Dimensioning Methods.Limits.Tolerances - Ivy Tech -

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8.4 DIMENSIONING METHODS
The choice of the most suitable dimensioning
method depends on how the part will be
produced
 Unit production: each part is made separately
 Mass production: parts are produced in
quantity

RECTANGULAR COORDINATE DIMENSIONING
Linear dimensions are measured paralle or
perpendicular to reference axes or datum
planes that are perpendicular to one another
(fig.8-36)
 Arbitrary points (fig.8-37,8-38)
 Without dimension lines (fig. 8-39)
 Tabular dimensioning (fig.8-40)

POLAR COORDINATE DIMENSIONING
Circular planes or circular configurations of
features (fig.8-42a)
 Position is determined by a linear dimension
and angle
 Chordal dimensioning (fig.8.42b)

CHAIN DIMENSIONING
A series of dimensions is applied on a point-topoint basis (fig. 8-44)
 Undesirable accumulation of tolerances
between individual features can result

DATUM OR COMMON-POINT DIMENSIONING
Several dimensions emanate from a common
reference point or line (fig.8-45)
 Parallel method
 Superimposed method-dimensions should be
placed near the arrowhead in line with the
corresponding extension line and the origin is
indicated by a circle (fig.8-46)

8-5 LIMITS AND TOLERANCES

Why?
 Exact
dimensions and shapes can not be attained
in the manufacture of materials and products
 Slight variations in size can be tolerated without
impairing its function
 Interchangeable parts need not be identical
 Restrict the variations with limits and tolerances
TOLERANCES
Definition: Tolerances are the permissible
variations in the specified form, size, or
location of individual features of a part from
that shown in the drawing.
 Definition: Limits are the largest and smallest
permissible sizes

TERMINOLOGY OF TOLERANCES AND LIMITS
Actual size-the measured size
 Basic size-the theoretical size from which the
limits are derived
 Nominal size-the designation used for the
purpose of general identification
 Tolerance-tolerance of a dimension is the total
permissible variation in size of a
dimension…the mathematical difference
between the limits of size

TERMINOLOGY CONTINUED
Bilateral tolerance-variation is permitted in both
directions from the specified dimension
[1.5±.004]
 Unilateral tolerance-variation is permitted in
only one direction from the specified dimension
[1.5+.004]
 Maximum material size-the limit of size of a
feature that results in the part containing the
maximum amount of material (GDT)

TOLERANCING-ALL DIMENSIONS REQUIRED IN
MANUFACTURE OF A PRODUCT HAVE A
TOLERANCE
Limits of tolerance directly on the drawing
(fig.8-48)
 +/- tolerancing (equal and unequal)
 General tolerance note referring to all
dimensions or specific dimensions

LIMIT DIMENSIONING
The high limit (maximum value) is placed above
the low limit (minimum value)
 When expressed in a single line, the low limit
precedes the high limit and they are seperated
by a dash
 The digits to the right of the decimal place
should match for both limits (English & SI)

PLUS-AND-MINUS TOLERANCING (FIG.8-50)
Specified size given first followed by a +/expression of tolerancing.
 Plus value placed above minus value

METRIC TOLERANCING RULES

The dimension need not be shown to the same number
of decimal places as its tolerance



1.5±0.04 NOT 1.50 ±0.04
10 ±0.1 NOT 10.0 ±0.1
Metric bilateral tolerances-both +/-must have the same
number of decimal places using zeros if necessary

30+0.15
-0.10

NOT 30+0.15
-0.1
When a unilateral tolerance is used and either value is
nil, a single zero is used

40 0
-0.15
INCH TOLERANCING RULES

The dimension is given to the same number of
decimals as its tolerance
 .500
±.004 NOT .50 ±.004
 .750+.500 NOT .750+.500
-.000
-0
TOLERANCE ACCUMULATION
A chain of tolerances can build up a cumulative
tolerance between surfaces that have an
important relation to one another (fig.8-52)
 Chain (greatest tolerance accumulation)
 Datum (lesser tolerance accumulation)
 Direct (least tolerance accumulation)

ADDITIONAL RULES FOR DIMENSIONING
Design intent
 Completely describe part geometry
 Avoid unnecessary accumulation of tolerances
 Manufacturing methods are not specified
 Dimension to visible lines (NOT hidden lines)
 In general, place dimensions outside the outline of
the part and between views
 Dimensions should be aligned, if practicable, and
should be grouped for uniform appearance

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