A comprehensive knowledge of part machining accuracy that must be mastered in machining

Processing accuracy refers to the degree to which the actual size, shape, and position of the machined part surface meet the ideal geometric parameters required by the drawing. The ideal geometric parameters, in terms of size, are the average size; For surface geometry, it refers to absolute circles, cylinders, planes, cones, and straight lines; For the mutual position between surfaces, it means absolute parallelism, verticality, coaxiality, symmetry, etc. The deviation between the actual geometric parameters of the part and the ideal geometric parameters is called machining error.
1. The concept of machining accuracy
Processing accuracy is mainly used to evaluate the degree of product production, and both processing accuracy and processing error are terms used to evaluate the geometric parameters of the processed surface. The machining accuracy is measured by tolerance level, and the smaller the level value, the higher the accuracy; The machining error is represented by numerical values, and the larger the value, the greater the error. High machining accuracy means small machining errors, and vice versa.
There are a total of 20 tolerance levels from IT01, IT0, IT1, IT2, IT3 to IT18. IT01 represents the highest machining accuracy of the part, while IT18 represents the lowest machining accuracy. Generally, IT7 and IT8 are of medium machining accuracy.
The actual parameters obtained by any machining method will not be absolutely accurate. From the perspective of the function of the part, as long as the machining error is within the tolerance range required by the part drawing, it is considered to ensure machining accuracy.
The quality of a machine depends on the machining quality of the parts and the assembly quality of the machine. The machining quality of the parts includes two major parts: machining accuracy and surface quality.
Mechanical machining accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of a part after machining match the ideal geometric parameters. The difference between them is called machining error. The magnitude of machining error reflects the level of machining accuracy. The larger the error, the lower the machining accuracy, and the smaller the error, the higher the machining accuracy.
2. Content related to machining accuracy
(1) Dimensional accuracy
The degree of conformity between the actual size of the processed part and the center of the tolerance zone of the part size.
(2) Shape accuracy
The degree to which the actual geometric shape of the processed part surface matches the ideal geometric shape.
(3) Position accuracy
The actual positional accuracy difference between the surfaces of the processed parts.
(4) Interrelationships
Usually, when designing machine parts and specifying the machining accuracy of parts, attention should be paid to controlling shape errors within positional tolerances, and positional errors should be smaller than dimensional tolerances. The shape accuracy requirement of precision parts or important surfaces of parts should be higher than the positional accuracy requirement, and the positional accuracy requirement should be higher than the dimensional accuracy requirement.
3. Adjustment method
(1) Adjusting the process system
(2) Reduce machine tool errors
(3) Reduce transmission chain transmission errors
(4) Reduce tool wear
(5) Reduce the stress deformation of the process system
(6) Reduce thermal deformation of the process system
(7) Reduce residual stress
4. Reason for impact
(1) Processing principle error
Processing principle error refers to the error generated by using approximate blade profiles or approximate transmission relationships for processing. The machining principle error often occurs in the machining of threads, gears, and complex surfaces.
In machining, approximate machining is generally used to improve productivity and economy, provided that the theoretical error can meet the machining accuracy requirements.
(2) Adjustment error
The adjustment error of a machine tool refers to the error caused by inaccurate adjustment.
(3) Machine tool error
Machine tool error refers to the manufacturing error, installation error, and wear of the machine tool. This mainly includes the guiding error of the machine tool guide rail, the rotation error of the machine tool spindle, and the transmission error of the machine tool transmission chain.
5. Measurement method
The machining accuracy adopts different measurement methods based on different machining accuracy content and accuracy requirements. Generally speaking, there are several types of methods:
(1) According to whether the measured parameters are directly measured, they can be divided into direct measurement and indirect measurement.
Direct measurement: Directly measure the measured parameters to obtain the measured size. For example, measuring with calipers and comparators.
Indirect measurement: measuring geometric parameters related to the measured size, and obtaining the measured size through calculation.
Obviously, direct measurement is more intuitive, while indirect measurement is more cumbersome. Generally, when the measured size or direct measurement cannot meet the accuracy requirements, indirect measurement has to be used.
(2) According to whether the reading value of the measuring instrument directly represents the value of the measured size, it can be divided into absolute measurement and relative measurement.
Absolute measurement: The reading value directly represents the size of the measured dimension, such as using a vernier caliper for measurement.
Relative measurement: The reading value only represents the deviation of the measured size from the standard quantity. If using a comparator to measure the diameter of a shaft, it is necessary to first adjust the zero position of the instrument with a measuring block, and then proceed with the measurement. The measured value is the difference between the diameter of the side shaft and the size of the measuring block, which is called relative measurement. Generally speaking, the accuracy of relative measurement is higher, but measurement is more complicated.
(3) According to whether the measured surface is in contact with the measuring head of the measuring tool, it can be divided into contact measurement and non-contact measurement.
Contact measurement: The measuring head is in contact with the surface being contacted and there is a measuring force acting mechanically. If measuring parts with a micrometer.
Non contact measurement: The measuring head is not in contact with the surface of the measured part, and non-contact measurement can avoid the influence of measurement force on the measurement results. Such as using projection method, optical interference method for measurement, etc.
(4) According to the number of parameters measured in one measurement, it can be divided into single measurement and comprehensive measurement.
Single item measurement: measure each parameter of the tested part separately.
Comprehensive measurement: measures the comprehensive indicators that reflect the relevant parameters of the parts. When using a tool microscope to measure threads, the actual pitch diameter, half angle error of the thread shape, and cumulative error of the pitch can be measured separately.
Comprehensive measurement generally has high efficiency and is more reliable in ensuring the interchangeability of parts. It is commonly used for the inspection of completed parts. Single item measurement can determine the error of each parameter separately, and is generally used for process analysis, process inspection, and measurement of specified parameters.
(5) According to the role of measurement in the machining process, it can be divided into active measurement and passive measurement.
Active measurement: The workpiece is measured during the machining process, and the results are directly used to control the machining process of the part, thereby preventing the generation of waste in a timely manner.
Passive measurement: measurement performed after workpiece processing. This type of measurement can only determine whether the processed parts are qualified, and is limited to discovering and removing waste products.
(6) According to the state of the measured part during the measurement process, it can be divided into static measurement and dynamic measurement.
Static measurement: measures relative stillness. Measure the diameter with a micrometer.
Dynamic measurement: During measurement, the measured surface moves relative to the measuring head in a simulated working state.
The dynamic measurement method can reflect the condition of the parts close to their usage state, which is the development direction of measurement technology.