An ELLIPSE is a plane curve generated by a point moving so that the sum of the distance from any point on the curve to two fixed points, called foci, is a constant  (fig.  2-12).  Ellipses  represent  holes  on oblique and inclined surfaces. CIRCLES on drawings most often represent holes or a circular part of an object. An  IRREGULAR  CURVE  is  an  unlike  circular arc where the radius of curvature is not constant. This curve is usually made with a French curve (fig. 2-6). An OGEE, or reverse curve, connects two parallel lines or planes of position (fig. 2-13). BASIC COMPUTER AIDED DRAFTING (CAD) The process of preparing engineering drawings on a  computer  is  known  as  computer-aided  drafting (CAD), and it is the most significant development to occur recently in this field. It has revolutionized the way we prepare drawings. The drafting part of a project is often a bottleneck because  it  takes  so  much  time.  Drafter’s  spend approximately  two-thirds  of  their  time  “laying  lead.” Figure 2-12.—Example of an ellipse. Figure 2-13.—A reverse (ogee) curve connecting two parallel planes. But on CAD, you can make design changes faster, resulting in a quicker turn-around time. CAD also can relieve you from many tedious chores such as redrawing. Once you have made a drawing you can store it on a disk. You may then call it up at any time and change it quickly and easily. It may not be practical to handle all of the drafting workload on a CAD system. While you can do most design and drafting work more quickly on CAD, you still  need  to  use  traditional  methods  for  others.  For example,  you  can  design  certain  electronics  and construction projects more quickly on a drafting table. A CAD system by itself cannot create; it is only an additional and more efficient tool. You must use the system to make the drawing; therefore, you must have a good background in design and drafting. In manual drawing, you must have the skill to draw lines and letters and use equipment such as drafting tables and machines, and drawing aids such as  compasses,  protractors,  triangles,  parallel  edges, scales,  and  templates.  In  CAD,  however,  you  don’t need  those  items.  A  cathode-ray  tube,  a  central processing unit, a digitizer, and a plotter replace them. Figure 2-14 shows some of these items at a computer work station. We’ll explain each of them later in this section. GENERATING DRAWINGS ON CAD A CAD computer contains a drafting program that is a set of detailed instructions for the computer. When you  bring  up  the  program,  the  screen  displays  each function or instruction you must follow to make a drawing. The CAD programs available to you contain all of the   symbols   used   in   mechanical,   electrical,   or architectural drawing. You will use the keyboard and/or mouse to call up the drafting symbols you need as  you  need  them.  Examples  are  characters,  grid patterns, and types of lines. When you get the symbols you want on the screen, you will order the computer to size, rotate, enlarge, or reduce them, and position them on the screen to produce the image you want. You probably will then order the computer to print the final product and store it for later use. The computer also serves as a filing system for any drawing symbols or completed drawings stored in its memory or on disks. You can call up this information any time and copy it or revise it to produce a different symbol or drawing. 2-8