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Solid Modeling and CAD/CAM

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.

Modeling Techniques

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.

Extrusion

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.

Constructive Solid Geometry
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.

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Read More On Solid Modeling: Model Exchange Formats
Read More On Solid Modeling: Model Verification and Application Requirements




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