Machining must master the parts machining accuracy knowledge

2021/11/07



Machining accuracy is the actual size, shape, position of the surface of the part after processing three geometric parameters and the drawing requirements of the degree of compliance with the ideal geometric parameters. The ideal geometric parameters, for size, is the average size; for surface geometry, is the absolute circle, cylinder, plane, cone and straight line; for the mutual position between the surface, is the absolute parallel, perpendicular, coaxial, symmetry, etc.. The deviation value between the actual geometric parameters of the part and the ideal geometric parameters is called machining error.

1, the concept of machining accuracy
Machining accuracy is mainly used to produce the degree of product, machining accuracy and machining error are both terms to evaluate the geometric parameters of the machined surface. Machining accuracy is measured by tolerance level, the smaller the value of the level, the higher the accuracy; machining error is expressed by the value, the larger the value, the larger 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, of which IT01 means that the part has the highest machining accuracy, IT18 means that the part has the lowest machining accuracy, and generally IT7 and IT8 are medium machining accuracy levels.

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 diagram, it is considered to ensure the processing accuracy.

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 contains two major parts: the machining accuracy of the parts 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 larger the error, the lower the machining accuracy, the smaller the error, the higher the machining accuracy.
2, the related content of machining accuracy
(1) dimensional accuracy
Refers to the actual size of the processed part and the center of the tolerance zone of the size of the part in line with the degree.

(2) shape accuracy
Refers to the actual geometry of the machined surface of the part and the ideal geometry of the degree of conformity.

(3) Position accuracy
Refers to the actual position accuracy difference between the relevant surface of the machined parts.

(4) interrelationship
Usually in the design of machine parts and parts processing accuracy, should pay attention to the shape error control within the position tolerance, position error and should be less than the size tolerance. That is, precision parts or parts important surface, the shape accuracy requirements should be higher than the position accuracy requirements, position accuracy requirements should be higher than the size accuracy requirements.

3, adjustment method
(1) the process system to adjust
(2) reduce the machine tool error
(3) reduce the drive chain transmission error
(4) reduce tool wear
(5) reduce the process system force deformation
(6) reduce the thermal deformation of the process system
(7) reduce residual stress

4, the impact of causes
(1) machining principle error
Machining principle error refers to the error caused by the use of an approximate edge profile or an approximate transmission relationship for processing. Machining principle error is mostly found in threads, gears, complex surface machining.

In the processing, the general use of approximate processing, in the premise of theoretical error can meet the processing accuracy requirements to improve productivity and economy.

(2) Adjustment error
The adjustment error of the machine tool is the error caused by inaccurate adjustment.

(3) machine tool error
Machine tool error refers to the machine tool manufacturing errors, installation errors and wear. Mainly including machine guidance error, machine tool spindle rotation error, machine tool drive chain transmission error.

5, measurement methods
Machining accuracy according to the different processing accuracy content and accuracy requirements, using different measurement methods. Generally speaking, there are the following types of methods.

(1) according to whether the direct measurement of the measured parameters, can be divided into direct measurement and indirect measurement.
Direct measurement: direct measurement of the measured parameters to obtain the measured size. For example, using calipers, comparator measurement.

Indirect measurement: measuring the geometric parameters related to the measured size, after calculation to obtain the measured size.

Obviously, direct measurement is more intuitive, indirect measurement is more tedious. Generally when the measured size or with direct measurement can not meet the accuracy requirements, it has to use indirect measurement.

(2) according to the gage gauge reading value is directly expressed by the measured size of the value, 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 vernier calipers.

Relative measurement: the reading value only indicates the deviation of the measured size relative to the standard amount. If you use the comparator to measure the diameter of the shaft, you need to use the block to adjust the zero position of the instrument first, and then measure, the measured value is the difference between the diameter of the shaft and the size of the block, which is the relative measurement. Generally speaking, the accuracy of relative measurement is higher, but the measurement is more troublesome.

(3) According to the measured surface and the measuring head of the gauge whether contact, divided into contact measurement and non-contact measurement.
Contact measurement: the measuring head and the contacted surface contact, and the presence of mechanical action of the 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 measurement force on the measurement results. Such as the use of projection, light wave interferometry measurement.

(4) according to the number of parameters measured at one time, divided into single measurement and comprehensive measurement.
Single measurement: each parameter of the part under test is measured separately.

Comprehensive measurement: measurement reflects the comprehensive index of the relevant parameters of the part. For example, when measuring threads with a tool microscope, the actual median diameter of the thread, the half-angle error of the thread type and the cumulative error of the pitch can be measured separately.

Comprehensive measurement is generally more efficient, more reliable to ensure the interchangeability of parts, often used in the inspection of completed parts. Single measurement can determine the error of each parameter separately, and is generally used for process analysis, process inspection and measurement of the specified parameters.

(5) according to the role of measurement in the processing, divided into active measurement and passive measurement.
Active measurement: the workpiece in the process of measurement, the results are directly used to control the processing of the parts, so as to prevent and control the generation of scrap in a timely manner.

Passive measurement: the workpiece is measured after processing. This kind of measurement can only judge whether the machined parts are qualified, limited to the discovery and rejection of scrap.

(6) according to the measured parts in the measurement process in the state, divided into static measurement and dynamic measurement.
Static measurement: the measurement is relatively static. Such as micrometers to measure the diameter.

Dynamic measurement: measurement of the measured surface and the measuring head simulates the relative movement in the working state.

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