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1 PTT 105/3: Engineering Graphics TAXONOMY OF PLANAR GEOMETRIC PROJECTIONS Planar Geometric Projections Parallel 2 Perspective Multiview orthographic Axonometric Oblique Isometric Dimetric Trimetric PTT 105/3: Engineering Graphics 1 point 2 point 3 point Parallel Projection 3 PTT 105/3: Engineering Graphics Perspective Projection 4 PTT 105/3: Engineering Graphics Orthographic Projection Projectors are orthogonal to projection surface 5 PTT 105/3: Engineering Graphics Orthographic Projection Advantages and disadvantages Preserves both distance and angle - shapes are preserved - can be used for measurements: Building plans Manufactured parts • Cannot see what object really looks like because many surfaces hidden from view. - often we add the isometric 6 PTT 105/3: Engineering Graphics Multiview Orthographic Projection Projection plane parallel to principle face In CAD and architecture, we often display three multiviews plus isometric. 7 PTT 105/3: Engineering Graphics Axonometric Projections Move the projection plane relative to object Classify by how many angles of a corner of a projected cube are the equal to each other. Types of axonometric projections: Three angle equals: isometric Two angle equals: dimetric None angle equal: trimetric 8 PTT 105/3: Engineering Graphics 1. ISOMETRIC PROJECTION The isometric projection has a standard orientation that makes it the typical projection used in CAD. In an isometric projection, the width and depth dimensions are sketched at 30° above horizontal. In an isometric projection all angles between the axonometric axes are equal. This results in the three angles at the upper front corner of the cube being equal to 120°. The three sides of the cube are also equal, leading to the term iso (equal) -metric (measure). 9 PTT 105/3: Engineering Graphics Isometric drawings work quite well for objects of limited depth. However, an isometric drawing distorts the object when the depth is significant. In this case, a pictorial perspective drawing is better. 10 PTT 105/3: Engineering Graphics 2. DIMETRIC PROJECTION In a dimetric projection two of the objects axes make equal angles with the plane of projection and the third angle is larger or smaller than the other two. 11 PTT 105/3: Engineering Graphics 3. TRIMETRIC PROJECTION In general, the trimetric projection offers more flexibility in orienting the object in space. In a trimetric projection no two axes make equal angles with the plane of projection. The width and depth dimensions are at arbitrary angles to the horizontal, and the three angles at the upper front corner of the cube are unequal. 12 PTT 105/3: Engineering Graphics This makes the three sides of the cube each have a different length as measured in the plane of the drawing; hence the name tri-metric. In most CAD software, the trimetric projection fixes one side along a horizontal line and tips the cube forward. Generally, small manufactured objects are adequately represented by isometric or trimetric views. 13 PTT 105/3: Engineering Graphics Axonometric Projections Advantages and Disadvantages Lines are scaled (foreshortening) – can find scaling factor but can’t measure distance directly. Parallel lines preserved but angles are not Can see three principal faces of box-like object Some optical illusions possible - parallel lines appear to diverge Does not look real because far objects are scaled the same as near objects Used in CAD applications 14 PTT 105/3: Engineering Graphics Oblique Projection An OBLIQUE projection is one in which the projectors are other than perpendicular to the plane of projection. Figure below shows the same object in both orthographic and oblique projections. The oblique projection: - shows the front surface and the top surface, which includes three dimensions: length, width, and height. Therefore, an oblique projection is one way to show all three dimensions of an object in a single view. 15 PTT 105/3: Engineering Graphics PERSPECTIVE PROJECTION A simply perspective (pictorial perspective) projection is drawn so that parallel lines converge towards a single point, unlike isometric or trimetric projections where parallel lines remain parallel. This has the effect that distant objects appear smaller than nearer objects. A perspective projection is quite useful in providing a realistic image of an object when the object spans a long distance such as the view of a bridge, railways or aircraft from one end. 16 PTT 105/3: Engineering Graphics PERSPECTIVE PROJECTION 17 PTT 105/3: Engineering Graphics Differences between 3D and 2D Criteria 1. Viewing Direction 3D - Look at your object from an angle 2D - Look straight down on the drawing plane 2. Object Created - have depth - No depth 3. Command related - 80 AutoCAD commands related primarily to 3D -Yes - however, are also useful in 2D 4. Constructing object 18 PTT 105/3: Engineering Graphics - No. - Drawing picture from different views 3D Term Frequently Used • 3D objects made in AutoCAD called models • 2D work referred to as drawing/drafting 19 PTT 105/3: Engineering Graphics 4 types of model used in 3D Wireframe Model Surface Model Model Solid Model 20 PTT 105/3: Engineering Graphics Rendering • Object by its edges only • Wireframe cannot hide object that are behind them • Hole has no meaning in a wireframe model because there is nothing in which to make a hole 21 PTT 105/3: Engineering Graphics • Surface model often use wireframe models as a frame for their surfaces • Surface model: part wireframe + part surface 22 PTT 105/3: Engineering Graphics • Wireframe, surface model & computercalculated mass; • eg: volume, centre of gravity, mass moment of inertia • Solid models look like wireframe unless a hidden line removal command is in effect 23 PTT 105/3: Engineering Graphics • Shaded, realistic-looking picture of a surface solid model is called a rendering • Two type: • Grayscale rendering • Fully capable of colour rendering 24 PTT 105/3: Engineering Graphics Reasons For Using 3D Closer to representing real object than a 2D Show design more clearly Good for verifying design as well as for use in presentation and documentation as well. 25 PTT 105/3: Engineering Graphics 3D Capabilities of AutoCAD 1. AutoCAD has complete 3D coordinate system for specifying points and drawing objects anywhere in space 2. To assist in point input and for working in local area, AutoCAD has a movable user coordinate system 3. It can set viewpoints from any location in space that can look in any directions 4. The computer screen can be divided into multiple viewports for simultaneously viewing 3D space from different viewpoints and different direction 26 PTT 105/3: Engineering Graphics 5. AutoCAD has a good assortment of surface entities for making surface models that have a variety of shapes 6. Solid models of most objects typically manufactured in machine shop can be made within AutoCAD 7. 3D models can be transformed into standard multiview, dimensioned production drawing 8. AutoCAD has a built-in renderer with lights and surface materials, capable of making realistic-looking shaded picture from 3D models 27 PTT 105/3: Engineering Graphics 28 EPT 112/3: Introduction to Engineering