In the manufacturing industry, someone once estimated that there are more circular workpieces than flat workpieces, ranging from screws, nuts, gaskets, to cylinders and bearings. The application of circular shapes is indeed very high.

For these rotating parts, the most urgent problem is how to evaluate their real round "shape". Today I will introduce the symbols and definitions related to roundness (reference standards: ISO/DIS 1101:2017, ISO 5459).

Roundness~The extraction circle shall be confined within the tolerance zone between two coplanar concentric circles whose radii differ by t.

Straightness~The edges extracted from the cylindrical surface should be limited to the tolerance zone between two parallel straight lines in the specified direction and with a distance of t.

Flatness~is the area between two parallel planes whose distance is the tolerance value t.

Cylindricity~The extracted cylindrical surface should be limited within the tolerance zone formed between two coaxial cylindrical surfaces with a radius difference of t

Concentricity~The extraction center of the circle should be limited within the tolerance zone formed by the circle with the diameter t concentric with the datum

Concentricity~The extraction axis should be limited within the tolerance zone formed by a cylindrical surface with a diameter t coaxial with the datum

Verticality~The extraction axis should be limited to the tolerance zone formed by the cylindrical surface with diameter t and perpendicular to the datum plane

The extraction surface shall define a tolerance zone formed between two parallel planes spaced apart by t and perpendicular to the datum axis