lect9- dimen and tolerancing

8/11/2019 LECT9- Dimen and Tolerancing

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CAD/CAM Ir. ZAMBRI HARUN

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

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