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.
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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
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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
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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.
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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. Draw the Layout Draw the bounding blocks of the top and front views.. 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.
2. Draw Visible Features Choose a base view (top, front, right). Draw all visible edges and shapes moving from left to right by visual inspection. You do not need to complete everything on a view to continue on the next view. Decide on the next view. Use projection lines and visual inspection to identify and locate features and shapes on other views. Complete all visible features in all views.
3. Add Hidden features Hidden features arise from steps, slots, and holes not directly visible Use projection lines and visual inspection to identify and locate hidden features and shapes in all views. Change hidden features linestyle to hidden or create them in the hidden layer
4. Add Center Lines Place centerline or center mark on all circles and arcs. Auxiliary/section views may be needed.
5. 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 or verse versa. 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 must be applied. Views must be aligned.
6. Add dimensions. 7. And notes and specifications. 8. Check and correct drawing. 9. Make check print(s) and review drawing. 10. Make final corrections. 11. Print/archive drawing
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AutoCAD Set-Up for Multiview Drawings Load linestyles 1. Hidden 2. Center 3. DOT (Stitch)
Create layers 1. Visible (lineweight = 0.4 mm) 2. Hidden (lineweight = 0.15 mm) 3. Center (lineweight = 0.15 mm) 4. Stitch (Projection line layer: lineweight = 0.15 mm) 5. Dimensions (lineweight = 0.15 mm) 6. Text (lineweight = 0.15 mm)
Develop the habit of using layers quickly. It helps to organize your work, saves time and makes changes on layer properties easy. It is a professional skill!
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