How many types of datums can be selected?

In GD&T, datums are reference points, lines, or surfaces that serve as the foundation for the measurement and control of a part's features. There are three primary types of datums that can be selected:

1. Plane datums - These are flat surfaces used as references for other features. They provide a stable and consistent point for measuring.

2. Axial datums - These involve the use of axes as references, which are important for cylindrical or rotational features.

3. Point datums - These are specific points that serve as reference locations for measurement.

The selection of these three types of datums is critical in defining the tolerances for manufacturing parts and ensuring that they meet design specifications accurately. Understanding the different types of datums helps in effectively communicating the required tolerances and ensuring parts fit together correctly during assembly. This comprehensive approach is essential for ensuring quality and precision in manufacturing processes.