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DATUM IDENTIFICATION AND DATUM TARGETS A datum is a feature or group of features of a part selected for use as a base from which other features or points are located within specified limits. Datum planes aid in achieving consistency in all phases of casting design, production, machining and assembly. They provide a common reference for dimensional layouts and control the accumulation of tolerances. It is strongly recommended that datum points or planes be indicated on the drawing. If they are not they will be selected by the foundry. They should not be on surfaces affected by parting lines, gating, risers, cored or tapered surfaces. They should be located close to the extremities of the casting to eliminate variations in alignment due to small surface irregularities. The designer and the foundry should work together in establishing target points or datum planes in order to minimize costs. Datum plane A is established by three points not in a straight line, Datum plane B is established by two points and perpendicular to plane A. Datum plane C is established by one point and perpendicular to planes B and C. The amount of draft must be taken into consideration when locating the "Datum Targets" on applicable surfaces. Datum targets define points, lines, or areas on a part used in establishing datum planes. To apply the three plane concept to certain parts, specific points, lines or areas on features are designated to establish datum planes. Datum targets may vary from the conventional three point, two point , one point orientation to assure a fixed position of the part. |
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