Machining accuracy knowledge that must be mastered in machining

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 parameter, for the size, is the average size; for the surface geometry, it is the absolute circle, cylinder, plane, cone and straight line, etc.; for the mutual position between the surfaces, it is the absolute 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 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 the highest machining accuracy of the part, and IT18 indicates that the machining accuracy of the part is the lowest. Generally speaking, IT7 and IT8 have medium machining accuracy. level.


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 that the machining accuracy is guaranteed.


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 surface of the machined part 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 process 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 process system

(6) Reduce the thermal deformation of the process 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.


5. Measurement method

Machining accuracy According to different machining accuracy content and accuracy requirements, different measurement methods are used. 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, measure with calipers and comparators.


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 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 indicates the size of the measured size, such as measuring with a vernier caliper.


Relative measurement: The reading value only represents the deviation of the measured size relative to the standard quantity. If a comparator is used to measure the diameter of the shaft, it is necessary to adjust the zero position of the instrument with a measuring block first, and then measure. The measured value is the difference between the diameter of the side shaft and the size of the measuring block, which is 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 to be contacted, 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, and the non-contact measurement can avoid the influence of the measurement force on the measurement results. Such as the use of projection method, light wave interferometry and so on.


(4) According to the number of parameters measured at one time, 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 measuring the thread with a tool microscope, the actual pitch diameter of the thread, the half-angle error of the tooth profile and the cumulative error of the pitch can be measured separately.


Comprehensive measurement is generally more efficient and more reliable for ensuring 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 the 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: Measurements taken after the workpiece is machined. This kind of measurement can only judge whether the workpiece is qualified or not, and is limited to finding and rejecting waste products.


(6) According to the state of the measured part during 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 measurement, the surface to be measured and the measuring head move relative to the simulated working state.


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

Post time: Aug-26-2022