need help on reference dimension

[jethru_perater]

hi evryone...

can somebdy clearly explain when to use a dimension wherein it`s nominal value is enclosed with parenthersis in drawings? is it called reference dimension? Coz I`m really confused when to have a dimension enclosed with parenthesis. pls helpppp

 

[Dvortygirl]

You are correct that dimensions in parentheses (or followed by the notation REF) is a reference dimension. Reference dimensions are used for informational purposes only. They do not govern the manufacture of the part.

Perhaps the best way to explain is to give a couple of examples. If a dimension duplicates information given elsewhere, I might use a reference dimension. For instance, if I already dimensioned the radius that governs the width of an obround slot, I might include the width dimension as a reference. If I were modifying an off-the-shelf part or beginning with raw material in a standard size (such as tubing or sheet metal) I might include reference dimensions to indicate the nominal size of the original material.

One suggestion, if you're just learning this. Don't include reference dimensions without a reason, just to add extra dimensions. Do include them if they will clarify, inform, or save your machinist a hunt.

You can find additional discussions here and here.

--DG

 

[jethru_perater]

thank you very much dvortygirl...i now understand what that type of dimension really means.