Mechanical knowledge 123

Release time:

2023-05-20

Machining accuracy is the degree of conformity between the actual size, shape and position of the surface of the processed parts and the ideal geometric parameters required by the drawings. The ideal geometric parameters, 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.

Machining accuracy is the degree of conformity between the actual size, shape and position of the surface of the processed parts and the ideal geometric parameters required by the drawings. The ideal geometric parameters, 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 for evaluating the geometric parameters of the machined surface. The machining accuracy is measured by the tolerance level, the smaller the level value, the higher the accuracy; the machining error is expressed by the value, the greater the value, the greater the error. High machining accuracy means that the machining error is small, and vice versa.
There are a total of 20 tolerance grades from IT01,IT0,IT1,IT2,IT3 to IT18. Among them, IT01 indicates that the machining accuracy of the part is the highest, and IT18 indicates that the machining accuracy of the part is the lowest. Generally, IT7 and IT8 are medium grades of 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 two parts: the processing accuracy and the surface quality.
Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the parts are consistent 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 greater the error, the lower the machining accuracy, and the smaller the error, the higher the machining accuracy.
 
2. Relevant contents of machining accuracy
(1) Dimensional accuracy
Refers to the degree of conformity between the actual size of the processed part and the tolerance zone center of the part size.
(2) Shape accuracy
Refers to the degree to which the actual geometry of the machined part surface conforms to the ideal geometry.
(3) Position accuracy
Refers to the actual position accuracy difference between the relevant surfaces of the machined part.
(4) Mutual relations
Usually in the design of machine parts and parts processing accuracy, attention should be paid to the shape error control in the position tolerance, the position error should be less than the dimensional tolerance. That is, the shape accuracy requirements of precision parts or important surfaces of parts 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 thermal deformation of process system

(7) Reduce residual stress

4. Causes of impact
(1) error of machining principle
Machining principle error refers to the use of an approximate blade profile or an approximate transmission relationship for processing errors. The error of machining principle appears in thread, gear and complex surface machining.
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 the 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 machine tool guide rail, the rotation error of machine tool spindle and the transmission error of machine tool transmission chain.
5. Methods of measurement
Processing accuracy according to different processing accuracy content and accuracy requirements, using different measurement methods. In general, there are the following types of methods:
(1) According to whether the measured parameters are directly measured, they can be divided into direct measurement and indirect measurement.
Direct measurement: directly measure the measured parameter to obtain the measured size. For example, with a caliper, comparator measurement.
Indirect measurement: measure the geometric parameters related to the measured size, and obtain the measured size after calculation.
Obviously, direct measurement is more intuitive, indirect measurement is more cumbersome. Generally, when the measured size or the use of direct measurement can not meet the accuracy requirements, we have to use indirect measurement.
(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 indicates the deviation of the measured dimension 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 measure it. The measured value is the difference between the diameter of the side shaft and the gauge block size, which is the relative measurement. Generally speaking, the accuracy of relative measurement is relatively high, 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 effect of the measuring force. If you use a micrometer to measure the part.
 
Non-contact measurement: the measuring head is not in contact with the surface of the part to be measured, and non-contact measurement can avoid the influence of measurement force on the measurement result. Such as the use of projection method, light wave interferometry measurement.
(4) According to the number of parameters measured at one time, it is divided into single measurement and comprehensive measurement.
Single measurement: each parameter of the tested part is measured separately.
Comprehensive measurement: measuring the comprehensive index reflecting the relevant parameters of the part. For example, when measuring the thread with a tool microscope, the actual pitch diameter, half angle error and pitch cumulative error of the thread can be measured respectively.
Comprehensive measurement is generally more efficient, to ensure the interchangeability of parts is more reliable, often used in the inspection of finished parts. Single measurement can determine the error of each parameter separately, 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: measurement after the workpiece is processed. This kind of measurement can only judge whether the processed parts are qualified or not, and is limited to finding and eliminating waste products.
(6) According to the state of the measured parts in the measurement process, it is divided into static measurement and dynamic measurement.
Static measurement: The measurement is relatively static. Measure the diameter as a micrometer.
Dynamic measurement: when measuring the measured surface and the measuring head to simulate the relative motion in the working state.
The dynamic measurement method can reflect the situation when the part is close to the use state, which is the development direction of measurement technology.

Focus on hot spots

Application Prospect of Diamond in Aerospace Field

The main application of diamond powder

As a kind of super hard abrasive, diamond powder has incomparably superior grinding ability, which has been paid more and more attention by the industrial developed countries. The grinding ability of diamond powder includes the grinding ability of the workpiece and its own wear resistance and crushing resistance, depending on its microhardness, particle size, strength, particle size, and thermal and chemical stability. To sum up, diamond powder used as abrasive has three aspects; (1) Used in the manufacture of resin-bonded tools, metal-bonded tools, and plating tools. (2) for industrial, scientific and medical precision or polishing of various precision components. (3) as a fine abrasive for mold processing, gem polishing processing, gem bearing processing and manufacturing. In addition to the application of diamond powder as a grinding material, another major use is used as a functional material. For example, the use of its thermal properties and electrical properties. By mixing diamond fine powder into thermosetting resin polymer, cellulose, phenolic resin or ceramic sheet, a new material having improved thermal conductivity and reduced thermal expansion can be produced. The diamond powder mixed to the metal sheet, such as sheet or stainless steel sheet can be made with high thermal conductivity, low thermal expansion and light weight of the new sheet material, in the electronics industry can be made into high-density high-energy devices for thermal control. From the form of diamond used as grinding materials, according to the relevant data published figures, the world's industrial consumption of more than 60% of diamond powder is used to make diamond plaster, diamond powder mixture and diamond powder grinding fluid.

Ultra-precision polishing of precision instruments for the use of nanodiamonds

We add nano-diamond to the medium to make nano-diamond polishing liquid, which is now mainly divided into aqueous polishing liquid and oily polishing liquid. Because the nano-diamond polishing liquid nano-diamond particles super hard super hard, with it can remove the material surface small uneven, polished material surface roughness can be reduced to nano-level, so that the material surface finish is very high. At present, nano-diamond polishing liquid can be used in semiconductor silicon wafer polishing, computer head polishing, precision ceramic processing, gem polishing, optical glass polishing and other fields. Nowadays, due to the development of computers, people's requirements for computer storage capacity continue to increase, which makes us want to manufacture larger disks. In recent years, the storage capacity of computer disks has been increasing, but with the increase of storage capacity, the problem is that the gap between the disk and the head is getting smaller and smaller, which is close to 10NM. The surface roughness, scratches and impurity particles of the head and the disk will cause damage to the computer disk. As the distance between the disk and the head has reached the nanometer level, the micron-level polishing liquid has failed to meet the polishing requirements, so only nano diamond polishing liquid can meet the requirements of this aspect. We used nano-diamond polishing liquid to polish the computer magnetic head, and found that the surface smoothness of the magnetic head was greatly improved after polishing. The measured surface roughness at this time was 0.09~0.40nm, while the surface roughness of the magnetic head was 0.45~0.80nm after polishing with ordinary polishing liquid. Compared with nano-diamond polishing liquid, the effect of nano-diamond polishing liquid is much better, thus the quality of the computer magnetic head is also improved, and we can produce more precision, A computer disk with a larger storage capacity. It is worth noting that when polishing, because the computer and other electronic products most avoid corrosion, so be sure to control the pH of the polishing liquid, the study found that the PH value of the polishing liquid is more appropriate to control between 6.5 and 8. Since the content of anions Cl-and SO2-will determine the pH of the polishing solution, the content of Cl-and SO2-should be controlled when preparing the polishing solution for computer head polishing, and the PH value should be observed at the same time.

Characteristics of Shandong Metal Powder Hydraulic Press

Shandong metal powder hydraulic press is suitable for automatic control molding of powder dry pressing. It is widely used in electronics, powder metallurgy, soft magnetic industry, conductive dry pressing molding, chemical industry, automobile manufacturing, medicine, energy and environmental protection industry, toy industry, etc. Suitable dry-pressed powders include ferrous and non-ferrous metal powders, electronic ceramic powders, ferrites, rare earth powders, cemented carbide powders, PTFE, diamond and graphite powders, and the like. The hydraulic transmission system of Shandong metal powder hydraulic press is composed of power mechanism, control mechanism, actuator, auxiliary mechanism and working medium. The power mechanism usually uses an oil pump as the power mechanism, and is generally an integrated oil pump. One or more oil pumps are selected in order to meet the speed of movement of the actuator. Low pressure gear pump; medium pressure vane pump; high pressure piston pump. The pressure processing and forming of various plastic materials, such as the extrusion, bending, drawing of stainless steel plates and steel plates, and the cold pressing of metal parts, can also be used for the pressing of powder products, grinding wheels, bakelite and resin thermosetting products.

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