Reference dimensions on blueprints

Many times I see a dimension on an engineering drawing such as 3.5 REF. What does a reference dimension refer to and is it considered critical?

 

[DenverEngineer]

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REF - usually means that the dimension is not used to specify the part but may be used in some other construction or quality control function.

 

[BML]

Another situation where you might see it is when you are working with an assembly or machined part, and the dimension was called out at the previous level. The operation you are at has nothing to do with the dimension, but it is included so that you don't have to go look up the other print.

 

[MarkRPflow]

In the days of hand drawings this was the way to show a dimensions for &quot;reference&quot; informations only. It was not to be used for the layout or machining of a piece of work. The use of CAD systems has replaced this with parentheses.<br>
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ie ---- 3.5 REF --- can be shown as ---- (3.5) -----. This is easier and &quot;smaller&quot; to use with CAD drawings.

 

[MikeVV]

Reference dimensions are often combinations of other dimensions that &quot;add-up&quot; to a value that is useful, but not one that should be verified thru inspection.&nbsp;&nbsp;&quot;Dual dimensioning&quot; occurs when two or more dimensions (that do have tolerances and need to be inspected) define the same attribute.&nbsp;&nbsp;Often, one of the two is made a reference dimension to avoid dual dimensioning.&nbsp;&nbsp;The careful use of reference dimensions can enhance use of a drawing.&nbsp;&nbsp;A reference dimension can also be used to provide users with the part scale so that they can derive other feature dimensions.