Editing Datum reference (section)

==Engineering== 
[[File:Annotation 2019-10-08 224945.png|thumb|Screenshot from [[AutoCAD]] with various [[geometric dimensioning and tolerancing]] datum reference symbols ([[total indicator reading]], [[Perpendicular|perpendicularity]], and [[Parallel (geometry)|parallelism]] ]]

An engineering datum used in [[geometric dimensioning and tolerancing]] is a feature on an object used to create a reference system for [[measurement]].<ref>ANSI Y14.5M ({{ISBN|0-7918-2223-0}}) for engineering datums.</ref>
In [[engineering]] and [[Engineering drawing|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 [[geometric dimensioning and tolerancing]], datum reference frames are typically [[Three-dimensional space|3D]]. 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-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. However, 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]] may be needed in many cases to convert between the functional datums and the manufacturing datums. Computer software can be purchased for [[dimensional analysis]].

There are typically 6 [[degrees of freedom (mechanics)|degrees of freedom]] that need to be considered 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 (the face) controls 3 degrees of freedom, B (first edge) controls 2 degrees of freedom, and C (second edge) 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]]. The face datum A controls 3: translation along the z-axis, rotation about the x-axis, and rotation about the y-axis. The edge datum B controls 2: translation along the y-axis and rotation about the z-axis. Finally, The edge datum C controls 1 degree of freedom, namely the translation along the x-axis.<ref>{{cite book|title=Dimensioning and tolerancing : engineering drawings and related documentation practices : an international standard.|date=2009|publisher=American Society of Mechanical Engineers|location=New York, NY|isbn=978-0791831922}}</ref>