CHAPTER  2 TECHNICAL  SKETCHING When you have read and understood this chapter, you should be able to answer the following learning objectives: Describe  the  instruments  used  in  technical sketching. Describe the types of lines used in technical sketching. Explain basic computer-aided drafting (CAD). Explain computer numerical control (CNC) design techniques used in machining. The ability to make quick, accurate sketches is a valuable  advantage  that  helps  you  convey  technical information or ideas to others. A sketch may be of an object, an idea of something you are thinking about, or a combination of both. Most of us think of a sketch as a freehand drawing, which is not always the case. You may sketch on graph paper to take advantage of the lined squares, or you may sketch on plain paper with or without the help of drawing instruments. There is no MIL-STD for technical sketching. You may draw pictorial sketches that look like the object,  or  you  may  make  an  orthographic  sketch showing  different  views,  which  we  will  cover  in following  chapters. In  this  chapter,  we  will  discuss  the  basics  of freehand   sketching   and   lettering,   drafting,   and computer aided drafting (CAD). We will also explain how  CAD  works  with  the  newer  computer  numerical control (CNC) systems used in machining. SKETCHING  INSTRUMENTS Freehand sketching requires few tools. If you have a pencil and a scrap piece of paper handy, you are ready  to  begin.  However,  technical  sketching  usually calls for instruments that are a little more specialized, and we will discuss some of the more common ones in the following paragraphs. PENCILS AND LEADS There are two types of pencils (fig. 2-1), those with conventional  wood  bonded  cases  known  as  wooden pencils and those with metal or plastic cases known as mechanical pencils. With the mechanical pencil, the lead is ejected to the desired length of projection from the clamping chuck. There  are  a  number  of  different  drawing  media  and types of reproduction and they require different kinds of pencil leads. Pencil manufacturers market three types that  are  used  to  prepare  engineering  drawings;  graphite, plastic, and plastic-graphite. Graphite lead is the conventional type we have used for years. It is made of graphite, clay, and resin and it is available in a variety of grades or hardness. The harder grades are 9H, 8H, 7H and 6H. The medium grades are 5H, 4H, 3H, and 2H. The medium soft grades are H and F. The soft grades are HB, B, and 2B; and the very soft grades are 6B, 5B, 4B, and 3B. The latter grade is not recommended for drafting. The selection of the grade of lead is important. A harder lead might penetrate the drawing, while a softer lead may smear. Plastic  and  graphite-plastic  leads  were  developed as a result of the introduction of film as a drawing medium, and they should be used only on film. Plastic lead has good microform reproduction characteristics, but it is seldom used since plastic-graphite lead was developed. A limited number of grades are available in these leads, and they do not correspond to the grades used for graphite lead. Plastic-graphite lead erases well, does not smear readily, and produces a good opaque line suitable for Figure 2-1.—Types of pencils. 2-1