CNC machining accuracy is the degree of conformity between the actual size, shape and position of the surface of the part after machining and the ideal geometric parameters required by the drawing.
The ideal geometric parameter for the size is the average size; for the surface geometry, it is the absolute circle, cylinder, plane, cone and straight line, etc.; for the mutual position between the surfaces, it is the absolute parallel, vertical, coaxial, symmetrical, etc. The deviation of the actual geometric parameters of the part from the ideal geometric parameters is called the machining error.
There are different accuracy measurement methods of CNC precision machining according to different machining accuracy content and accuracy requirements. Generally speaking, there are the following types of methods:
Direct measurement: directly measure the measured parameter to obtain the measured size. For example, measure with calipers and comparators.
Indirect measurement: measure the geometric parameters related to the measured size, and obtain the measured size through calculation.
Obviously, the direct measurement of CNC machining is more intuitive, and the indirect measurement is more cumbersome. Generally, when the measured size or the result of direct measurement cannot meet the accuracy requirements, indirect measurement has to be used.
Absolute measurement: the reading value directly indicates the size of the measured size, such as measuring with a vernier caliper.
Relative measurement: the reading value only represents the deviation of the measured size relative to the standard quantity.
If a comparator is used to measure the diameter of the shaft, the zero position of the instrument should be adjusted with a measuring block first, and then the measurement can be carried out. The measured value is the difference between the diameter of the measured shaft and the size of the measuring block, which is relative measurement. Generally speaking, the results of relative measurement of CNC machining are more accurate, but the measurement is more troublesome.
Contact measurement: the measuring head is in contact with the contacted surface, and there is a measurement force which may affect the results, such as measuring parts with a micrometer.
Non-contact measurement: the measuring head is not in contact with the surface of the measured part, and the non-contact measurement can avoid the influence of the measurement force on the measurement results.
For example, the use of projection method, optical interference method and so on.
Single measurement: measure each parameter of the measured part separately.
Comprehensive measurement: measure the comprehensive index reflecting the relevant parameters of the part.
For example, when the thread with a tool microscope is measured, the actual pitch diameter of the thread, the half-angle error of the tooth profile and the cumulative error of the pitch can be measured separately.