Coordinate system and tolerance basis in precision measurement

In precision measurement, the establishment of a coordinate system and the understanding of tolerances are the basis for correct measurement and result judgment.

Measurement coordinate system

In image measuring instruments and three-dimensional measuring instruments, the Cartesian coordinate system (X, Y, Z axis) is usually used. The main ways to establish a coordinate system are:

1. surface-line-point method: Take a plane of the workpiece as the datum plane (Z-axis direction), one edge as the datum line (X-axis direction), and a point as the origin.
2. three point method: Take three points on the datum plane to determine the plane, and take two points on the datum line to determine the direction.
3. best fit: Fit the best coordinate system through multiple measurement points.

Common Tolerance Types

1. Dimensional tolerance: Control the allowable variation range of linear dimensions, such as ±0.05mm.
2. Geometric tolerance: Including shape tolerance (straightness, flatness, roundness, cylindricity) and position tolerance (parallelism, perpendicularity, coaxiality, position, etc.).
3. surface roughness: Control the error of the micro-geometric shape of the workpiece surface.