Beam Compass The BEAM COMPASS (fig. 221) is used for
drawing circles with radii larger than can be set
on a pivot joint or bow compass. Both the needlepoint
attachment and the pen or pencil attachment
on a beam compass are slidemounted on a metal
Figure 221.Beam compass.
Figure 222.Proportional dividers.
bar called a beam. The slidemounted attachments can
be locked in any desired position on the beam. Thus,
a beam compass can be used to draw circles of
any radius up to the length of the beam. With one
or more beam extensions, the length of the radius
of a beam compass ranges from about 18 in.
to 70 in.
PROPORTIONAL DIVIDERS
PROPORTIONAL DIVIDERS (fig. 222) are used
for transferring measurements from one scale
to another. This capability is necessary when drawings
are to be made to a larger or smaller scale.
They can also be used to divide lines or circles
into equal parts.
Proportional dividers consist of two legs of equal
length, pointed at each end, and held together
by a movable pivot. By varying the position
of the pivot, you can adjust the lengths of
the legs on opposite sides of the pivot so that the
ratio between them is equal to the ratio between
two scales. Therefore, a distance spanned by
the points of one set of legs has the same relation
to the distance spanned by the points of the
other set as one scale has to the other. On
the proportional dividers, a thumb nut moves
the pivot in a rackandgear arrangement. When
the desired setting is reached, a thumbnut clamp
on the opposite side of the instrument locks the
pivot in place. A scale and vernier are provided
on one leg to facilitate accurate setting. On
less expensive models, the movable pivot is not
on a rack and gear, and there is no vernier. The
dividers may be set by reference to the table of
settings that is furnished with each pair; they will
accommodate varying ranges of scales from 1:1
to 1:10. However, it is better not to depend entirely
on the table of settings. You can check the
adjustment by drawing lines representing the desired
proportionate lengths, and then applying the
points of the instrument to them in turn until,
by trial and error, the correct adjustment is
reached.
To divide a line into equal parts, set the divider to
a ratio of 1 to the number of parts desired on the
scale marked Lines. For instance, to divide a
line into three parts, set the scale at 3. Measure off
the length with points of the longer end. The span
of the points at the opposite ends will be equal
to onethird the measured length. To use proportional
dividers to transfer measurements from
feet to meters, draw a line 1 unit long and another
line 3.28 units long and set the dividers by
trial and error accordingly.
Some proportional dividers have an extra scale for
use in getting circular proportions. The scale marked
Circle indicates the setting for dividing the
circumference into equal parts.
The points of the dividers are of hardened steel,
and if they are handled carefully, these points
will retain their sharpness during long use.
If they are damaged, they may be sharpened and
the table of settings will still be usable, but the
scale on the instrument will no longer be accurate.
SCALES
In one sense, the term scale means the succession
of graduations on any graduated standard
of linear measurement, such as the graduations
on a steel tape or a thermometer. In another
sense, when we refer to the "scale of a drawing,"
the term means the ratio between the dimensions
of the graphic representation of an object
and the corresponding dimensions of the object
itself.
Suppose, for example, that the top of a rectangular
box measures 6 in. by 12 in. If you draw
a 6in. by 12in. rectangle on the paper, the dimensions
of the drawing would be the same as those
of the object. The drawing would, therefore, be
a fullscale drawing. This scale could be expressed
fractionally as 1/1, or it could be given as
1 in. = 1 in.
Suppose that instead of making a fullscale drawing,
you decided to make a halfscale drawing.
You should then draw a 3in. by 6in. rectangle
on the paper. This scale could be expressed
fractionally as 1/2, or it could be given as
1 in. = 2 in., or as 6 in. = 1 ft.
In this case, you made a drawing on a smaller scale
than the scale of the original object, the scale of
an original object being always 1/1, or unity. The
relative size of a scale is indicated by the fractional
representation of the scale. A scale whose
fractional representation equals less than unity
is a lessthanfull scale. One whose fractional representation
is greater than unity (such as a scale of
200/1) is a largerthanfull scale. A scale of 1/10,000
is, of course, smaller than a scale of 1/100.
A scale expressed as an equation can always be
expressed as a fraction. For example, the scale of
1 in. = 100 ft, expressed fractionally, comes to
1 over (100 x 12), or 1/1,200.
It is obvious that any object that is larger than the
drawing paper on which it is to be represented must
be "scaled down" (that is, reduced to lessthan full
scale) for graphic representation.
Conversely, it is often desirable to represent a very
small object on a scale larger than full scale
for the purpose of clarity and to show small
details. Because the drawings prepared by
an EA frequently require scaling down, the
following discussion refers mostly to that procedure.
However, scaling up rather than down
simply means selecting a largerthanfull scale
rather than a smallerthanfull scale for your drawing.
You could, if necessary, determine the dimensions
of your drawing by arithmetical calculation;
for example, on a halfscale drawing, you
divide each of the actual dimensions of the object
by 2. However, this might be a timeconsuming process
if you were drawing a map of a
certain area to a scale of 1 in. = 1,000 mi, or 1/6,335,000
ft.
Consequently, you will usually scale a drawing
up or down by the use of one or another of
a variety of scales. This sense of the term scales refers
to a graduated, rulerlike instrument on which
scale dimensions for a drawing can be determined
by inspection.
Scales vary in types of material, shapes, style of
division, and scale graduations. Good quality scales
are made of highgrade boxwood or plastic, while
inexpensive scales are sometimes made of
Figure 223.Types of scales in cross section.
yellow hardwood. The boxwood scales have white plastic
scale faces that are permanently bonded to
the boxwood. The graduation lines on the boxwood
scales are cut by a highly accurate machine.
Plastic scales, while less expensive than boxwood
scales, have clear graduations and are reasonably
accurate.
Scales are generally available in four different shapes,
as shown in figure 223. The numbers in the
figure indicate the location of the scale face. The
triangular scale provides six scale faces on one
rule. The twobevel flat scale provides two scale
faces on one side of the rule only. The oppositebevel
flat scale provides two scale faces, one
on each side of the rule. And the fourbevel flat
scale provides four scale faces, two on each side
of the rule. The most common types of scales are
the architect’s, the engineer’s, the mechanical engineer’s,
and the metric. All of these scales are
found in the EA’s draftsman kit with the
exception of the mechanical engineer’s scale, which
is primarily used by machine draftsmen. To
gain a better understanding of the
architect’s and engineer’s scale, which will be described
in the following sections, it may be helpful
to have the actual scales at hand as you study.
