Machining accuracy is the degree to which the actual size, shape, and position of the surface of the machined parts conform to the ideal geometric parameters required by the drawings. The ideal geometric parameters, in terms of size, are the average size; in terms of surface geometry, they are circles, cylinders, planes, cones, straight lines, etc.; in terms of mutual positions between surfaces, they are parallel and perpendicular. , coaxial, symmetrical, etc. The deviation of the actual geometric parameters of the part from 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 terms used to evaluate the geometric parameters of the machined surface. The machining accuracy is measured by the tolerance level. The smaller the level value is, the higher the precision is; the machining error is represented by a numerical value, and the larger the numerical value is, the greater the error is. High machining accuracy means small machining errors and vice versa.
There are 20 tolerance grades from IT01, IT0, IT1, IT2, IT3 to IT18. IT01 indicates that the machining accuracy of the part is high, and IT18 indicates that the machining accuracy of the part is low. Generally, IT7 and IT8 are medium machining accuracy. level.
The actual parameters obtained by any machining method will not be accurate. From the function of the part, as long as the machining error is within the tolerance range required by the part drawing, the machining accuracy is considered.
The quality of the machine depends on the machining quality of the parts and the assembly quality of the machine. The machining quality of the parts includes 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 are in line with the ideal geometric parameters. The difference between them is called machining error. The size 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. Contents related to machining accuracy
(1) Dimensional accuracy
Refers to 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
Refers to the degree of conformity between the actual geometry of the processed part surface and the ideal geometry.
(3) Position accuracy
Refers to the actual position accuracy difference between the relevant surfaces of the parts after machining.
(4) Interrelationship
Usually, 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 smaller than the dimensional tolerance. That is to say, for precision parts or important surfaces of parts, the shape accuracy requirements 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 processing system
(2) Reduce machine tool error
(3) Reduce the transmission error of the transmission chain
(4) Reduce tool wear
(5) Reduce the force deformation of the processing system
(6) Reduce the thermal deformation of the processing system
(7) Reduce residual stress
4. Reasons for influence
(1) Processing principle error
Machining principle error refers to the error caused by the use of an approximate blade profile or an approximate transmission relationship for processing. Machining principle errors mostly occur in the machining of threads, gears, and complex surfaces.
In processing, approximate processing is generally used to improve productivity and economy under the premise that the theoretical error can meet the requirements of processing accuracy.
(2) Adjustment error
The adjustment error of the 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. It mainly includes the guide 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.