What is Geometric Dimensioning and Tolerancing ?
Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used in engineering and manufacturing to communicate design specifications and tolerances for the production of parts and assemblies. It provides a standardized way of representing the dimensions, tolerances, and relationships between features on a part.
Where is Geometric Dimensioning and Tolerancing is used?
GD&T is widely used in industries such as aerospace, automotive, and manufacturing, where precise and consistent communication of design requirements is crucial for ensuring the interchangeability and functionality of components. It allows engineers, designers, and manufacturers to convey complex geometric information in a concise and standardized manner, leading to more accurate and efficient production processes.
The following information of this post is based on the ASME Y14.5M-1994. It is the standard for dimension and tolerancing. ASME stands for American Society of Mechanical Engineers. Y14.5 is the number of the standard. M points out that is metric and 1994 is the year it was approved. Other standards exist, for example, the International Standards Organization (ISO).
Definitions in Geometric Dimensioning and Tolerancing
- Dimension: numerical value expressed in the appropriate measure units and used to define form, dimension, orientation and location of a part or characteristic.
- Tolerance: total quantity of variation allowed in a dimension. The tolerance in a drawing defines the maximum and minimum limits of a dimension.
- Geometric tolerance: used to control the dimensions, form, profile, orientation and location.
- Exists three types of dimensions:
- Nominal Dimensions –> Used to define form, dimension, orientation an location. In case it is not explicit, it is shown at GTOL.
- Basic dimensions -> It is a dimension theoretically perfect. They are the base of the geometric tolerance and appear in the drawing with a box. Determined by the feature control frame.
- Reference Dimension -> Only for informational purposes. Appears in brackets (). Means that it is not required.
What is a datum in GD&T?
A datum defines a theoretically perfect surface, axis, plane or point. Datums are reference surfaces or points on a part that are used as a basis for dimensioning and tolerancing. They establish a coordinate system from which other features are measured. Datum features are specified in GD&T to ensure consistent measurement references.
- Dimension and validation.
- Reference.
A datum characteristic could be:
- A surface
- Multiple surface
- A dimension characteristic
- A pattern dimension characteristic
Modifiers in Geometric Dimensioning and Tolerancing
The modifiers are used in dimensioning and geometric tolerancing. There are two types as shown in the table below.
Maximum material condition (MMC)
This condition states that the dimensional figure has the maximum material inside the dimensional limits specified. This refers that the exterior diameters and with are at the mayor limit of the dimension, and the inside diameter and thickness are at the minimum.
The maximum amount of material exists within its dimension tolerance. The goal of MMC is that when the part is in its worst tolerances, it always assembles. It is commonly used for straightness, parallelism, perpendicularity and angularity.
Least material condition (MMC)
Least material condition is a feature of size simble in which defines the dimension with the mínimum material cuantity that exisst within its dimensional tolerance. For internal features or holes the least material condition is largest hole size and for external the smallest size.
The reason for using this feature is when a hole is close to an edge, so using the minimum material condition, ensures that the position of the hole is correct.
True position
The true position defines the total allowable variation that a feature can have from “its” true position. The true positionis the exact coordinate, or location defined by basic dimensions or other means that represents the nominal value.
Feature control frame
The feature control frame is a feature that gives information about the especified requirements.
A typical feature control frame is divided into three parts: symbol of the geometric feature, geometric tolerance and reference datum.
- The symbol of the geometric feature is a feature that it is being controlled and appears in first place.
- The geometric tolerance appears in the second position. It defines the numerical value of the tolerance that is being controlled.
- Finally, in third place, the reference datums are written in capital letters and are positioned in the next frames (er,4th, 5th). Each one remove degrees of freedom, as it is shown below:
- Primary: removes 3 degrees of freedom.
- Secondary: removes 2 degrees of freedom.
- Tertiary: removes 1 degree of freedom.
Composite feature control frame
A composite feature control frame is a feature that is characterized by:
- Two lines of specifications
- A symbol of a geometric characteristic
