Datum references
Encyclopedia
Datum reference are a concept used in carpentry
, metalworking
, needlework
, Geometric dimensioning and tolerancing
(GD&T) and similar activities.
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
. 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.
. Datum reference frames are used as part of the feature control frame 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-1994 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 than the functional datums to save cost, improve process speed, and repeatability. A tolerance analysis
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 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
. 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.
Carpentry
A carpenter is a skilled craftsperson who works with timber to construct, install and maintain buildings, furniture, and other objects. The work, known as carpentry, may involve manual labor and work outdoors....
, metalworking
Metalworking
Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewelry. It therefore includes a correspondingly wide range of skills,...
, needlework
Needlework
Needlework is a broad term for the handicrafts of decorative sewing and textile arts. Anything that uses a needle for construction can be called needlework...
, Geometric dimensioning and tolerancing
Geometric dimensioning and tolerancing
Geometric dimensioning and tolerancing is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models for explicitly describing nominal geometry and its allowable variation...
(GD&T) and similar activities.
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
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
. 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.
Engineering
In GD&T, datum reference frames are typically 3DThree-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
. Datum reference frames are used as part of the feature control frame 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-1994 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 than the functional datums to save cost, improve process speed, and repeatability. A tolerance analysis
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 electrical systems. Engineers analyze tolerances for the...
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 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
Coordinate system
In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element. The order of the coordinates is significant and they are sometimes identified by their position in an ordered tuple and sometimes by...
. 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.