Knowledge Of Machining Accuracy Of Machined Parts

Machining accuracy is the degree to which the actual size, shape and position of the machined part surface conform to the ideal geometric parameters required by the drawing. The ideal geometric parameter, for size, is the average size. For surface geometry, it means absolute circle, cylinder, plane, cone, straight line, etc. The mutual positions between surfaces are absolutely parallel, vertical, coaxial, symmetric, etc. The deviation between the actual geometric parameters and the ideal geometric parameters of a part is called machining error.

Concept Of Machining Accuracy

Machining accuracy is mainly used for the degree of product production. Both machining accuracy and machining error are terms for evaluating geometric parameters of machined surfaces. Machining accuracy is measured by tolerance grade. The smaller the grade value, the higher the accuracy. The machining error is expressed by 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 classes from IT01, IT0, IT1, IT2, IT3 to IT18, of which IT01 represents the highest machining accuracy of the part, and IT18 represents the lowest machining accuracy of the part. Generally, IT7 and IT8 are of medium machining accuracy.

The actual parameters obtained by any processing method will not be absolutely accurate. From the function of the part, as long as the processing error is within the tolerance range required by the part drawing, it is considered that the processing accuracy is 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 the parts includes the processing accuracy and surface quality of the parts.

Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of a part after machining conform to the ideal geometric parameters. The difference between them is called machining error. The machining error reflects the machining accuracy. The greater the error, the lower the machining accuracy, and the smaller the error, the higher the machining accuracy.

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Type Of Machining Accuracy

(1) Dimensional Accuracy

It refers to the conformity between the actual size of the part after machining and the tolerance zone center of the part size.

(2) Shape Accuracy

It refers to the degree to which the actual geometric shape of the machined part surface conforms to the ideal geometric shape.

(3) Position Accuracy

It refers to the actual position accuracy difference between the relevant surfaces of the parts after machining.

(4) Interrelationship

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 size tolerance. That is, the shape accuracy of precision parts or important surfaces of parts shall be higher than the position accuracy, and the position accuracy shall be higher than the dimension accuracy.

Adjustment Method

(1) Adjust the process system

(2) Reduce machine error

(3) Reduce transmission error of transmission chain

(4) Reduce tool wear

(5) Reduce stress deformation of process system

(6) Reduce thermal deformation of process system

(7) Reduce residual stress

Common Error Types

(1) Machining Principle Error

The error of machining principle refers to the error caused by machining with approximate blade profile or approximate transmission relationship. Machining principle errors often 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 requirements of processing accuracy.

(2) Adjustment Error

The adjustment error of the machine tool refers to the error caused by inaccurate adjustment.

(3) Machine Error

Machine tool error refers to manufacturing error, installation error and wear of machine tools. It mainly includes the guide error of the machine tool guide, the rotation error of the machine tool spindle, and the transmission error of the machine tool transmission chain.

Measuring Method

The machining accuracy adopts different measurement methods according to different machining accuracy contents 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 parameters to obtain the measured dimensions. For example, measure with caliper and comparator.

Indirect Measurement:

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

Obviously, direct measurement is more intuitive, while indirect measurement is more cumbersome. Generally, when the measured size or direct measurement cannot meet the accuracy requirements, indirect measurement must be adopted.

(2) It can be divided into absolute measurement and relative measurement according to whether the reading value of the measuring instrument directly represents the value of the measured size.

Absolute Measurement:

Reading the value directly indicates the size of the measured dimension, such as measuring with vernier caliper.

Relative Measurement:

Reading the value only indicates the deviation of the measured dimension from the standard quantity. If a comparator is used to measure the diameter of the shaft, first adjust the zero position of the instrument with a gauge block, and then measure. The measured value is the difference between the diameter of the shaft to be side and the size of the gauge block, which is called relative measurement. Generally speaking, the relative measurement accuracy is higher, but the measurement is more troublesome.

(3) It can be divided into contact measurement and non-contact measurement according to whether the measured surface contacts the measuring head of the measuring instrument.

Contact Measurement:

The measuring head is in contact with the contact surface, and there is a measuring force with mechanical effect. For example, use a micrometer to measure parts.

Non Contact Measurement:

The measuring head does not contact the surface of the measured part. Non contact measurement can avoid the influence of measuring force on the measurement results. For example, projection method and light wave interference method are used for measurement.

(4) According to the number of parameters measured at a time, it can be divided into single measurement and comprehensive measurement.

Single Measurement:

Measure each parameter of the measured part separately.

Comprehensive measurement: 

Measure the comprehensive indicators reflecting the relevant parameters of parts. 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 respectively.

The general efficiency of comprehensive measurement is relatively high, and it is more reliable to ensure the interchangeability of parts. It is often used for the inspection of finished parts. Individual measurement can determine the error of each parameter, which is generally used for process analysis, process inspection and measurement of specified parameters.

(5) According to the role of measurement in the processing, it can be divided into active measurement and passive measurement.

Active Measurement:

The workpiece is measured during the processing, and the results are directly used to control the processing of parts, so as to timely prevent the generation of waste products.

Passive Measurement:

Measurement after workpiece processing. This kind of measurement can only judge whether the machined parts are qualified or not, and is only limited to finding and removing waste products.

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

Static Measurement:

The measurement is relatively static. For example, the micrometer measures the diameter.

Dynamic Measurement:

relative movement between the measured surface and the measuring head in the simulated working state during measurement.

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

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