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After studying this unit, you will be able to:
Identify and understand dimensions on a
drawing.
Identify and understand direct tolerancing
methods.
Determine tolerances on fractional, decimal,
metric, and angular dimensions.
Calculate tolerances on limits.
Define terms that apply to dimensioning and
tolerancing.
The United States (US) customary system is the
measurement system used for engineering drawings
in the United States. The linear measurement used for
length, width, and height is the inch. The measurement
used for angles is the degree. Unit 4 explains how this
system applies to dimensions, tolerances, and limits.
This unit also explains the metric system.
DIMENSIONS
A DIMENSION is a definite value of measurement
for distance between two given points of an object.
Dimensions describe the size and shape of a
part as well as the location of its features. Typical
dimensions include length, width, height, angle,
radius, and diameter. Dimensions on a drawing can
be fractional, decimal, metric, and angular values.
DECIMAL INCH DIMENSIONS
DECIMAL INCH DIMENSIONS are measure-
ments based on decimal inches. DECIMAL INCH is
a fraction, or decimal equivalent, of an inch. A decimal
inch is expressed in bases of ten, such as .1 =1/10
(tenths), .01= 1/100 (hundredths), and .001=1/1000
(thousandths) and so forth. See Fig. 4-1. Dimensions
expressed with more decimal places are more
precise than dimensions with fewer decimal places.
A part requiring high precision machining will have
decimal dimensions expressed in thousandths or ten-
thousandths of an inch. See Fig. 4-2. The decimal inch
is the standard unit of linear measurement in the US.
Fig. 4-1. Chart showing the placement of whole numbers and
decimal fractions in relationship to the decimal point.
Fig. 4-2. Decimal inch dimensioning provides precise
measurements in decimal inches.
FRACTIONAL DIMENSIONS
FRACTIONAL DIMENSIONS are measurements
based on fractional numbers. The ASME Y14.5-
2009 Dimensioning and Tolerancing standard does
not recommend fractional dimensioning except
for specifying nominal sizes. NOMINAL SIZE is a
= ONE AND THREE TENTHS
= ONE AND THREE HUNDREDTHS
= ONE AND THREE THOUSANDTHS
= ONE AND THREE-TEN
THOUSANDTHS
FOUR DECIMAL PLACES
DECIMAL
POINT
DECIMAL FRACTIONS WHOLE NUMBERS
(UNITS)