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LOCATION TOLERANCES


Location Tolerances can be state by three tolerance zone. These are position, concentricity, and symmetry tolerances. Concentricity and symmetry are used to control the center distance of feature elements. Position is used to control coaxiality of features, the center distance between features, and the location of features as a group. position, concentricity, and symmetry tolerances are associated with datums. LMC or MMC can apply to feature of size


Symmetry:


Symmetry Tolerance controls the median points of a feature of size, symmetry tolerance is applied to non circular features. Symmetry Tolerance is a three-dimensional geometric tolerance that controls how much the median points between two features may deviate from a specified center plane or axis. This tolerance is similar to concentricity, and the verification of symmetry tolerance is likewise time-consuming and difficult. It is generally recommended that to use position, parallelism and straightness instead of it. Datum is used for this Tolerance. Symmetry is much like concentricity, except that it controls rectangular features and involves two imaginary flat planes, much like parallelism.




The median plane of the slot shall be contained between two parallel planes, which are 0.004 apart and symmetrically disposed about the median plane with respect to the datum feature A. The two plane are equally disposed about datum plane -A-. The symmetry of the part must be within specified tolerance limit. The tolerance zone is limited by two parallel planes a specified distance apart and disposed symmetrically to the median plane with respect to the datum axis or datum plane.


Positional:


Positional tolerance is a three-dimensional geometric tolerance that controls how much the location of a feature can deviate from its true position. Positional tolerances are probably used more than any other geometric control. Positional tolerances is used to locate features of size from datum planes such as a hole or keyway and used to locate features coaxial to a datum axis. A position tolerance is the total permissible variation in the location of a feature about its exact true position. Positional tolerances for cylindrical features, the position tolerance zone is typically a cylinder within which the axis of the feature must lie. Positional tolerances for other features, the center plane of the feature must fit in the space between two parallel planes. The tolerance defines a zone that the axis or center plane of a feature of size may vary from. The concept is there is an exact or true position that the feature would be if it was made perfect however since nothing is made perfect a tolerance zone allows deviation from perfection. The exact position of the feature is located with basic dimensions. Datums are required. The true/exact location of a feature of size is defined by basic dimensions which is shown in a box and are established from datum planes or axes.  When a material condition modifier is specified a boundary named virtual condition is established. It is located at the true position and it may not be violated by the surface or surfaces of the considered feature. Its size is determined by adding or subtracting depending on whether the feature is an external or an internal feature and whether the material condition specified. LMC or MMC can apply to feature of size apply to feature of size.





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