Processing accuracy refers to the degree to which the actual size, shape and position of the three geometric parameters of the processed part surface conform to the ideal geometric parameters required by the drawing. The ideal geometric parameters, in terms of size, are average size; in terms of surface geometry, these are absolute circles, cylinders, planes, cones, straight lines, etc. ; in terms of mutual positions between surfaces, these are absolute parallelism; , vertical, coaxial, symmetrical, etc. The deviation between the actual geometric parameters of the part and the ideal geometric parameters is called machining error.
1. The concept of processing precision
Machining precision is mainly used to manufacture products. Machining accuracy and machining error are two terms for evaluating the geometric parameters of the machined surface. Machining accuracy is measured by the tolerance grade. The smaller the note value, the higher the accuracy; the larger the numerical value, the greater the error. High processing accuracy means small processing errors, and vice versa.
There are a total of 20 tolerance levels ranging from IT01, IT0, IT1, IT2, IT3 to IT18. Among them, IT01 represents the highest machining accuracy of the workpiece, IT18 represents the lowest machining accuracy, and generally IT7 and IT8 have average machining accuracy. . level.
The actual parameters obtained by any processing method will not be absolutely accurate from the perspective of part operation, as long as the processing error is within the tolerance range required by the part drawing, the processing accuracy is considered guaranteed.
The quality of the machine depends on the processing quality of the parts and the assembly quality of the machine. The processing quality of parts includes the processing precision and surface quality of parts.
Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the workpiece after processing are consistent with the ideal geometric parameters. The difference between them is called a machining error. The size of the machining error reflects the level of machining precision. The larger the error, the lower the processing accuracy, and the smaller the error, the higher the processing accuracy.
2. Related content on machining accuracy
(1) Dimensional accuracy
It refers to the degree to which the actual size of the part being processed matches the center of the part size tolerance zone.
(2) Shape accuracy
It refers to the degree to which the actual geometric shape of the machined part surface matches the ideal geometric shape.
(3) Position accuracy
Refers to the difference in actual position accuracy between the relevant surfaces of the processed parts.
(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 less than the dimensional tolerance. That is, for precision parts or large areas 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. Setting method
(1) Adjust the process system
(2) Reduce machine tool errors
(3) Reduce transmission chain transmission errors
(4) Reduce tool wear
(5) Reduce stress deformation of process system
(6) Reduce thermal deformation of the treatment system
(7) Reduce residual stress
4. Causes of impact
(1) Error in processing principle
Machining principle error refers to the error caused by using an approximate blade profile or approximate transmission relationship for processing. Processing principle errors mainly occur in the processing of threads, gears and complex surfaces.
In processing, approximate processing is generally used to improve productivity and economy on the premise that the theoretical error can meet the processing accuracy requirements.
(2) Setting error
Machine tool setting error refers to the error caused by inaccurate setting.
(3) Machine tool error
Machine tool errors refer to manufacturing errors, installation errors, and wear and tear on machine tools. 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.
5. Measuring method
The processing precision adopts different measurement methods according to the processing precision content and precision requirements. Generally speaking, we distinguish the following types of methods:
(1) Depending on whether the measured parameter is directly measured, it can be divided into direct measurement and indirect measurement.
Direct measurement: directly measure the measured parameters to obtain the measured dimensions. For example, use calipers and dial gauges to measure.
Indirect measurement: measure the geometric parameters related to the measured size and obtain the measured size by calculation.
Obviously, direct measurement is more intuitive, while indirect measurement is more cumbersome. Generally, when measured size or direct measurement cannot meet the accuracy requirements, indirect measurement should be used.
(2) Depending on whether the value read from the measuring instrument directly represents the value of the measured size, it can be divided into absolute measurement and relative measurement.
Absolute measurement: The value read directly represents the size of the measured waist, such as measuring with a vernier caliper.
Relative measurement: the value read only indicates the deviation of the measured size from the standard quantity. If you use a dial gauge to measure the diameter of a shaft, you must first adjust the zero position of the instrument with a gauge block, and then measure the measured value is the difference between the side shaft diameter and the size of. the gauge block. This is a relative measurement. Generally speaking, the relative accuracy of measurements is higher, but measurement is more difficult.
(3) Depending on whether the measured surface is in contact with the measuring head of the measuring instrument, it is divided into contact measurement and non-contact measurement.
Contact measurement: the measuring head is in contact with the contacting surface and there is a mechanical measuring force. For example, use a micrometer to measure parts.
Non-contact measurement: The measuring head does not come into contact with the surface of the measured workpiece, and the non-contact measurement can avoid the influence of the measuring force on the measurement results. Such as using projection method, measuring light wave interference method, etc.
(4) According to the number of parameters measured at the same time, it is divided into single measurement and comprehensive measurement.
Single measurement: Each parameter of the tested part is measured separately.
Comprehensive measurement: Measure comprehensive indicators that reflect relevant room parameters. For example, when measuring threads with a tool microscope, the actual pitch diameter, profile half-angle error and cumulative pitch error can be measured respectively.
Comprehensive measurements are generally more efficient and reliable in ensuring part interchangeability and are often used for 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 treatment process, it is divided into active measurement and passive measurement.
Active measurement: The workpiece is measured during processing and the results are directly used to control the processing of the workpiece, preventing timely generation of waste.
Passive measurement: Measurement carried out after machining the part. This type of measurement can only determine whether the processed parts are qualified and is limited to finding and rejecting scrap.
(6) According to the condition of the measured workpiece during the measurement process, it is divided into static measurement and dynamic measurement.
Static measurement: The measurement is relatively stationary. Like a micrometer measuring diameter.
Dynamic measurement: During measurement, the measured surface and the measuring head move relative to each other in a simulated operating state.
Dynamic measurement methods can reflect the condition of parts close to use and are the development direction of measurement technology.
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