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Machine Trades Print Reading
Fig. 4-3. Fractional dimensioning provides measurements in
fractional inches. Fractions can indicate nominal sizes that
require less precision than decimal inches.
Fig. 4-5. The unit of measurement for angles is the degree.
An angle is measured from a plane, origin, or reference point.
A—Circumference of a circle divided into 360 degrees. B—An
angle is any measurement from 0 degree to 360 degrees.
Fig. 4-4. Metric dimensioning uses millimeters as the base unit
of measurement.
CIRCLE
A
B
VERTEX
VALUE IS 12 MM
VALUE IS 42 AND 45 HUNDREDTHS MILLIMETERS.
OR 42 MM
45
100
general size or stock size used for the identification
of a part. It may not be the actual size of the part. For
example, a 3/4—10 UNC bolt has a 3/4″ nominal
size diameter. However, the actual diameter size
may vary from .7288 to .750.
Fractions are common on welding and casting
drawings. Common fractional values found on
a drawing include 64ths, 32nds, 8ths, 4ths, and
halves. Fig. 4-3 shows typical fractional dimensions.
METRIC DIMENSIONS
The International System of Units (SI) is the
system of measurement used by most international
countries. METRIC DIMENSIONING uses SI units
for its measurement system. The millimeter (mm)
is the linear unit of measurement used for metric
dimensioning of engineering drawings. The millimeter
is equal to 1/1000 of a meter. In addition, decimals
express fractions of a millimeter. See Fig. 4-4.
Due to the growing global economy, the use of
metric dimensioning is expanding in the United
States. Some drawings will require converting US
customary units to SI units. The following example
shows how to convert US customary units to SI units
by using the ratio of one inch to 25.4 millimeters.
Convert Inches to Millimeters:
1 inch = 25.4 millimeters
Example 4-1:
Convert .9375 inch to millimeters.
Formula for calculating work:
Millimeters = inches
×
25.4 mm/in
Solution:
Where
Millimeters = .9375 in.
×
25.40 mm/in
Millimeters = 23.81 mm
ANGULAR DIMENSIONING
ANGULAR DIMENSIONING is a measurement
of the angle of a line, a surface, or an origin from a
given reference point. The reference point can be the
vertex of an angle (the point where two lines intersect
or meet), or an intersecting line, ray, or plane. The
DEGREE, represented by the symbol °, is the unit of
measurement for an angle based on 360 divisions of
a circle’s circumference. Each division, 1/360th of a
circle, is a degree. See Fig. 4-5.
A degree can be broken down into minutes (sym-
bol ′) and seconds (symbol ″). There are 60 minutes
to one degree and 60 seconds to one minute.
Circle = 360 degrees (360°)
Degree = 60 minutes (60′)
Minute = 60 seconds (60″)