule

A simple measuring instrument consisting of a long, thin metal strip with a graduated scale. The steel rule, or machinist's rule, comes in many sizes and forms and can be rigid or flexible.

stock

Processed raw material that is used to make manufactured parts. Stock is available in standard shapes such as long bars, plates, or sheets.

surface plate

A hard, flat surface, usually made of granite, that is used as a base for mounting gages. A height gage must be mounted on a surface plate.

ten-to-one rule

The inspection guideline that states that the accuracy and resolution of an inspection instrument must be equal to one tenth of the required tolerance of the inspected part feature. According to the ten-to-one rule, a feature with a tolerance to the thousandths place must be measured by an instrument with an accuracy rating and resolution to the ten-thousandths place.

thermal expansion

An increase in size as a material increases in temperature. Thermal expansion of measuring instrument components is undesirable and can cause inaccurate measurements.

thimble

A ring or cylinder that fits around the spindle of a micrometer and contains a smaller scale than the main scale. The thimble rotates along with the rotation of the spindle.

thread pitch

The distance from one thread to the next, measured from crest to crest. The thread pitch determines the distance that the micrometer spindle moves with each rotation.

thread plug gage

A cylinder of heavy metal with threaded ends used to gage threaded hole features. Inspectors screw thread plug gages into threaded holes to determine whether the hole threads are within tolerance.

thread ring gage

A disk of heavy metal with a central, threaded hole used to gage threaded shafts. Inspectors screw thread ring gages onto threaded shafts to determine whether the shaft threads meet the required tolerance.

threads

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

three-point micrometer

A type of internal micrometer with a ring-shaped head opposite the handle and three spindle ends that expand from the ring. On a three-point micrometer, each spindle end contacts the internal surface of a round hole or other feature.

tolerance

An unwanted but acceptable variation or deviation from a desired dimension of a part. A dimension that is within an acceptable tolerance range will still meet specifications.

touch probes

A type of contact probe that detects a feature on a part and generates an electronic signal to record its dimension. Touch probes are used on some height gages to optimize measurement precision.

tubular micrometer

A type of internal micrometer with an anvil on the handle end and a spindle end that increases or decreases the length of the micrometer as it extends and retracts. The anvil and the spindle end on tubular micrometers contact two opposing surfaces of an internal part feature, such as a hole or slot.

variable inspection

A type of inspection that reveals the degree of variation from a given standard or expected measurement. Unlike gaging, variable inspection does not result in a pass/fail decision, but instead gives the actual measurement of a part feature that can then be compared with its specification.

variance

The degree of deviation between one measurement result and the next when the same object is measured. High variance between consecutive inspection measurements indicates low measurement precision.

vernier calipers

A type of caliper with a main graduated scale along the beam and a vernier scale on the movable jaw. Vernier calipers are accurate to within 0.001 in. (0.0254 mm).

vernier micrometers

A type of micrometer that uses a vernier scale with markings that run vertically along the sleeve in addition to the horizontal main scale. Vernier micrometers are the most accurate and precise manual inspection tools.

vernier scale

A smaller scale used alongside a larger graduated main scale that divides the smallest units of the main scale into even smaller units. A vernier scale increases the resolution of the measuring instrument for greater measurement accuracy.