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Datum Elements and Datum Systems
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Datum elements and datum systems are only
specified in case of geometrical tolerancing
(not on dimensional tolerancing and except on form tolerancing). You can
specify simple datum elements, common datum elements, datum targets or
specified datum systems. The toleranced feature is positioned relatively
with the tolerance zone, and this set is positioned relatively with the
datum or the datum system. These related
positions will be specified using basic dimensions and they are displayed or
not.
For more information about datum constitution and specification, see ISO
5459, ASME Y15.5M and ISO 1101. |
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Datum System Composition
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When the identifiers
are specified separately in each frame of the tolerance frame, the datum
elements represent a datum system. A hierarchy is established between the
datum elements. Reference A is the primary datum and reference B is the
secondary datum. The datum system fitting would be performed first on datum
A, then on datum B, with respect to A. By the way, a tertiary datum
can also be specified. |
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The datum elements have to be fitted
successively (following their specification order) with respect to the
hierarchy for the measurement computations. |
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When only one identifier is specified in the
tolerance frame, the datum is a single datum. |
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When two identifiers separated by a dash are
specified in the tolerance frame, the datum is a common datum.
The two datum elements are to be considered simultaneously. The datum system
fitting would be performed in the meantime on datum A and on datum B (both
datum elements have to be fitted simultaneously for measurement
computations). |
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Datum writing rules
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A capital letter is used to identify the datum
element in the tolerance frame. The datum triangle
may be filled or non-filled. When the datum triangle is placed on the
outline of the element or on its extension line, the datum element
represents the surface itself or the 2D representation of the surface, which
is a line.
When the datum triangle is placed in the alignment of the dimension line,
the datum element represents the median element (usually an axis or a median
plane).
When the datum triangle points directly on a median element, the datum
element represents either the median element itself (usually an axis or a
median plane) or the resulting median element of the collection of the
considered elements.
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