A datum reference or just datum (plural: datumsThe plural of this

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...

of the word ''

datum In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted. ...

'' is ''datums'' by convention, in contrast with the other senses of the word in which ''

data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...

'' usually serves as both the plural form and the

mass noun In linguistics, a mass noun, uncountable noun, non-count noun, uncount noun, or just uncountable, is a noun with the syntactic property that any quantity of it is treated as an undifferentiated unit, rather than as something with discrete elemen ...

counterpart.) ) is some important part of an object—such as a

point Point or points may refer to: Places * Point, Lewis, a peninsula in the Outer Hebrides, Scotland * Point, Texas, a city in Rains County, Texas, United States * Point, the NE tip and a ferry terminal of Lismore, Inner Hebrides, Scotland * Point ...

line Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Arts ...

plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''Planes' ...

, hole, set of holes, or pair of

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is t ...

s—that serves as a reference in defining the

geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...

of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the

nominal Nominal may refer to: Linguistics and grammar * Nominal (linguistics), one of the parts of speech * Nominal, the adjectival form of "noun", as in "nominal agreement" (= "noun agreement") * Nominal sentence, a sentence without a finite verb * Nou ...

value, which may be an ideal, standard, average, or desired value. For example, on a car's wheel, the

lug nut A lug nut or wheel nut is a fastener, specifically a nut, used to secure a wheel on a vehicle. Typically, lug nuts are found on automobiles, trucks (lorries), and other large vehicles using rubber tires. Design A lug nut is a nut fastener wi ...

holes define a

bolt circle A bolt circle diameter or pitch circle diameter (PCD), sometimes simply called bolt circle or pitch circle, is a common term for when a number of screw holes for bolts are evenly distributed with their centers along an imaginary circle with a giv ...

that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be

concentric In geometry, two or more objects are said to be concentric, coaxal, or coaxial when they share the same center or axis. Circles, regular polygons and regular polyhedra, and spheres may be concentric to one another (sharing the same center point ...

to within close limits (or else the wheel will not roll smoothly). The concept of datums is used in many fields, including

carpentry Carpentry is a skilled trade and a craft in which the primary work performed is the cutting, shaping and installation of building materials during the construction of buildings, ships, timber bridges, concrete formwork, etc. Carpenters tr ...

metalworking Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale ...

needlework Needlework is decorative sewing and textile arts handicrafts. Anything that uses a needle for construction can be called needlework. Needlework may include related textile crafts such as crochet, worked with a hook, or tatting, worked with a ...

geometric dimensioning and tolerancing Geometric Dimensioning and Tolerancing (GD&T) is a system for defining and communicating engineering tolerances and relationships. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that exp ...

(GD&T),

aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air ...

surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...

geodesy Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...

(

geodetic datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other plan ...

s), and others.

Uses

In carpentry, an alternative, more common name is "face side" and "face edge". The artisan nominates two straight edges on a workpiece as the "datum edges", and they are marked accordingly. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). For most work, the datum references of the workpiece need to be

square In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90-degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length adj ...

. If necessary they may be cut, planed or filed to make them so. In subsequent marking out, all measurements are then taken from either of the two datum references. In aviation, an aircraft is designed to operate within a specified range of weight and (chiefly longitudinal) balance; an airman is responsible for determining these factors for each flight under his or her command. This requires the calculation of moment for each variable mass in the aircraft (fuel, passengers, cargo, etc.), by multiplying its weight by its distance from a datum reference. The datum for light airplanes is usually the engine firewall or the tip of the spinner, but in all cases it is a fixed plane perpendicular to the aircraft's longitudinal axis, and specified in its operating handbook.

Engineering

An engineering datum used in

is a feature on an object used to create a reference system for

measurement Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared ...

. In

engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

and drafting, a ''datum'' is a reference point, surface, or axis on an object against which measurements are made. These are then referred to by one or more 'datum references' which indicate measurements that should be made with respect to the corresponding datum feature . In

, datum reference frames are typically 3D. Datum reference frames are used as part of the

feature control frame Geometric Dimensioning and Tolerancing (GD&T) is a system for defining and communicating engineering tolerances and relationships. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that exp ...

to show where the measurement is taken from. A typical datum reference frame is made up of three planes. For example, the three planes could be one "face side" and two "datum edges". These three planes are marked A, B and C, where A is the face side, B is the first datum edge, and C is the second datum edge. In this case, the datum reference frame is A/B/C. A/B/C is shown at the end of feature control frame to show from where the measurement is taken. (See the ASME standard Y14.5M-2009 for more examples and material modifiers.) The engineer selects A/B/C based on the dimensional function of the part. The datums should be functional per the ASME standard. Typically, a part is required to fit with other parts. So, the functional datums are chosen based on how the part attaches. Note: Typically, the functional datums are not used to manufacture the part. The manufacturing datums are typically different from the functional datums to save cost, improve process speed, and repeatability. A

tolerance analysis Tolerance analysis is the general term for activities related to the study of accumulated variation in mechanical parts and assemblies. Its methods may be used on other types of systems subject to accumulated variation, such as mechanical and elec ...

may be needed in many cases to convert between the functional datums and the manufacturing datums. Computer software can be purchased for dimensional analysis. A trained engineer is required to run the software. There are typically 6

degrees of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

that need to be considered by the engineer before choosing which feature is A, B, or C. For this example, A is the primary datum, B is the secondary, and C is the tertiary datum. The primary datum controls the most degrees of freedom. The tertiary datum controls the least degrees of freedom. For this example, of a block of wood, Datum A controls 3 degrees of freedom, B controls 2 degrees of freedom, and C controls 1 degree of freedom. 3+2+1 = 6, all 6 degrees of freedom are considered. The 6 degrees of freedom in this example are 3 translation and 3 rotation about the 3D

coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...

. Datum A controls 3: translation along the Z axis, rotation about the x axis, and rotation about the y axis. Datum B controls 2: translation along the y axis and rotation about the z axis. Finally, Datum C controls 1 degree of freedom, namely the translation along the x axis.

Notes

References

{{Woodworking Woodworking Geometry Coordinate systems

Uses

Engineering

See also

Notes

References