Technical difficulties and countermeasures of ultra-fine micromachining

1 Introduction Microfabrication technology refers to the manufacturing and processing technology of small-sized parts. With the development of aerospace, defense industry, modern medicine, and bioengineering technology, more and more miniaturized, miniaturized equipment, and micro-sized parts have emerged. Various micro-machineries manufactured using micro-machining technology such as micro-motors, micro Sensors, micro-pumps, etc. have increasingly broad application prospects. The requirements for microfabrication in modern manufacturing technologies are also increasing. They have developed into ultra-fine micromachining. They have challenged the processing limits of existing manufacturing technologies, and have developed super-precision machining, ultra-fine processing and nano-machining technologies, which have become modern manufacturing technologies. A direction of development. The micro-machining technology not only includes various conventional precision machining methods, but also includes special processing methods such as electron beam processing, ion beam processing, and chemical processing. These special processing methods are currently well applied in the micro-fabrication field. However, there are some technical difficulties in the micro- and ultra-fine micro-scale machining, which limits its wide application. Because even the traditional mechanical processing, the processing mechanism and method are not the same for the micro size and the normal size. This paper analyzes the mechanism of ultra-fine processing, analyzes the technical difficulties in ultra-fine processing and its impact on the processing process, and proposes solutions. 2 Mechanism of Ultrafine Machining Machining and fine cutting differ in the machining mechanism. In normal cutting, the allowable depth of cut and feed are large due to the large size of the workpiece. In the case of micro-cutting, due to the small size of the workpiece, large depths of cut and advances in strength and stiffness are not allowed. To give the quantity, at the same time to ensure the workpiece size accuracy requirements, the final finish of the surface cut-off layer thickness must be less than the precision value, so the amount of cutting must be very small. The general metal material is composed of crystal grains having a diameter of several micrometers to several hundred micrometers. Due to the very small cutting depth of fine cutting, especially submicron and nanometer ultrafine cutting, the depth of cut is usually smaller than the grain diameter of the material, so that the cutting can only be performed within the grains, and the cutting is equivalent to one at a time. Discontinuities are cut, so fine cutting is an interrupted cut. Due to the presence of micro-defects in the material and non-uniform material distribution, the cutting force of the cutting tool changes greatly, and the cutting edge will be subject to greater impact and vibration. Cutting force characteristics of micro-cutting Micro-cutting machining is an ultra-micro separation technology. The cutting force near the cutting edge of diamond cutting tools is sub-Newtonian or even smaller. The cutting force can clearly reflect the removal process of the chips. Therefore, it is helpful to study the cutting force model to understand the chip cutting characteristics. The cutting force characteristics in fine cutting are: the cutting force is small, the unit cutting force is large, and the cutting depth resistance is greater than the main cutting force; the cutting force increases with the decrease of the cutting depth, and the cutting force sharply increases when the cutting depth is small Big. This is the size effect of the cutting force. The physical model of the cutting force during fine cutting is closely related to the submicron structure of the cutting edge. Due to the presence of the arc radius of the cutting edge edge, the cutting edge has a large negative rake angle at the nanometer cutting stage, which increases the cutting deformation, so the unit cutting force during cutting is large; at the same time, fine cutting is often performed. Within the grain, the cutting force must be greater than the molecular and atomic bonding forces inside the crystal, and thus the cutting force on the unit cutting area increases sharply. The cutting force increases with the increase of the cutting depth when compared with the ordinary cutting, and the depth of cut and the feed amount during the fine cutting are very small. Due to the presence of the arc radius of the tool tip and the radius of the arc of the cutting edge, the cutting deformation is significantly increased. When the depth of cut is small, the additional deformation caused by the radius of the tool tip arc accounts for a large proportion of the total cutting distortion. Due to the size effect of the cutting force, the smaller the depth of cut, the greater the cutting force (the influence of the depth of cut on the cutting force at the time of fine cutting is shown in Fig. 1).

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Fig. 1 Effect of cutting depth on cutting force

Minimum cutting thickness for fine-cutting When the machine conditions are optimal, a very sharp diamond tool can be used to achieve a nano-scale continuous and stable cutting. The minimum effective cutting thickness for stable cutting is called the minimum cutting thickness, and the minimum cutting thickness that can be achieved for fine cutting is related to the radius of the arc of the diamond tool edge and the physical and mechanical properties of the material being cut.

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Fig. 2 Effect of the minimum cutting thickness on the arc radius of the blade

As shown in Fig. 2, at the point A on the workpiece, the main cutting force Fc and its vertical force Fd act to form a resultant force F (the resultant force F can also be decomposed into the normal force N and the friction force μN at the point A), and the resultant force F The direction is the direction of normal stress at point A. When the angle w between the direction of normal stress at point A and the direction of cutting speed is about 45° (for different materials, the required angle is also different. When machining aluminum alloy with diamond tools, the included angle is about 38°. ~45°), the processed material accumulates and forms chips at point A, and the processed material at point A is elastically and plastically deformed to form a machined surface. At this point, point A is the critical limit of the minimum cutting thickness. The limit minimum cutting thickness hDmin can be obtained by the following formula:
hDmin=r(1-cosq) (1) Where: r—cutting edge radius of the cutting edge is known from Fig. 2, q+w+b=90°, ie q=90°-(w+b) ( 2) Where: b—the friction angle between the tool and the workpiece material, tgb=μ(friction coefficient), the friction coefficient is approximately 0.12 to 0.26 w when cutting the aluminum alloy with a diamond tool—the clamp of the normal stress direction and the cutting speed direction The angle, w value is related to the strength of the workpiece material, elongation, friction coefficient, and the position of the point A. According to the experience w=38° to 45°, substituting formula (2) into formula (1) can be simplified:

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When w=45°, the above equation can be simplified as:

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Table hDmin value and r value relationship hDmin w = 38° w = 40° w = 42° w = 45° μ = 0.12 0.295r 0.271r 0.246r 0.214r μ = 0.26 0.206r 0.158r 0.165r 0.138r It can be seen that the limit minimum cutting thickness hDmin is related to the cutting edge arc radius r, the physical and mechanical properties of the material itself, and the friction coefficient between the tool and the workpiece. The relationship between hdmin value and r value is shown in the right table. According to experience, it is desirable that w=42°. When μ=0.12 to 0.26, hDmin=(0.165 to 0.246)r. It can be seen that to realize ultra-thin cutting with a nano-scale cutting thickness, the radius of the cutting edge arc of the diamond tool used should be 4 to 6 nm. This is a very sharp tool with a small cutting edge radius. Effect of cutting temperature Due to the extremely small cutting amount of micro-cutting and the high thermal conductivity of the diamond tool and workpiece material, the cutting temperature of micro-cutting is considerably lower than that of conventional cutting. However, for ultra-fine micro machining with extremely high precision, the effect of small machining temperature changes on machining accuracy cannot be ignored. At the same time, the cutting temperature has a great influence on the tool wear, and the cutting temperature has a very significant influence on the chemical wear of diamond tools. The complexity of the micro-cutting process Minimal depth of cut (nano-scale), limited edge radius, low ratio of cutting thickness to edge radius, cutting edge quality, and a small amount of tool wear occurring on the flank The microfabrication process is complicated. Deformation of the three deformation zones, especially the tool-to-workpiece friction in the third deformation zone and the wear of the tool due to the elastic recovery of the machined surface, generate cutting heat, affect the integrity of the machining surface, and cause subsurface damage. . When the cutting thickness and the cutting edge radius are in the same order of magnitude, the effects of the sliding and ploughing phenomena due to the change in the rake angle (negative rake angle) of the tool are also obvious to the cutting process. 3 Technical difficulties in ultra-fine micromachining Micro-cutting mainly refers to the processing of micro-sized parts with a part size of 1mm or less and a machining accuracy of 0.01 to 0.001mm. Ultra-micromachining refers to ultra-fine parts with a size of 1μm or less. Processing; Nano-scale ultra-fine processing refers to the processing of parts with a fineness of 1 nm or less. There are the following technical difficulties in achieving nano-scale ultra-fine micromachining: Effects of material micromachining The material removal process depends not only on cutting tools but also on the material being processed. The selection of ultra-fine micromachining materials is based on the premise of nanometer-scale surface quality, which is called "micro-processability" of the material (as defined by nanoscale surface roughness and negligible tool wear at a certain machining distance). The factors affecting the micromachining of the material include the internal affinity (chemical reaction) of the material to be cut to the diamond tool, the crystal structure of the material itself, defects, distribution, and heat treatment conditions (eg, the anisotropy of the polycrystalline material on the surface of the part machined. Completeness has a big impact.) Large cutting force per unit of cutting force is an extremely thin cutting, and the cutting thickness may be smaller than the size of the crystal grains. Therefore, the cutting force is characterized by a small cutting force, but the unit cutting force is very large. The physical essence of nano-scale ultra-micromachining is to cut off the binding between molecules and atoms and to achieve the removal of atoms or molecules. Therefore, the cutting force must exceed the molecular and atomic bonding forces inside the crystal. When the cutting depth and feed rate are extremely small, the cutting force on a unit cutting area will increase sharply, and at the same time, a large amount of heat will be generated, so that the temperature of the local area of ​​the tip of the blade will increase. Therefore, the requirement on the tool during the fine cutting is high. Need to use wear-resistant, heat-resistant, high temperature hardness, high temperature strength super hard tool material. When cutting non-ferrous metals such as aluminum alloys, the most common tool is a diamond tool. Cutting edge arc radius limits on ultra-microcutting thickness The cutting edge radius of the tool limits the minimum cutting thickness. The smaller the cutting edge radius is, the smaller the allowable cutting thickness is. From Table 1 we can see that hDmin = (0.165 ~ 0.246) r The current commonly used diamond tool sharpness is about r = 0.2 ~ 0.5μm, the minimum cutting thickness up to 0.03 ~ 0.15μm; after special cutting tool up to r = 0.1μm, the minimum cutting thickness of 0.014-0.026μm. If a workpiece with a cutting thickness of 1 nm is to be machined, the radius of the tool edge must be less than 5 nm, and it is very difficult to sharpen and apply this extremely sharp diamond tool. Tool wear and breakage Due to the micro-wear of the diamond tools, the tool wear will gradually increase after cutting for a period of time, sometimes even abruptly. There are two forms of diamond tool failure: chipping and wear. The mechanical wear and microscopic chipping of diamond tools are caused by the microscopic cleavage at the blade edge, and the nature of the wear is the accumulation of microscopic cleavage. Accumulated diamond tool wear occurs mainly on the front and back flank of the tool. After several hundred kilometers of cutting length, this wear becomes sub-micron wear. Diamond tools also produce thermochemical wear due to oxidation, graphitization, diffusion, and carbonization. The chipping occurs when the stress on the cutting edge of the tool exceeds the local bearing capacity of the diamond tool. It is the most difficult to predict and control damage, and the effect on the quality of the machining surface is greater than the wear of the front and back surfaces. Lowering the cutting temperature can effectively reduce tool wear. In addition, cutting in saturated carbon gas can also inhibit the carbonation of diamond tools. The micro-vibration of the workpiece surface during cutting is the result of the tool's contour mapping onto the workpiece. Therefore, the surface roughness is determined by the relative movement between the tool and the workpiece and the shape of the tool edge. In the case of fine cutting, cutting depths are often smaller than the grain diameter of the material, which is equivalent to the cutting of individual discontinuities. This microscopic intermittent cutting and the dynamic characteristics of the machine can cause micro-vibration during cutting. The influence of micro-vibration in fine cutting on the quality of the machined surface can not be ignored.

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Fig. 3 Effect of cutting speed V on the height of BUE

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Fig. 4 Effect of feed f on the height of BUE

The effect of BUE on the processing process The effect of BUE cannot be neglected in ultrafine cutting. The built-up edge affects the cutting force and cutting deformation, and the built-up edge on the cold edge can also affect the surface roughness. In addition to the micro-defects of the blade, which directly affect the build-up of the built-up edge, the effects of cutting speed and feed on the build-up edge are also obvious. In fine cutting, the built-up edge is present in all cutting speeds, but the cutting speed will affect the height of the built-up edge: the lower the cutting speed, the higher the built-up edge (the cutting speed V is the height of the built-up edge The influence is shown in Figure 3); the smaller the feed, the higher the built-up edge (the effect of the feed f on the height of the built-up edge is shown in Figure 4). 4 Measures and methods To solve the above technical difficulties in ultra-fine micromachining, the following technical measures and methods should be taken during processing: Reasonable material selection In order to improve the surface quality of ultra-fine micromachining, a reasonable selection of workpiece materials is required. The workpiece surface with good surface integrity can be obtained by selecting a workpiece material with a good micro-processability (such as an amorphous material or a material having a fine grain structure). Reducing the radius of the cutting edge The ultra-precision machining of non-ferrous metals and non-metallic materials with diamond tools can obtain a mirror surface with Ra0.02 to 0.002 μm. After grinding the tool finely, chips with a thickness of 1 nm can be cut. At present, the quality of the cutting edge of the diamond cutting tool is mainly obtained by grinding and polishing the edge of the diamond cutting tool on the rotating cast iron disc, while the ion cutting and chemical polishing processing can make the processed tool have sub-micron shape accuracy. The use of bevel cutting bevel cutting can increase the actual cutting rake angle, reduce the cutting edge arc radius and the minimum minimum cutting thickness, so as to achieve ultra-thin cutting and fine cutting. For fine-cutting, the choice of edge angle needs to consider the choice of diamond tool facets and sharpening. Choosing the right tool front and back face natural diamond has a series of excellent characteristics such as high hardness, high wear resistance, high temperature strength, good thermal conductivity, low friction coefficient with non-ferrous metals, and the ability to sharpen sharp edges. Therefore, although natural diamond is expensive, it is still an irreplaceable material for ultra-fine cutting tools. Diamond crystals have a strong anisotropy, so the choice of crystal faces for the front and back rake faces of diamond tools is particularly important. The diamond crystal planes commonly used as the front and back facets of the tool are (100) crystal faces and (11 0) crystal faces. The use of a (100) crystal plane with a small coefficient of friction as the front and back flank of the tool can reduce the cutting distortion and reduce the friction between the tool flank and the machining surface and the residual stress on the machining surface. At the same time, the (1 0 0) crystal plane is used as the front and back flank of the tool, which has good wear resistance and high micro-intensity of the cutting edge, so it is not easy to produce microscopic chipping. This is very advantageous for maintaining the sharpness of the cutting edge and prolonging the tool life. Stable machine dynamics and machining environment To achieve ultra-fine cutting, it is also important to select the machine's dynamic characteristics and maintain the stability of the machining environment. The processing machine tool should be equipped with high-precision micro-feed device, which can achieve accurate, stable, reliable and rapid micro-displacement; at the same time, the processing should have an ultra-stable processing environment to ensure that the processing process is strictly constant temperature, humidity, vibration Under ultra-clean conditions, minimize the effect of micro-vibration on the quality of the machined surface. Reducing the cutting temperature Because the cutting amount influences the height of the cutting edge of the cutter due to the change of the cutting temperature, the use of the cutting fluid to reduce the cutting temperature is an effective measure to suppress the built-up edge and reduce the tool wear. 5 Conclusion Superfine micromachining is a kind of microscopic interrupted cutting. The unit cutting force is large, the quality of the workpiece surface is restricted by many factors, and the cutting process is complicated. This paper quantifies the relationship between the minimum cutting thickness and the tool edge arc radius during ultra-fine cutting, and proposes to select a workpiece material with a small amount of machinability, adopts bevel cutting, and adopts a fine grinding tool to reduce the arc radius of the tool edge. , Choosing the right diamond face as the tool's front and back flank and reducing the cutting temperature and other technical measures to achieve ultra-fine cutting processing, also pointed out that high-precision machine tools and ultra-stable machining environment is also an important to achieve ultra-fine micro-cutting condition.