ITS Technology Knowledge: Several Main Factors Affecting the Surface Quality of Ultra-precision Machining

Influence of ultra-precision lathe and the influence of cutting tools.

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Abstract: This paper analyzes and discusses several key technical problems involved in obtaining the stable ultra-precision surface of the processed material, including the influence of ultra-precision machining machine tools, cutting tools and environmental conditions on the surface, as well as the analysis and testing of ultra-high precision surface. The mechanism of plus one and the error compensation of each link are briefly described, and the future of this research field is prospected.

 

Keywords: ultra-precision machining, diamond cutting tools, submicron lathes, surface morphology detection

 

1. Introduction

Precision machining and ultra-precision machining technology is the frontier of modern manufacturing technology, which marks the level of machining in an industrial country. It is a new process technology developed in the 1960s with the development of cutting-edge technologies of electronics, computers, aerospace and lasers. In the past 30 years, using modern advanced technologies and processes, the machining accuracy has been improved by an order of magnitude. At present, it is changing from micron Submicron precision is advancing towards nanometer level.

 

Precision and ultra-precision machining mainly include three fields.

 

1.1 Ultra-precision cutting: for example, ultra-precision turning with diamond tools can process non-ferrous or non-metallic parts requiring high surface quality and high shape accuracy. Such as the plane mirror of laser or infrared system, large parabolic mirror of celestial telescope, disk, optical disc, non-ferrous metal valve core, etc

 

1.2 Ultra-precision cutting and abrasive processing: mainly used for processing servo valves with high dimensional and shape accuracy, air bearing spindles, gyro bearings, coating surface processing of high-density hard disks, and processing of large-scale integrated circuit chips.

 

1.3 Precision and ultra-precision special machining: micro machining large-scale and ultra-large-scale integrated circuit chip graphics with precision and ultra-precision machining methods such as electron beam and ion beam etching to achieve minimum linewidth, etc.

 

In the above three fields, the application of diamond tools to ultra-precision cutting non-ferrous and non-metallic materials outside the iron group is a very important ultra-precision machining means. In order to obtain a stable mirror surface of materials, there must be good ultra-precision machine tools and tools, ultra-stable environmental conditions, and real-time detection and feedback compensation of the machined surface.

 

2. Influence of ultra-precision lathe

Ultra-precision machining lathe is the key to obtain high-precision machined surface. At present, this technology has become more and more mature at home and abroad, and different types of products have been successfully developed. In order to achieve ultra-precision machining with size and shape accuracy better than 0.1um and surface roughness better than 0.1um. In the overall index, the accuracy of the main shaft should reach 0.5um, and the straightness of the guide rail should be more than 0.1um/200mm. Generally speaking, in order to obtain a stable high-precision surface that meets the index requirements. The design of lathe should be comprehensively considered from the following aspects.

 

2.1 Spindle system

Ultra-precision turning requires that the spindle system has the characteristics of high stiffness, high rotation accuracy, wide speed regulation range and small temperature rise when rotating at high speed. Ultra-precision lathes generally use static spindle, and its axial and radial accuracy should reach 0.5um. In the early stage, the main shaft components mostly used manifold hydrostatic bearings. It has the characteristics of high stiffness and good dynamic performance. However, when rotating at high speed, its temperature rise is larger. The DTM-III ultra-precision lathe developed by LLL laboratory in 1983 is the manifold hydrostatic bearing. The temperature rise can be controlled by the circulation constant temperature grasping technique. Aerostatic bearing is more suitable for micro cutting, and has the characteristics of small temperature rise, which is the current development trend

 

2.2 Servo mechanism

The performance of the servo system of ultra-precision lathe will directly affect the positioning accuracy of the lathe workbench, and finally reflect the machining accuracy of the workpiece. Servo feed mechanism mainly refers to guide rail and guide rail drive mechanism. From the performance indicators provided by several typical lathes that have been commercialized in the world at present, it can be seen that the straightness of its guide rail has reached 0.5um/100mm. Hydrostatic guideway and aerostatic guideway can meet the above requirements. Both of them have the same level of motion accuracy, but the manifold hydrostatic guideway has large bearing capacity and stiffness. Therefore, more and more ultra-precision lathes use manifold hydrostatic guide rail. The driving mechanism mainly includes ball screw, hydrostatic screw, friction drive device and linear motor. Among them, the positioning accuracy of oil static pressure lead screw is the highest. In a long journey, the resolution can reach nanometer level or even sub nanometer level. There is no creeping and return clearance caused by friction, and the accuracy can be maintained for a long time. However, its axial stiffness and bearing capacity are small, and it is difficult to manufacture and assemble. Although the ball screw can achieve a positioning accuracy of 0.01um under closed-loop control, it has shortcomings such as return clearance, friction and nonlinear contact deformation; Although the friction drive structure is the simplest, its maximum positioning accuracy can also reach 0.01um, and there is no return clearance problem, but its thrust is limited, and it is prone to elastic deformation and friction loss. In recent years, with the rapid development of electronic technology, linear motor as a new servo drive mechanism has great development prospects.

 

2.3 High precision displacement measuring device

The displacement measurement devices of ultra-precision machine tools mainly include dual frequency laser interferometer and high-precision grating measurement system. The dual frequency laser interferometer has high resolution and accuracy, and uses the wavelength of the laser beam as the measurement benchmark. However, when the laser beam passes through the air, atmospheric pressure, ambient temperature and humidity will affect the laser wavelength. Therefore, when using dual frequency laser interferometer to achieve ultra-precision measurement, the environmental requirements are very high. In contrast, the high-precision grating measurement system has the advantages of high resolution, high precision and good stability. The maximum resolution can reach 0.001um, the measurement accuracy +0.1um, and the 'range can reach 70mm. It is reported that the LG100 photoelectric meter of Beijing photoelectric meter research center has a resolution of 0.001um, an accuracy of +0.1um and a measurement range of 100mm.

 

2.4 Bed structure materials

All materials are made of materials that are less affected by temperature. Such as Gu steel (4J36), granite, artificial granite, ceramics, glass ceramics, etc. In China, natural granite is mostly used for manual grinding to the required dimensional accuracy. At present, there are high-precision granite grinders in yinnei. Some domestic platforms also use glass ceramics with lower temperature coefficient (also known as glass ceramics, yellow glass or Zerodur, etc.). Even so, high-precision lathes also use thermostatic devices, which can be controlled at a maximum of 20+0.0025 ℃.

 

2.5 Vibration reduction technology of ultra-precision machine tools

The vibration of ultra-precision machine tools can be divided into two categories: one is the self-excited vibration of machine tool system. The other is the interference of external vibration on machine tools. At present, the main methods to reduce the self-excited vibration of machine tools are to increase the quality of machine tools, improve the connection stiffness of various parts, reduce the interference of power sources of machine tools, and sufficient dynamic balance. In order to eliminate the interference of external vibration on the machine tool, air vibration isolation support is generally used, and the machine bed is placed on the vibration isolation foundation.

 

3. Influence of cutting tools

Natural diamond tool is the most important ultra-precision cutting tool at present. Because its edge shape is directly reflected on the surface of the processed material, the grinding technology of diamond tool is an important problem in ultra-precision cutting. The grinding technology should include three aspects: the selection of crystal surface, the grinding technology and the measurement of the edge radius after grinding.

 

3.1 Crystal plane selection

Due to the anisotropy of natural diamond crystal, the physical and mechanical properties of each crystal surface are different. Its manufacturing difficulty and service life are different, and the 100, 110 and 111 crystal surfaces are reasonably selected as the front and rear cutting surfaces of the tool to be suitable for different applications. The durability, grinding performance, grinding condition and cutting performance of diamond tools made by accurate orientation have been improved to varying degrees. At present, there are three kinds of crystal orientation: artificial visual crystal orientation, X-ray crystal orientation and laser crystal orientation.

 

3.2 Grinding of cutting edge

The general requirements for diamond tool grinding are: the sharpness of the blade is good, the roughness and flatness of the front and rear blade surfaces are high, and the blade edge is straight, smooth and free of defects under the microscope. The grinding is generally carried out on the grinding machine after crystal orientation. At present, the level of Shanghai Instrument Factory, 303 Institute of Aviation Industry Corporation and 230 plant of Aerospace Industry Corporation with high grinding level in China has been able to grind the edge radius below 100~200nm.

 

In recent years, the research results show that when the cutting amount reaches the nanometer level, the surface quality, surface micro morphology, surface cold hardness and lattice structure dislocation of the cutting surface are directly related to the sharpness of natural diamond tools. It is estimated from the observation of previous electron microscope (SEM) micrographs and the fact that the stable chips are obtained from the nominal cutting thickness of 10mm. The cutting edge radius can be ground to less than 10nm; According to the energy balance principle between the elastic deformation energy of the workpiece surface and the blade, the cutting edge radius of diamond tools can be assumed to be calculated to about 2nm. From the results of these experiments and analysis, it can be considered that the radius of the diamond tool edge can be ground to several nanometers, which is consistent with the cutting edge radius of 2~4nm inferred from the minimum chip thickness abroad, that is, the minimum cutting edge radius is in the same order of magnitude as the minimum cutting thickness that can be achieved.

 

At present, experts at home and abroad are exploring new grinding methods. Someone has proposed to use thermochemical methods to replace the traditional mechanical grinding methods in order to obtain ultra-high sharpness edges.

 

3.3 Measurement of cutting edge radius

In the field of current experiments and theories, the conclusion is that the smaller the cutting edge radius is, the smaller the chip thickness is. The higher the surface processing quality. Therefore, in addition to pursuing good grinding technology, we hope to accurately measure the sharp edge radius. The traditional measurement methods of cutting edge radius include film printing method, optical method, chip analysis method and electron microscope measurement method (SEM method). The commonly used method is SEM method. Japanese scholars have added two secondary electronic detection devices on the basis of SEM method, and obtained the detection results within 50nm.

 

Due to the limitation of resolution, the traditional SEM method is no longer suitable for the detection of nano cutting edge radius. With the development of scanning tunneling microscopy and the application of atomic force microscopy (AFM) in various fields, American scholars put forward the idea of measuring cutting edge radius. Although there are still some problems in the direct application of AFM to measure, this is a development direction of diamond tool edge radius measurement at present. It is believed that in the near future, a measuring instrument based on AFM principle will appear to qualitatively evaluate the edge contour.

 

3.4 Other tools

In recent years, scholars at home and abroad are also looking for tools that are harder than diamond. At present, polycrystalline tools and other products are available at home and abroad. Due to the development of vapor deposition technology in recent years, diamond film coated tools have become a new type of tools with wide application prospects.

 

3.5 Interaction of machine tool, cutting tool and workpiece

The problem that is easy to be ignored in processing equipment is the coordination of machine tools, cutting tools and processed workpieces. The quality of workpieces, especially the tooling system as a conversion device, has a particularly important impact on the accuracy of workpieces. At present, the main problems are its stiffness and the additional vibration caused by the imbalance of tooling weight under high-speed cutting.

 

4. Strict control of environmental conditions

The environmental conditions of ultra-precision machining mainly include constant temperature, vibration isolation and ultra-purification.

 

The ultra-precision machining laboratory requires constant temperature, which has reached 0.5 ℃ of 20 soil, while the cutting parts can reach 0.5 ℃ of 20 soil under the spray of constant temperature liquid. In general processing occasions, multi-level temperature control can be adopted, such as preliminary temperature control in the laboratory and medium precision temperature control in the small processing environment. At this time, the space volume, air flow rate and other factors should be considered, and finally the accurate temperature control at the processing and measurement points.

 

As mentioned above, the anti-vibration system is divided into two categories, except that the foundation is established according to the standard. Vibration isolation springs and pneumatic tube devices are also widely used to reduce the vibration in various frequency ranges: high precision lathes also use the dynamic balance method of rotating parts to reduce the vibration.

 

Ultra-precision machining must also have a clean environment, which is gradually recognized by people in recent years. The maximum requirement is that the number of dust greater than 0.01um in 1 cubic meter of air is less than 10

 

5. Detection method of processed workpiece

The detection of parts mainly includes real-time online detection and offline measurement detection in the measurement room. With the gradual improvement of accuracy, online detection and error compensation technology have been paid more and more attention.

 

At present, most of the measurement work is still completed in the measurement room, and the main detection indicators are shape error and surface roughness, etc. we should also vigorously develop instruments for measuring ultra-precision surface micro defects, cracks and stress distribution. Three coordinate measuring machines and various surface roughness measuring instruments have always occupied a dominant position. With the birth of CNC three coordinate measuring machines, automatic measurement can move forward to a higher step.

 

In fact, a hard method to eliminate or reduce errors is to improve the manufacturing accuracy of machine tools and ensure the processing environment. The other is error compensation, which should be based on online measurement, real-time modeling and dynamic prediction of machining errors. This is a development trend in the field of ultra-precision machining detection.

 

There are many methods to detect the surface morphology, such as optical interferometry, stylus method, laser and X-ray. With the development of scanning tunneling microscopy, this technology has been gradually applied to the field of surface morphology detection. At the same time, nano-metrology and nano-metrology testing technology have been widely used in various detection fields of ultra-precision machining. The future technical direction is:

a. To achieve nano-precision surface topography measurement in the whole measurement range of parts

 

b. Establish a nano-scale benchmark for evaluating ultra-precision machined surfaces

 

c. The realization of high-precision on-line detection.

 

In addition to the above four aspects, we should also conduct in-depth research on the machining mechanism of superfinishing. Due to the particularity of many mechanical problems in ultra-precision machining, such as the formation and growth of chip nodules, the occurrence of scales and thorns, and the influence of cutting parameters on the cutting process and surface quality. The in-depth study of these problems will better provide us with some valuable information to obtain stable and high-precision surfaces; Foreign-countries have gradually recognized this and carried out a series of studies. Especially for ultra-precision cutting of ferrous metals. China's precision machining departments also realize that the research and solution of machine rationality problems will have a great impact on the improvement of ultra-precision machining accuracy. At present, some research has also been carried out in the laboratory stage.

 

More about ITS Thrust Ball Bearing:

ITS thrust ball bearing top quality brand has guaranteed production and highly active management.

 

A thrust ball bearing is a particular type of rotary ball bearing. Like other ball bearings, they permit rotation between parts, but they are designed to support a high axial load while doing this (parallel to the shaft). Higher speed applications require oil lubrication.

 

A thrust ball bearing is used to bear axial load mainly shaft, radial combined load, but the radial load shall not exceed 55% of the axial load. Compared with other thrust roller bearings, such bearings have a lower friction factor, higher speed, and have the ability to align.

 

Thrust Ball Bearing 


2022-07-20

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