CHAPTER 5 3-D Solid Modeling

MODEL DEFINITION

5.1

5.2

A primitive modeler allows you to build models using a limited set of simple geometric forms. Using the metaphor of 'building blocks' and demonstrations with physical geometric primitives, the concept of creating more sophisticated models through purely additive means can be demonstrated. Students can typically grasp this technique of visualizing the decomposition of more complex forms into primitives quite readily; an important skill for working with modelers of all levels of sophistication.

Working with purely additive techniques with primitives also allows there to be some focus on manipulation and arrangement of objects in 3-D space. Working in 3-D space on a computer screen often takes some practice.

Working with primitives also gives and opportunity to focus on construction techniques used in solid modelers. One tool that some primitive modelers have is parametric control over the geometry of the primitive. A number of the more powerful solids modelers more fully incorporate parametric techniques throughout their systems to allow for variational design capabilities.

5.3

This section on constructive solid geometry (CSG) introduces Boolean operations, and central tool for most of the more capable 3-D modelers. Review the figures in this section to familiarize yourself with the properties of the different Boolean operations. As is true with many concepts, the best way for students to come to understand Booleans is to do it themselves. If this is not possible, demonstrations with computer and physical models can also be effective. You may choose to begin with just union and difference and save the less used intersection operation for later. One use of demonstrating the intersection operation is that it helps drive home the fact that it is the overlap of the two bodies that is at the heart of Boolean operations.

One way of demonstrating the binary tree data structure (and its unevaluated data structure) is to create a model through a series of Boolean operations and then undo them, stepping back through the tree. Just as you may have had students decompose objects into a series of primitives which could be 'glued together' to create the final model, you can now have them go through the same exercise but allow them to incorporate Boolean operations.

5.4

HYBRID MODELING

5.5

CONSTRAINT-BASED MODELING

5.6

5.6.1

One of the critical early questions to ask before creating the model is where is the model data coming from and how is the model data going to be used – both in the short term and in the long run.

Ideally, the model will be built so that the model behaves as expected when features are modified. This behavior should reflect the design intent of the product being modeled. That is, changes in geometry of a feature should create model feedback or further changes in the model which reflect design performance or manufacturing constraints of the product.

5.6.2

5.6.3

5.6.4

FEATURE ANALYSIS

5.7

Capturing design intent in a model is a process of defining features and the relation of features within a model. The goal is to make sure that information extracted from the model or modified versions of the model for use in other parts of the product development process accurately reflects (as much as is possible) the original intent of the designers and engineers who specified the requirements of the product. 3D modeling is a process of transforming product requirements into geometry. In a constraint-based model this geometry is dynamic, since the size and location of features in the model can easily be changed to alter model’s geometry.

Figures in this section demonstrate how geometry is broken down into features and how these features can be constrained to represent the design intent of the product.

The building of the model begins with the creation of the base feature.

FEATURE DEFINITION

5.8.1

Many features in a model can be made through the use of sweeping operations. Most CAD systems use methods of automating object generation. In a sweeping operation, a closed polygon, called a profile, is drawn on a plane and is moved or swept along a defined path for a defined length.

Figures in this section give examples of how features can be swept along both linear and revolute paths.

5.8.2

All geometry in a model must be located and oriented relative to some 3D coordinate system.

A workplane, is the most common type of construction geometry used to support the creation of part geometry relative to the world coordinate system. Construction geometry does not represent any of the final geometry representing the part, but instead provides a framework for guiding the construction of this part geometry.

A workplane can be used in the same manner as a drawing surface. In a modeler, workplanes are typically used to orient the profile sketch used in feature generation. By adjusting the view of the model to be normal (perpendicular) to the workplane, the effect is that you can draw on the workplane as though you were looking directly down on a piece of paper.

Once the base feature is created, workplanes are often oriented using geometry of the model being built.

In addition to workplanes, construction axes and construction points can also be created.

5.8.3

Many features on a part model begin as a profile sketch on a workplane. This sketch will consist of a series of line elements such as straight lines, arcs, circles, or splines. Tools used for drawing this sketch will be very similar to the tools used for drawing such elements in a 2D CAD system.

One important difference concerns the accuracy with which the sketch needs to be drawn. Unlike a 2D CAD drawing, the sketch does not need to be dimensionally accurate. Instead, the sketch represents the overall shape, the topology, of the profile. That is, the sketch should represent the total number of sides of the final profile, the basic shape of the elements (curved or straight), and the order in which the elements are connected together. The sketch should also represent the basic geometric relationships between the elements (parallel, tangent, etc.) within a reasonable level of accuracy.

Depending on the modeler used other characteristics of the profile, the sketch might be either a closed loop or an open loop.

The definition of inside and outside is needed to specify how the profile is to interact with the existing geometry.

5.8.4

Going hand in hand with the sketching of the profile is applying constraints. Whereas the sketching defined the topology of the profile, constraining defines the geometry of the profile. This use of the word geometry refers to a more narrow definition of the term: the size, location, and orientation of geometric elements that give them an overall shape and form.

The types of constraints applied to the sketch profile can be roughly divided into two categories: explicit and implicit. These two types of constraints differ as to whether the modeling system infers the constraint based on the way the sketch was drawn, or whether the operator has to explicitly apply the constraint to the sketch.

Though the profile does not need to be sketched dimensionally accurate, how you sketch it will influence how implicit geometric constraints are applied.

Explicit constraints, unlike implicit constraints, are applied by the user to the profile sketch. The application of explicit constraints is very much like applying dimensions in a 2D CAD system, yet they behave very differently. Figures in this section show examples of how explicit dimensional constraints can be used to drive changes in model geometry.

Central to developing a strategy for constraining a profile is knowing when the profile is fully constrained, underconstrained, or overconstrained. A fully constrained profile has completely specified the geometry of a profile.

Dimensional constraint parameters can be set to something other than a constant value. The ability to link constraint parameters through algebraic equations or to control values based on logic statements provides tremendous power to the modeler to both embed design intent and to automate modifications of the model.

5.8.5

With the sweep profile drawn and constrained, there remain a few more elements of the sweep which need to be defined:

One part of the sweep definition that still needs to be defined is how the profile is going to be sweep out to create a form in 3D space. Typically it is swept out as a linear or circular sweep.

A less commonly used definition is a path-based sweep. With a path-based sweep, the profile is swept along a path defined either by an existing edge on the part model or by a path drawn by the operator.

The distance that a profile is swept can be determined in a number of ways, including: blind, through all, or to next.

The swept feature will either add or subtract material from the existing model depending on how the feature has been defined.

Most constraint-based modelers have tools that speed up the definition of commonly used features. Rather than having to define every variable of every feature, options can be given for common design or manufactured features which either have pre-defined certain feature parameters, bundled variables together in easy to use dialogue boxes, or otherwise automated the feature definition process.

5.8.6

Though it is impossible to come up with a definitive list of ‘rules’ which should be followed when planning the modeling of every part, there are still certain characteristics of the part geometry which should be evaluated and decisions which have to be made for most parts during the planning process.

One of the more important considerations is whether the parts contain lines of symmetry.

Another decision, which usually has to be made, is how geometric features are distributed across part features of the model.

Ultimately, the level of complexity of feature profile geometry comes down to what is a logical decomposition of the part geometry. This logic is driven by how features are defined in the design and manufacturing process.

Finally, good modeling practice calls for the user to avoid certain types of feature operation in order to preserve the integrity of the model geometry and to allow for easier management of the model.

EDITING PART FEATURES

5.9.1

Most constraint-based modelers record the features created for a part in a tree. This tree may or may not be directly visible to the operator of the system. Features, as they are created, are placed at the bottom of the feature tree. If a new feature is created as a copy or instance of another feature in the part model, the new feature on the tree may reference the original feature.

Because features can be moved to other locations up and down the feature tree, the tree cannot be considered a history of feature creation. With many modelers, however, the order in the tree is the order in which features are applied to the construction of the model. Each time a new feature is added to the model, the user explicitly rebuilds/regenerates the model, or the modeler is otherwise triggered to do a rebuild, the feature tree is traversed from top to bottom; modifying the part model with a succession of feature operations.

Closely related to the idea of feature ordering is the concept of parent-child relationships between features. As in a real parent-child relationship, the child feature is dependent on the existence of its parent feature.

Editing the order of features means moving features up or down in the feature tree. Dependencies between features means that features can’t be moved to every possible position on the feature tree.

5.9.2

In addition to changing the order of features within the feature tree, many of the parameters that initially defined the feature can be edited at a later time.

Within the sketch profile, elements of the profile can be deleted or modified.

Other parameters besides the sketch profile can also be altered. The possible parameters that might be modifiable are:

- The type of sweep path

- The distance of the sweep

- Whether the sweep in one or two sided

- The direction of a one-sided sweep

- The side of the profile a removal operates on.

Often the type of operation – removal or addition – cannot be changed.

DUPLICATING PART FEATURES

5.10

Constraint-based modelers typically allow the user to duplicate at the level of features. Whether all of the dimensional constraints are tied between the parent and the child copy is often determined by the options chosen.

A common tool is an array option. With a linear array the parent feature is copied in one or two dimensions with specifications given for distances between copies and the total number of copies.

With a radial array, an axis of revolution is specified along with a radius, angular displacement, and total number of copies

Another common copying process is a mirror. In this case a mirror plane is specified along with features to be copied/mirrored.

VIEWING THE PART MODEL

5.11

5.11.1

The view camera is a metaphor used to describe the viewing process with respect to 3-D models in various CAD systems. For each view, there is a camera, and there is an image plane onto which the model is projected. The camera records the image on the plane and broadcasts that image to the computer screen. The broadcasted image is contained within a viewport on the screen, and viewports may be resizable and relocatible or fixed, depending on the system.

A related issue in view specification is how to display the geometry of the model. The most common methods are:

- wireframe

- hidden lines rendered

- hidden lines removed

- shaded

In addition, the lines representing tangency can be:

- hidden

- displayed solid

- displayed as an alternate linetype

5.11.2

If you move the view camera, however, the camera has changed location relative to the world system, but the geometry of the model has remained untouched. Preserving the location and orientation of a model can be critical when multiple parts in an assembly are being coordinated. To get a new view of a part, rotate the camera and not the part.

Viewpoints should be chosen on the basis of the task to be performed. With multiple viewports, the views are often distributed between those used strictly for viewing and those used for constructing the model.

APPLICATION OF THE PART MODEL DATA

5.12

5.12.1

The dynamic linkage of data can be established in a number of different ways. With unidirectional associativity, the supporting application’s data can be altered by changing the 3-D model, but not vice versa. With bi-directional associativity, changes in either the 3-D model or data in the supporting application will affect the other.

In addition to the direction of the data flow, the updating can be set to be done automatically whenever there is a change, or done manually when an update command is chosen.

5.12.2

One of the most common types of data associativity is between the 3-D model and a 2-D production drawing. For example, if the 3-D modeling system also has a 2-D drafting module, then links can be set up between the model and the views represented in the production drawing.

Typically, a base view of the model (such as a front view) is anchored in the production drawing. Then, principal and auxiliary views are established relative to this base view. The scale established for the base view will typically hold for child views created from the base view. However, special detail or removed views can be created at alternate scales.

Once views are placed, notation can be added to the views. Since dimensional constraints should represent the design intent of the model, displaying these dimensions in the drawing should suffice for the majority of the dimensions shown in the documentation.

It is important to note that in documentation with bi-directional associativity with the model, dimensions in the document which represent constraints can be modified to drive changes in model geometry.

5.12.3

The assembly of parts into larger models uses many of the same techniques and concepts used in part modeling. In an assembly model, components are brought together to define a larger, more complex product representation. A component is either a part or another assembly brought into an assembly model and associated with other components.

Constructing an assembly begins with bringing in a base component. As with the construction of a base feature in a part, a base component will usually be selected because of its central role in defining the overall assembly.

Each successive component brought in needs to be oriented and located relative to other components in the assembly. Location and orientation is achieved by defining geometric relations between geometric elements of a component in the assembly and the elements of component being brought in.

An assembly modeler tracks the relationship between components through degrees of freedom. The establishment of each geometric relationship between two components reduces the degrees of freedom components have to move relative to one another.

Modification of dimensional constraints and performing geometry removal within the assembly modeler are both examples of top-down design, where the final geometry of the parts has not been defined before bringing them in as components within the assembly. The reverse of this approach would be bottom-up design, where all of the part geometry is defined before it is brought into an assembly.

An exploded view can be automatically created by having the model components move away from each other along the lines of the geometric constraints applied in the assembly. The location and orientation of the parts can then be adjusted to create a more optimal view.

5.12.4

The specification phase of the design process establishes the requirements for the needed product. Periodically, the various design concepts being developed should be evaluated against these requirements.

Visual inspection is an evaluation technique that is quick and easy, although very subjective. The visual inspection may involve making sure all the necessary parts are in an assembly model.

Prototyping techniques allow physical models to be made directly from a 3-D database. An increasingly common technique, called rapid prototyping, uses a number of technologies to ‘build’ models to match the form and geometry of critical features of the virtual computer model.

For these situations, as well as others, virtual reality (VR) systems offer a viable analysis approach. VR systems use the principles of perception to develop completely immersive environments in which the user can interact with the object through some or all of the senses.

Kinematics is an analysis technique used to evaluate the design of a mechanism, that is, an assembly of multiple parts, some of which move with respect to other parts.

Additional information about the model can be obtained by performing a mass properties analysis. When mass properties analysis is combined with kinematic information, a more sophisticated technique called a dynamic analysis can be performed on the model.

A process called discretization divides more complex geometries into simpler forms so that the response of a solid to forces can be estimated. The process of creating a model of primitive geometries is called finite element modeling (FEM), and the analysis done on the resulting model is finite element analysis (FEA).

Ergonomics examines the interaction between technology and humans.

Three-dimensional modeling techniques can be combined with computer-aided manufacturing (CAM) capabilities to ensure that a product design satisfies the desired manufacturability requirements as closely as possible.