![]() ![]() Datum targets are symbols that describe the shape, size, and location of gage elements that are used to establish datum planes, axes, and points. Coplanar datum features are two or more datum features that are on the same plane.Coplanar surfaces are two or more surfaces that are on the same plane.The 3-2-1 Rule defines the minimum points of contact, with the primary datum as 3, the secondary datum as 2, and the tertiary datum as 1.A part may have as many datum reference frames as needed to define its functional relationships.A Datum reference frame is a set of three mutually perpendicular datum planes, which provide direction and origin of measurements.Datum Features are often the features that orient (stabilize) and locate the part in its assembly.Datum Features are selected on the basis of part function and assembly requirements.A true geometric counterpart is the theoretical perfect boundary or best fit tangent plane of a specified datum feature.A datum feature is a part feature that contacts a datum. ![]() A datum is a theoretically exact plane, point, or axis from which a dimensional measurement is made.One-of-a-kind part specify your tolerances RFS. MMC is very useful when designing a part destined for mass production (reduce rejection rates).RFS means that the geometric tolerance applies regardless of the actual produced size of the feature. Otherwise MMC or LMC must be specified on the Feature Control Frame. Rule #2: Regardless of Feature Size (RFS) is the default condition of all geometric tolerances.With multiple FOS, rule #1 does not control location, orientation or relationship between them. Note that rule #1 does not apply to Stock size parts or parts made of flexible materials and, in a part.There is no perfect form boundary requirement at LMC. The form tolerance increases as the actual size of the feature departs from MMC toward LMC. No element of a feature shall extend beyond the MMC boundary of perfect form. In its form, as well as its size, are allowed. Rule #1: Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extend to which variations.Used to control the form, orientation, and cylindricity of a diameter relative to one or more datum axis. Used to control the form, orientation, and circularity of individual circular elements of a part feature relative to one or more datum axis. Used on planar FOS to ensure balanced distribution of mass and consistent wall thickness. Used on rotational parts to ensure balanced distribution of mass and consistent wall thickness. With respect to true position (theoretically exact location). Tolerance of position (TOP) is used to define the variation of location of a feature of size (FOS) Used to limit the amount a surface, axis, or centerplane is permitted to vary from being parallel to the datum. Used to control the amount a surface, axis, or centerplane is permitted to vary from 90° to the Datum. Used to control the amount a surface, axis, or centerplane is permitted to vary from its specified angle. Surfaces of revolution, or cones and coplanar surfaces. Used to control the size, location, orientation and form of planar, curved, or irregular surfaces, polygons, cylinders Used to limit the amount of error for line elements relative to their true profile. Used to limit the surface conditions (out of round, taper, and straightness) of a shaft diameter. Used to limit the lobing (out of round) of a shaft diameter, which can cause bearing or bushing failure. Used to control flatness for a gasket or seal, to attach a mating part or for better contact with a datum simulation surface.Īpplies without regard for the size of the feature (RFS). ![]() Straightness tolerance specifies a tolerance zone within which the line element of a feature must lie. (click the control name for more details) GD&T Guide Geometric Dimensioning and Tolerancing ![]()
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