The machining accuracy is the degree to which the actual size, shape and position of the surface of the part after processing are consistent with the ideal geometric parameters required by the drawing. The ideal geometric parameter, in terms of size, is the average size; For surface geometry, it is an absolute circle, a cylinder, a plane, a cone, a straight line, etc.; For the mutual position between surfaces, it is absolutely parallel, perpendicular, coaxial, symmetrical, etc. The deviation between the actual geometric parameters of the part and the ideal geometric parameters is called the machining error.
1. The concept of machining accuracy
Machining accuracy is mainly used to produce products, and machining accuracy and machining error are both terms for evaluating the geometric parameters of the machined surface. The machining accuracy is measured by the tolerance grade, and the smaller the grade value, the higher the accuracy; The machining error is expressed as a numerical value, and the larger the value, the greater the error. High machining accuracy means that the machining error is small, and vice versa.
There are 20 tolerance grades from IT01, IT0, IT1, IT2, IT3 to IT18, of which IT01 indicates that the machining accuracy of the part is the highest, and IT18 indicates that the machining accuracy of the part is the lowest, and generally IT7 and IT8 are medium levels of machining accuracy.
The actual parameters obtained by any machining method will not be absolutely accurate, from 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 the machining accuracy.
The quality of the machine depends on the processing quality of the parts and the assembly quality of the machine, and the processing quality of the parts includes two parts: the machining accuracy and the surface quality of the parts.
Machining accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of the part after machining conform to the ideal geometric parameters. The difference between them is called machining error. The magnitude of the 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. The relevant content of machining accuracy
(1) Dimensional accuracy
It refers to the degree to which the actual size of the machined part is consistent with the center of the tolerance zone of the part size.
(2) Shape accuracy
Refers to the degree to which the actual geometry of the machined part surface conforms to the ideal geometry.
(3) Position accuracy
Refers to the difference in actual position accuracy between the relevant surfaces of the part after machining.
(4) Mutual relationship
Generally, when designing machine parts and specifying the machining accuracy of parts, attention should be paid to controlling the shape error within the position tolerance, and the position error should be less than the dimensional tolerance. That is, the shape accuracy requirements of precision parts or important surfaces of parts should be higher than the position accuracy requirements, and the position accuracy requirements should be higher than the dimensional accuracy requirements.
3. Adjustment method
(1) Adjust the process system
(2) Reduce the error of the machine tool
(3) Reduce the transmission error of the transmission chain
(4) Reduce tool wear
(5) Reduce the force deformation of the process system
(6) Reduce the thermal deformation of the process system
(7) Reduce residual stress
4. Influencing causes
(1) Error in processing principle
The error of the machining principle refers to the error caused by the use of an approximate blade profile or an approximate transmission relationship for machining. Machining principle errors mostly occur in threads, gears, and complex surface machining.
In machining, approximate machining is generally adopted, and productivity and economy are improved under the premise that the theoretical error can meet the requirements of machining accuracy.
(2) Adjustment error
The adjustment error of the machine tool refers to the error caused by the inaccurate adjustment.
(3) Machine tool error
Machine tool error refers to the manufacturing error, installation error and wear and tear of the machine tool. It mainly includes the guidance error of the guide rail of the machine tool, the rotation error of the spindle of the machine tool, and the transmission error of the transmission chain of the machine tool.
5. Measurement method
Machining accuracy adopts different measurement methods according to different machining accuracy content and accuracy requirements. In general, there are the following types of methods:
(1) According to whether the measured parameters are directly measured, it can be divided into direct measurement and indirect measurement.
Direct Measurement: Measure the measured parameters directly to obtain the measured dimensions. For example, it is measured with calipers and comparators.
Indirect measurement: Measure the geometric parameters related to the measured size, and calculate the measured size.
Obviously, direct measurements are more intuitive, while indirect measurements are more cumbersome. Generally, when the measured size or direct measurement does not 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 size, e.g. with a vernier caliper.
Relative measurement: The reading value only represents the deviation of the measured size from the standard. If the diameter of the shaft is measured with a comparator, the zero position of the instrument needs to be adjusted with a measuring block first, and then the measurement is carried out, and the measured value is the difference between the diameter of the side shaft and the size of the measuring block, which is the relative measurement. Generally speaking, the relative measurement accuracy is higher, but the measurement is more troublesome.
(3) According to whether the measured surface is in contact with the measuring head of the measuring instrument and measuring instrument, it is divided into contact measurement and non-contact measurement.
Contact measurement: The measuring head is in contact with the surface to be contacted, and there is a measuring force that acts mechanically. For example, use a micrometer to measure parts.
Non-contact measurement: The measuring head is not in contact with the surface of the part to be measured, and the non-contact measurement can avoid the influence of the measuring force on the measurement result. For example, the use of projection method, light wave interferometry measurement, etc.
(4) According to the number of parameters of a measurement, it is divided into single measurement and comprehensive measurement.
Single measurement: Each parameter of the part under test is measured separately.
Comprehensive measurement: Measure the comprehensive index that reflects the relevant parameters of the part. For example, when measuring the thread with a tool microscope, the actual middle diameter of the thread, the half-angle error of the thread and the cumulative error of the pitch can be measured separately.
The comprehensive measurement is generally more efficient, more reliable to ensure the interchangeability of parts, and is often used in the inspection of finished parts. A single 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 processing process, it is 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, so as to prevent the generation of scrap in time.
Passive measurement: Measurement of the workpiece after machining. This measurement can only determine whether the workpiece is good or not, and is limited to the detection and rejection of scrap.
(6) According to the state of the measured parts in the measurement process, it is divided into static measurement and dynamic measurement.
Static measurement: The measurement is relatively stationary. Such as a micrometer to measure the diameter.
Dynamic measurement: During the measurement, the measured surface moves relative to the measuring head in the simulated working state.
Dynamic measurement methods can reflect the situation of parts close to use, which is the development direction of measurement technology.