In how many directions should flat surfaces be checked with a straight edge?

When checking flat surfaces with a straight edge, it is standard practice to evaluate in six different directions to ensure an accurate assessment of the flatness. This method involves measuring in two perpendicular directions on the surface, typically referred to as the X and Y axes, and then also checking the diagonal directions across the surface. This comprehensive approach allows for the identification of any irregularities or deviations from flatness in both the primary and secondary directions. By assessing the surface in all six orientations, you can accurately determine the overall geometric condition of the surface, ensuring that it meets required tolerances and specifications. This thorough evaluation is crucial in the precision-driven environments where millwrights operate.