![]() A trained engineer is required to run the software. Computer software can be purchased for dimensional analysis. A tolerance analysis may be needed in many cases to convert between the functional datums and the manufacturing datums. The manufacturing datums are typically different from the functional datums to save cost, improve process speed, and repeatability. ![]() Note: Typically, the functional datums are not used to manufacture the part. So, the functional datums are chosen based on how the part attaches. Typically, a part is required to fit with other parts. The datums should be functional per the ASME standard. The engineer selects A/B/C based on the dimensional function of the part. (See the ASME standard Y14.5M-2009 for more examples and material modifiers.) A/B/C is shown at the end of feature control frame to show from where the measurement is taken. In this case, the datum reference frame is A/B/C. 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. ![]() For example, the three planes could be one "face side" and two "datum edges". A typical datum reference frame is made up of three planes. Datum reference frames are used as part of the feature control frame to show where the measurement is taken from. In geometric dimensioning and tolerancing, datum reference frames are typically 3D. 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 engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made. Screenshot from AutoCAD showing usage of T.I.R., perpendicularity, and parallelism GD&T datum reference symbols.Īn engineering datum used in geometric dimensioning and tolerancing is a feature on an object used to create a reference system for measurement. 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. 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. 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. In subsequent marking out, all measurements are then taken from either of the two datum references. If necessary they may be cut, planed or filed to make them so. ![]() For most work, the datum references of the workpiece need to be square. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). The artisan nominates two straight edges on a workpiece as the "datum edges", and they are marked accordingly. In carpentry, an alternative, more common name is "face side" and "face edge". The concept of datums is used in many fields, including carpentry, metalworking, needlework, geometric dimensioning and tolerancing (GD&T), aviation, surveying, geodesy ( geodetic datums), and others. This matters because the hub and rim need to be concentric to within close limits (or else the wheel will not roll smoothly). For example, on a car's wheel, the lug nut holes define a bolt circle that is a datum from which the location of the rim can be defined and measured. JSTOR ( November 2015) ( Learn how and when to remove this template message)Ī datum reference or just datum (plural: datums ) is some important part of an object-such as a point, line, plane, hole, set of holes, or pair of surfaces-that serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value.Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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