Flatness refers to the deviation of the macro concave convex height of the substrate from the ideal plane. A tolerance zone is the area between two parallel planes with a distance of tolerance value t.

Flatness belongs to the form error in form and position error, and it belongs to macroscopic geometric form error.

Surface roughness refers to the unevenness of the machined surface with small gaps and small peaks and valleys.

The distance (wave distance) between its two peaks or valleys is very small (below 1mm), which belongs to micro geometric shape error. The smaller the surface roughness, the smoother the surface.