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# How to Calculate Coaxiality by Using CMM

Nov 16, 2017

Coaxiality is a common kind of tolerance in mechanical product testing. It is indicated that the axis and axis of the part, hole and hole, the axis and the hole, it can also be understood as: control the deviation degree of the actual axis and the reference axis. In measurement, coaxiality is often encountered in the measurement, the commonly used measuring equipment mainly is 3D coordinate measuring machine(CMM), CMM is recognized as precision testing equipment with better form error.

The method of measuring the Coaxiality by using coordinate measuring machine are the common axis-line method, the straightness method and the distance method, the common axis method is the most widely used method

1.  Common axis-line method

Measuring multiple circular cross section on the measured element and reference elements, then insert the round circle constructing a 3D line, as a common axis, the diameter of each circle can be inconsistent, and then calculate the benchmark measured cylindrical columns and the common axis of coaxiality, take the maximum as the coaxiality of the parts. This common axis is similar to a simulated mandrel, so the measurement of the common axis-line method is the closest to the actual assembly process of the parts.

2.  Straightness method

The circle of multiple cross sections is measured on the elements and reference elements, and then selects these elements to construct a 3D straight line. The coaxiality is approximately twice as the degree of straightness. The collected circles are best whole-circle measured, and if they are measured in a sector, the error calculated by measurement software can be large.

3.  Distance method

The coaxiality is twice as the maximum distance between the measured element and the base element axis. When the maximum distance between the measured element and the reference element is calculated, multiply it by 2. The distance method is calculated by projecting maximum distance onto a plane at, so this plane is better perpendicular to the axis used as the benchmark. The condition is more suitable for measuring concentricity.