Engineering design is a partial realization of the designer's concept. The designer can communicate with others using engineering drawings, and the information from these drawings can be used by the machinist for the manufacture of the product. As parts become more complex, designers require a more sophisticated modeling approach and tools for design development and analysis. A computer-aided design system will provide the necessary environment for the designer to create, analyze, modify, and optimize an engineering design. CAD systems are categorized based on the system hardware (PC, workstation, and mainframe), the application areas
(mechanical, architectural, etc.), and the modeling techniques (2-0, 3-D, etc.). Figure 5.1 illustrate the classifications among geometric models.
Geometric Models
Wire-frame Surface Models Solid
l Feature-Based Parametric Constraint-Based PM and CBM Figure 5.1. Geometric Modeling Classification
Geometric models are classified as two- and three-dimensional. A 2-D model is usually a wire-frame model. Wire-frame models require less computing time and memory and provide no information regarding the surfaces of the part. However, they do contain an accurate geometric description of the object being modeled. Three-D geometric modeling includes wire-frame, surface, and solid modeling. Surface models can illustrate the mathematical description of the object being designed. They also provide the capability to visually inspect the model in a 3-D coordinate system. They are best suited for the representation of complex surface contours (e.g., an automobile body). The concept of solid modeling is similar to the construction of an object as if it were actually being fabricated. The solid-modeling approach is considered to be both iconic (mock-ups) and symbolic (mathematically descriptive) [2]. It is the most comprehensive approach and contains all necessary information and data regarding the product's features, which are stored in the design database. This information can be used for the development of a comprehensive knowledge-based system for product design purposes.
In a feature-based design environment, features can be used to illustrate associativity between design and manufacturing using "standard features,"
although from a designer's point of view the feature is defined as a specific design functionality, whereas from a manufacturing point of view a feature could illustrate a certain manufacturing process. In a feature-based design
system, features are designed using sets of parameterized data [67]. In this approach, the designer defines a set of geometric constraints and engineering relationships that are used for creating the geometry of the object and establishing the associativity among the objects. Table 5.1 illustrates sample associativites between design and manufacturing based on standard features.
Table 5 1 .. Product-to-Process Features Associativities
Sam Die Process and Machine Features Standard Design Features
Gun-drill machine hole
Broach machine flat face
Slotter machine kevwav
OD-grinder machine outer cylinder
Turn-broach machine flat face/outer cv linder Induction heating machine outer cylinder/filletlchamfer Draw furnace machine hole/outer cylinderlfilletlkeyway Wire brush machine hole/outer cvlinder/filletlchamferlkevway CNC lathe machine hole/outer cylinderlfilletlchamferltlat face
A set of expressions and variables is used to define the dimensions of the object. When the numerical quantities of parameters are changed, the characteristics of the features are also updated concurrently. Although it is considered to be a complex approach, it provides the necessary flexibility and increased design efficiency, creating a new design by altering existing models.
Parameterized features of mechanical parts are grouped into three classes.
The first class contains standard dimensions such as a keyway on a shaft.
The data necessary to define these features include width, height, and length.
The second class includes features such as chamfer and formed shapes.
These do not have standard dimensions and must be defined by the designer.
Finally, the third class includes special and unique features associated with the part and/or its family . A lighting hole on a crankshaft is an example of such a feature.
Geometric dimensioning and tolerancing (GD&T) is another example of design features that can be associated to specific process features. It is a standard technique for the dimensioning and tolerancing of a design with respect to the actual function or relationship of part features, allowing for a more efficient and economical approach to production. It is a major factor in controlling the quality of the part and is used significantly during the development of the process plan. For an integrated CAD/CAM environment, standardization is the key. GD&T can provide such standardization and therefore has the ability to adapt to automation and computerization in an integrated design and manufacturing environment. Table 5.2 lists the GD&T standards specified by ASME Y14.5-1994.
Table 5 2 GD&T Classifications . .
Geometric Category Characteristics Datum References
form flatness Never uses a datum reference
Straightness Circularity Cylindricity
Orientation Perpendicularity Always uses a datum reference Angularity
Parallelism
Location Position
Concentrity
Runout Circular
Total
Profile Line May use a datum reference
Surface
The features in GD&T are categorized into three groups. Individual features are those that are related to a geometric counterpart of itself and have no data for reference. The data are referenced surfaces that are used to make part measurements. The related features are those defined as using one datum or several data. The third category of GD&T features are those that can be considered both as individual and as related. Table 5.3 lists sample associativites between GD&T standards and process features. Tables 5.1 and 5.3 can be used to select a machine that is capable of processing the standard design features and to achieve the specified GD&T features.
Table 5.3. Sample GD&T and Process Machines Associativity
GD&T Features for outer cylinder tolerances (mm) Sample Process Machine Size Tolerance Roundness Finish (f.lm)
Grinder 0.5-1.000 0.004-0.4 0.005-1.000 0.2
Turn-broach 0.2-1.000 0.17-0.47 0-7 1
Induction heating 0.7-1.007 0.17-0.47 0-7 1.7
Draw furnace 0.7-1.117 0.07-0.7 0-7 1.7
CNC lathe 0.2-1.000 0.04-1.000 0.02-1.000 1