MECHANICAL SUBJECTS

Computer-aided design

Computer-aided design (CAD) is the use of computer technology to aid in the design and especially the drafting technical drawing and engineering drawing) of a part or product, including entire buildings. It is both a visual (or drawing) and symbol-based method of communication whose conventions are particular to a specific technical field. (

Drafting can be done in two dimensions ("2D") and three dimensions ("3D"). Drafting is the integral communication of technical or engineering drawings and is the industrial arts sub-discipline that underlies all involved technical endeavors. In representing complex, three-dimensional objects in two-dimensional drawings, these objects have traditionally been represented by three projected views at right angles.

Current CAD software packages range from 2D vector-based drafting systems to 3D solid and surface modellers. Modern CAD packages can also frequently allow rotations in three dimensions, allowing viewing of a designed object from any desired angle, even from the inside looking out. Some CAD software is capable of dynamic mathematic modeling, in which case it may be marketed as CADD — computer-aided design and drafting.

CAD is used in the design of tools and machinery used in the manufacture of components, and in the drafting and design of all types of buildings, from small residential types (houses) to the largest commercial and industrial structures (hospitals and factories).

CAD is mainly used for detailed engineering of 3D models and/or 2D drawings of physical components, but it is also used throughout the engineering process from conceptual design and layout of products, through strength and dynamic analysis of assemblies to definition of manufacturing methods of components.

CAD has become an especially important technology within the scope of computer-aided technologies, with benefits such as lower product development costs and a greatly shortened design cycle. CAD enables designers to lay out and develop work on screen, print it out and save it for future editing, saving time on their drawings.

About this Chapter

This chapter provides an overview of CADD functions. It describes what various functions will accomplish, how they are organized into CADD menus and how to use them.

You will learn to use basic functions to draw lines or circles, enter text, etc. A brief exercise walks you through the use of these functions. You will also learn how to measure distances and how to locate exact points. This chapter illustrates how to use coordinate methods, such as absolute coordinates, relative coordinates and polar coordinates to measure distances.

In addition, this chapter highlights important principles in setting up CADD drafting standards. It demonstrates how to set up a prototype CADD drawing for a quick start.
Finally, this chapter shows you how to save drawings and where to store them. In this section, you will learn about data saving functions and safety measures to prevent data loss.

Key Terms in this Chapter

Term	Description
Aperture	Effective diameter of the cursor on the screen.
Cartesian coordinates	A rectangular system of measurement to locate points in the drawing area.
Isometric	A view in 3D geometry when viewing an object from the top at a 45° angle.
Object snaps	A method for indicating point locations using existing drawing objects as a reference.
Origin point	The 0,0 location of the coordinate system.
Polar coordinates	A system to locate the points using an angle and distance.
Prototype drawing	A template drawing that has a number of preset defaults.
User-defined coordinate system	A mode of measurement that allows the user to set up a customized coordinate system.

Components of a CADD Drawing

CADD drawings are created by drawing individual components (also called drawing objects or entities) of the drawing such as lines, arcs, dimensions and symbols. On a drawing board, you may not notice how many lines and arcs you need to complete a diagram, but with CADD you need to be specific. You need to specify exactly what you want to draw and how you want to draw it. Consider this scenario: drawing a square with rounded edges. Using a drawing board, you would quickly draw four lines and round off the edges with a template. Using CADD to draw the similar square, you will have to draw four lines and four arcs separately. You must specify where each line and arc starts, where it ends, the center point of the arc, the radius, etc.

CADD’s drawing functions are used to create each component of a drawing. Fig. 2.1 illustrates the basic drawing components of CADD. As shown, there are many different kinds of lines, arcs, circles, ellipses, symbols, text, dimensions pointers, polylines, borders and patterns. You can complete most of the drawings by using these components of CADD.

To draw any component of the drawing, you need to access a particular function from the Draw menu. There are separate functions to draw text, dimensions, patterns, symbols, and other elements of a drawing. To draw a symbol, such as an octagon, you don't need to draw each side one by one. The Symbol function allows you to instantly insert a symbol into the drawing. Similarly, when you need to draw hatch patterns, borders, or arrows, separate CADD functions enable you to do that.

Drawing a component is quite straightforward. When you enter a command, the corresponding menu displays a prompt that guides you through steps to take. To complete a task, you will need to enter data using the keyboard, make selections from sub-menus, and enter points with the help of a pointing device to indicate where you want to position a drawing component.

The Coordinate System

The coordinate system is a method of locating points in the drawing area. It enables you to locate points by specifying distances from a fixed reference point. You can locate a point by giving its distance in the horizontal direction, vertical direction, measuring along an angle, etc.

The coordinate system is available when a function requires data input in the form of point locations. You may use it while drawing, editing or any time you need to locate a point. The most common coordinate systems are as follows:

· Cartesian coordinates

· Polar coordinates

Note:

The method of entering coordinates varies from one program to another. For example, in AutoCAD, the coordinates can be entered directly in the command window with the keyboard. In MicroStation and Cadkey, special dialog boxes let you enter the coordinates.

Cartesian Coordinates

Cartesian coordinates is a rectangular system of measurement that enables you to locate points with the help of horizontal and vertical coordinates. The horizontal values, called X-coordinates, are measured along the X-axis. The vertical values, called Y-coordinates, are measured along the Y-axis. The intersection of the X- and Y-axes is called the origin point, which represents the 0,0 location of the coordinate system.

The positive X values are measured to the right and the positive Y values are measured above the origin point. The negative X and Y values are measured to the left and below. To enter a coordinate, you need to enter both the X and Y values separated by a comma (X, Y).

Example: To locate a point two units to the right and one unit above the origin point, enter X = 2, Y = 1 or (2,1). Similarly, to locate a point two units to the left and one unit below, you need to enter X = -2, Y = -1 or (-2, -1). You can use fractions as needed to enter the exact coordinate values.

Polar Coordinates

Polar coordinates allow you to define a point by specifying the distance and the direction from a given point. This mode of measurement is quite helpful in working with angles. To draw a line at an angle, you need to specify how long a line you want to draw and specify the angle.

The Format to Enter Coordinates

Cartesian or polar coordinate values can be entered in two formats:

· Absolute

· Relative

Absolute format is a way of measuring distances from a fixed reference location (origin point), which is the 0,0 location of the coordinate system. Consider this point to be stationary at all times. In some CADD programs this point remains visible at the left bottom corner of the drawing area, while in others it is invisible. You can use this point as a reference to measure any distance in the drawing. Absolute coordinates are primarily used to adjust the alignment of diagrams in a drawing, to align one drawing with another or to make plotting adjustments.

Relative format is a way of measuring distances from the last point entered. All measurements are taken the same way as the absolute coordinates, with the only difference being that the relative coordinates are measured from the last point entered instead of the origin point. When a point is entered, it becomes the reference for entering the next point and so on. This mode of measurement is frequently used for drawing because it is always convenient to place the drawing components relative to each other rather than a fixed reference point.

Note:

Each CADD program uses its own annotations to enter absolute or relative coordinates. The task table located at the end of the chapter shows some examples from leading CADD programs.