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Geometric Dimensioning and Tolerancing

Geometric dimensioning and tolerancing is an international language used to accurately describe a part in engineering drawings. It consists of a well-defined set of symbols, rules, definitions and conventions that can be used to describe the size, form, orientation and location tolerances of part features. G D & T is an exact language that enables designers to “say what they mean” through a drawing.

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Benefits of using GD&T

• Functional dimensioning is the design philosophy of GD&T

• GD&T's datum system clearly conveys one set up for inspection

• GD&T allows round tolerance zones

• GD&T allows a “bonus” tolerance

• GD&T reduces assembly problems

• GD&T supports the use of SPC

• GD&T is supported by national and international standards


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Strategy 1 Linear Dimensioning


Strategy 2 Combination of Linear & Geometric Dimensioning


Strategy 2 Combination of Linear & Geometric Dimensioning

80

0.1A

± 0.1


Strategy 3 Geometric Dimensioning

10


GD&T Applied to Sheet Metal Part


35

36

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COORDINATE V/s ROUND TOLERANCE ZONE

100±0.1

68±0

.1

Ø20.28

Ø20.28


ROUND TOLERANCE ZONE


Feature:

A feature is a specific characteristic portion of a part such as a surface, hole, slot, screw, thread or profile.

Feature of Size:

A feature of size (FOS) is one cylindrical or spherical surface or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension.

Feature & Feature of Size


Actual Mating Envelope

Internal Feature External Feature

Minimum Circumscribed circle

Maximum Inscribed circle


Ø 20.008

Ø 20.003

Precision Cylindrical Fit

Should be less than 20.008


Feature Control Frame


Geometric Characteristic Symbols

Type of Tolerance

Characteristic Symbol Applicable For

Form

Straightness

Individual Features

Flatness

Circularity

Cylindricity

Profile

Profile of a Line

Individual or Related Features

Profile of a surface


Type of Tolerance

Characteristic Symbol Applicable For

Orientation

Angularity

Related Features

Parallelism

Perpendicularity

Location

Position

Concentricity

Symmetry

Runout

Circular Runout

Total Runout

Geometric Characteristic Symbols


Modifying Symbols


STRAIGHTNESS

Straightness of surface element of a cylindrical surface

The tolerance zone is limited by two parallel planes 0.1mm apart.


Straightness of Axis at RFS


Straightness of Axis at MMC


Functional Gage


Tangent Plane

A plane contacting the high points of the surface shall lie within two parallel planes 0.1 apart


ANGULARITY The tolerance zone is limited by two

parallel planes a distance t apart and

inclined at the specified angle to the

datum

The condition of a surface, center plane or axis being exactly at a specified angle

0.08


Position tolerance of Multiple Holes


Projected Tolerance Zone

Fastener and Hole


Profile Between Basis


Type of Tolerance

Characteristic Symbol Datum Required

Yes / No MMC/LMC Applicable

Yes / No

Form

Profile

Orientation

Location

Runout

Exercise Complete the following table:


If you have any queries please contact: A Raghunath

Aditi Consultancy Services Email: [email protected] Tel / Fax: 91 80 26580711; Mobile: 9449612671


Web http://www.aditiconsultancy.com

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