Computer Aided Engineering

Applications

6.Geometric Dimensioning and

Tolerancing (GD&T)5.1 Basic tolerancing

5.2 Surface control

Engi 6928 - Fall 2014

5.2 Surface control

5.3 GD&T

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5.1 Basic 5.1 Basic TolerancingTolerancing

An objective of an engineering drawing is to convey

the shape and size of an object.

Dimensioning specifies the required size of features.

Tolerancing specifies the required precision of the

size.

Surface control specifies the required surface finish.

Geometric tolerancing is a specification used to

control the shape of parts

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5.1 Basic 5.1 Basic TolerancingTolerancing

The tolerance determines the manufacturing process

to use.

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5.1 Basic 5.1 Basic TolerancingTolerancing

Cost increases exponentially with the specified

tolerance.

Tighter tolerances should be used only where

necessary for the functionally critical features.

5.1 Basic 5.1 Basic TolerancingTolerancing

Unilateral tolerance

Bilateral tolerance

5.1 Basic 5.1 Basic TolerancingTolerancing

MMC maximum material condition

LMC least material condition

5.1 Basic 5.1 Basic TolerancingTolerancing

Fit refers to how tightly or loosely mating parts must fit

together when assembled

Categories:

Clearance Fit The internal member is always

smaller than the external member so that the parts

will slide together.will slide together.

Interference Fit The internal member is always

larger that the external member so that the parts

must be forced together.

Transition Fit Compromise for applications

where accuracy of location is important but either a

small amount of clearance or interference is

permissible.

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5.1 Basic 5.1 Basic TolerancingTolerancing

The Basic Hole System The

external member (often a hole) is

used as the basic dimension from

which the toleranced dimensions to

achieve the fit are determined.

The Basic Shaft System - Takes The Basic Shaft System - Takes

the size of the shaft as the basic

dimensions from which toleranced

dimensions are derived. Not

commonly used

Allowance The difference

between the smallest hole size and

the largest shaft size.

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5.1 Basic 5.1 Basic TolerancingTolerancing

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5.1 Basic 5.1 Basic TolerancingTolerancing

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5.1 Basic 5.1 Basic TolerancingTolerancing

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5.1 Basic 5.1 Basic TolerancingTolerancing

1. Select basic size 40mm

2. Select shaft or hole basis - Hole

3. Select type of fit - Sliding

4. Select ANSI preferred tolerances

H7/g6

Example

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5.1 Basic 5.1 Basic TolerancingTolerancing

Example

Type of fit MMC Hole MMC Shaft Allowance Loosest fit

A Interference 50 50.1 -.1 -.03

B Transition 49.950 50 -.05 +.15

C Clearance 50 49.95 +.05 +.15

The basic dimension of the hole/shaft is 50 mm

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5.2 Surface control5.2 Surface control

Surface finish is another crucial factor which affects

the cost of parts

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5.2 Surface control5.2 Surface control

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5.2 Surface control5.2 Surface control

Roughness: Measure of the irregularities of the

surfaces finish

Waviness: Measure of the more widely spaced

irregularities of the surface due to warping, machine

vibrations, heat treating or other similar factors

Lay: Direction of the primary surface patternLay: Direction of the primary surface pattern

Roughness Width Cut-Off: Maximum width of surface

irregularities to be included in the measurement of

roughness height

Roughness Width: Maximum allowable spacing

between repetitive units of surface pattern

5.2 Surface control5.2 Surface control

Perthometer

5.2 Surface control5.2 Surface control

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5.2 Surface control5.2 Surface control

Example

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5.3 GD&T5.3 GD&T

Uses symbols to specify geometric characteristics

and dimensional requirements

Current GD&T standard: ANSI/ASME Y14.5M-2009

Symbols for:

Form and profile tolerances, orientation

tolerances, location tolerances, runout tolerances

5.3 GD&T5.3 GD&T

5.3 GD&T5.3 GD&T

Straightness

The line feature has a tolerance zone which is specified by two

parallel lines spaced 0.002 apart. The tolerance zone is not

necessarily parallel to any datum feature

5.3 GD&T5.3 GD&T

Flatness

The plane feature has a tolerance zone which is specified by two

parallel planes spaced 0.002 apart.

5.3 GD&T5.3 GD&T

Parallelism

The line feature has a tolerance zone which is specified by two

parallel lines spaced 0.002 apart. The tolerance zone is parallel to the

Datum feature A.

5.3 GD&T5.3 GD&T

Circularity

The circle feature has a tolerance zone which is specified by two

concentric circles spaced 0.002 apart. The center is the best fit

center for the profile of the irregular circle.

5.3 GD&T5.3 GD&T

Cylindricity

The cylindrical face feature has a tolerance zone which is specified by

two concentric cylinders spaced 0.002 apart.

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5.3 GD&T5.3 GD&T

Coordinate Measurement Machines (CMM) allows

automated part inspection

Least squares fit for line, plane, circle and cylinder

Defines the median line of the tolerance zones for

straightness, flatness, circularity and cyclindricity respectively.

5.3 GD&T5.3 GD&T

Cocentricity

The center of the circle feature has a tolerance zone which is specified

by a circles with diameter 0.005. The tolerance zone is centered with

datum A.

5.3 GD&T5.3 GD&T

Runout

The circle feature has a tolerance zone which is specified by two

concentric circles spaced 0.005 apart. The tolerance zone is

centered with datum A.

5.3 GD&T5.3 GD&T

Positional tolerance

5.3 GD&T5.3 GD&T

Datum preference specifies the order in which the

part is assembled w.r.t a datum reference frame.

5.3 GD&T5.3 GD&T

Bonus tolerance - This is an extra tolerance zone

applicable to a feature due to the use of feature

frame modifiers.

5.3 GD&T5.3 GD&T

Example:

5.3 GD&T5.3 GD&T

Example:

0.02

ReferencesReferences

ASME Y14.5-2009 Dimensioning and Tolerancing.