The concept of feature and feature of size is very basic and very important for understanding GD&T, because in later stage these terms are used quite frequently.
What is Feature?
Feature is general term used for representing literally all physical portion of a part. In engineering drawing or CAD drawing, feature is termed for the cuts, protrusions, rounds, fillets etc. In general every surface (curved or flat) of a engineering or CAD drawing is termed as a feature.
The above drawing of the plate with hole in centre and a chamfer and a fillet has nine features. How? Four sides, one fillet, one chamfer, one cylindrical hole and top and bottom surfaces, total nine.
What is Feature of Size?
The feature of size is either a cylindrical or a spherical surface or a set of two opposite parallel surface with a dimension. So, a feature of size (FOS) dimension must fulfill the following two conditions:
- It should be a spherical, cylindrical or a parallel set surfaces for which a centre axis or plane can be calculated from the surfaces.
- The feature should be associated with a dimension.
It the figure above we have three feature of sizes dimensions, namely, the dimension 8, 4 and the cylindrical hole inside of radius 1.
The feature of size dimensions are of two types: internal feature of size and external feature of size dimension. In the above drawing the dimension of the centre hole is internal feature of size dimension and the other two feature of size dimensions are external feature of size dimensions.
What is non feature of size Dimension?
In an engineering drawing, the dimensions other than the feature of size are called non feature of size dimensions. In the above figure, we have two non feature of size dimensions (the fillet and the chamfer).
The terms feature and feature of size are referred very frequently for explaining various GD&T concepts. The feature of size dimensions are the dimensions attached to the cylindrical or spherical surfaces or a set of parallel surfaces.
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