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A reference dimension is a dimension on an engineering drawing provided for information only. Reference dimensions are provided for a variety of reasons and are often an accumulation of other dimensions that are defined elsewhere (e.g. on the drawing or other related documentation). These dimensions may also be used for convenience to identify a single dimension that is specified elsewhere (e.g. on a different drawing sheet).
Reference dimensions are not intended to be used directly to define the geometry of an object. Reference dimensions do not normally govern manufacturing operations (such as machining) in any way and, therefore, do not typically include a dimensional tolerance (though a tolerance may be provided if such information is deemed helpful). Consequently, reference dimensions are also not subject to dimensional inspection under normal circumstances.
Prior to use of modern Computer-Aided Design (CAD) software, reference dimensions were traditionally indicated on a drawing by the abbreviation "REF" written adjacent to the dimension (typically to the right or underneath the dimension). However, with the assistance of modern CAD software, the abbreviation "REF" has often been replaced with the use of parentheses around the dimension. As an example, a distance of 1500 millimeters might be denoted by "(1500 mm)" instead of "1500 mm REF." Although both of these methods of denoting a reference dimension are accepted and still in use today, it is increasingly common to see the use of parentheses instead of the abbreviation "REF" primarily due to widespread use of modern CAD software that makes use of parentheses as the default denotation method whenever reference dimensions are "automatically" created by the software.
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