3-D GEOMETRIC TRANSFORMATION AND PROJECTION LECTURE 3: 3-D GEOMETRIC TRANSFORMATION AND PROJECTION PTT 105/3: Engineering Graphics

3D PROJECTION Three different types of 3-D projections are available in most CAD software: isometric, trimetric, and perspective. These three views of a cube are shown in figure below. 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. 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). 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. PTT 105/3: Engineering Graphics

2. TRIMETRIC PROJECTION In general, the trimetric projection offers more flexibility in orienting the object in space. 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. 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. PTT 105/3: Engineering Graphics

3. 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. PTT 105/3: Engineering Graphics

But the oblique projection is usually even easier to sketch. Two types of pictorial sketches are used frequently in freehand sketching: isometric and oblique. The isometric projection is often used in freehand sketching because it is relatively easy to create a realistic sketch of an object. But the oblique projection is usually even easier to sketch. The figure compares an isometric and an oblique projection of a cube with a hole in it. oblique freehand sketching easier than isometric sketching. PTT 105/3: Engineering Graphics

Differences between 3D and 2D Criteria 3D 2D 1. Viewing Direction - Look at your object from an angle - Look straight down on the drawing plane 2. Object Created - have depth - No depth 3. Command related - 80 AutoCAD commands related primarily to 3D - however, are also useful in 2D 4. Constructing object - Yes No. Drawing picture from different viewpoints PTT 105/3: Engineering Graphics

3D Term Frequently Used PTT 105/3: Engineering Graphics Model 3D objects made in AutoCAD called models 2D work referred to as drawing/drafting PTT 105/3: Engineering Graphics

4 types of model used in 3D PTT 105/3: Engineering Graphics Model Wireframe Model Surface Model Solid Model Rendering PTT 105/3: Engineering Graphics

1. Wireframe Model 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 PTT 105/3: Engineering Graphics

2. Surface Model Surface model often use wireframe models as a frame for their surfaces Surface model: part wireframe + part surface PTT 105/3: Engineering Graphics

3. Solid Model Wireframe, surface model & computer-calculated mass; eg: volume, centre of gravity, mass moment of inertia Solid models look like wireframe unless a hidden line removal command is in effect PTT 105/3: Engineering Graphics

4. Rendering Shaded, realistic-looking picture of a surface solid model is called a rendering Two type: Grayscale rendering Fully capable of colour rendering PTT 105/3: Engineering Graphics

Closer to representing real object than a 2D Show design more clearly 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. 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 PTT 105/3: Engineering Graphics

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 multi-view, 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 PTT 105/3: Engineering Graphics

EPT 112/3: Introduction to Engineering