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Dimensioning and Tolerancing
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Tolerancing
No manufacturing process can produce parts with
exact dimensions
Allowable variations or tolerances must bespecified by the designer, with two objectives:
ensure fit and function
minimize manufacturing cost
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Example - shaft in hole fit
Shaft in hole fits are very common
Both shaft and hole diameter vary
Dimensions and tolerances must be specified toguarantee the desired fit
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Shaft in Hole
Tolerance
Minimumclearance
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Types of Fit
Clearance fit
largest shaft diameter is smaller than smallest hole
diameter
there is always clearance
Interference
smallest shaft diameter is larger than largest hole
diameter
there is always interference
Transition
there could be either interference or clearance
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Clearance Fits
Loose running
lots of play, where accuracy is not important
Free running less play, good for moving parts
Close running
close fit for moving parts, high accuracy required
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Transition Fits
Used to accurately locate parts during assembly
Tradeoff between ease of assembly/disassembly
and accuracy of location Example: locating dowels or pins
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Interference Fits
Used for force or press fits
Results in permanent assembly without need for
fasteners or other joining operations High locational accuracy
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Multiple holes and shafts
Often parts are assembled with multiple shafts
mating with multiple holes
Examples?
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Objectives
Close fit with no play and good locational
accuracy
High probability of parts fitting despite variationsin dimensions
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Types of Variations
Size tolerances on hole diameters
Size tolerances on shaft diameters
Positional tolerances on hole locations
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Example
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Must Fit in Worst Case
Holes at minimum size
Shafts (e.g. bolts) at maximum size
Holes at minimum distance on one part Holes at maximum distance on the other part
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Worst Case
Minimum
MaximumSmallest
hole
Largest
shaft
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Geometric Dimensioning and
Tolerancing (GD&T) GD&T symbols specify additional tolerancing
information for 3D geometry
Used in addition to standard +/- dimensioning Used properly, GD&T can allow looser tolerances
to minimize manufacturing cost
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Datums
A datum is a plane, centerline or point used as a
reference starting point for dimensions
Often flat faces of a part or centerlines of holesare used as datums
There can be several datums, labeled A, B, C,
etc.
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Maximum Material Condition (MMC)
MMC is the condition where a feature has the
maximum volume or material
For a hole, it is the smallest size For a shaft, it is the largest size
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Departure from MMC
As a feature departs from MMC, it moves away
from the worst case
This may permit tolerances on other dimensionsto be increased while still achieving fit
For example, if the holes in the previous example
are larger than the minimum, the tolerance on the
spacing can be increased and the parts will still fit
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Geometric Controls
Form controls
compare feature to ideal geometry
Orientation controls compare orientation of features to datums
Location controls
compare location to datums
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Form Controls
Straightness
compares a line or axis to a perfectly straight line
Circularity compares a circular cross section to a perfect circle
Flatness
compares a flat surface to a perfect plane
Cylindricity compares a cylindrical feature to a perfect cylinder
includes axis straightness, circularity and taper
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Orientation Controls
Parallelism
a line or surface must be parallel to a datum
Perpendicularity a line or surface must be perpendicular to a datum
Angularity
a line or surface must be at an angle to a datum
Line profile and Surface profile line and surface profiles compare features to ideal
profiles
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Location Controls
Concentricity
controls deviation of concentric axes of cylindrical
elements
Runout
measures wobble in surface of cylindrical feature as it
is rotated about an axis
Position
Locates features relative to datums
allows larger bonus tolerances as features depart from
MMC
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Feature Control Frames
GD&T controls are added to drawings as feature
control frames, using standard symbols
In I-DEAS, they are defined by completing a form
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Tolerancing in I-DEAS
Tolerances can be applied to constraints
I-DEAS tolerance analysis uses variational
geometry to analyze the effect of tolerance stack-up
GD&T symbols can be applied as annotations in
modelling mode
Annotations are automatically included indrawings
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FRONT ISORIGHT
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A
VIEWA
B
DETAIL B
1:2.50
CC
SECTIONC-C
1515
242 X
37.5
75
C
45
35
39 X 50 `R 40
7562.5
D
R 254 X
242 X
25
25
90
200
B
A
65
125
l 0.3 MA B D
l 0.3 MA B C
l 0.3 MB A D