Chapter 9 Graphical Communication mmm Becoming a fully competent engineer is a long yet rewarding process that requires the acquisition of many diverse skills and a wide body of knowledge. Learning most of the concepts in engineering requires the use of visual information and visual thinking. This, in turn, requires graphical communication skills, not only as an engineering student, but also as a practicing engineer. Communicating by way of graphics requires the ability to visualize information and translate it into visual products such as sketches and drawings. This allows you to represent a three-dimensional object in a two-dimensional medium. Studying this chapter and working through the activities will help you improve your visualization ability and your understanding of common graphical communication techniques used in the engineering profession. The purpose of this chapter is to illustrate the visualization and graphical communication skills essential for all engineering disciplines. Visual experiences such as the sketching of simple objects are used in this chapter to exercise your ability to create and edit visual information, and thereby strengthen your ability to visualize new and unfamiliar designs. You will be asked to produce sketches using standard graphical techniques to reinforce the common language engineers use for communicating technical information. Traditionally, visualization skills have been taught implicitly through engineering graphics or drafting classes; however, some disciplines no longer require drafting classes. The need for visualization and graphical communication skills has not diminished. Indeed, the prevalence of computers in the workplace has increased it. This chapter seeks to provide at a minimum a basic understanding of visualization and engineering graphics that will be useful to all engineering disciplines. It must be stressed that this chapter is only an introduction to these subjects, and is not intended to be a comprehensive reference. Many engineering disciplines rely upon more sophisticated graphical techniques and a deeper understanding of the topics described here which are more effectively covered in a course dedicated solely to engineering graphics. The motivation for this chapter, then, is twofold. First, visualization is critical to all subjects of a graphical nature. Any medium that provides a visual input to be interpreted by the observer requires comprehension of this information. If the observer is then required to respond to this information in a graphical form, the ability to create and edit the visual input is required. The second reason is the fact that all engineering disciplines rely on graphics to communicate information both technical and non-technical. Such skills are essential for communication with colleagues or decision makers. Often, the quality of the graphical infor- 229
230 Chapter 9: Graphical Communication mation can be the determining factor as to whether a project wins approval. If engineering students are to become effective at graphical communication, they must also have proficient visualization ability. In order to use a sheet of paper or the computer screen which only has two dimensions to display a three-dimensional object in an accurate way, we must develop a method where we can represent the three dimensions so that someone who has the ability to read technical drawings can interpret the design and build the three-dimensional object. This is done through a method known as orthographic projection, which incorporates a series of twodimensional views of the object. Then when the views are looked at together, the drawing will give a true representation of the three-dimensional object. To understand orthographic projection, we need to look at the theory of projection from which it is derived. Projection theory involves four specific components: (1) an object, (2) an observer, (3) a projection plane or picture plane, and (4) visual rays. Figure 9.1 shows these four components. We start with an object. In this case our object is a tree. At some measurable (finite) distance from the tree we place the observer. Between the observer and the object we place a projection plane or picture plane. From the object to the observer we show visual rays passing through the picture plane. When these visual rays come in contact with the picture plane, a two-dimensional image of the object is shown on the picture plane. This is known as a projected image. Now, imagine the observer and the object are stationary and unable to move, but the picture plane can move either closer to the observer or closer to the object. What happens to the image on the picture plane as the picture plane moves closer to or farther away from the observer? Of course, the image gets smaller as the picture plane gets closer to the observer and it gets larger as it moves away from the observer. More importantly, the size of the image on the picture plane is never the true size of the real object. It is always a bit smaller. This is due to the fact that the observer is a finite or measurable distance from the object and the image on the picture plane is called a central perspective. Central perspectives are very common in our world since they are called pictures or photographs and we see them every day when we read the newspaper or spend time on the internet. It would seem that using this type of image for our design documentation would not be beneficial since the size of the image depends on how close the picture plane is to the observer. In order to get the image on the picture plane to be the same size as the real object, we must move the observer to a position where he/she is an infinite (can t be measured) distance from the object. See Figure 9.2. When this occurs, the visual rays become parallel to each other and are perpendicular to the picture plane. Now the image on the picture plane is always the same size as the object. The image on the picture plane is called an orthographic view or an orthographic projection of the object. The prefix ortho means perpendicular so the visual rays are perpendicular (ortho) to the picture plane and the image is a twodimensional image that is being viewed by the observer. Orthographic projection is the fundamental principal upon which all engineering drawing is based.
236 Chapter 9: Graphical Communication Lines drawn to represent three-dimensional objects can have various meanings with regard to the object. The question becomes What does a line represent? When dealing with threedimensional objects, a line can represent the edge view of a surface. Figure 9.8 shows the step block. Each line in each view represents the edge view of a specific surface. For example, surface ABCD is represented by a single line in the front view. It is also represented by a single line in the right side view. All the lines of the step block in each of the orthographic views represent the edge of a surface. Figure 9.8 A line of an object defined as an edge view of a surface. A line can also represent the intersection of two surfaces. This is very different from a line that represents an edge of a surface. In Figure 9.9 we see the orthographic views of a block that has an inclined surface (surface ABCD). Surface ABCD is represented in the front view by a single line that is the edge view of the surface. In the top view, line AB is not the edge view of a surface, but it is the intersection of two surfaces. In a similar manner, line CD in the right side view is the intersection of two surfaces. Finally, a line can represent the limiting element of a curved surface. Figure 9.10 shows a cylinder. We can draw an infinite number of lines on the outside of the cylinder from the top to the bottom. These lines are parallel to the axis of the cylinder and are called elements. In the front view of the cylinder, note the lines AB and CD. Are these lines either the edge of a surface or the intersection of two surfaces? The answer to this question is obviously no. The lines represent the limiting element of a curved surface. The limiting element is the last element that one can see before the curve begins to turn back on itself. Thus a line is generated to show the limit of the cylinder.
Chapter 9: Graphical Communication 237 Af, Bf Figure 9.9 A line of an object defined as the intersection of two surfaces. Figure 9.10 A line of an object defined as the limiting element of a curved surface.
238 Chapter 9: Graphical Communication When dealing with real objects and representing them with orthographic views, a line can be defined as the edge view of a surface, the intersection of two surfaces, or the limiting (extreme) element of a curved surface. There are no other definitions of lines on an orthographic view of a three-dimensional object. When representing three-dimensional objects orthographically, we not only must show all the lines that are visible to the observer, but we must represent the lines that are hidden or not visible as well. Generally, the object itself blocks our view of the details that appear hidden, so we need a way to represent these features. In order to distinguish these hidden lines from visible lines, we display them as dashed lines. Visible lines are solid lines that are generally considered thick lines (0.7mm thick), while hidden lines are dashed lines that are thinner than visible lines (0.5mm thick). Being thinner than visible lines as well as being dashed and not solid, makes hidden lines significantly different and thus easy to interpret. Figure 9.11 shows a step block with a notch cut out of the back. Two lines in the front view that represent the notch are shown as hidden (dashed) lines. Also, the right side view has a hidden line which shows the notch. Figure 9.11 Hidden lines of an object represented with thin dashed lines.
240 Chapter 9: Graphical Communication but are something less. In the view where there is a partial circle or arc, the centerline is represented as a cross hair. In the longitudinal view(s) the centerline is a long, broken line and is an axis of symmetry. See Figure 9.13. Centerline for Radius Semi-Circular View Figure 9.13 Treatment of centerlines of objects with radii. Sometimes, especially in complex shapes, various line types will get superimposed on each other in a specific view. For example, a visible line and a hidden line may occupy the exact same space in a view. This creates a small dilemma. In cases such as this, we rely on the alphabet of lines and line precedence to tell us what should be shown. Various line types (visible, hidden, centerline) have a priority with regard to the orthographic drawing. The highest priority belongs to the visible line, the next priority belongs to the hidden line, and the lowest priority belongs to the centerline. Figure 9.14 shows the alphabet of lines from the highest priority to the lowest priority. Note that priority is given in reverse alphabetical order.
Chapter 9: Graphical Communication 241 Visible Line (Highest Priority) Hidden Line (Second Priority) Centerline (Lowest Priority) Figure 9.14 Three common standard line types used in engineering drawing and their priority. Using this guideline, the visible line has the highest priority which means when it competes for space in a view with either a hidden line or a centerline, it has precedence and is shown before the other two line types. A hidden line has precedence over a centerline only. A centerline has no precedence. Figure 9.15 shows an example of line precedence in views. Study this figure carefully and refer to it when you have questions regarding line precedence. Over Centerline Figure 9.15 Orthographic views of an object showing line precedence.
Chapter 9: Graphical Communication 243 + A. B. C. D. Figure 9.17 Example technique for sketching circles. When creating a sketch, keep proportions in mind. We do not measure any distances on the sketch, but we want the width, depth, and height to remain in their proper proportions. Real world measurements will come later when working drawings, also known as production drawings, are produced. Line weights are important. For example, earlier in this chapter we discussed line thickness with regard to visible lines, hidden lines, and centerlines. When sketching, it may be rather difficult to keep the line thicknesses precise; however, your visible lines should be thick and your hidden lines and centerlines should be thin. This does not mean that the hidden lines and centerlines are light. They should be dark, but thin. There will be additional discussion with regard to sketching as we begin pictorial drawings in the sections that follow. I c i i In previous sections of this chapter we learned that orthographic drawings were composed of multiple views where each view represented two dimensions. Pictorial sketching involves creating a view of the object in which all three dimensions are shown. Pictorials are used as a method to help us with visualization which enables us to formulate in our mind s eye what the object looks like. Also, pictorials can be used to present one s case for a design recommendation among various non-technical professionals. Pictorials are relatively easy to understand and can be extremely important in selling a design. There are generally three types of pictorial sketches that can be used. These types of sketches are known as axonometric sketches, oblique sketches, and perspective sketches.
244 Chapter 9: Graphical Communication Examples of each type of pictorial are shown in Figure 9.18. The most popular form of axonometric pictorial is the isometric, so we will limit our discussion of axonometric pictorials to isometric pictorials. The oblique pictorial is also a popular method of describing threedimensional pictorials in engineering; therefore, we will discuss this form of pictorial as well. Since perspective is widely used in the technical illustration field as well as in the field of architecture and not engineering, we will not be discussing it. (Isometric) Figure 9.18 Examples of pictorial sketches. 2 4 1 3 Figure 9.19 Orthographic views of the slotted block.
Chapter 9: Graphical Communication 251 1 VISUALIZATION When you see a multi-view two-dimensional orthographic drawing with your eyes and then try to read this information in your mind s eye as the three-dimensional object, it can get confusing. This requires skill that takes practice to master. Sometimes you can look at the orthographic drawing and see the three-dimensional object right away. Other times you have to think about it and perhaps employ some pictorial sketching technique as mentioned in the last section in order to visualize the object in its three-dimensional form. What we want to present in this section are methods that can be used to help you to develop your visualization skills. There is no substitute for practice, and by practicing, you can achieve the goal of seeing the object in your mind as a three-dimensional object without having to sketch it. To be able to read and interpret shapes from orthographic drawings and to properly visualize them, it is important to understand the meaning of surfaces as they appear in the orthographic views. Recall earlier in this chapter, we discussed the meaning of lines and discussed how a line is generated in orthographic views from a three-dimensional object. Surfaces can be defined in a similar manner. There are three types of plane surfaces that can be used to create a three-dimensional object. These include principal surfaces, inclined surfaces, and oblique surfaces. A principal surface is a surface that is parallel to one of the principal planes (picture planes) shown on the glass box. Therefore we can have a frontal surface which is parallel to the frontal plane, we can have a horizontal surface which is parallel to the horizontal plane (top view), and we can have a profile surface that is parallel to the profile or right side plane. Figure 9.27 shows an object that is composed of all principal surfaces. Surface A is a frontal surface because it is parallel to the frontal plane and appears true size in the frontal view. Note that the right side view of surface A is an edge view and the horizontal view of surface A is also and edge view. This is always true of principal surfaces. One view will appear in true size, while the other two views will be edge views. The same holds true for surface B. Surface B is true size in the horizontal view and appears as an edge in the front view and the right side view. Figure 9.27 Example of an object with principal surfaces.
252 Chapter 9: Graphical Communication An inclined surface does not appear true size in any of the principal views (front, top, side), but appears foreshortened in two of the views and is an edge view in the other. Figure 9.28 shows surface A as an inclined surface. Note that surface A in the horizontal view and the right side view appears as a rectangle. This rectangle is foreshortened in both views in other words it is not true size. Lines 1-3 and 2-4 are true length in the horizontal view, but lines 1-2 and 3-4 appear foreshortened, therefore the view of the plane is not true size. Note that the front view of surface A appears as an edge. This is always true of inclined surfaces. Two views will show a foreshortened view of the surface and the remaining view will appear as an edge. ----------------------------- V 1 Edge View Surface A \ Surface A \ Foreshortened -----------------------------------------------------------------A 2,4 2 Figure 9.28 Example of an object with an inclined surface. An oblique surface will never appear as an edge in any of the principal views. An oblique surface will always maintain its characteristic shape and show up as foreshortened in all three principal views. Figure 9.29 shows an oblique surface A in the orthographic views. Note that surface A is a foreshortened triangle in all three views.
Chapter 9: Graphical Communication 253 Figure 9.29 Example of an object with an oblique surface. Regarding visualizing surfaces, there is one hard and fast rule that applies to any plane surface. A surface will either appear as an edge or it will maintain its characteristic shape in any view in which it appears. This does not mean that the surface will appear in true size, but it only has to maintain its characteristic shape. If it is a rectangle, it will either be an edge or it will be a rectangle. Note that in the examples of Figures 9.27 and 9.28 that the surface is either an edge or it appears as a rectangle. In Figure 9.29, the surface appears as a triangle (its characteristic shape) in all three views. This rule is very useful when visualizing objects. Another type of surface you need to be familiar with while practicing visualization is the curved surface. Earlier in the chapter, we defined what was meant by a limiting element. Recognizing limiting elements will enable you to visualize curved surfaces. Figure 9.30 shows an object composed of plane surfaces and a cylinder. Note the limiting elements and how they appear in defining the cylindrical feature. Also note the plane surface intersects the cylinder tangentially and creates what is referred tel as a run out.
254 Chapter 9: Graphical Communication Figure 9.30 An object composed of a cylinder and a prism showing limiting elements of a curved surface and a run out where the prism meets the cylinder. 9.12 SCALES AND MEASURING Most technical drawings are drawn to some scale. This means that there is a ratio between the length of the lines on the drawing representing the object and the length of the same line on the real world object. For example a 1:1 (one to one) scale means that one linear unit of measure on the drawing is equal to one linear unit on the object. Use of scale primarily depends on the size of the drawing media. If your object is relatively small and you are showing the orthographic views on an 8.5 x 11 (A size) page, then you probably could use a full scale or 1:1. If your object is large, and you are going to draw the orthographic views on an A size sheet, then you will have to scale it down some. The larger the object being shown on the drawing, the smaller the drawing scale has to be on the sheet. Common drawing scales used in engineering are full scale (1:1), half scale (1:2), quarter scale (1:4), and tenth scale (1:10). The scale denotes the size of the drawing. For example, a half scale drawing means the drawing appears at half the size of the real world object. Sometimes it is advantageous to make drawings larger than they are in the real world. For example, when dealing with extremely small objects, you might draw them at double scale (2:1) or larger in order to see more detail. Drawing scale refers to the ratio that the drawing has to the real world object; however, there is an instrument called a scale as well. Sometimes this instrument is called a ruler. It is a calibrated drawing instrument used to measure linear distance according to the speci-