Table of Contents
GD&T is a set of rules and symbols describing a part’s nominal geometry and the permissible variation range. It emphasizes the part’s function and parts assembly, allowing for larger tolerances on less critical design features. This approach can help save money during manufacturing. GD&T enables designers to communicate their intended design accurately and efficiently. It allows for engineering information that cannot be expressed by traditional tolerancing.
The system is based on a library of symbols, rules, definitions, and conventions to describe the part dimensioning, inspection, and acceptance. Although it has been used for decades, it is still common to find professionals in the industry who “are afraid of”, or misinterpret it.
Tolerances
There are two main types of tolerances:
Size Tolerance, also known as dimensional tolerance, is the simple tolerance we all know that specifies the allowable variation in the size of a physical dimension (For example, a bore’s diameter or a block’s height). It defines the upper and lower limits between which a dimension can vary and still be acceptable. The size tolerance is generally expressed as a range of values, such as +/- 0.001 inches (or millimeters).
Geometric tolerance, also known as form tolerance, specifies the allowable variation in the geometric shape, orientation, or location of a feature or set of features (For example, flatness and perpendicularity).
GD&T is a method of expressing geometrical tolerance and their connection with size tolerances on a drawing
Datums
A datum is a reference point on a drawing used to define other geometric characteristics relative to it. A datum can be a point, line, or plane. A datum is indicated with a capital letter inside a square connected by a line to the datum symbol (▲). In most cases, the GD&T symbols (that we will discuss later) are related to datums.
A datum can be one of the below 3 options:
- Datum Point: A point in space. It can be a center of a hole, a corner of a part, or any other feature that can be defined as a point.
- Datum Axis: A straight line that can be a centerline of a cylindrical or conical feature.
- Datum Plane: A surface of a part or an established plane from multiple points or features.
Basic Dimensions
Basic Dimensions describe a feature’s theoretically exact location, orientation, or size. Therefore, they have no size tolerance associated with them.
A Basic Dimension is denoted in a box around it and must relate to a Datum Feature (Or another basic dimension related to a datum). These dimensions are not inspected directly. However, they are measured indirectly by GD&T position callouts.
Feature of size
A feature of size is a geometric feature on a drawing that can be measured by “two points”. For example, a bore’s diameter or the height of a block. It should have a dimension with limits (like 25+/-0.1) associated with it. Opposed to that, elements such as chamfers and angles are not considered a “feature of size”.
Tolerance Zone
To ensure that a feature meets a specified geometric tolerance, it must be contained within a two or three-dimensional region called the Tolerance Zone. It takes various shapes depending on the type of geometric feature it should control.
Examples:
- For the flatness of a plane, the tolerance zone is the space between two parallel planes.
- The circularity of a circle, it will be 2 coaxial circles, where the measured circle should be contained.
Geometric Symbols
The foundation of the GD&T “language” is a set of geometric symbols that define a Tolerance Zone for geometric features on the part. They are divided into 5 Control Categories.
| Category | Symbol | Name | Tolerance Zone | Requires Datum | Works With MMC |
|---|---|---|---|---|---|
| FORM | ⏤ | Straightness |
|
No | Yes |
| ⏥ | Flattnes | 2 parallel planes | No | Yes | |
| ⌭ | Cylindicity | 2 coaxial cylinders | No | No | |
| ○ | Circularity | 2 coaxial circles | No | No | |
| PROFILE | ⌒ | Profile of a line | 2 Offset lines | Optional | No |
| ⌓ | Profile of a surface | 2 offset surfaces | Optional | No | |
| ORIENTATION | ⟂ | Perpendicularity |
|
Yes | Yes |
| ∠ | Angularity |
|
Yes | Yes | |
| ∥ | Parallelism |
|
Yes | Yes | |
| LOCATION | ⌖ | True Position |
|
Yes | Yes |
| ⌯ | Symmetry |
|
Yes | No | |
| ◎ | Concentricity | Datum Axis | Yes | No | |
| RUN-OUT | ➚ | Circular Run-out | 2 coaxial circles | Yes | No |
| ⌰ | Total run-out |
|
Yes | No |
Material Condition Modifiers
Material Condition Modifiers are a set of letter codes used within GD&T to refine the geometric tolerance based on additional conditions. They extend the tolerance zone and, thus, may reduce the manufacturing cost of a part.
- Maximum Material Condition: Nickinamed MMC and marked by ⓜ. When indicated, it means that the feature must have the maximum amount of material within its tolerance in order to be acceptable. When it has “less material”, the tolerance zone can be extended accordingly.
- Least Material Condition: Nickinamed LMC and marked by Ⓛ. When indicated, it means that the feature must have the minimum amount of material within its tolerance in order to be acceptable. It is helpful in the design of thin-walled parts.
(*) There are 6 more modifiers; however, in 99% of the cases, only MMC and LLC are used.
Feature Control Frame (FCF)
All the above definitions are the foundations for understanding the feature Control Frame, which is the method used to indicate the GD&T instructions on the drawings.
- GD&T Symbol: The controlled characteristic (e.g., Faltness, Prepedicularoty, etc.)
- Diameter Symbol: If present, it indicates that the feature and tolerance are diametrical.
- Tolerance Zone: A number indicating the limits for the controlled controlled characteristic.
- Material Condition: Optional modifier (LMC/MMC).
- Datums: Up to 3 datums can be indicated in order of importance (Primary / Secondary / Tertiary). The datum indicator is mandatory for some symbols and optional for others (See above table).
