Transcript Tolerancing

Tolerancing
Chapter 10
Objectives
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Describe nominal size, tolerance, limits,
and allowance of two mating parts
Identify a clearance fit, interference fit,
and transition fit
Describe the basic hole and basic shaft
systems
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Objectives (cont.)
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Dimension mating parts using limit
dimensions, unilateral tolerances, and
bilateral tolerances
Describe the classes of fit and give
examples of each
Draw geometric tolerancing symbols
Specify geometric tolerances
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Understanding Tolerance
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Tolerancing is an extension of
dimensioning
It allows you to specify a range of
accuracy for every feature of a product
so the parts will fit together and
function properly when assembled
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Understanding Tolerance
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To provide tolerances in CAD, you must:
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Understand the fit required between
mating parts
Have a clear picture of how inspection
measurements are performed
Be able to apply tolerance symbols to a
drawing
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Tolerance
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Tolerance is the total amount a specific
dimension is permitted to vary
Use generous tolerances when possible
because increased precision makes
parts more expensive to manufacture
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Quality Control
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Large batches of parts may use
statistical methods to control quality
where a sample of parts are inspected
Specific tolerances are based on the
part’s function and fit
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Variations in Form
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Acceptable parts must not extend
beyond boundary limits
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Implied 90 Degree Angles
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When lines intersect on a drawing at
angles of 90 degrees, it is customary
not to dimension the angle
Implied 90 degree angles have the
same general tolerances applied to
them as any other angles covered by a
general note
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Implied 90 Degree Angles
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Fits Between Mating Parts
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Fit is the term for the range of tightness
or looseness resulting from the
allowances and tolerances in mating
parts
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Clearance Fit
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Interference Fit
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Transition Fit
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Definitions for Size Designation
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Nominal size – used for general
identification and usually expressed in
decimals
Basic size (basic dimension) – the
theoretically exact size from which
limits of size are determined
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Definitions for Size Designation
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Actual size – the measured size of a
finished part
Allowance – the minimum clearance or
maximum interference specified to
achieve a fit between two mating parts
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Basic Hole System
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Toleranced dimensions are commonly
determined using the basic hole system
in which the minimum hole size is taken
as the basic size
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Basic Shaft System
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In this system, the maximum shaft is
taken as the basic size and is used only
in specific circumstances
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Specifying Tolerances
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The primary ways to indicate tolerances
in a drawing are:
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A general tolerance note
A note providing a tolerance for a specific
dimension
A reference on the drawing to another
document that specifies the required
tolerances
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Specifying Tolerances
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(cont.)
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Adding limit tolerances to dimensions
Adding direct plus/minus tolerances to
dimensions
Geometric tolerances
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General Tolerance Notes
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Limit Tolerances
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Plus-or-Minus Tolerances
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Tolerance Stacking
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Tolerances and Machining
Processes
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Metric System of Tolerances
and Fits
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The preceding material on limits and fits
applies to both systems of measurement
The ISO has a system of preferred
metric limits and fits
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The system is specified for holes, cylinders,
and shafts and has similar definitions of
terms
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Geometric Tolerancing
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Geometric tolerances state the
maximum allowable variations of a form
or its position from the perfect
geometry implied in the drawing
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The term “geometric” refers to forms such
as planes, cylinders, squares, etc.
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Geometric Tolerancing
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Tolerances of form and position (or
location) control such characteristics as:
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Straightness
Flatness
Parallelism
Perpendicularity (squareness)
Concentricity
Roundness
Angular displacement, etc.
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Geometric Characteristics and
Modifying Symbols
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Symbols for Tolerances of
Position and Form
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Symbols for Tolerances of
Position and Form
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Datum Surfaces and Features
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Datum surfaces and features are used
as references to control other features
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Maximum Material Condition
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Maximum material condition (MMC)
means that a feature of a finished
product contains the maximum amount
of material permitted by the toleranced
dimensions for that feature
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Tolerances of Angles
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Bilateral tolerances have traditionally
been given on angles
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Using Geometric Dimensioning
and Tolerancing
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Geometric dimensioning and tolerancing
(GDT) considers an individual part’s
dimensions and tolerances and that
part’s relation to its related parts
GDT also simplifies the inspection
process
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Using Geometric Dimensioning
and Tolerancing
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Applying GDT principles requires:
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Defining the part’s functions
Listing the functions by priority
Defining the datum reference frame
Control selection
Calculating tolerances
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