Figure 2-27.--Layout of a perpendicular from a point on a
Figure 2-26.--Layout of a perpendicular from a point to a
line.
line.
Use a divider to lay out a perpendicular FROM a
natural trigonometric functions and find the sine of
point TO a line, as shown in figure 2-26. Lightly
305. Then, multiply the sine value by 10 inches
(0.50126 10 = 5.0126) to find the height of the
prick-punch point C, then swing any arc from C that
gauge blocks. The second method is to use a table of
will intersect line AB. Prick-punch intersections D
sine bar constants. These tables give the height
and E, as in the figure. With D and E as centers,
setting for any given angle (to the nearest minute) for
scribe two arcs that intersect at a point such as F.
a 5-inch sine bar. Tables are not normally available
Place a straightedge on points C and F. The line
for 10-inch bars because it is easy to use the sine of
drawn along this straightedge from point C to line AB
the angle and move the decimal point one place to the
will be perpendicular (90) to line AB.
right.
Use a divider to lay out a perpendicular FROM a
Although sine bars appear to be rugged, you
point ON a line, as shown in figure 2-27. Lightly
should give them the same care as gauge blocks.
prick-punch point C on line AB. Then, set the divider
Since they are used with other tools or parts that are
to any distance to scribe arcs that intersect AB at D
heavy, they are subject to rough usage. Remove or
and E with C as the center. Punch C and E lightly.
repair scratches, nicks, and burrs. Keep them clean
With D and E as centers and with the setting of the
from abrasive dirt and sweat and other corrosive
divider increased somewhat, scribe arcs that cross at
agents. Make regular inspections to locate defects
points such as F and G. The line drawn through F and
before they affect the accuracy of the bar. When you
G will pass through point C and be perpendicular to
store sine bars for extended periods, clean all bare
line AB.
metal surfaces and then cover them with a light film
of oil. Place a cover over the sine bar to prevent
To lay out parallel lines with a divider, set the
accidental damage and to discourage corrosion.
divider to the selected dimension. Then, referring
to figure 2-28, from any points (prick-punched)
such as C and D on line AB, swing arcs EF and GH.
Then, draw line IJ tangent to these two arcs and it
GEOMETRIC CONSTRUCTION OF
LAYOUT LINES
will be parallel to line AB and at the selected
distance from it.
Sometimes you will need to scribe a layout that
cannot be made with conventional layout methods.
For example, you cannot readily make straight and
angular layout lines on sheet metal with irregular
edges by using a combination square set. Neither can
you mount sheet metal on angle plates in a manner
that permits scribing angular lines. Geometric
Figure 2-28.--Layout of a parallel line.
construction is the answer to this problem.
2-13
