Which datum type primarily assures the stability of parts in a stack?

The correct choice is planar datums, which play a crucial role in ensuring the stability of parts within an assembly or stack. This type of datum provides a flat reference surface that can efficiently support and locate multiple components, allowing for consistent and repeatable stacking. When parts are oriented or constrained in relation to a planar datum, it provides a stable foundation that minimizes movement and potential misalignment, which is vital for maintaining the integrity of the assembly.

Planar datums also contribute to better control of geometric features, enabling precise measurements and adjustments. By defining key features of a part in relation to a flat surface, planar datums ensure that parts can be consistently assembled in a predictable manner, facilitating effective manufacturing and quality control processes.

In contrast, other datum types might not provide the same level of stability or support. Centerline datums are primarily used for features that are symmetric but may not provide the necessary stability for stacked assemblies. Centerplane datums, while useful for symmetrical features, do not offer the same flatness and surface area needed for stacking stability. Auxiliary datums are generally applied for specific geometric control and do not substantively enhance stacking stability like planar datums do.