A reference dimension is just what it sounds like. It is a dimension shown for reference. In other words it is there for informational purposes only.
They are not a requirement in any way.
Reference dimensions can be used to clarify other dimensions on a drawing. In some instances, they make a drawing easier to understand.
Reference dimensions on blueprints
How are reference dimensions shown on a drawing?
There is no GD&T symbol for a reference dimension. Reference dimensions are shown on a drawing as a value enclosed in parentheses.
An alternate method is to follow the dimension with “Reference” or “Ref”. The use of “Ref” or enclosing the dimension inside parentheses are by far the most common notations used. These notations are specified in ASME Y14.5 the Dimensioning and Tolerancing standard.
When to use a reference dimension
Reference dimensions are useful for clarification purposes. Their inclusion can make it clear how another dimension should be inspected or manufactured.
At other times they are included to make the drawing easier to read. It isn’t always immediately clear what a part looks like by looking at the blueprint.
A very common use of reference dimensions is to provide a conversion of the length units of the drawing from either metric to inches or vice versa.
Watch out for these conversions! Too often they are rounded excessively and not accurate. Reference dimensions should never be used for acceptance..
Reference dimension examples
These examples show some of the variety you might see on your blueprints to call out reference dimensions.
Reference dimension measurement
Are reference dimensions measured?
Reference dimensions can be measured and the results recorded but this is not a requirement. Often reference dimensions will be recorded more as a note.
Reference dimension vs basic dimension
Shown in parentheses or with Ref notation
Shown enclosed in a box
Controlled by another tolerance (GD&T)
Do not need to be measured or recorded
Will need to be measured for calculation
Basic dimensions are used in GD&T tolerancing. associated with another tolerance or dimension.
While they don’t have a tolerance tied to themselves, they are used to calculate the tolerance of another feature such as the true position of a hole. If the location of a hole was controlled by basic dimensions, you would never reject it for the hole location but instead for violating a GD&T requirement such as true position.
In other words, basic dimensions don’t have their own +/- tolerance but they are controlled by a different tolerance requirement.
Reference dimensions do not have a +/- tolerance and are not controlled by another requirement. They have no tolerance associated with them. No matter how far off the given value a reference dimension is, it would never be cause for rejection.
A basic dimension being far off its nominal value would not be cause for rejection itself, but its effect on another feature referencing the basic dimension could be cause for rejection.
Basic dimensions are identified with a rectangular frame around them such as in the example below.
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