Multiview Drawings STANDARD MULTIVIEW DRAWINGS Introduction Orthographic views are 2D images of a 3D object obtained by viewing it from different orthogonal directions. They are created using orthographic projection principles. 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 a projection, picture or image plane. This allows a 3D object to be accurately represented on a 2D plane. The image plane is an imaginary transparent flat surface that coincides with the drawing surface. In practice the drawing surface 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.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. 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. While the observer is in one position in perspective projection, several positions are needed for parallel projection. 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. Projection line Object Projection line Object Projection Plane Projection Plane Image Image Observer Observer a) Parallel projection b) Perspective projection Fig. 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 (orthoviews for short) make it possible to describe a 3D object in 2D 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 manufacturing and inspection 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. 1 Osakue, E. Multiview Drawings Principal Views Six principal planes are all that are needed in orthographic projection because six viewing directions are possible as shown in Fig. 2. The planar surfaces of the image box shown in Fig. 3 are called principal planes and the images created on its planes are called principal views. The six (6) principal views in orthographic projection are Top, Bottom, Front, Rear, Left, and Right views. In multiviews, a line in one view may represent one or more edges in an object. So care is needed in interpretation of faces. Think about the object in terms of faces, features, and edges. Remember that an edge may be a curved or straight line. Top view Image box Front view Right view Fig. 2 Principal view directions Fig. 3 Views and image box 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. 4. With the object inside the image box in Fig. 3, then Fig. 5 is what is obtained for the principal views in paper space. Fig. 4 Image box faces and principal planes 2 Osakue, E. Multiview Drawings Fig. 5 Layout of six principal views on flat paper Standard Views Though there are six principal views, three have been chosen as standard views. The U.S. standard views are Top, Front and Right-side views. 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. These are based on the first angle projection in which the object is assumed to be located in the first quadrant. Fig. 6 shows the layout of 3 rd and 1 st angle projections while Fig. 7 shows the symbols for the two standards of projection. Adhering to these standards is a professional issue. Fig. 6a European standard views Fig. 6b U.S. standard views 3 Osakue, E. Multiview Drawings a) First angle b) Third angle Fig. 7 Standard projection symbols Choose Front View. Choosing a correct front view is very important in multiview drawings. The following points should be considered when making a front view choice in multiview drawings. a) Most stable or natural position of use. b) Contains the least hidden features. c) Best shape or most descriptive profile. d) Shows the longest dimension. Necessary Views A drawing in standard orthographic views requires three views. However, some objects may need less or more views for complete description. For example, spheres need only one view for representation. Components of uniform shape (e.g. square, circular, triangular, and rectangular) or that have relatively complex profiles but very small thickness (e.g. sheet metal components) may be described by one view. Such drawings normally include notes specifying the object thickness. Objects with axial symmetry and without complicated features may be represented with two views. Examples are cylindrical, conical and pyramidal objects. Irregular objects generally need two or more views for representation. Sometimes auxiliary and section views are necessary in detail drawings. Auxiliary views are needed when a feature is distorted in one or more principal views. Features are distorted in a principal view whenever they appear on an inclined or oblique face on an object. A section view is an orthographic projection view drawn to reveal internal or hidden features in an object. They improve visualization of designs, clarify multiviews and facilitate dimensioning of hidden features. Section and auxiliary views can substitute for standard orthographic views, and this helps to keep the number of views down. Partial views in cases where auxiliary views are needed give clearer presentation. PRINCIPAL DIMENSIONS AND LAYOUT 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) for an object; see Fig. 8a. For example, the front view, can only show the width and height dimensions, see Fig. 8b. The heights of faces on a top view can only be known from the front view or right view . The depth dimension is not shown. In single view drawings, the missing dimension is usually included as a general note. W D H H H W D W D a) Object b) Layout of standard views Fig. 8 View layout and principal dimensions 4 Osakue, E. Multiview Drawings The Table1 below summarizes the views and principal dimensions. Views Principal Dimensions Top, Bottom Width, Dept Front, Rear Width, Height Right, Left Height, Depth Table 1: Principal views and dimensions Placement of Views Views in multi-view drawings should be properly placed on the layout; see Fig. 9. Only Fig. 9a is acceptable because the views are correctly placed and aligned in 3 rd Angle projection. a) Correct placement and alignment b) Top View not aligned c) Front View not aligned d) Right View not aligned Fig. 9 Placement and alignment of multiviews CONSTRUCTING MULTIVIEW DRAWINGS Multiview drawings are 2D images of a 3D object obtained by viewing it from different orthogonal directions. They are generated using orthographic projection principles and are used to completely describe the shape and size of objects. There are two phases in constructing multiview drawings. These are the planning and the drawing phases. The planning phase is more of mental activity than the drawing phase which is basically mechanical. Good planning reduces the time and effort for the drawing phase. Planning Phase 1. Choose paper size (manual/scaled sketching). You can skip this step in CAD environment. 2. Envision the bounding and determine principal dimensions. 3. Choose the front view of object. 4. Determine plot scale (manual/scaled sketching). You can skip this step in CAD environment Envision the Bounding Box and Determine Principal Dimensions. Mentally picture the bounding box around the object. Determine the principal dimensions of the object. You may need to add or subtract dimensions along the principal directions to obtain the principal sizes of the object. 5 Osakue, E. Multiview Drawings Choose Front View. Choosing a correct front view is very important in multiview drawings. 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 dimension (when profile is simple) • Contains the least hidden features Drawing Phase: 1.