To establish a primary datum, a minimum of three datum points are required. Datum points are used to establish a virtual datum for the measurement. Datum targets are used to establish datum on an irregular surface where planner datum is not readily possible. You can think of them as anchors for the entire part. Datum Target: Establish datum on irregular surfaces. By placing the part against these six datum points, generally a Tool Point Fixture, a casting then can be measured from the theoretical planes using either a height gage or CMM probe. A datum feature is the tangible surface or feature of size (comprised of multiple surfaces or revolved surfaces) that is indicated by the datum feature symbol. Number of Places The X is used along with a value to indicate the number of times a dimension or feature is repeated on the drawing. Once the datum surfaces are determined, Datum Point locations, with three points on the Primary, two points on the secondary and one on the tertiary need to be designated on each of the datum planes. The movable datum target symbol may be used to indicate movement of the datum target datum feature simulator. Although most castings are not as straightforward as a Shoe Box, the same logic applies. DATUM TARGETS SYMBOLS(contd.) A datum target point is specified by an X shaped symbol, consisting of a pair of lines intersecting at 90. For example envision a Shoe Box, with the floor of the Box being Primary Surface the longest side as being Secondary and the short end as being Tertiary. The second largest perpendicular surface would then be then designated as the Secondary (-B-) and the Tertiary (-C-) a surface perpendicular to the first two. Generally the longest & largest surface of a part is designated as the primary (-A-) datum surface.
The selection of datum surfaces, and assignment of the datum point (also called Tool Points) locations, is a critical consideration for good casting design, so here are a few helpful tips: Previously only Datums were available, but now in MBD 2021, Datum Targets can be added to a Datum. In SOLIDWORKS MBD 2021, Datum Targets are now available. Moreover, Datum Points allow a casting to be machined from the same datum structure. Datum Targets now available in SOLIDWORKS MBD 2021. As castings are never perfectly flat, straight or square, this Geometric fundamental is often used by Casting Designers to establish a datum structure from which to obtain a consistent measurement of cast parts. Recalling basic Geometry, three points are required to define a plane (-A-), two additional points to define a perpendicular, secondary (-B-), plane to the primary and one additional point to define a tertiary plane perpendicular to the first two (-C-). CSNAME: LL84 DESCNM: WGS84 datum, Latitude-Longitude Degrees DTNAME: WGS84.