Engineering designs today are frequently

Transcription

1 Basic CAD Engineering designs today are frequently constructed as mathematical solid models instead of solely as 2D drawings. A solid model is one that represents a shape as a 3D object having mass properties. Solid models are useful in several ways. For one, solid-modeling software can serve as a means of portraying parts for study by cross-functional engineering teams. Sales and marketing teams can also more easily understand a solid model. In certain circumstances, solid models are useful because the geometry can represent not only the parts being designed but the intent of the designer. No matter what dimensions the designer types in after he or she has built the model, the nature of the software means that the designer constructs a part so that it is composed of individual features that describe how the geometry is supposed to behave. Today s software is increasingly sophisticated and comprises different varieties of feature-based, parametric, and so-called direct or explicit tools that let designers push and pull models as if they were made of clay. It may first be helpful to describe earlier non-feature-based solid modelers where the designers would, say, define a simple cylinder having the diameter of the desired hole and which was long enough to go through the part. Designers would then tell the software to perform a Boolean difference operation between the part and the cylinder. The result: a hole in the part having the diameter of the cylinder. But say the designer later modifies the part and makes it thicker. If the designer didn t happen to make the cylinder long enough to extend through the new, thicker part, the result is the model of a blind hole. The model captured the geometry the designer specified, but it did not capture the design intent of the designer. A designer working in feature-based software, on the other hand, would approach Presented by Sponsored by Solid operators Solid models can be built with either primitive or boundary representations. Many packages provide both methods. In the primitive approach, elementary shapes are combined in a building-block fashion to create a new shape. So-called Boolean logic commands, such as union, difference, and intersection, aid in forming new shapes. With boundary definitions, 2D surfaces are swept through space to trace out volumes. Most packages provide several types of sweeps to help create a variety of shapes. 1 May 2012

2 Wire frames A wire frame model is created on the terminal display of a typical CAD package. the through-hole differently. The designer would define a feature called a through-hole such that no matter what the dimension of the part, the hole extends all the way through it. In other words, once the topology of the design has been called out in terms of features, any changes to the design always keep these features operational unless the designer specifies otherwise. Typically, the software prompts the user for inputs during the definition of the feature. These may include positional constraints, algebraic definitions, and other factors. Another type of solid modeler that captures more than just part geometry is the variational type. Variational modelers are generally known for their ability to depict underspecified models, where the designer has not described all the part dimensions. These models solve a set of simultaneous nonlinear equations to determine dimensional or geometric constraints. So-called parametric modeling is somewhat analogous to feature-based modeling. Parametric modeling is the term used to describe the capturing of design operations as they take place, as well as the editing that takes place on the design. Here, the ordering of the design operations can be significant. Here is an example. Suppose a designer specifies that two surfaces are parallel such that surface two is parallel or coplanar to surface one. So if one surface moves, surface two moves along with it to maintain the arrangement. But the designer can t move surface two directly when this definition is operative. Fortunately, parametric modelers incorporate means of undoing operations. Some versions incorporate features that let users change previously defined relationships, and the order in which operations take place, so that it is easier to handle knotty constructions problems. Surface modeling Wire-frame models represent 3D part shapes with interconnected line elements. Wire frames are the simplest 3D geometric representation, though not necessarily the easiest to create. However, wire-frame models use little computer time and memory, and provide precise information about the location of surface discontinuities on the part. Wire frames, though, contain no information about the surfaces themselves nor do they differentiate between the inside and outside of objects. Thus, wire frames can be ambiguous in representing complex physical structures and often leave much interpretation to users. Wire-frame models are created by specifying points and lines in space. To create the model, the computer screen is usually divided into sections showing various model views. Designers draw lines to create top, bottom, side, isometric, and other views of the model. Designers need not manually 2 May 2012

3 the vital link Diverse plastic and composites are now standard engineering materials, with new materials constantly being invented. As a result, manually searching for a material that would do the job at the lowest cost would be difficult at best. That is why online databases have become so popular. They eliminate information overload by organizing material properties into a manageable format. In a typical search, engineers enter material requirements in order of importance. Each successive requirement reduces the number of candidate materials until a final list is produced. These finalists are further evaluated for performance and cost. Beside a fast and efficient materials search, some databases also integrate with CAD/ CAM packages to support molding and finite-element purposes. This arrangement optimizes the productivity of capital-intensive CAD/CAM software while supplying rheological data for design-related software. Databases can be said to form a symbiotic link between materials selection and CAD. Properties of several candidate materials can be tested in optimized designs, increasing the chances of selecting the best material. On the other hand, designers using databases together with design software can expect to get optimum designs more quickly. draw each line in a wire frame. Rather, the CAD package constructs the lines based on user-specified points and commands chosen from an instruction menu. It is important to remember, however, that not all models that look like 3D wire frames are wire-frame models. Some software lets users build isometric models hat appear to have Z-axis depth, but in reality do not. This software is usually called 2 ½D software. Although wire-frame models are the simplest form of geometric model, the term is usually associated with surface and solid modeling. Surface models define the outside part model precisely and help produce NC machining instructions where the definition of the structure boundaries is critical. However, surface models represent only an envelope of part geometry, even though tools such as automated hidden-line removal make the model appear as a solid. Surface models, in turn, are created by connecting various types of surface elements to user-specified lines. The entire model may comprise different types of interconnected surfaces. With surface modeling, though, an entire structure may provide more detail than necessary for many applications. So some models combine surfaces for detailed faces, with wire frames representing the rest of the part. Typical CAD surface menus from which to choose include planes, tabulated cylinders, ruled surfaces, and surfaces of revolution along with sweep, fillet, and sculptured surfaces. Of Surfaces Surfaces available for geometric modeling range from simple planes to complex sculptured surfaces. These surfaces usually are represented as a set of ruled lines. However, the computer recognizes these lines as continuous surfaces. Users select surface types from a menu to model individual details or fully envelope parts. 3 May 2012

4 Solid modelers Variational modelers are known for their insensitivity to the order in which the designer placed constraints on the model. Feature-based models let designers define features pertaining to geometry as well as to steps in downstream analysis and manufacturing. Parametric modeling is the term used to describe the capturing of design operations as they take place, as well as the editing that takes place on the design. course, the plane is the most basic surface type. The software merely creates a flat surface between two user-specified straight lines. A tabulated cylinder is the projection of a free-form curve into the third dimension. A ruled surface is produced between two different edge curves. The effect is a surface generated by moving a straight line through space with the end points resting on the edge curves. A surface of revolution is created by revolving an arbitrary curve in a circle about an axis. This capability is useful in modeling turned parts and parts with axial symmetry. The sweep surface is an extension of the surface of revolution. Sweep curves, however, sweep an arbitrary curve through another arbitrary curve instead of a circle. The fillet surface is a cylindrical sur- face connecting to other surfaces in a smooth transition. Previously, this was a tedious and manual process. But over time CAD has solved the problem with the precise mathematical continuity required by many applications. Sculpted surfaces are the most complex surface representation. There are many kinds of sculptured surfaces, including curve-mesh, freeform, B-spline, and cubic patch surfaces. Curves need not even be parallel. The two curve families intersect one another in crisscross fashion, creating a network of interconnecting patches. Building blocks for solids With this information in hand, it is helpful to know that solid models are constructed in two ways: with primitives, or with boundary definition. Both 4 May 2012

5 methods develop complex geometries from successive combinations of simple geometric operations. The primitive approach lets elementary shapes such as blocks and cylinders to be combined in a buildingblock fashion. Users position the primitives and then create a new shape with the proper Boolean command. With boundary definition, tow-dimensional surfaces are swept through space to trace out volumes. A linear sweep translates the surfaces in a straight line to produce an extruded volume. A rotational sweep produces a part with axial symmetry, while a compound sweep moves a surface through a specified curve to generate a more complex solid. Each of these construction methods is best suited to a particular class of shapes. Most industrial parts, for example, consist of planar, cylindrical, or other simple shapes and are readily modeled with primitives. But components with complex contours such as automobile exhaust manifolds and turbine blades are more easily modeled with boundary definition. Some advanced software combines both methods in on package. That s why the advice to think like a machinist when creating solid models often makes a lot of sense. As Milton Florest President of Tooling Research Inc. in Walpole, Mass., says, Most designers and engineers learn solid modeling in colleges or technical schools and then launch into the business world to start designing machines and components. But some newer (and even some moreexperienced) designers and engineers never consider whether the 3D CAD component they have just created is actually manufacturable. The assumption is, if I can create it on the computer, it can be made. In most cases this is true, but at what cost? The result: The machinist is often confronted with unnecessarily difficult or sometimes impossible machining scenarios. 5 May 2012