GD&T (Geometric Dimensioning & Tolerancing) is an international language used to accurately describe a mechanical part's geometry. It comprises symbols, rules, and definitions. GD&T provides benefits like uniform drawings, better designs, and avoids assumptions. The standard is ASME Y14.5-2009, which revised concepts like FOS, datum references, and composite tolerances from the 1994 version. GD&T defines dimensions, features, datums, and symbols for form, profile, orientation and location tolerances. An example part drawing demonstrates applying GD&T controls.

1. GD&T
(Geometric Dimensioning & Tolerancing)
-Sreelakshmy (B.E)

2. 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)

3. 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

4. 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

5. 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

6. Definitions Contd..
• Actual Local Size
It is the value of any individual distance at a cross section of a FOS. It
is a point-to-point measurement
• Actual Mating Envelope
Measure 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 Tolerance
It is the additional tolerance permitted only when MMC or LMC is
specified in the feature control Frame
• Regardless of FOS
It is the Geometric tolerance applied to any size the part is
produced

7. Symbols & its characteristics
There are 14 geometric characteristic symbols. They are categorized with
the type of geometric control (Form, Profile, Orientation, Location Run-Out)

8. Modifiers and its symbols
They 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

9. Additional Symbols

10. 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

11. Datum symbols

12. 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

13. GD&T in a part design (An example)

14. Thank you!!
(Referred from Wikipedia and concerned websites)