reasons. A machinist must be able to make the part from the detailed drawings without need-
ing to go back to the engineer or the draftsperson who drew the drawings to ask questions
regarding the size, or the tolerances, or what type of material the part should be made from.
There are basically two concepts that you need to keep in mind when specifying dimensions in
an engineering drawing:sizeandlocation. As shown in Figure 16.6, not only do you need to
specify how wide or how long an object is but you must also specify the location of the center
of a hole or center of a fillet in the part. Moreover, a drawing is dimensioned with the aid of
dimension lines,extension lines,centerlines, andleaders.
Dimension linesprovide information on the size of the object; for example, how wide it
is and how long it is. You need to show the overall dimensions of the object because the machin-
ist can then determine the overall size of the stock material from which to make the piece. As
the name implies,extension linesare those lines that extend from the points to which the
dimension or location is to be specified. Extension lines are drawn parallel to each other, and
the dimension lines are placed between them, as shown in Figure 16.6. Theleadersare the
arrows that point to a circle or a fillet for the purpose of specifying their sizes. Often the draw-
ings are shownNot To Scale(NTS), and therefore a scaling factor for the drawing must also be
specified. In addition to dimensions, all engineering drawings must also contain an informa-
tion box with the following items: name of the person who prepared the drawing, title of the
drawing, date, scale, sheet number, and drawing number. This information is normally shown
on the upper- or lower-right corner of a drawing. An example of an information box is shown
in Figure 16.7.
Let us now say a few words about fillets, which are often overlooked in engineering
drawings, a shortcoming that could lead to problems.Filletrefers to the rounded edges of
an object; their sizes, the radius of roundness, must be specified in all drawings. If the size
of fillets is not specified in a drawing, the machinist may not round the edges; consequently,
the absence of fillets could create problems or failure in parts. As some of you will learn
later in your mechanics of materials class, mechanical parts with sharp edges or a sudden
reduction in their cross-sectional areas could fail when subjected to loads because of high
stress concentrations near the sharp regions. As you will learn later, a simple way of reduc-
ing the stress in these regions is by rounding the edges and creating a gradual reduction in
cross-sectional areas.512 Chapter 16 Engineering Drawings and Symbols
Location
Size
Size
Location Size Location
(^12)
3
4
■Figure 16.6
The basics of dimensioning
practices: (1) dimension line,
(2) extension line, (3) centerline,
and (4) leader.
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