In GD&T they are used to orient and locate tolerance zones.Дatum Reference Frames and 6 Degrees of FreedomД®very Datum exists within the context of some Datum Reference Frame. You may also come across the term Datum Target, which is defined as, вЂњA specified point, line, or area on a part used to establish a datum.вЂќ Definition of a Datum Reference FrameДЄ Datum Reference Frame is a coordinate system, and preferably it is a Cartesian coordinate system.Д¬oordinate systems are valuable because theyвЂ™re used to locate objects. The standard itself defines a Datum Feature as, вЂњAn actual feature of a part that is used to establish a datum.вЂќ Edges on parts are wavy and are not perfect lines. Surfaces on parts are not perfectly flat planes. It is because real features are never perfect abstract geometrical concepts. The reason theyвЂ™re not the same is perhaps small, but significant. When we refer to the tangible feature on the part that the Datum is associated with, we call it a Datum Feature. They correspond to points, lines, and planes. Datums are abstract geometrical concepts. Datums vs Datum Features and Datum TargetsДЄt this point it is important to talk about the difference between a DatumВ and a Datum FeatureвЂ“they are not the same though it is tempting to refer to them as such at first. In addition, the datum symbol could be shown on a side view by denoting the axis. вЂ“ It can be placed on the Feature Control Frame for the hole. вЂ“ It can be placed on the leader pointing to the hole as it is in the middle case. It is of course referring to the axis and not the surface of the hole. вЂ“ It can be placed directly on the hole, as in the leftmost case. Referring to the example above, the вЂњAвЂќ Datum can appear in these three different places on a drawing: The вЂњAвЂќ Datum can appear in these three different places on a drawingвЂ¦ To establish a datum axis on a feature such as a hole, there are a number of ways to place the datum symbol: Placing the Datum on an axis is common with GD&T symbols that can have axis control like runout, perpendicularity, or concentricity. Remember: this places the Datum on the central axis through the feature, not the surface of the feature. Place the datum symbol on the dimension of a diametric tolerance for Axis control: However, for a round cylinder, the datum is the entire round surface. For any surface other than a round cylinder, the datum is strictly on the side where the symbol is shown. We can place the datum symbol either on the surface or on one extension line from the surface. Two examples of Datum Sybols on a Surface LetвЂ™s go through each one to see how it looks: Datum Symbol On DrawingsДЄs we said in the definition, Datums are planes, axes, or point locations, and thatвЂ™s exactly how they show up on a drawing. Nearly every GD&T symbol except for form tolerances (straightness, flatness, circularity, and cylindricity) can use datums to help specify the geometric control that is needed on the part. Typically, multiple features will be referenced by each datum, so theyвЂ™re a very important part of the whole thing. Definition of a DatumДЄ Datum is a plane, axis, or point location that GD&T dimensional tolerances are referenced to. For that reason, we want to start early with a solid grounding on Datums and Datum Reference Frames. Some say the DRF is the most important concept in geometric positioning and tolerancing because it provides the skeleton or frame of reference to which all requirements are connected. GD&T positions every part within a Datum Reference FrameВ (DRF). Calling them out clearly and labelling each Feature Control Block with the applicable Datums makes it completely clear how things are to be measured.В In the drwaing below, A and B are the Datums used to establish the center of the hole using the Position Symbol and the Feature Control Block for the hole center.ДЄ simple drawing with GD&T datum symbols. BeginnerвЂ™s Guide to GD&T: Datums Introduction to DatumsДatumsВ tell us where to measure from.

Catia hole callout