GD&T(Geometric Dimensioning & Tolerancing)

-Sreelakshmy (B.E)

Context

• Introduction• Revised changes made in ASME Y14.5 2009• Definitions• Symbols• Modifiers• Additional symbols• Datum• Datum symbols• Feature control Frame• GD&T in a part design (An example model)

Introduction

• GD&T is an international language used to describe a part accurately.

• It comprises of symbols, rules and definitions for defining a geometry of mechanical part.

• It has many benefits like uniformity in drawings, Better designs, No assumptions, large tolerances

• It is used especially in the manufacturing sector for dimensioning and Tolerancing

• The standard used was ASME Y14.5M-1994 and now, ASME Y14.5-2009 is used

Revised changes made in ASME Y14.5 2009

Important changes that has been made in 2009 version from 1999 are:

• Concept of FOS

• Datum References

• Degree of Freedom

• Composite Position Tolerances

• Surface Boundaries

• Axis Method of Interpretation

• Profile Tolerances

• Modifiers Tool

• Symbology

Definitions• Dimension

It is the numerical value expressed in units of measure to define SIZE, LOCATION, ORIENTATION, FORM or any other CHARACTERISTICS of a part

• Feature

• It is a general term applied to a physical portion of part such as surface, hole or slot

• FOS (Feature of Size)

It is a cylindrical/Spherical surface or set of two opposed elements associated with size dimensions

• Datum

It is a co-ordinate system used to locate places

• Datum Shift

It is the allowable movement between the part feature and the gage

• Nominal Value

It is the specified value on a drawing

• Tolerance

It is the total amount that part features are permitted to vary from their specified dimension

• Types of tolerances

Limit Tolerance, Plus-Minus Tolerance, Equal Bilateral Tolerance, Unequal bilateral tolerance, Unilateral Tolerance

Definitions Contd..

• Actual Local SizeIt is the value of any individual distance at a cross section of a FOS. It is a point-to-point measurement

• Actual Mating EnvelopeMeasure of distance between the highest points in a part design

• MMC (Maximum Material Condition)It is the maximum amount of material within the limits

• LMC (Least Material Condition)It is the least amount of material within the limits

• Bonus ToleranceIt is the additional tolerance permitted only when MMC or LMC is specified in the feature control Frame

• Regardless of FOSIt is the Geometric tolerance applied to any size the part is produced

Symbols & its characteristicsThere are 14 geometric characteristic symbols. They are categorized with

the type of geometric control (Form, Profile, Orientation, Location Run-Out)

Modifiers and its symbolsThey are symbols which defines conditions for the tolerance zone of a geometric tolerance. There are six modifiers. The symbols of modifiers and their meaning is shown

Additional Symbols

Datum

• It is a theoretically exact plane from which a dimensional measurement is made

• Datum system is a set of symbols which is used to describe the function and dimensions to the user

• Datum plane, Datum axis and Datum center plane are used to make measurements of a part

Datum symbols

Feature Control Frame

It is a rectangular box which is divided into compartments within which symbols, modifiers, tolerances, etc… are placed. Below is an example for feature control frame

GD&T in a part design (An example)

Thank you!!

(Referred from Wikipedia and concerned websites)