What is the minimum number of contact points required for a secondary datum?

The minimum number of contact points required for establishing a secondary datum is two. This is because a secondary datum is used to provide additional control and reference for part features in relation to the primary datum. With two contact points, the secondary datum can effectively define a plane or axis, allowing proper geometric control in the design and inspection processes.

When using only one contact point, it would be challenging to define a stable or repeatable orientation, making it difficult to establish the necessary reference for other features on the part. Three contact points would be more than sufficient to establish a surface or a location, but is not the minimum requirement for defining a secondary datum. It should also be noted that four contact points are unnecessary, as two points are sufficient to achieve the desired stability and accuracy in positioning the secondary datum. Thus, two contact points are essential to properly define and use a secondary datum in GD&T.