What is Geometric Dimensioning and Tolerancing?

Geometric Dimensioning and Tolerancing, abbreviated as GD&T, is a system for defining and communicating design intent and engineering tolerances in manufacturing. GD&T is a language of symbols and standards that describes any part dimensions and its allowable variation in size, form, orientation, and location.

Why Use GD&T in Complex CNC Machining?

There are 3 benefits of using GD&T in complex CNC machining.

1. Enhanced Communication: 

GD&T allows you to convey substantial information about your part’s design using a relatively small amount of letters, numbers, and symbols. The use of a single and standardized GD&T language ensures engineering, design, production, and quality teams are able to communicate clearly and accurately with each other, saving time and increasing efficiency.

2. Accurate Production:

Since GD&T is such a precisely articulated system of language, it minimizes ambiguity in the description of a part’s design. Part production becomes more repeatable and consistent – simplifying inspection, reducing the part rejection rate, and improving quality. 

3. Increased Flexibility: 

GD&T specifies the design intent instead of the specific geometry, providing the appropriate tolerances that maximize production. It results in more manufacturing flexibility and larger tolerances, especially for intricate or complex designs!

Geometric tolerancing reference chart

Within GD&T, “geometric” refers to various form, such as a plane, cylinder, or square.Below are examples of the geometric characteristics and corresponding symbols as well as the nomenclature commonly used with GD&T.

Geometric Tolerance Symbol

Types of Geometric Symbols Datum Sample Graphs
Form Straightness Straightness No Straightness
Flatness Geometric-tolerancing-Flatness No Geometric-tolerancing-Flatness-1
Circularity Circularity No Circularity
Cylindricity Cylindricity No Cylindricity
Profile rofile of a Line Profile-of-a-Line Yes or No Profile-of-a-Line-1
Profile of a Surface Profile-of-a-Surface Yes or No Profile-of-a-Surface-1
Orientation Parallelism Parallelism Yes Parallelism
Perpendicularity Perpendicularity Yes Perpendicularity
Angularity Angularity Yes Angularity
Location Position Position Yes or N Position
Concentricity Geometric-tolerancing-Concentricity Yes Geometric-tolerancing-Concentricity-1
Symmetry Geometric-tolerancing-Symmetry Yes Geometric-tolerancing-Symmetry-1
Run-out Circular Run-out Geometric-tolerancing-Circular-Run-out Yes Geometric-tolerancing-Circular-Run-out-1
Total Run-out Total-Run-out Yes Total-Run-out-1
Additional Symbols Datum Symbol and code Datum Symbol and code Yes Geometric-tolerancing-Datum-Symbol-and-code-1
Datum Target Geometric-tolerancing-Datum-Target Yes Geometric-tolerancing-Datum-Target-1
Maximum Material condition Geometric-tolerancing-Maximum-Material-condition   Geometric-tolerancing-Maximum-Material-condition-1
Envelope Requirement Geometric-tolerancing-Envelope-Requirement   Geometric-tolerancing-Envelope-Requirement-1
Projected Tolerance Zone Projected-Tolerance-Zone   Projected-Tolerance-Zone-1

Theoretically Exact Dimension

Theoretically-Exact-Dimension   Theoretically-Exact-Dimension-1
Not concave,
Not convex,
Only allowed to decrease in small end direction.
Not concave,   Geometric-tolerancing-Not-concave-1

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