Graphics Reference
In-Depth Information
Feature-based modeling dates back to the mid-1970s when attempts were made
to get data on manufacturing features for NC programming. Kyprianou [Kypr80]
was the person who first introduced automated feature recognition into a modeler
(BUILD). [PraW85] was one of the earliest studies of “design by features.” So what
exactly is a “feature?”
The term “feature” refers to a high-level abstract entity. It refers to some inter-
esting aspect of, or associated to, an object and is usually a combining of details into
one entity that is more meaningful for manipulation than the individual parts. For
example, in a b-rep modeler a block with a hole through it might consist of a collec-
tion of surface patches with no explicit notion of the center and radius of the hole.
Moving the hole might then involve moving and changing a subset of these patches -
a tedious task. The term “feature” was first used in manufacturing but has since taken
on a broader meaning. Machined parts typically can be described by things like holes,
slots, pockets, and grooves. A relatively small collection of such features might have
been adequate to describe a part in a particular manufacturing environment and with
them one might then be able to create a manufacturing plan. Features are important
to automating the design to manufacturing process because they help define the func-
tionality of objects. [ShaM95] lists the following characteristics of a feature:
(1) It is a physical constituent of a part.
(2) It is mappable to a generic shape.
(3) It has engineering significance. (This may involve describing its function or
how it “behaves.”)
(4) It has predictable properties.
A
feature model
is a data structure representing a part or assembly mainly in terms
of its constituent features. It is convenient to subdivide features into the following
subtypes:
(1) Geometric features
(a) Form features:
They describe some idealized geometry.
(b) Tolerance features:
They describe variance constraints from the ideal-
ized geometry.
(c) Assembly features:
This is a grouping of various feature types into a
larger entity and includes kinematic relationships.
(2) Functional features:
These describe the overall function of the part.
(3) Material features:
These give material names, specify treatments such
as painting, etc.
Figure 5.32 shows a standard example of what one means by form features. It is
a slightly modified version of the CAM-I ANC101 part that is not the picture of any
real functioning object but is simply used to test geometric capabilities of modelers.
(CAM-I is an abbreviation for Computer Aided Manufacturing, Inc., a nonprofit
consortium in Arlington, Texas.) Form features can be primitive or compound. For
example, one can talk about a specific pattern of holes rather than just an individual
