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Solid Modeling and Computer-Aided Drafting

Simulation, estimation, brainstorming, and other creative processes play important roles in engineering design. In cases where the ultimate goal is a real, physical product, formal sketches and drawings of the object also must be produced. These documents form a key link between design engineers and technicians, fabricators, marketing specialists, customers, and other individuals. Pictorial documentation may appear in one of several forms at various stages of the design process. These forms include informal sketches, isometric views, orthographic projections, exploded views, and solid models. Each of these graphic formats serves a particular design need.

Various methods for generating engineering drawings or graphics, have evolved over the years. Before the days of computers, the skill of manual drafting (sometimes called “technical drawing”) was taught to engineers and technicians in all disciplines. Courses on drafting were common in high school and college curricula, and no self-respecting engineering student would be without a complete set of drafting tools. A typical engineer's drafting kit would have included T-squares, triangular rules, mechanical pencils, erasers, inking pens, and drawing templates. Drafting skills learned in school carried over to the workplace, as the practice of manual drafting was a mainstream activity in most engineering companies. In any given engineering firm, entire rooms would be filled with drafting tables and engineers at work.

Nowadays, computers have virtually eliminated the need for manual drafting skills. Much as the word processor has replaced the manual typewriter, so have numerous computer aided design (CAD) software tools replaced the need for engineers with manual drafting skills. Popular CAD software packages, including ProEngineer™and SolidWorks™, are used by engineering companies everywhere. In this section, we review the key steps involved in using these types of CAD tools for generating engineering drawings.

1 Why an Engineering Drawing?

Consider the engineering drawing of the chassis plate shown below. This object might be used, for example, as part of a battery-powered, scale-model vehicle. Contrast the detailed engineering drawing of the figure with the following written description:

“The plate should be made from 0.4-mm thick aluminum stock and should be a rectangle 25-mm long by 20-mm wide. It should be drilled with four holes. The first should be located 2.0 mm from the right-hand, 20-mm edge of the plate and 2.5-mm from the upper 25-mm side. The second hole should be located 1.9-mm to the left of the first. These dimensions should be held to a tolerance of 0.1 mm. Both holes should be drilled to a diameter of 0.2 mm with a tolerance of 0.001 mm. These holes should be duplicated using the same dimensions at the other corner of the plate, but located 2.8 mm from the right-hand, 15-mm edge of the plate, and 2.5-mm from the lower 25-mm side.”

For most people, the diagram conveys the information much more succinctly than does the written version. The human brain is an extremely efficient image processor, and drawings will almost always surpass written prose an a means for conveying information. The superiority of human imaging power motivates the well-known saying, “A picture is worth a thousand words.”

2 Types of Drawings

As noted previously, engineering drawings come in several widely accepted forms. Categories include hand sketches, isometric projections, orthographic projections, exploded views, and solid models. Each type of drawing has its own particular use in the engineering design process. During the idea-generation phases of a project, hand sketches are extremely useful. By quickly drawing things on paper, an engineer can rapidly convey a design concept to other team members. The very act of producing a hand sketch can act as a catalyst for ideas. Hand sketches also are the medium of choice for entering ideas into engineering logbooks.

When a commitment has been made to pursue a particular design concept, more formal types of drawings are in order. The drawing of Figure 24 shows the isometric view of a simple part. An isometric view is a three-dimensional rendition in which the parallel sides of the actual object are drawn as parallel lines on the page. Isometric views differ from perspective drawings, commonly found in classical art and in advertising graphics, in which parallel lines point to a distant “vanishing point.” An isometric projection becomes a slightly distorted rendition of the object, but if the part is small and distances are short, the differences will be minor. The isometric view of Figure 24, for example, differs little from its equivalent perspective drawing, whereas the isometric and perspective views of, say, a long narrow box would differ significantly. The principle advantage of the isometric view is that it is much easier to draw than a perspective drawing. Also, compared with its related counterpart, the orthographic projection, the isometric drawing provides a “birds eye view” of the object that conveys many of its features at a single glance.

24. Isometric view of cylindrical collar with rectangular tab and pin hole.
The orthographic projectionof an object consists of set of two-dimensional projections of the object's front, side, and end views. In some cases, a fourth view may be necessary. Figure 25 shows an orthographic projection of the same part described by the isometric view of Figure 24. Orthographic projections are principally used by machinists for whom such drawings provide all the information needed to fabricate the actual part. For example, dimensions, tolerances, and machining details are very easy to convey on an orthographic projection. Compared with other types of drawings, orthographic projections are exceptionally easy to draw but require more interpretation on the part of the person trying to read the drawing.

25. Orthographic projection of the part of Figure 24.
An exploded view, or assembly drawing, is used to describe the way in which multiple parts are to be put together to form the working whole. Dotted lines often are used to convey a path to connection or attachment. The diagram in Figure 26, for example, shows how the part of Figure 25 is to be assembled with other related parts. Although exploded views sometimes can be difficult to draw, they are very useful for conveying information about complex structures.

26. Exploded view of several parts shows the way in which they are to be assembled.
The most computationally sophisticated type of drawing produced by a CAD system is called a solid model. Unlike isometric and orthographic projections, which depict just the surfaces of an object, a solid model rendition includes information about the surfaces and the interior details of the object. A solid model is much more than a simple visualization. It contains a complete mathematical description of the object's material properties as well as its interior and exterior dimensions. This additional information makes the solid model useful for many applications besides viewing. For example, the solid model can be used to predict the object's deformation under applied force, or stress, the object's reaction to temperature changes, and its interaction with other parts in the system. At the core of solid modeling lies a computational method known as finite element analysis (FEA) in which an object is represented by a large number of interconnected cells, or “elements.” A finite element analysis keeps track of the mutual interaction between each cell and its neighbors and computes the behavior of each cell in response to internal and external stimuli. The popular CAD tools ProEngineer and SolidWorks, for example, incorporate finite-element analyses into the solid models of parts and objects.

When an object has been rendered in a CAD tool as a solid model, the latter becomes invaluable when the part is ready for manufacturing. Sophisticated software linked to computer-guided machining tools—lathes and milling machines, for example—can be instructed to fabricate the part directly from its solid model representation. The language used by this class of machines is called computer numeric control, or CNC. The CNC system enables an engineer to design a part on the computer screen, then send its CNC code directly to a computer-controlled machine for fabrication from metal, plastic, or other machinable materials. Another method for fabrication directly from solid models is called rapid prototyping. In this technique, a rendition of the part suitable for prototype needs is produced using a laser beam that shapes the part from very thin cross sections of plastic resins or paper. The prototype part is assembled, literally layer by layer, by stacking the cross sections.

EXAMPLE
Producing a Simple Part
Suppose that you wished to machine a prototype version of the part shown in Figures 25 and 26. In this example, we illustrate the steps involved in producing the solid model of the part that you could send to a machinist for fabrication. (Alternatively, perhaps you might make the part yourself if you have been properly trained in the use of machine tools.) The steps for producing the solid model drawing are summarized here in a generic way, but they are similar to those one would follow when using specific CAD tools such as ProEngineer and SolidWorks.

Step 1. Open up a new drawing screen: Open a new part screen in the CAD software by choosing NEW from the software's FILE pull-down menu. A blank screen appears on which the part description will be drawn.

Step 2. Sketch the principal cross section (Figure 27): Using the mouse and keyboard cursors, sketch the part's basic cross section on the screen. Even though the part may have features in all three-dimensions, only its principal cross section (e.g., its top view) need be drawn at this stage. The other views of the part will be generated automatically in a later step. This initial cross section of the part can include some of its machined features. For example, the part in Figure 25 includes a drilled hole in its center whose location and dimensions are specified in the initial cross section. The radii of corners and bends in the cross section also can be specified at this stage. This step is sometimes desirable because no machining process can produce perfect angles from solid materials.

27. The principal cross section of the part of Figure 25 is drawn on the computer screen.
Step 3. Dimension the sketch (Figure 28): Major features of the part—lines, circles, and arcs, for example—are selected on the screen and their dimensions are specified in the units chosen for the drawing (e.g., mm, cm, or inches) This step also provides a reference scale for all the other lines and curves that will make up the finished drawing.

28. Dimensions are added to the cross-sectional sketch.
Step 4. Extrude the cross section (Figure 29): The defined and dimensioned cross section from the previous step is extruded, or “stretched,” in the direction perpendicular to the drawing plane to form a three-dimensional version of the part. The extrusion of a circle produces a cylinder, while the extrusion of a rectangle produces a rectangular solid. The extrusion operation leading to Figure 29 thus results in a cylindrical solid with a hole in its center plus a rectangular appendage called a tab.

29. The cross section is extruded to form a solid model.
Step 5. Add features to the extruded part (Figure 30): Once the cross section has been extruded to form a three-dimensional solid model, the three orthographic projections of the part will be available to the designer. Features that are not part of the principal cross section, including perpendicular holes, material cuts, and rounded corners, are added to the object at this stage using the appropriate orthographic view. In this case, a hole whose axis is parallel to the cross-sectional plane is drawn through the rectangular tab, and the top and bottom faces of the tab are trimmed.

30. Other features are added to the extruded model that were not created during the first extrusion. In this case, a hole whose axis is parallel to the cross-sectional plane is drawn through the rectangular tab, and the top and bottom faces of the tab are trimmed.
Step 6. Save the file: The file is saved for future use, printing, and so forth. The solid model rendition of the part is now complete and can be sent as a drawing to other engineers for review in either hard copy or electronic form. Additionally, it can be sent to a CNC-equipped machine tool or to a rapid prototyping machine for computer-controlled fabrication.