What type of gauge is generally used for checking flatness?

The correct answer is the thickness gauge. A thickness gauge is specifically designed to measure the thickness of materials and can also be utilized to check flatness by assessing the uniformity of thickness across a surface. When checking flatness, measurements can be taken at various points on a surface, and any irregularities in thickness can indicate deviations from a flat plane.

In contrast, a feeler gauge is primarily used to measure gaps and clearances, a dial gauge is typically used for precision measurement of small distances or displacement, and a snap gauge is employed in industrial settings for quick, comparative measurements of parts against set limits, rather than direct flatness evaluations. Therefore, the thickness gauge is best suited for assessing flatness as it provides the necessary measurements to evaluate the consistency of a flat surface.