Machining Accuracy Knowledge That Must Know in Machining

Machining accuracy is the degree of conformity between the actual size, shape and position of the three geometric parameters of the surface of the processed part and the ideal geometric parameters required by the drawing. Ideal geometric parameters are the average size for size, for surface geometry, they are absolute circles, cylinders, planes, cones, and straight lines, for the mutual positions of surfaces, they are absolutely 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.

1. The Concept Of Machining Accuracy

Machining accuracy is mainly used for the degree of product production. Machining accuracy and machining error are both terms used to evaluate the geometric parameters of the machined surface. Machining accuracy is measured by tolerance grade, the smaller the grade value, the higher the accuracy. The machining error is expressed by a numerical value. The larger the numerical value, the greater the error. High machining accuracy means small machining error, and vice versa.

There are 20 tolerance levels from IT01, IT0, IT1, IT2, IT3 to IT18. If IT01 indicates the part has the highest machining accuracy, IT18 indicates the part has the lowest machining accuracy. Generally, IT7 and IT8 have medium machining accuracy level.

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 that the machining accuracy is guaranteed.

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The quality of the machine depends on the processing quality of the parts and the assembly quality of the machine. The machining quality of the parts includes two major parts, the machining accuracy and the surface quality.

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 size of the machining error reflects the level of machining accuracy. The greater the error, the lower the machining accuracy, and the smaller the error, the higher the machining accuracy.

2. Related Content Of 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 geometric shape of the surface of the processed part and the ideal geometric shape.

(3) Position Accuracy

Refers to the actual position accuracy difference between the relevant surfaces of the parts after processing.

(4) Mutual Relations

Generally, when designing machine parts and stipulating the machining accuracy of parts, attention should be paid to control the shape error within the position tolerance, and the position error should be less than the dimensional tolerance. That is, 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.

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3. Adjustment Method

(1) Adjust the process system

(2) Reduce machine tool error

(3) Reduce the transmission error of the transmission chain

(4) Reduce tool wear

(5) Reduce the stress and deformation of the process system

(6) Reduce the thermal distortion of the process system

(7) Reduce residual stress

4. Reasons For Impact

(1) Error Of Processing Principle

Machining principle error refers to the error produced by processing with approximate blade profile or approximate transmission relationship. The machining principle errors mostly appear in the machining of threads, gears, and complex curved surfaces.

In machining, approximate processing is generally used to improve productivity and economy on the premise that the theoretical error can meet the machining accuracy requirements.

(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. Mainly include the guide error of the machine tool, the rotation error of the spindle of the machine tool, and the transmission error of the machine tool transmission chain.

5. Measuring Method

The processing accuracy adopts different measurement methods according to different processing accuracy content and accuracy requirements. Generally speaking, there are the following types of methods:

(1) According to whether the measured parameter is directly measured, it can be divided into direct measurement and indirect measurement.

Direct measurement: directly measure the measured parameter to obtain the measured size. For example, use calipers and comparators to measure.

Indirect measurement: Measure the geometric parameters related to the measured size, and obtain the measured size through calculation.

Obviously, direct measurement is more intuitive, and indirect measurement is more cumbersome. Generally, when the measured size or direct measurement fails to meet the accuracy requirements, indirect measurement has to be used.

(2) According to whether the reading value of the measuring tool directly represents the value of the measured size, it can be divided into absolute measurement and relative measurement.

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 indicates the deviation of the measured size from the standard quantity. If you use a comparator to measure the diameter of the shaft, you need to adjust the zero position of the instrument with a gauge block first, and then perform the measurement. The measured value is the difference between the diameter of the side shaft and the size of the gauge block. This is a 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, it is divided into contact measurement and non-contact measurement.

Contact measurement: The measuring head is in contact with the surface being touched, and there is a mechanical measuring force. Such as measuring parts with a micrometer.

Non-contact measurement: The measuring head is not in contact with the surface of the measured part. Non-contact measurement can avoid the influence of the measuring force on the measurement result. Such as the use of projection method, light wave interferometry and so on.

(4) According to the number of measurement parameters, it is divided into single measurement and comprehensive measurement.

Single measurement: measure each parameter of the tested part separately.

Comprehensive measurement: measure the comprehensive index reflecting the relevant parameters of the part. For example, when using a tool microscope to measure the thread, the actual pitch diameter of the thread, the half-angle error of the tooth profile, and the cumulative error of the thread pitch can be measured separately.

Comprehensive measurement is generally more efficient and more reliable to ensure the interchangeability of parts, and is often used for inspection of finished parts. 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 processing, and the result is directly used to control the processing of the part, so as to prevent the generation of waste in time.

Passive measurement: the measurement performed after the workpiece is processed. This kind of measurement can only judge whether the processed part is qualified, and is limited to discovering and rejecting waste products.

(6) According to the state of the measured part in the measurement process, it is divided into static measurement and dynamic measurement.

Static measurement: The measurement is relatively static. Such as a micrometer to measure the diameter.

Dynamic measurement: The measured surface and the measuring head move relative to each other in the simulated working state during measurement.

The dynamic measurement method can reflect the situation of the part close to the use state, which is the development direction of measurement technology.

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