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
