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CADD PRIMER
Chapter 7: An Introduction to 3D CAD

 Contents  
About this Chapter
Key Terms in this Chapter
Why 3D CAD?
Pictorial Views Concept
  • Oblique Views
  • Isometric Views
  • Steps to Draw an Isometric
Isometric Drawing-Aid Functions
  • Isometric Grid
  • Fixed Cursor Direction
  • Isometric Circles, Text and Dimensions
  • 2D Drawings to Isometrics Conversion
3D Modeling
  • Working with 3D Coordinates
  • Cartesian Coordinates
  • Spherical Coordinates
  • Cylindrical Coordinates
Steps to Draw a 3D Model
User-defined Coordinate System
Displaying Views
  • View Coordinate Geometry
  • Object Coordinate Geometry
  • Displaying Isometric Views
  • Displaying Plans and Elevations
  • Displaying Perspective Views
3D Drawing-Aid Functions
  • 3D Ready-made Shapes
  • Extruding Objects in the Linear Direction
  • Extruding Objects in the Circular Direction
  • Shading and Rendering
AutoCAD, MicroStation and Cadkey Terms
About this Chapter This chapter introduces you to the general principles of 3D (three-dimensional) drawing that are commonly used in CADD. It describes how to make use of simple 2D functions to create a 3D effect, as well as how to create actual 3D models. You will learn how to measure distances in 3D, how to enter 3D coordinates and how to draw 3D shapes.

This chapter describes a number of 3D drawing techniques that are commonly used by CADD professionals. You will learn how to extrude 3D objects from simple 2D shapes, how to take advantage of 3D ready-made objects and how to make the views look realistic.

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You will also learn how to display 3D CAD views of a model from different angles.

Although the actual working of 3D CADD varies from program to program, the principles described here can be applied to most programs.
 

Key Terms in this Chapter
 
Term
Description
Isometric A view of an object tilted at 30° on both sides.
Oblique view A view of an object drawn by taking parallel projections from an elevation.
Perspective A view of an object showing true angles as they would appear from a specific point.
3D modeling A CADD capability that allows you to draw objects as physical objects having length, width and height.
3D coordinates The mode of measurement used to specify the length, width and height of objects created by 3D modeling.
Viewpoint The point from where a 3D model is viewed.
Linear extrusion A 3D technique that allows you to form 2D shapes into 3D shapes along a linear path.
Radial extrusion A 3D technique that allows you to form 2D shapes into 3D shapes along a circular path.

  Why 3D CAD? AutoCAD Forum

3D capabilities allow you to draw pictorial views such as isometrics, oblique views and perspectives. The views drawn with CADD have a number of advantages as compared to views drawn on a drawing board. The views drawn with CADD are very accurate and provide a lot of flexibility in terms of editing and display. You can rotate a model on the screen just like an actual model, and display views from different angles.

Designers often use 3D to visualize designs and to make presentations. It helps them understand how an object will appear from different angles. Using additional rendering programs can further enhance the drawings.

Although working with 3D CADD programs is quite complex, it is worth the extra effort to make use of them. Many users never take the time to learn the 3D capabilities of CADD. This can be a major disadvantage because the full potential of CADD is never explored.

Pictorial Views Concept How do we create 3D views on paper or on a computer screen, which are only two-dimensional media? Is an isometric or perspective 2D or 3D?

The views that we draw on two-dimensional media are a 2D representation of 3D images. We create isometrics and perspectives on paper by drawing objects as they would appear from a specific angle and distance. The same concept is used in CADD to draw pictorial views.

There are two distinct ways to draw 3D views with CADD: You can draw views using simple 2D functions or using CADD's special 3D functions.

The 2D functions allow you to draw views just like on a drawing board. You can draw a view using lines, arcs, or other 2D objects. This is the quickest method to draw simple isometric and oblique views. However, a view created this way is static; just like a view created on a drawing board. If you need to view the object from a different angle, you will have to draw it again from scratch.

CADD provides special 3D functions that allow you to create 3D drawings that are true representations of an actual model. These drawings can be viewed from any angle just like a physical model. That is why 3D CADD drawings are called 3D models.

The major distinction between a 2D drawing and a 3D model is that a 2D drawing is defined only with two coordinates (X and Y). A 3D model is defined with three coordinates (X, Y and Z). The Z-coordinate determines the height of an object. To make a 3D model, you need to consider all the objects of the model in 3D space and enter the X, Y and Z coordinates for all drawing objects.

 
3D Modeling CADD's 3D modeling capabilities allow you to create 3D images that are as realistic as the actual objects. These images are called 3D models because, just like a physical model, they can be rotated on the screen. You can display views from a 3D model, such as isometrics or perspectives, from any angle with a few simple steps.

3D modeling is usually a separate CADD module that has its own set of functions. Some manufacturers market 2D programs and 3D programs as separate packages while others combine them into a single program.

The 3D models fall into the following categories:
 

  • Wire-frame models
  • Surface models
  • Solid models

  • When you draw a model with lines and arcs, they are called wire-frame models. These models appear to be made of wires and everything in the background is visible. This does not create a very realistic effect.

    Surface models are more realistic than wire-frame models. They are created by joining 3D surfaces rather than bare lines and arcs. A 3D surface is like a piece of paper that can have any dimension and can be placed at any angle to define a shape. Just like a paper model, you join surfaces to form a surface model. The views displayed from these models are quite realistic, because everything in the background can be hidden.

    Solid models are considered solid inside and not hollow like a surface model. They appear to be the same as a surface model but have additional properties, such as weight, density and center of gravity, just like that of a physical object. These models are commonly used as prototypes to study engineering designs.

    Example: You can draw a 3D model as a wire-frame, a surface model or a solid. To draw a 3D model of a cube as a wire-frame, you need to draw twelve lines by specifying 3D coordinates for each of its points. To draw it as a surface model, you need to draw all six surfaces of the cube. Although you see only three planes of the cube in front, it is essential to draw all the planes when drawing a 3D model. This ensures that a realistic view is displayed when it is rotated to display a view from any angle. When drawing a solid you can also specify its material.

    Important Tip:

    For general 3D drawings, wire frames and surface models are used. You start with a wire-frame model and then fill in spaces with 3D surfaces to make it more realistic.

    Working with 3D Coordinates 3D coordinates are measured with the help of three axes: X, Y, and Z. The axes meet at a point in the shape of a tripod as shown in Fig. 7.9. This point is called the origin point, which is the 0,0,0 location of all coordinates. All distances can be measured using this point as a reference.

    The three axes form three imaginary planes: XY plane, XZ plane and YZ plane. The XY plane is the horizontal plane and the XZ and YZ are the two vertical planes. When you need to draw something horizontal, such as the plan of a building, you draw it in the XY plane using X and Y coordinates. This generates a plan view. When you need to draw something vertical, such as an elevation of a building, you draw it using the XZ or YZ planes.

    Example: To draw a line in 3D, enter two end points defined with X, Y and Z coordinates. If you need to draw the line lying flat on the ground (XY plane), the Z coordinate for both the end points of the line is zero. If you want to draw the same line at 10'-0" above the XY plane, enter the Z-coordinate for both the end points as 10'-0".

    The 3D coordinates can be entered using the following formats:
     

  • Cartesian coordinates
  • Spherical coordinates
  • Cylindrical coordinates
  • Cartesian Coordinates  
    Cartesian coordinates are based on a rectangular system of measurement. In Chapter 2 "CADD Basics", we discussed how Cartesian coordinates are used in 2D drawings. The same principle is applied to enter 3D coordinates with the exception that you need to enter an additional Z coordinate. Positive Z-coordinate values are used when you need to measure distances above the XY plane; negative values are used for the distances below the XY plane.

    Coordinate values are entered separated by commas (X,Y,Z). The coordinates can be measured from the origin point (absolute coordinates) or from the last reference location of the cursor (relative coordinates).
     

    Spherical Coordinates  
    Spherical coordinates are based on the longitude and latitude system of measurement (Diagram A, Fig. 7.11). Consider the origin point of the coordinate system at the center of the earth or a transparent globe. Then consider a horizontal plane (XY plane) passing through the center of the globe. To locate a point in 3D, first locate a point in the XY plane by specifying a radius and an angle (polar coordinates). To specify the height, enter an angle up or down from the XY plane (latitude).

    Note:

    Spherical coordinates are not very efficient for drawing purposes. They are commonly used to view a model from different angles.
     

    Cylindrical Coordinates  
    Cylindrical coordinates are commonly used to draw cylindrical shapes. They are based on a cylindrical system of measurement. Consider a cylinder placed vertically and the origin point at the center of the cylinder (Fig. 7.12, Diagram A). Cylindrical coordinates are quite similar to spherical coordinates, the difference being that the Z-coordinate is specified by height and not angle.

    To locate a point with the cylindrical coordinates, first you need to locate it in the XY plane just like polar coordinates. Then indicate an exact height at that point.
     

    Displaying Views You can rotate a 3D model on the screen and display different views by specifying an exact viewpoint. The viewpoint represents the position of the camera from where a picture of the view is to be taken. You can define a viewpoint with the help of any of the coordinate methods discussed earlier.

    There are two main protocols used to display views:
     

  • View coordinate geometry
  • Object coordinate geometry

  • View Coordinate Geometry

    View coordinate geometry assumes that the camera (viewpoint) remains stationary and the 3D model is rotated to display a desired view. The model can be rotated around the X, Y, or Z axis. You need to specify around which axis the rotation will take place and by how much. When you rotate the model around the Z-axis, the model rotates in the XY plane; when you rotate it around the Y-axis, the rotation takes place in the XZ plane.

    Object Coordinate Geometry

    Object coordinate geometry assumes that the model remains stationary and the camera (viewpoint) is moved to a display a desired view. You can use any of the coordinate methods to specify an exact viewpoint. Spherical coordinates are particularly helpful to indicate a viewpoint.

    Comparison: View coordinate geometry can be compared to holding a small model in your hand and rotating it on its sides to get a desired view. Object coordinate geometry can be compared to viewing a building from the sky. The building remains stationary, while the camera is moved to get a desired view.

    Note: Most CADD programs provide both view coordinate geometry and object coordinate geometry options to display views. Depending on how you want to view a model, you can use either method.
     

    Displaying Isometric Views  
    To display an isometric, you need to specify the direction from which the view is to be taken. The most appropriate method to indicate direction is with the help of spherical coordinates. You need to specify two angles: an angle in the XY plane (longitude) and an angle from the XY plane (latitude). The longitude determines the orientation of the model in the XY plane and the latitude determines the height of the viewpoint.

    Important Tips:

    You can convert this model into a surface model by drawing 3D surfaces for each of the planes. You can draw a surface for the top of the model by indicating the points I,J,K and L. This model requires a total of seven surfaces for all the sides: top, bottom, four sides and one inclined surface.

    You can manipulate the views of a 3D model in a number of ways. You can rotate the view, cut a section of the view along a plane, reduce or enlarge the view, change the focus of the view, hide and display certain lines, etc.

    You can display more than one view of a model on the screen at the same time. You can create a number of viewing windows (viewports) that can be used to display different views. For example, you can display a plan view of the model in one viewport, elevation in another and a perspective view in another. When you draw something in one viewport, it is automatically shown in all the viewports.

    Advanced CADD programs enable you to create animated images. You can create perspective views and store them in the computer memory. You can display a number of these images within seconds to create an animated effect. You can create an effect that simulates walking through a building or the functioning of a machine.

    Refer to CADD PRIMER for details on the topics listed above.

    Note: CADD PRIMER is illustrated with more than 100 diagrams. The above diagram is an example from CADD PRIMER illustrating the concept of spherical coordinates used in CADD.

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