Coaxiality (◎) is used to control the degree of different axes between the measured axis and the reference axis which should be coaxial in theory. Because the different points between the measured axis and the reference axis may appear in all directions of space, the tolerance zone is a cylinder with the reference axis as the axis, and the tolerance value is the diameter of the cylinder, and the symbol "φ" is always added before the tolerance value.

The purpose of coaxiality:

1, accurately measure roundness, concentricity, circumferential runout and section difference of shaft parts;

2. Accurate measurement of the parameters of the outer circle and the inner circle of shaft parts at the same time;

3. Simultaneous and accurate measurement of multi-point parameters of shaft parts;

4, rapid measurement, fault plane, inner circle and outer circle can be measured at the same time.

Extended data:

In the national standard, the definition of coaxiality tolerance zone refers to the area within the cylindrical surface with a diameter tolerance of t and coaxial with the reference axis.

It has the following three control elements: ① axis and axis; (2) shaft and male shaft; (3) the center and the center.

Therefore, the main factors affecting coaxiality are the central position and axial direction of the measured element and the reference element, especially the axial direction. If two cross-sectional circles are measured on a reference cylinder, the connecting line is used as the reference axis.

Baidu encyclopedia-coaxiality

Baidu encyclopedia-form and position tolerance