Feature-Based Design

FlJ:Uft3.2iJ Lshaped bracker rfrorn C~om~tric••nd Solid Mod~Ung:A"

Introduction by Christoph M. Hoffman,

© 1989. Morgan Kaufmann Publishers.)

FiJ:Uft3.21 BooleantreeofCSG operations and individual features from the previous figure.

An extension of CSG is to create special cases of building blocks thatin fact corre-spond to manufactured features. The designer in this case calls upon a more restricted menu of features, such as holes and pockets, which correspond to the physical actions of a "downstream" manufacturing process. A menu that includes holes and pockets will correspond to machining." This idea sets the stage for the destructive solid geom-etry (DSG)-5ection 3.II-further on in this chapter. First, the text revisits the joy-stick example using solid modeling.

3.10 SECOND TUTORIAL: SOLID MODELING USING

CONSTRUCTIVE SOLID GEOMETRY ICSG)

The joystick base willnow be constructed using solid modeling techniques for comparison with the wire frame tutorial. The process begins by developing three-dimensional blocks of material to which other three-dimensional objects willbe subtracted or added.

~Feature-based design is not just machining oriented. Sheet metal forming, forging, casting, and so on all exhibit process specific, standard shapes that can easily be made with a standard die set If the designer knows which process is going 10 be used, it makes sense to design with this die set and corre-sponding shapes in mind. Of course, this is also the key philosophy of the MOSIS service for the design and rapid delivery of integrated circuits (MOSIS, 2000).

3.10 Second Tutorial: Solid Modeling Using Constructive Solid Geometry(C5G} 105

From a CAD/CAM viewpoint, the distinct advantage of CSG is that models are developed intuitively and give insights into manufacturability provided the designer is sympathetic to the manufacturing operations. Process planning, fixturing, and ori-entations also become much clearer. This is because the Boolean nature of CSG requires that features be added and subtracted in a logical manner. If done well, the ordering of the features can anticipate manufacturing. For communications, CSG models can berendered: a process that takes the internal representation and creates a shaded, attractive picture on the screen.

Here is an important subtlety before the tutorial:in terms of graphic representa-tion, solid models are capable of being depicted as either wire frames or rendered solids.

At the time of writing, the more expensive CAD tools running on high-end workstations show fully shaded solid blocks during every stage of the CAD proce-dures. On the other hand, in many other CAD packages, a solid might well be cre-ated with CSG methods buttemporarily displayed as a wire frame. The previous sections indicate why this is in fact desirable: CSG programs incorporate compli-cated computer algorithms and therefore require considerable computer power.

Thus while CSG procedures (even adding a simple hole) are being done on the par-tially completed CAD object, the computer will do the calculations faster if the tem-porary object is displayed as a wire frame. Once the CSG procedure is finished, the object can be rerendered.

For the purposes of illustrating CSG, Figures 3.22 onward are provided. Figures 3.22 and 3.23 show the base and the individual objects that are to be subtracted from it. Figure 3.22 is a wire frame representation of the solid models, while Figure 3.23 is a solid representation of the same model.

Flgure 3.22 Wire frame drawing of VRjoystick primitives.

Internal cavities

Mai0 base of Joystick

For the baseplate Screw holes Intersection block ~ool)

Intersection block (tool)

106 Product Design, Computer Aided Design (CAD), and Solid Modeling Chap. 3

Flame 3.23Solid model drawing ofVR joystick parts with hidden lines removed

These figures also include thecutting blocks orintersection blocks for the slopes on the front edges of the joystick. These are labeled bandtin the upper right of the figure. These are special geometric blocks that are created as separate operational toots by the user. They are used to cut or intersect with a virtual starting stock and create the sloping angles on the joystick.

The creation of the base on the left of the figures begins by tracing the outline of the block. Trace segments are then joined together as a polyline. Theextrude command is then executed, and the block is extruded to the appropriate height. It is important to note that for the extrude command to work, the polyline has to be closed. At the bottom center of Figure 3.22, the outline of the base plate cavity is con-structed in an identical manner with that of the wire frame. The outline of the plate is made into a polyline and extruded. Because the base plate cavity is constructed with Boolean subtraction, the height of the extrusion is arbitrary as long as its thick-ness is greater than the thickthick-ness of any intended recesses.

Since the design is for the bottom of the object, it makes sense to also construct the internal cavities for the sensors and printed circuit hoard at this time. The outline of the cavities is constructed in exactly the same way as in the ~frame. As before, the common edges are removed from the rectangular cavities. Like the base plate, the outlines are made into polylines and extruded to an arbitrary height.Two poly-lines, one for the rectangular part of the cavity and one for the trapezoidal part, are created and extruded.

For the base plate intersection Joystick base

3.10 Second Tutorial: Solid Modeling Using Constructive Solid Geometry (CSG) 107

The extrusion of the cavities remains sensitive to the differences in height between the rectangular and trapezoidal cavities.For simplicity.these two objects are then combined with the "union" command.

The next features to construct are the bores and counter-bores. Since the bore and the counter-bore are coaxial, it makes sense to construct them at the same time.

Both holes are essentially cylinders and can be generated with the cylinder com-mand. The command asks for the location of the center of the base, the radius of the cylinder, and then the height. The bore-eounter-bore combination is created by defining the two cylinders and then unioning them with the union command used earlier.

To this point, all of the objects have been built from the world (or global) coor-dinate system. For example, this allows polylines to be built up on the specificx-y plane shown earlier in Figure 3.8. However, thex-yplane does not always have to be defined by the world coordinate system (WeS) shown in that figure. It can be rede-fined by a user coordinate system (UeS). A ues is a user-defined or local coordi-nate system.

The sloping sections of the front of the object are good candidates for con-struction with intersection blocks along the worldxaxis. However, it is desirable to perform the construction from anx-y plane that is at right angles to the original wes, x-y plane shown in Figure 3.8. It is therefore desirable to change the coordinate system from the wes to a ues. The command to do this is simply ues.After entering Des, the user is prompted to describe the change of coordinate system in Figure 3.8.

This new reference plane corresponds to the small triangular face labeled b and the quadrilateral labeled t at the top (back) of Figure 3.22. In this particular case, the coordinate system is rotated 90 degrees about the original x axis and then 90 degrees about the new or modified y axis (because of thexrotation by 90 degrees) in Figure 3.8. It is emphasized that the order is important: when prompted to determine the axis of rotation, x is chosen first. Following the x rotation, the ucs command is chosen again to rotate the system 90 degrees about y.

Once the ues has been adjusted, thecutting orintersection blocks labeled b andtcan be extruded. As mentioned, these will be used as intersection blocks to create the angled faces on the front portion of the base. One polyline, t, is developed for the top slope of the base and another, b, for the bottom slope of the base.The top piece has been made from a quadrilateral, while the bottom has been made from a triangle. Once the polylines band t are constructed, they can be extruded. A charac-teristic of the eSG subtraction process is that the height of the extrusion does not always have to be exact. It only has to be of sufficient height to cross the width at the largest part of the intersection between the two objects. Once the two intersection blocks have been extruded, they can be used to intersect the main block. These inter-sections give the main block its more "artsy," slanted faces at the front.

In Figures 3.24 and 3.25 the individual objects are shown in the appropriate locations for solid subtraction. The former is the wire frame rendition, while the latter is the solid. Figure 3.26 is the final part. Rendering can then be initiated with the render command. Be sure to compare Figure 3.26 with Figure 3.14.

108 Product Design, Computer Aided Design (CAD), and Solid Modeling Chap. 3

FIgure 3.24 Wire frame drawing of parts positioned for CSG subtraction operations.

FIgure 3.25 Solid model drawing of CSG method.

Intersection block Intersection block

Base

...Intersection block . Intersectionblock

Jovstick base

Intersection to fonn cavity