Chapter 4 Measurement
67
A universal bevel protractor has several parts: a
dial, a base or stock, and a sliding blade. The dial is
graduated into degrees, and the blade can be
extended in either direction and set at any angle to
the stock. The blade can be locked against the dial
by tightening the blade clamp nut. The blade and
dial can be rotated as a unit to any desired position,
and locked by tightening the dial clamp nut.
The protractor dial is graduated into 360° and
reads from 0° to 90° and then back down to 0°.
Every ten degree division is numbered, and every
five degrees is indicated by a fine line longer than
those on either side. The Vernier scale is divided
into twelve equal parts on each side of the “0.”
Every third graduation is numbered (0, 15, 30, 45,
60), representing minutes. Each division equals five
minutes. Since each degree is divided into 60 min-
utes, one division is equal to 5/60 of a degree.
To read the protractor, note the number of
degrees that can be read up to the “0” on the Vernier
plate. To this, add the number of minutes indicated
by the line beyond the “0” on the Vernier plate that
aligns exactly with a line on the dial.
• Never force the tool when you are making
measurements.
• Use a magnifying glass or a jeweler’s loupe to
make Vernier readings. Hold the tool so the
light is reflected on the scale.
• Handle the tool as little as possible. Sweat and
body acids cause rusting and staining.
• Periodically check for accuracy. Use a mea-
suring standard, Jo-block, or ground parallel.
Return the tool to the manufacturer for
adjustments and repairs.
• Lay Vernier height gages on their side when
not in use. Then there will be no danger that
they will be knocked over and damaged.
4.4 GAGES
It is impractical to check every dimension on
every manufactured part with conventional mea-
suring tools. Specialized tools, such as plug gages,
ring gages, and optical gages are used instead.
These gaging devices can quickly determine
whether the dimensions of a manufactured part are
within specified limits or tolerances.
Measuring requires the skillful use of precision
measuring tools to determine the exact geometric
size of the piece. Gaging involves checking parts
with various gages. Gaging simply shows whether
the piece is made within the specified tolerances.
When great numbers of an item with several
critical dimensions are manufactured, it might not
be possible to check each piece. It then becomes nec-
essary to decide how many randomly selected
pieces must be checked to ensure satisfactory qual-
ity and adherence to specifications. This technique
is called statistical quality control.
Always handle gages carefully. If dropped or
mishandled, the accuracy of the device could be
affected. Gages provide a method of checking your
work and are very important tools.
4.4.1 Plug Gage
Plug gages are used to check whether hole
diameters are within specified tolerances. The
double-end cylindrical plug gage has two gaging
members known as go and no-go plugs, Figure 4-28.
The go plug should enter the hole with little or no
interference. The no-go plug should not fit.
The go plug is longer than the no-go plug.
A progressive plug gage, or step plug gage, has the
go and no-go plugs on the same end. This gage is
able to check the dimensions in one motion. See
Figure 4-29.
0°20′
50°00′
50°00′
0°20′
Reading is 50°20′
In this example the “0” is past the 50° mark,
and the Vernier scale aligns at the 20′ mark.
Therefore, the measurement is 50°20′.
4.3.5 Care of Vernier Tools
Reasonable care in handling these expensive
tools will maintain their accuracy.
• Wipe the instrument with a soft, lint-free cloth
before using. This will prevent dirt and grit
from being ground in, which could eventually
affect the accuracy of the tool.
• Wipe the tool with a lightly oiled, soft cloth
after use and before storage.
• Store the tool in its case.