What Is Datum In GD & T: Definition, Types, Uses, And Examples [PDF]


Datum plays an important role in GD & T. Except for form tolerances like Straightness, Flatness, Circularity, and Cylindricity, all other geometrical tolerancing can refer a datum to explain the design intent. So in this article, we will learn more about ” What is a datum in GD & T with definition, type, uses, and examples.

At the end of this article, you will get a link to download Datum PDF copy, which you can download for your ready reference. So let’s get into the article.

Before going ahead into this article, you can consider reading through the below list of articles as this will make things easy for you to understand datum in a better way.

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What Is MMC and LMC modifier in GD and T
What is regardless of feature size (RFS)
Basic GD & T rules as per ASME standard

What Is A Datum: Datum definition

Datum is a theoretical exact plane, axis, or point that GD & T or geometrical tolerances use to reference dimensions. So datum acts as a reference point or an anchor for almost all geometrical tolerances and dimensions. Datum needs to be chosen carefully, because choosing a wrong datum may cause the part getting rejected considering form, fit, and function.

See this below example where all the hole positions are referenced from a datum named “A

Example of Datum

Why datum is required?

To understand why datum is required, we need to look into the below image. In the example, the left side image shows a typical dimension of a block. Now the manufacturer can measure those surfaces in two different ways as shown on the right side.

Lets assume that the bottom surface is important for mating with other part and need to be flat. But if the supplier measure the part like” Scenario-1″, then the part will not get assembled and will be rejected. Same goes true if the left side surface is important and supplier measure the part like “Scenario-2”

So we noticed that without datum, there are high chances that a part may get rejected. The reason is supplier has to assume which surface is important. Now if we show the datum in the drawing, the supplier will understand which surface is important, and accordingly, they will measure.

What Is Datum Feature

A datum feature can be defined as an integral feature of a part that can be established as a datum. If you see the below example, the left side surface of the block is a datum feature as it is establishing datum “A”. Also, the second hole is the datum feature as the axis of that hole is the datum ” B”

Datum Feature

What Is Datum Feature Simulator

Datum feature simulator is inspection equipment like the surface plate, gauge surface which comes in contact with the datum of a part. The reason for using the datum feature simulator is that no part can have a perfect surface to establish itself as a datum. In contrast, surfaces of inspection equipment are considered to be perfect surfaces. That is why the datum feature simulator is used to establish a datum.

In this example, the green color body is the desired part which has datum A, B, and C. The other body act as a datum feature simulator to establish datum A, B, and C.


What is Simulated Datum

Datum established by a datum feature simulator is called a simulated datum. In the below image, the dotted line is the simulated datum as it is established by contacting the datum feature simulator surface and the theoretical perfect part datum surface.


Datum Symbol

Below are the two standards of how datum should be shown in a typical engineering drawing.


Types Of Datum shown in engineering drawing

Here are a couple of notation of datum that is shown in engineering drawings.

Datum On A Surface

Here in this example, the datum A is the top surface govern by the dimension 100.000 inches. This type of datum is useful to control the tolerances of that particular surface. Please make a note that this type of datum is only applicable to the surface having the notation.


Datum On an Axis

In this example, the center of the hole is established as Datum A. This type of datum is essential for controlling the position,


Datum on a center plane

In this example, the center plan of the slot is the datum A which essentially controls the position and parallelism of opposite surfaces.


Datum Precedence: 3-2-1 Rule

Datum precedence decides which datum is more important for a part function point of view. Not all datum is equally important. To find out the level of importance datum precedence is used. Based on datum precedence, datum’s are divided into the following types.

  • Primary Datum
  • Secondary Datum
  • Tertiary Datum

There are 6 degrees of freedom a body can move. Three transnational ( X, Y, and Z) and three rotational against X, Y, and Z. Unless we lock all 6 degrees of freedom, a body is not locked and not considered stable.

To understand datum precedence, let us watch this below video. In this video, you may notice that, as soon as we assemble the part with the first datum plane ( Primary datum), 3 degrees of freedom is locked. When we assemble with the second datum, two more degrees of freedom are locked. As soon as we assemble with the third datum, the last degrees of freedom is also locked.

So in this case, the first datum is called Primary datum, the second datum is called secondary datum and the last datum is called a tertiary datum. This whole phenomenon of locking 6 degrees of freedom is called the 3-2-1 rule.

Primary Datum

The primary datum should make three-point of contact with the datum feature simulator and should lock three degrees of freedom. In the above video, you noticed that the first datum locks three degrees of freedom. Primary datum is usually shown as Datum A

Secondary Datum

Secondary datum should make two points of contact with the datum feature simulator and should lock two degrees of freedom. In the above video, the second plane locks two degrees of freedom. Secondary datum is usually shown as datum B

Tertiary Datum

Tertiary datum should make one point of contact with the datum feature simulator and should lock one degree of freedom. In the above video, the third plane locks one degree of freedom. Tertiary datum is usually shown as datum C

How to decide datum precedence?

The designer should consider the following aspect before deciding datum precedence. They should ask themselves below question before making a decision.

  • Is the datum is a functional surface?
  • Does the datum surface have enough datum points to touch the datum simulator?
  • The datum will make any difference in the assembly?
  • Is the datum is readily accessible for measurement?

How datum’s are shown in the feature control frame?

The below image shows how datum is usually shown in a typical drawing. In the feature control box, datum A is primary, Datum B is secondary and Datum C is tertiary. For example, if you want to make datum B as primary then the sequence should be B|A|C instead of A|B|C

Datum Target: Establish datum on irregular surfaces

Datum targets are used to establish datum on an irregular surface where planner datum is not readily possible. Datum points are used to establish a virtual datum for the measurement. Here again, the 3-2-1 rules apply. To establish a primary datum, a minimum of three datum points are required. Similarly, with two datum points, a secondary datum is established and one datum point for the tertiary datum.

Please see the below image where we need to establish a datum on an irregular surface (Green color) as this surface will mate with the other parts. Three datum point is decided to establish the datum. The measurements are also shown.

Please read this below article to learn more about Datum Points and different types of datum points.

Must Read: What is datum target? Definition with types


That’s all I have in this article. I hope that this article could able to give you all the possible information about the datum. Datum is the basics of GD and T. Unless you have a clear idea about datum’s, it very difficult to learn GD & T. If you still have any questions about datum’s, then please do write in the comment section and I will be happy to respond you back.

As you may be noticed that I took enormous effort with numerous graphics to explain this subject. I would appreciate if you can share this article among your friends so that I can feel good about my effort.

Frequently Asked Questions ( FAQ)

What is datum?

Datum is a theoretically exact plane, surface, axis or point that GD & T use to anchor dimensions and tolerances.

What are types of datum?

Planner Datum
Axis Datum
Point Datum

What are types of datum based on Datum Precedence?

Primary Datum
Secondary Datum
Tertiary Datum

What are datum points?

Datum points are used to establish a virtual datum on irregular surfaces where theoretical exact datum plane is not possible

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