GD&T Feature of size

Elaboration on the concept of Feature of size in GD&T . What is the difference between a regular feature and feature of size?

A mechanical component can have multiple features which can be categorized as

Planar surfaces
Curved surfaces
Features of size

If we take the example of a block with a center hole

The list of features would be included.

The bottom surface
Side surface
Top surface
Hole
Horizontal width of block
Vertical width of block

4,5,6 are called as Feature of size of the part.

The feature of size is any feature which has two opposing elements of geometry which can be measured using a measurement tool like Vernier caliper or micrometer.

Here is a closeup view of a pair of calipers holding a cylindrical feature ,

The cylinder has two opposing elements which are lines of contact to the caliper . That makes it a feature of size

When we consider the square block then the there are multiple points of contact and hence there are multiple opposing elements of geometry.

The feature of size can be internal or external . Above shown examples are external features of size. Holes and rectangular slots are internal feature of size.

Features of size are always accompanied with a nominal size and then there is an associated size tolerance for the feature of size as shown.

Normal features like surfaces need to be differentiated from features of size in GD&T as this is crucial to the application of various geometric controls.

True position geometric control cannot be applied to Non- Features of size .

Also it is very important to specify the diameter symbol when there is a cylindrical feature of size (internal or external)