What is used as a reference point for checking the alignment of machine parts?

The gauge block is the correct choice for use as a reference point for checking the alignment of machine parts because it is a precise measuring tool known for its accuracy and stability. Gauge blocks, also referred to as gage blocks or Jo blocks, are standardized blocks of metal or ceramic that can be stacked together to create precise lengths. They are specifically manufactured to very tight tolerances, which makes them ideal for ensuring that parts align correctly with each other within the desired specifications.

By using gauge blocks, a technician can set up reference points that serve as a baseline for alignment checks. This accuracy is crucial in machine design and fabrication, where even minor misalignments can lead to significant operational issues or failures.

While other tools listed, such as a leveling tool, caliper, and Vernier scale, serve their purposes in measurements and aligning tasks, they do not provide the same level of precision and standardization that gauge blocks offer when it comes to establishing primary reference points for the alignment of components. Leveling tools, for example, are more suited for ensuring that surfaces are horizontal or vertical, while calipers and Vernier scales are primarily for direct measurement rather than establishing a precise reference standard.