Basic dimension

[Pac-man]

Hi all!

Not much is shown in ASME 14.5 about basic dimensions “boxed dimensions” and how to use/put them on a drawing?
People put them everywhere on the drawing? What is ASME 14.5 telling us?

Is it wrong to put them on the drawing where no datum feature symbols exist?
Is it wrong to put them on the drawing where no feature control frame exist?

Regards
Pac-man

 

[TheTick]

Basic dimensions are meaningless without feature control frames to give them tolerance and form. They are nothing more than reference dimensions without related FCFs.

 

[3DDave]

It typically isn't useful to use basic dimensions without other tolerances. It's not possible to make a blanket statement saying no, but I'd have to see an example drawing to know what it is that is being discussed.

 

[drawoh]

Pac-man,

A dimension is basic when a feature control frame controls the variance of the feature.

--
JHG

 

[ewh]

An exception to requiring a FCF for basic dimensions would be on inspection tooling drawings, which may default to shop tolerances; I'm not aware of any other exceptions.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[Burunduk]

A basic dimension is a theoretcally exact dimension.
There are other uses of basic dimensions beside providing a basis for a tolerance zone related to a value noted in feature control frame. Examples are: defining size and location for a limited length or area indication such as you may need for a flatness control on a partial area of the surface, or for defining datum targets - locating datum targets to each other, or specifying dimensions for datum target areas size and form.