Introduction of Datum in GD&T

Geometric dimensioning and tolerancing (GD&T) is a language of symbols used to communicate information on technical drawings. One important aspect of GD&T is the use of datums, which are reference points or surfaces used to establish a common frame of reference for specifying tolerances and measurements on a part. In this blog post, we'll take a closer look at what datums are, how they are used in GD&T, and some common use cases for datums.

What are datums?

A datum is a reference point or surface that is used as the basis for establishing a common frame of reference for specifying tolerances and measurements on a part. Datums are typically specified on a drawing using special symbols and may be based on a physical feature of the part, such as a surface or an axis, or may be defined as a virtual condition.

Datums are an important aspect of GD&T because they allow designers to specify tolerances and measurements relative to a known reference point or surface, rather than relying on the actual physical dimensions of the part. This allows designers to more accurately specify the required tolerances and dimensions of a part, which can help to improve the quality and consistency of the finished product.


How are datums used in GD&T?

Datums are used in GD&T to establish a common frame of reference for specifying tolerances and measurements on a part. When specifying tolerances on a drawing, designers will often specify a datum or set of datums to which the tolerance applies. For example, a designer might specify that a particular feature of a part must be "within 0.1 mm" of a particular datum.

Datums are typically specified using special symbols on a drawing and may be accompanied by additional symbols and notes to clarify their meaning. For example, a designer might use a datum symbol followed by a letter to specify a particular datum, such as "Datum A". Datum symbols may also be accompanied by additional symbols to indicate the orientation or position of the datum relative to other features on the part.

Common use cases for datums There are many different use cases for datums in GD&T, and they are often used in combination with other geometric tolerances to specify the required dimensions and tolerances of a part. Some common use cases for datums include:

  • Specifying orientation and location tolerances: Datums can be used to specify orientation and location tolerances on a part, such as a parallelism or position. For example, a designer might specify that a particular surface of a part must be parallel "within 0.1 mm" to a datum surface. 

What are datum systems?

In the field of geometric dimensioning and tolerancing (GD&T), a datum is a theoretical exact location, axis, or plane used as a reference for the purpose of dimensioning, tolerancing, and inspecting a part. A datum system is a set of three or more datums that are used to establish the orientation of a part.

In GD&T, a datum system is usually established by defining a set of primary datums (also known as "A-B-C" datums), which are typically chosen from the most stable and symmetrical features of a part. These primary datums are then used to establish the orientation of the part, and all other dimensions and tolerances on the part are referenced to this orientation.

In addition to primary datums, a datum system may also include secondary datums, which are used to establish the orientation of features that are not directly related to the primary datums. These secondary datums are usually established using features such as holes, slots, or other geometric elements that are directly related to the primary datums.

By establishing a datum system and referencing all dimensions and tolerances to this system, it is possible to ensure that a part will meet its design requirements and function properly, regardless of variations in the manufacturing process.


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