UNIT 5a STANDARD ORTHOGRAPHIC VIEW DRAWINGS

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1 UNIT 5a STANDARD ORTHOGRAPHIC VIEW DRAWINGS 5.1 Introduction Orthographic views are 2D images of a 3D object obtained by viewing it from different orthogonal directions. Six principal views are possible and are named top, bottom, front, rear, left, and right views. However, three of the six views are regarded as standard views. The U.S. standard views are the top, front, and right views and are based on third angle orthographic projection. The European standard views are the front, top, and left views and based on first angle orthographic projection. 2D orthographic views can be generated directly from solid models and is much faster than drawing the views. Multiview drawings consist of two or more views with appropriate annotations arranged in some preferred pattern. They include standard orthographic views, auxiliary views and section views. Detail component drawings are most often 2D engineering drawings of parts with necessary information for constructing, manufacturing, or inspection. 2D assembly drawings are extensively used in the building of equipment and structures. Multiview drawing guidelines are prescribed by ASME Y14.3M in the U.S standards. 5.2 Projection Types Projection is the graphic technique of extending points on a 3D object by straight lines (linear projection) so as to create its image(s) on an image plane. They are created using the principles of orthographic projection. Orthographic projection allows a 3D object to be accurately represented on a 2D plane. In orthographic projection, the views of the object are obtained by viewing it from different orthogonal directions. Natural objects are in 3D solid form. They are bounded by vertices, edge, and faces. These and other geometric entities that make the solid are called features. Since points are the most basic graphic entities, images of objects may be created from the points on it. In projection, points on a 3D object are extended by straight lines (linear projection) to create its image(s) on a projection or picture or image plane. The image plane is an imaginary transparent flat surface that coincides with the drawing surface which in may be a paper or computer screen. A projection relates an observer and an object to an image plane through the lines of sight or projection. There are two types of projections: parallel and perspective projections. Fig. 5.1 illustrates the principles of parallel and perspective projections. In parallel projection, the projection lines are always parallel but in perspective projection, the projection lines converge at a point. While the observer is in one position in perspective projection, several positions are needed for parallel projection. Parallel projection is used in orthographic, axonometric and oblique projection methods. Axonometric projections have three variants of isometric, dimetric, and trimetric projections. Perspective projection is used to generate one-point, two-point, and three-point perspective drawings. Perspective, axonometric and oblique projections are used to generate pictorial drawings. Whether the projection is parallel or perspective, the image of object vertices are constructed on the image plane at the intersections of lines of sight and the image planes. a) Parallel projection b) Perspective projection Fig. 5.1 Basic types of Projection Orthographic projection is a parallel type of projection technique in which the view directions are parallel but perpendicular to the image planes. Orthographic views or orthoviews make it possible to describe a 3D object in 2D 1

2 multiple views. For manufacturing and inspection purposes, information about shape, size and location for each feature on an object must be precisely described to avoid problems. By viewing the object from different directions, it is possible to completely describe the shape, size and location of features on it and hence provide precise information for manufacturing and inspection. Though 2D views are easier to create but reading and interpreting them require drafting skills because they are abstract or conceptual form of representation. Standard orthographic views are 2D views selected by national and international standard bodies that are used for formal design documentation. Projections are true representations of objects on appropriate scales. However, true projections sometimes distort the view of objects. Hence in some situations, practical judgment is applied and a representation deviating from a true projection is substituted. These modified projections are called drawings, not projections. For example, the isometric scale is about 18% shorter than true size. For convenience, the actual dimensions of the object are shown in isometric views and such views are, therefore, called isometric drawings and not isometric projections. 5.3 Object Planes and Features Real objects are 3D in nature, so the representation of objects in 3D gives the most realistic model. Features are segments of 2D shapes and 3D forms that make up objects. 3D features could be main segments of basic 3D forms such as cylinders, boxes, cones, pyramids, or auxiliary segments such as holes, screws, etc that are part of objects. 2D features are basic shapes such as rectangles, circles, ellipses, etc. or segments of shapes like lines, arcs and points. Points on objects are known as vertices. Edges are lines or curves on objects and are formed from the intersection of two planes or surfaces. A face is a surface on an object that may be flat or curved. Faces are defined relative to an object, but surfaces and planes need not be referenced to any object. There are vertical and horizontal planar faces as shown in Fig Other types of planar faces are inclined and oblique faces. Fig. 5.3 shows examples of planar, curved, and inclined faces and Fig. 5.4 shows examples of planar and oblique faces. A Horizontal face A Planar face A Planar face B Frontal face B Curved face B Planar face C Profile face C Inclined face C Oblique face Fig. 5.2 Normal faces Fig. 5.3 Non-normal faces Fig. 5.4 Planar and oblique faces Features on normal faces of objects appear as true size and shapes in orthographic projection. Features on inclined and oblique faces do not appear as true size and shapes in orthographic projection. They are described as foreshortened because the apparent size or shape on the face is not equal to the true size or shape. The true size and shape of a feature on an incline face is obtained on an auxiliary plane perpendicular to the inclined plane. The true size and shape of a feature on an oblique face is obtained on an auxiliary plane perpendicular to a plane which shows a true length of a foreshortened edge in the original face. At least one auxiliary projection is required to develop the true size and shape of a feature on an inclined face. At least two auxiliary projections are required to develop the true size and shape of a feature on an oblique face. The next chapter discusses auxiliary views. 5.4 Orthographic Projection Concepts and Assumptions In orthographic projection, an observer looks at an object in a view direction of interest. When the view direction is normal to a flat face, only two of the principal dimensions can be seen. By changing positions in steps of 90 o, 2

3 multiple 2D views of the object can be generated. Using multiple 2D views arranged in well-defined pattern provide an easy means of adequately describing 3D objects. In orthographic projection, the observer is assumed to be at infinite distance from the object. Lines of sight from the observer to the object will then appear parallel. The projection or lines of sight are parallel and perpendicular to the image plane. The views created by orthographic projection are called orthographic views and they are drawn based on what we know about objects. Therefore, orthographic views do not match optical reality. Hence, 3D visualization skills are needed to combine orthographic views into a pictorial view. Multiview drawings are combinations of two or more orthographic views. One view of orthographic drawing reveals only two of the three principal dimensions of an object. Therefore, two views are normally required in a multiview drawing to define the third dimension. The concepts, assumptions and principles of orthographic projection are summarized below: Concepts 1. Line of sight: Direction of light travel from observer to object and image plane. 2. Image plane: Flat surface where image is constructed. 3. Object: Abstract or real entity of interest. 4. Observer: Imagined person looking at object. Assumptions 1. Observer is at infinite distance from object. 2. Image planes are orthogonal. 3. Lines of sight meet image planes at right angle. 4. Points on objects are projected on image planes. 5. Lines of sight are represented by projection lines. 5.5 Bounding Box Concept The box volume an object occupies in space is defined by its principal dimensions. Principal dimensions are the limits of size or overall size in the principal axes of X, Y, and Z in 3D space. These are often designated as W, D, and H respectively as shown in Fig In Fig. 5.5 the object consists of two 3D segments (features) of a top cylinder and rounded box at the base. The bounding box (Bbox) is indicated with phantom linestyle. In general the bounding box of an object can be constructed, no matter how complicated. It gives the minimum volume of a box that can accommodate the object. Also, it provides a natural basis for constructing a local or object rectangular coordinate system. In orthographic projection, projection planes are assumed to be imaginary, but the B-box seems to be an intuitive framework for visualizing image planes. In this regard, image planes assume physical significance on the basis of a B-box. This concept seems to be one not previous realized. Fig. 5.5: Bounding box and principal dimensions 5.6 VISUALIZING ORTHOGRAPHIC VIEW PROJECTION Image Box and B-box Considering the B-box, the image box can be seen as an enlarged B-box for the object as shown in Fig Thus, the B-box provides a conceptual basis for constructing an image box. Now position the image box, such that the object is at the center. The image box is made up of six planar surfaces and the surfaces are known as principal planes. Consequently, six 2D images of the object can be created. The image box is considered transparent so that the object inside it is observable from outside the box. The box is sometimes called the picture box. With the image box in place, the observer is positioned in a view direction perpendicular to an image plane; say the front side as shown in Fig Vertices on the object are the key points used for projection. Lines of sight are imagined to project or run from a vertex to a point on an image plane. Because the view direction is perpendicular to the image plane, the lines of sight will intersect the image plane at right angles. A piercing point is the point of intersection of a line and a plane. Hence the point on an image plane representing the object vertex is a piercing point on the image plane. A 3

4 straight edge or line segment on an object is defined by two vertices. Therefore, the projection of these two vertices on an image plane defines the end points of the straight edge on the image plane. Fig. 5.6: Image box and object Fig. 5.7: Object views on principal planes Connecting the image points by a line defines the image line for the object on the image plane. When the shape on one image plane is completed, then the observer can turn through 90 o (a rotational movement) to view the object from another principal direction. In Fig. 5.6, six view directions of front, rear; top, bottom; right and left are indicated. This process is repeated so as to generate the six view images on the image planes. Fig. 5.7 shows three of the view images of the object of Fig Note the dotted projection lines in Fig They are shown to help in the visualization of the projection concept. If an object has curved edges or contours, then the edges are divided into tiny line segments and the projection described above carried out. In this case, it is clear that constructing the image of a contour in an image plane will be is tedious task. This should help appreciate the availability of curve templates and instruments in traditional drafting and of special curves in CAD systems such as Bezier curves and NURBS. In Fig. 5.7 and Fig. 5.8; the intersection of two principal planes is an edge known as the fold line. Lines of sight from a point on a principal plane to a fold line must be at right angle. Principal Views Though an infinite number of view directions are theoretically possible, six principal planes are all that are needed in orthographic projection. As mentioned earlier, the planar surfaces of the image box are called principal planes. Consequently, the images created on these planes are called principal views. The six (6) principal views in orthographic projection are top, bottom, front, rear, left, and right views. The views of the image box can be laid out on a flat surface or paper space. These are obtained by considering the fold-lines (intersection of image planes) in the image box to be imaginary hinges on which the views can swing about. Therefore, for the image box, the faces can be opened up as depicted in Fig With the object inside the image box in Fig. 5.7, then Fig. 5.9 is what is obtained for the principal views in paper space. It is seen then that the 3D object of Fig. 5.5 has now been converted to multiple 2-D views in Fig. 5.9 through the principles of orthographic projection. Apart from the advantage of simplicity of the 2-D views, there is the ability to clearly and completely describe the shape and size of the 3D object by multiple 2-D views. Fig. 5.8: Image box faces and principal planes 4

5 Fig. 5.9: Layout of six principal views on flat paper Projection Standards Fig 10a shows spatial quadrants 1, 2, 3, and 4 as they are conventionally assumed. The horizontal and frontal principal planes are indicated in Fig. 5.10a. The third principal plane, called the profile plane is omitted for clarity. Fig. 5.10b shows the planar representations of the spatial quadrants. In projection theory an object can be assumed to be in any of the four quadrants, however, the first and third quadrants are the preferred. In first angle projection (Fig. 5.11), the a) Spatial layout b) Planar layout Fig. 5.10: Spatial and planar quadrants object is in front of the principal planes relative to the view direction or position of the observer. Hence the projected views of the object are placed behind the object. For example, the top view lies below the object, the front view is behind the object, and the right view is to the left of the object. However, in the third angle projection (Fig. 5.12), the principal planes are in front of the object relative to the view direction. Hence the projected views of the object are 5

6 placed before the object. For example, the top view lies on top of the object, the front view is in front of the object, and the right view is to the right of the object. Symbol Symbol Fig. 5.11: First angle projection Fig. 5.12: Third angle projection First angle projection is the standard in Europe while the third angle projection is the standard in the United States and Canada. The object views generated based on these two standards must be placed in the correct relative positions. Standard symbols are used in drawings to indicate first and third angle projections as shown in Figs. 12 and 13 respectively. In either standard, the observer s position defines the view direction and name. Standard Views Though there are six principal views, three are chosen as standard views. The U.S. standard views are Top, Front and Right-side views and are shown in Fig These are based on the third angle projection in which the object is assumed to be located in the third quadrant. The European standard views are Front, Top and Left-side views and are shown in Fig These are based on the first angle projection in which the object is assumed to be located in the first quadrant. The front view may be used as the reference view in both standards. In the U.S. standard, the top view is located on top of the front view and the right view is located to the right of the front view. Fig. 5.13: U.S. standard views Fig. 5.14: European standard views This arrangement seems logical and intuitively natural. In the European standard, the top view is located below the front view, while the left view is located to the right of the front view. This arrangement appears counter-intuitive. When more details about an object is desired, auxiliary and section views may be created. Auxiliary views are 6

7 employed to reveal the true shape and size of features on inclined and oblique faces. Section views are created to reveal hidden details. Principal Dimensions and Layout As mentioned earlier, a principal view reveals only two principal dimensions. Therefore, a minimum of two principal views are usually required to show all three principal dimensions of width (W), height (H) and depth (D) as shown in Fig. 5.15a. For example, the front view shows the width and height dimensions, see Fig. 5.15b. The depth dimension is not shown. a) Object principal dimensions b) Layout of standard views Fig. 5.15: Principal dimensions and drawing layout In single view drawings, the missing dimension is usually included as a general note. Table 5.1 summarizes the views and principal dimensions. Table 5.1: Principal views and dimensions Principal View Top, Bottom Front, Rear Right, Left Principal Dimensions Width, Dept Width, Height Height, Depth 5.7 Non-Unique Views An issue in orthographic projection is that projected views of different shapes may look alike. Hence singe views are not necessarily unique. It takes a minimum of two views to establish the shape or form of an object. Therefore every effort must be made to avoid ambiguity in drawing representations by generating the minimum views necessary to uniquely describe an object. Fig shows examples of non-unique side views. Fig. 5.16: Non-unique side views 5.8 Required Views and Placement in Drawings A drawing in standard orthoviews requires three views. However, some objects may need less or more views for complete description. For example, a sphere needs only one view for representation because of its symmetry about two axes. Components of uniform thicknesss (e.g. sheet metal components) may be described by one view. Such drawings normally include notes specifying the object thickness. Objects with one line of symmetry and without complicated features may be represented with two views. Examples are cylindrical, conical and pyramidal objects. Irregular objects are without lines of symmetry; they generally need two or more views for representation. Similarly, 7

8 very complicated objects with inclines and oblique faces need two or more views for representation, often including auxiliary and section views. a) Correct placement and alignment b) Top View not aligned c) Front View not aligned d) Right View not aligned Fig. 5.17: Placement and alignment of multiviews Views in multiview drawings should be properly placed on the layout. If the views are created with the aid of bounding blocks and miter line, the views will be aligned as will be shown in the next section. If views are generated from a solid model (many CAD packages can do this), then proper placement and alignment of views is a concern. The width dimensions on the top and front views should be aligned vertically. Similarly, the height dimensions on the front and right views should be aligned horizontally. The offset of the views from the fold lines should be made equal. However, this is not a critical requirement. Be sure to place them correctly according to the desired standard (U.S. or European). Fig illustrates these points. In the U.S. standard (3 rd Angle projection), the top view is placed on top of the front view and the right view is placed on the right of the front view. In the E.U. standard (1 st Angle Projection), the top view is placed below the front view and the left view is placed on the right of the front view. Only Fig. 5.17a is acceptable because the views are correctly placed and aligned in 3 rd Angle projection, the others are not acceptable. 5.9 CONSTRUCTING STANDARD MULTIPLE VIEWS Standard orthoviews can be constructed from isometric sketches and drawings or generated from solid models. The construction or generation process uses orthographic projection principles discussed in the preview sections. It is strongly recommended that the student invests the time and effort necessary for understanding the concepts and principles of orthographic projection. The temptation of short cuts should be avoid; short cuts seem to produce professionals who do not understand the whys of what they do! Understanding concepts and principles pay off handsomely with time as the hard working student soon develops his or her short cuts from them. The following steps may be used to construct a standard multiview drawing. 1. Envision the image boxes as enlargement of the B-box. 2. Choose the front view of object and define axes. 3. Construct the layout. 4. Draw the visible features. 5. Draw hidden features. 6. Add center lines. 7. Check and Correct views. 8. Make check print(s) and review drawing. 9. Make final corrections. 8

9 10. Print/archive drawings. Steps 1 to 10 are required for non-dimensioned drawings. If drawings are dimensioned, for example, when preparing working drawings, dimensions, notes and specifications are necessary and step 8 is done after annotation. In this case, after step 7; then 8. Add dimensions. 9. And notes and specifications. 10. Check and correct drawing. 11. Make check print(s) and review drawing. 12. Make final corrections. 13. Print/archive drawing. Envision the Bounding Box. Mentally picture the bounding box around the object. Enlarge the bounding box to get the image box. The faces of the image box form the principal planes and become portions on the view layout. The bounding box helps to establish the principal dimensions for layout construction. Fig. 5.18a is an object that the standard views are required and Fig. 5.20b shows the bounding box is added. Fig. 5.18a: Object Fig. 5.18b: Bounding Box Choose Front View. Choosing a correct front view is very important in multiview drawings. The front view for the object in Fig. 5.18a is chosen in Fig. 5.19a and the axes and view directions are added in Fig. 5.19b. The following points should be considered when making a front view choice in multiview drawings. Best shape or most descriptive profile. Most natural position of use. Most stable position. Shows the longest principal dimension. Contains the least hidden features. a: Front View Choice b: Axes and View Directions Fig. 5.19: Front view choice, local axes, and view directions Define Axes and View Directions Once the front view is chosen, the local axes are defined to establish the other view directions. The axes are laid out using the right-hand rule. In most CAD packages the X-Y plane is horizontal (Fig. 5.19b) and the Z-axis is vertical. In orthographic projection, the coordinate axes are imaginary. The origin of the coordinate axes can be placed anywhere relative to the object. However, it is easier to work with an origin located at some point on the object. It is suggested that the origin of the coordinate axes should be positioned at the front bottom left corner of the bounding box as shown in Fig. 5.19b. This makes every point on the object to have positive coordinate values. Keep in mind that the coordinate system is a local coordinate system. That is, one coordinate system is needed for each object of 9

10 interest. A global coordinate system will have a fixed origin that applies to every object. It is assumed here that the local axes are parallel to the global axes for a CAD system. The view directions must be correctly identified. The front view direction is parallel to the Y-axis, the top view direction points downward in the negative Z-axis and the right view direction points leftward along the negative X-axis. Draw the Layout Draw the bounding blocks of the top and front views, see Fig. 5.20a. Draw the miter line at 45 o beginning at the top right corner of the front view bounding block. Use projection lines to construct the right view bounding block, see Fig. 5.22b. a) Top and Front Views Boundaries b) Bounding Blocks for Views Fig. 5.20: View layout Draw Visible Features (see Fig. 5.21a) Choose a base view (top, front, right). Front view is chosen as base view. Use visual inspection to identify all visible features. Draw edges moving from left to right by visual inspection. (Use the offset button in CAD package). Draw edges moving from bottom to top by visual inspection. Draw visible shape features moving from left to right by visual inspection. You do not need to complete everything on a view to go on to the next view. Decide on the next view. a) Visible features development b) Hidden features development Fig. 5.21: Development of Views 10

11 Draw Hidden Features (see Fig. 5.21b) Use projection lines and visual inspection to identify and locate hidden features. Use hidden linestyle for hidden features or move hidden features to hidden layer. Add hidden features to the views. Auxiliary/section views may be needed. Remember linestyle precedence. Object lines have precedence over hidden lines and centerlines. Hidden lines have precedence over centerlines. Cutting-plane lines have precedence over centerlines. Fig. 5.22: Completed views Add Center Lines Place centerline or center mark on all circles and arcs (see Fig. 5.22). The completed standard multiviews drawing of the object in Fig. 5.18a are shown in Fig Check your drawings. All vertical lines in top view must be projected to front view or verse versa. All feature size limits in top view must be projected to front view or verse versa. All horizontal lines in top and front views must be projected to right view. All feature size limits in top and front views must be projected to right view or verse versa. All features must be represented in all views. Hidden lines must be properly represented. Centerlines must be properly represented. Precedence of lines should be applied. The following constitute the main principles in orthographic projection: 1. Lines of sight are parallel in orthographic projection, see projection lines in Fig Fold lines are the intersections of image planes and are placed midway between adjacent views. They are generally omitted for clarity, see Fig and compare with Fig There are only 6 possible image planes, see Fig Every feature in one view must be aligned on a parallel projector in an adjacent view, see Fig Distances between any two points of a feature in related views must be equal, see center distance between holes in top and front views or top and right views in Fig Features are true length or true size when the lines of sight are perpendicular to the feature planes, see circle view in Fig

12 7. Features are foreshortened when the lines of sight are not perpendicular to the feature planes, that is, features on inclined and or oblique faces. 8. Parallel features will always appear parallel in all views. 9. Surfaces that are parallel to the lines of sight will appear as lines or edge views. 10. No two contiguous areas can lie in the same plane. 11. The projection of an oblique line or oblique plane to the image plane is foreshortened. 12. An edge view line in one view of a multiview drawing represents a face on the 3D object. 13. Horizontal lines in front and top views have either visible or hidden line representations in side view. 14. Projection lines between views help in correctly locating features in the views and minimize errors. 15. The orthographic view of a curve is determined by first drawing the shape of the curve in one view. Then, divide the shape into segments with a segment bounded by two key points. Project the key points to the adjacent plane. Smaller segments give more accurate representation Summary In documenting and communicating design intent, information about shape, size and location for each feature on an object must be precisely described to avoid manufacturing and inspection problems. Graphic projection techniques provide means of accurately representing the forms and shapes of 3D object models on 2D media. They involve extending points on a 3D object by straight lines (linear projection) to projection, picture or image planes. The image plane is an imaginary transparent flat surface that coincides with the drawing surface which in practice may be a paper or computer screen. There are two types of projections, namely parallel and perspective projections. In parallel projection, the lines of sight are always parallel but in perspective projection, the lines of sight converge at a point. Parallel projection is used in orthographic, axonometric, and oblique projection methods. In orthographic projection, the view directions are perpendicular or orthogonal and six principal view directions are possible. These view directions give rise to six principal views that are named top, bottom, front, rear, left, and right views. The lines of sight are perpendicular to plane of projection. Faces and features on an object parallel to plane of projection also appear in true size and shape. Though there are six principal views, only three of them are regarded as standard views. The U.S. standard views are the top, front, and right views and are based on third angle orthographic projection. The image or glass box for the object is placed in third spatial quadrant in 3 rd angle projection. The projection plane is between the observer and the object. The European standard views are the front, top, and left views and based on first angle orthographic projection. The image or glass box for the object is in the spatial first quadrant in 1 st angle projection. The object is between the observer and projection plane. Orthographic views are easier to create but reading and interpreting them require drafting skills because they are abstract or conceptual forms of representation. The top view shows the principal dimensions of width and depth. The front view shows the principal dimensions of width and height. The right view shows the principal dimensions of height and depth. The front view is generally the most important view. Other views are referenced from the front view. Factors to consider in choosing the front view are: a) Most natural position of use; b) Best shape description; c) Longest principal dimension; d) Fewest hidden features; e) Most stable position; f) View with most contours. Multiview drawings have two or more views arranged in one drawing. They can combine standard, auxiliary, and section views. Adding dimensions, notes, and specifications to multiviews produces a document of design intent. Hence with multiview drawings, it is possible to completely describe the shape, size and location of features on an object. They provide precise information for design documentation, manufacturing, and inspection. Multiview drawing guidelines are prescribed by ASME Y14.3M. In multiview drawings, object lines have higher precedence over hidden lines and centerlines. Hidden lines have higher precedence over centerlines and cutting-plane lines have higher precedence over centerlines. Multiview drawings can be constructed from sketches and isometric drawings or generated from 3D models. The number of required views in a drawing depends on the complexity of the object. CAD packages with 3D modeling capabilities have utilities that can be used to create 2D views. A 2D view of a 3D model can be a pictorial view or an orthographic view and one can generate as many views as desired of the model object by changing the viewing 12

13 direction. Detail component drawings in mechanical engineering are prepared in multiviews with dimensions, tolerances and notes. All views should be properly aligned in either first- or third-angle projection. Alignment of views is critical to integrity and credibility of the drawing and the design drafter. When features appear on inclined and oblique faces of an object, the principal orthographic views give distorted images of the faces and features. Auxiliary views can be employed to reveal the true shape and size of the features. When there is interest in revealing the internal features of an object, a section view can be created. Auxiliary and section views are also used to supplement standard views in order to clarify views, improve visualization of designs, and facilitate dimensioning of drawings. Both part and assembly sections can be created. The next two chapters address the topics of auxiliary and section views. 13