The solid modeling of a cast product is the backbone
for various CAD/CAM programs that help in improving the accuracy and
speed of different tasks in casting development. Solid modeling enables
economical manufacture of products with intricate geometry to near net
shape. Components having complex curved surfaces, including blends
between sculptured surfaces, intricately shaped pockets, and meandering
holes besides features such as bosses, slots, steps and ribs can be
produced. Several geometric elements or features can be economically
combined together in a single casting, which otherwise require joining
or assembly of multiple parts.
The two most common approaches to solid modeling include extrusion of a
2D section and constructive solid geometry. Another approach, called
features-based modeling, is also available in most solid modeling
systems today. These techniques are useful to define the base feature of
a casting as well as its local features.
Extrusion or sweep: This technique involves sketching a
cross-section on a plane and sweeping it through a straight or curved
path. The cross-section can have inner loops, leading to hollow shapes.
Depending on the path, we can have straight extruded shapes and solids
of revolution. For straight extrusion, the distance has to be provided.
Revolution can be along the complete circle or a part of it. It is also
possible to have negative extrusion, referred to as cut extrude, which
produces depression features in an existing solid. For example, a cut
extrude circle produces a hole.
Constructive Solid Geometry: This technique uses Boolean
operations: union, intersection and difference on pairs of simpler
models to create the desired shape. A library of solid primitives such
as cube, cylinder, sphere, cone and torus are provided to initiate the
modeling. Each of these is instantiated by supplying the values of its
dimensions, location and orientation. The model created by extrusion or
by combining primitive solids is then combined with other primitives or
other solids to eventually obtain the designed shape of the component.
The final as well as intermediate models are represented in a CSG tree
that shows the primitives and steps used for making the
model.Features-based modeling: Standard features include hole, pocket,
slot, boss, rib and fillet. These can be modeled by Boolean operations;
for example a hole can be produced by subtracting a cylinder from the
base solid. Features-based modeling makes this operation more intuitive
to engineers by adding a hole feature (instead of subtracting a
cylinder), which is essentially a negative cylinder. The user selects a
face (on the base feature) or a set of edges/faces (as in the case of a
fillet), and specifies the feature parameters. The features tree is
preserved, so that the user can easily move, modify or delete a specific
feature.
A modern solid modeling system has graphical icons, context
sensitive menus, features tree (can be expanded or collapsed) and shaded
display of solid model.
Special techniques: This includes analytical surfaces, lofting
and shelling. Analytical surfaces include Coon, Bezier, B-spline or
non-uniform rational B-spline. These are useful for defining intricately
curved surfaces. Lofting involves defining a number of cross-sections
normal to a straight or curved path, and joining them using an
analytical surface. Shelling involves creating an offset surface,
usually inside, and removing the inside part to obtain a shell.
Most of the recent systems also support parametric modeling. The user
can specify relationships between geometric elements (for example, the
distance of a hole from a given rib). The software automatically
re-evaluates these relationships when the relevant parameters are
modified (for example, hole diameter or rib thickness), and positions
the feature appropriately. This also implies that standard shapes of
castings (such as gear wheels and crankshafts) as well as tooling
features (such as feeders) can be created in terms of a few selected
parameters. The appropriate shape can be automatically created by
supplying the values of the parameters involved.
The majority of castings require a combination of all techniques. For
example, the overall shape of a cylinder block or exhaust manifold is
created by sweeping a few sections, then combining them with other
primitives using Boolean operations, followed by subtracting the volume
corresponding to the internal ducts, which are themselves modelled by
complex sweep and CSG techniques. Local features like holes, bosses and
ribs are modelled next. Feature modifiers, such as fillets and taper are
applied last.
The designer requires training and experience in deciding the strategy
for modeling, in particular, combinations of primitive solids that will
lead to the final shape in the least number of steps. Sometimes it
becomes necessary to retrace the steps during modeling and take a
different approach to complete the shape. Evan an experienced designer
may take several days for creating the model of a complex casting. The
benefits of downstream applications however, amply justify the initial
modeling effort.
Solid
Modeling and CAD/CAM