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 213; and the very soft grades are 613, 513, 413, 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.

Figure 2-4.-The triangles

microform reproduction. There are two types: fired and extruded. They are similar in material content to plastic fired lead, but they are produced differently. The main drawback with this type of lead is that it does not hold a point well.

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