![]() There are 6 degrees of freedom a body can move. Based on datum precedence, datum’s are divided into the following types. To find out the level of importance, datum precedence is used. 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 in the datum ” B.”ĭatum precedence decides which datum is more critical from a part function point of view. What Is Datum FeatureĪ datum feature can be defined as an integral feature of a part that can be established as a datum. If we show the datum in the drawing, the supplier will understand which surface is important, and accordingly, they will measure. The reason is supplier has to assume which surface is important. So we noticed that without datum, there are high chances that a part may get rejected. The same goes true if the left side surface is important and the supplier measures the part like “Scenario-2.” But if the supplier measures the part like” Scenario-1″, then the part will not get assembled and will be rejected. Let us assume that the bottom surface is important for mating with other part and need to be flat. ![]() Now the manufacturer can measure those surfaces in two different ways, as shown on the right side. In the example, the left side image shows a typical dimension of a block. To understand why datum is required, we need to look into the below image. GD&T Tutorial – Datum, Datum Feature and Datum Target See this below example where all the hole positions are referenced from a datum named “ A“ Datum needs to be chosen carefully because choosing the wrong datum may cause the part to get rejected considering form, fit, and function. So datum acts as a reference point or an anchor for almost all geometrical tolerances and dimensions. So in this article, we will learn more about ” What is a datum in GD & T with definition, type, uses, and examples.īefore 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.īasic GD & T rules as per ASME standard What Is A Datum: Datum definitionĭatum is a theoretical exact plane, axis, or point that GD & T or geometrical tolerances use to reference dimensions. ![]() Except for form tolerances like Straightness, Flatness, Circularity, and Cylindricity, all other geometrical tolerancing can refer to a datum to explain the design intent.
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