Current situation and development prospect of the

Current situation and development prospect of precision machining technology

1 Introduction to precision machining technology

at present, precision machining refers to the machining technology with machining accuracy of 1~0.1 m and surface roughness of ra0.1~0.01 m, but this boundary is constantly changing with the progress of machining technology. Today's precision machining may be tomorrow's general machining. The problems to be solved in precision machining are: first, machining accuracy, including geometric tolerance, dimensional accuracy and surface condition. Sometimes, whether there are surface defects is also the core of this problem; The second is processing efficiency. Some processing can achieve better processing accuracy, but it is difficult to achieve high processing efficiency. Precision machining should include micro machining, ultra micro machining, finishing and other processing technologies

2 several commonly used precision machining methods and characteristics

traditional precision machining methods include cloth wheel removal of prosthesis, emptying of sweat and moisture accumulated in the socket part, polishing, abrasive belt grinding, ultra-fine cutting, fine grinding, honing, grinding, ultra-fine grinding and polishing technology, magnetic particle finishing, etc

abrasive belt grinding is to use the blended cloth with abrasive as the abrasive tool to process the workpiece, which belongs to the category of coated abrasive grinding. It has the characteristics of high productivity, good surface quality, wide range of use and so on. Foreign countries have made great achievements in abrasive belt materials and manufacturing technology. They have abrasive belt series suitable for different occasions, and have produced universal and special abrasive belt grinders, and the degree of automation has been continuously improved (there are fully automatic and self-adaptive control abrasive belt grinders). However, there are few varieties of abrasive belts in China, and the quality needs to be improved. The machine tool is still in the stage of transformation

precision cutting is also known as diamond tool cutting (SPDT). High precision machine tools and single crystal diamond tools are used for cutting. The growth of main exports is stable and promising. It should be used for the precision processing of soft metals such as copper and aluminum that are not suitable for grinding, such as magnetic drums for computers, magnetic disks and metal reflectors for high-power lasers, which are 1-2 levels higher than the general cutting accuracy. For example, the cylindricity of the plug hole of the hydraulic motor rotor column processed by precision turning is 0.5~1 m, and the dimensional accuracy is 1~2 m; The surface roughness of the infrared reflector is ra0.01~0.02 m, and it also has good optical properties. In terms of cost, the cost of optical mirrors processed by precision cutting is about half or a fraction of that of products processed by chromium plating and grinding in the past

but many factors affect the effect of precision cutting, so it is not easy to achieve the desired effect. At the same time, diamond tools wear faster when cutting hard materials. For example, the wear speed when cutting ferrous metals is 104 times faster than that when cutting copper, and the surface roughness and geometric accuracy of the machined workpiece are not ideal

ultra precision grinding is a micro grinding process carried out on a precision grinder with a precision trimmed grinding wheel. The amount of metal removal can be sub micron level or even smaller, which can achieve high dimensional accuracy, form and position accuracy and low surface roughness value. The dimensional accuracy is 0.1~0.3 m, the surface roughness is ra0.2~0.05 m, and the efficiency is high. It has a wide range of applications, from soft metals to hard cutting materials such as quenched steel, stainless steel and high-speed steel, and hard and brittle non-metallic materials such as semiconductors, glass and ceramics. Almost all materials can be processed by grinding

however, after grinding, the metallographic structure of the machined surface will change under the action of grinding force and grinding heat, and it is easy to produce defects such as work hardening, quenching hardening, thermal stress layer, residual stress layer and grinding cracks

the honing head composed of oilstone sand bars for honing moves back and forth along the surface of the workpiece under a certain pressure. The surface roughness after processing can reach ra0.4~0.1 m, preferably Ra0.025 m. It is mainly used for processing cast iron and steel, not for processing non-ferrous metals with low hardness and good toughness

precision grinding and polishing is a processing method that enables the workpiece to reach the required size and accuracy through the mutual mechanical friction between the workpiece and tool, abrasive and processing fluid, workpiece and grinding tool. Precision grinding and polishing can achieve the precision and surface roughness that cannot be achieved by other processing methods for metal and non-metal workpieces. The roughness of the ground surface RA ≤ 0.025 M. the processing metamorphic layer is very small, the surface quality is high, and the precision grinding equipment is simple. It is mainly used for the processing of flat surfaces, cylindrical surfaces, gear tooth surfaces and mating parts with sealing requirements, as well as gauges, gauge blocks, fuel injection nozzles Finishing of valve body and valve core

but the efficiency of precision grinding is low. For example, the dry grinding speed is generally 10~30m/min, and the wet grinding speed is 20~120m/min. The processing environment is strictly required. If there is large abrasive or foreign matter mixed, the surface will have scratches that are difficult to remove

polishing is a kind of micro machining that uses mechanical, chemical and electrochemical methods to carry out the specific use methods of the wear-resistant testing machine on the surface of the workpiece. It is mainly used to reduce the surface roughness of the workpiece. The commonly used methods are: manual or mechanical polishing, ultrasonic polishing, chemical polishing, electrochemical polishing and electrochemical mechanical composite processing

manual or mechanical polishing is to use a polisher coated with grinding paste to make relative movement with the workpiece surface under certain pressure, so as to realize the finishing of the workpiece surface. After machining, the surface roughness Ra of the workpiece is ≤ 0.05 M, which can be used for the polishing of plane, cylindrical surface, curved surface and mold cavity. The processing effect of manual polishing is related to the proficiency of the operator

ultrasonic polishing uses the end face of the tool to do ultrasonic vibration, and finishes hard and brittle materials through abrasive suspension. The machining accuracy is 0.01~0.02 m, and the surface roughness is ra0.1 M. Ultrasonic polishing equipment is simple, easy to operate and maintain. Tools can be made of soft materials without complex movement. It is mainly used to process hard and brittle materials, such as non-conductive non-metallic materials. When processing conductive hard metal materials, the productivity is low

chemical polishing is to oxidize the surface of the processed metal under certain conditions through oxidants such as nitric acid and phosphoric acid, so as to make the surface smooth and glossy. The chemical polishing equipment is simple and can process workpieces of various shapes with high efficiency. The machined surface roughness is generally RA ≤ 0.2 M. however, the corrosive liquid is harmful to human body and equipment and pollutes the environment, which needs to be properly handled. It is mainly used for bright finishing of stainless steel, copper, aluminum and their alloys

electrochemical polishing is a method to improve the surface brightness of parts by using electrochemical reaction to remove the micro unevenness left by cutting. It has higher productivity and less surface roughness than mechanical fixture polishing: generally it can reach Ra0.2 M. if the original surface is ra0.4~0.2 m, it can be increased to ra0.1~0.08 m after polishing. After processing, the workpiece has better physical and mechanical properties and long service life, but electrochemical polishing can only process conductive materials. With the development of electrochemical machining technology, a variety of new composite machining methods have been produced, such as ultra precision electrochemical grinding, electrochemical mechanical composite finishing, electrochemical ultra precision machining and so on. They are mainly aimed at reducing the surface roughness value of the workpiece, and the removal amount is very small, generally 0.01~0.1mm. For the outer circle with a surface roughness of ra0.8~1.6 m, the plane, inner hole and free-form surface can be processed to the mirror in one process, and the surface roughness ra0.05 m or even lower. Electrochemical machining is a precision machining method with very small machining units. In principle, the machining accuracy can reach the atomic level, so the machining accuracy has great potential. However, because the factors that affect its machining accuracy are not very clear at present, its machining shows a certain instability in practical application, which greatly limits its application in industrial production

3 development trend of ultra precision machining

the development of foreign precision machining technology began in the early 1970s, mainly concentrated in the United States, Japan, Britain and other countries, and achieved remarkable results in the mid-1980s. In 1977, the precision machine tool research professional committee of the Japanese precision engineering society proposed two levels of supplementary it-1 and it-2 for the machining accuracy standard of machine tools. The attached table is the content of the supplemented standard, which can be seen that the accuracy is much higher than the original it0 level. Attached table precision machine tool branch of precision society of Japan precision machining grade

precision

grade parts machine tool size

Precision Roundness cylindricity flatness surface

roughness spindle

jumping motion

straightness it22.500.71.251.250.20.71.25it11.250.30.630.630.070.30.63it00.750.20.380.050.20.38it- 10.300.120.250.030.120.25it it- 20.250.060.130.130.010.060.13

currently, precision machining can reach The machining accuracy is still far from the limit of machining. Some people abroad claim to have developed the processing method of removing units at the atomic level, but it has not been applied in actual production at present. In order to promote the development of precision machining technology, the following problems should be deeply studied and discussed

1. The machining method based on the new principle strives to develop the precision machining method with a very small machining unit, and its error must be dispersed below 1nm in the machining mechanism itself. At present, the processing methods of relatively small processing units mainly include elastic damage processing, chemical processing, ion beam processing, electron beam processing, plasma processing, etc. At present, the processing units of diamond cutting and diamond wheel precision grinding are large in terms of their processing mechanism

2. The development of precise mechanical mechanisms, whether processing devices or measuring devices, requires precise mechanical mechanisms, including guide rails, feed mechanisms and bearings. Ultra precision aerostatic guide rails are the best guide rails at present, and their straightness can reach (0.1~0.2) m/250mm. The straightness can be further improved through compensation technology, but it is not as rigid as hydrostatic guide rails. At the same time, since the air film thickness of the aerostatic guide rail is only about 10 m, pay attention to dust prevention during use. In addition, in the design of guide rail, multiple guide rails can be connected in parallel to homogenize the error of gas film. The systematic error of the rolling guide made of high elastic alloy and ruby is about 0.5 m, and the random error may exceed 0.1 M

at present, the accuracy of magnetic bearing spindle used in ultra precision machining is lower than that of aerostatic bearing spindle. The rotation accuracy of aerostatic bearing spindle can reach 0.05 M and 0.03 m abroad, but this still cannot meet the accuracy requirements of nano machining for spindle. In order to improve the rotation accuracy of the aerostatic bearing spindle, it is necessary to improve the rotation accuracy of the bearing. The rotation accuracy of the aerostatic bearing is affected by the roundness of the bearing parts and the air supply conditions. Due to the effect of the pressure film, the rotation accuracy of the bearing is 1/15~1/20 of the roundness of the bearing parts. Therefore, in order to obtain the rotation accuracy of 10nm, the roundness of the shaft and the shaft sleeve should reach 0.15~0.20 M. at the same time, for the uniformity of the gas outflow, For the spindle of nanometer scale, it mostly adopts

3 Develop high-precision testing system in the current ultra precision machining field, there are two main methods to measure the machining accuracy; Laser detection and grating detection, and grating is the most widely used. At present, the measurement accuracy of grating can reach nm level. For example, the grating system of Beijing photoelectric instrument research center can reach 0.1 M, the holographic grating system of Russia can reach 10nm, the resolution of LG100 grating system can reach 0.1 M, and the measurement range is 100mm

before developing testing instruments and control devices with small system error, high precision and high reliability