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. 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. What are datum points? What Is A Datum: Datum definitionĭatum is a theoretical exact plane, axis, or point that GD & T or geometrical tolerances use to reference dimensions.What are types of datum based on Datum Precedence?.Datum Target: Establish datum on irregular surfaces.

How datum’s are shown in the feature control frame?.Types Of Datum shown in engineering drawing.

Datum line