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1. Simulation of micro-cutting mechanism
Mainly using finite element technology and molecular dynamics method, finite element technology is based on continuum mechanics, so molecular dynamics method is more suitable for micro-cutting. Molecular dynamics simulation of micro-cutting mechanism has been carried out for more than ten years in the world. The research work is mainly to establish cutting models at the atomic and molecular scales to understand the chip and surface formation process from an atomic and molecular perspective. Explain the effects of material properties, tool geometry and process parameters on micro-cutting stress and strain distribution, cutting forces, cutting temperatures and machined surface quality.
2. Minimum cutting thickness
The minimum effective cutting thickness that can be used for stable cutting is called the minimum cutting thickness. Chip shape, cutting force, cutting stability, micro-machining of workpiece materials, reasonable selection of cutting amount, and surface quality of machining are all affected by minimum cutting thickness. Therefore, the study of minimum cutting thickness is of great significance for micro-cutting. . The minimum cutting thickness that can be achieved by micro-cutting is related to the arc radius of the cutting edge of the tool, the physical and mechanical properties of the workpiece material, the microstructure and the third deformation zone tool—the friction coefficient between the workpieces. Since the minimum cutting thickness has many influencing factors, it is difficult to determine the minimum cutting thickness. In the actual production, the minimum cutting thickness is generally determined according to the radius of the cutting edge of the cutting edge. The research shows that the minimum cutting thickness is proportional to the radius of the cutting edge of the tool. The proportional coefficient is related to the tool and the workpiece material. It is generally 0.165~0.246. If the cutting edge radius is 50nm, it is necessary to achieve ultra-thin cutting thickness. Micro-cutting, the minimum cutting thickness at this time is about 10nm.
3. Chip form
Chips can only be produced when the depth of cut of the micro-cut is greater than the minimum cut thickness. Similar to conventional cutting, micro-cutting chips have three forms: continuous chips, non-continuous chips, and chips associated with built-up edges. The shape of the chip is related to the performance of the workpiece material, the cutting speed, the cutting deformation, and the like.
4. Micro cutting force
The cutting force during micro-cutting is small, but the unit cutting force is large, and the depth-of-depth resistance is greater than the main cutting force. The cutting force increases as the depth of cut decreases, and the cutting force increases sharply when the depth of cut is small, which is the size effect of the cutting force. The existence of the cutting force size effect makes the cutting force model of ordinary cutting not suitable for micro-cutting. The size effect of the cutting force is closely related to the cutting edge radius. Due to the radius of the cutting edge, the cutting edge forms a large negative rake angle during micro-cutting, which increases the cutting deformation and increases the unit cutting force during cutting. Big. If the depth of cut is further reduced, the cutting may occur inside the crystal grains. At this time, the cutting force must be larger than the molecular and atomic bonding force inside the crystal, so that the cutting force per unit cutting area is sharply increased. The cutting force during micro-cutting is also related to the crystal orientation and grain boundaries.
5. Cutting temperature
Due to the small amount of cutting used for micro-cutting, the cutting temperature of micro-cutting is lower than that of conventional cutting. For micromachining with high precision requirements, the influence of the change of machining temperature on the machining accuracy can not be ignored, and the influence of cutting temperature on the wear of micro-cutting tools can not be ignored.
6. Micro-processability of workpiece materials
The removal process of the workpiece material depends not only on the cutting tool, but also on the workpiece material itself. The micro-machining properties of micro-cut workpiece materials can be defined by nano-scale surface roughness and negligible tool wear over a certain machining distance. Factors affecting the micro-machineability of the workpiece material include the affinity (chemical reaction) of the workpiece material and the tool material, the crystal structure of the workpiece material itself, dislocations, defect distribution, and heat treatment state (such as the anisotropy of the polycrystalline material). Part processing surface integrity has a greater impact).
7. Tool deformation
The rigidity of the tool has a considerable influence on the micro-machining process. For example, when the rigidity of the tool is insufficient in the milling process, the machining accuracy will be deteriorated during the machining process, and the micro-cutter will be broken when it is severe. The tool deformation of the micro-milling cutter is d=F·L3/(3·E·I) where d is the radial deformation of the end mill; F is the radial cutting force; L is the tool extension length; E is The modulus of elasticity of the tool material; I (I = p D 4/64, D is the equivalent diameter of the end mill) is the pole moment of inertia of the tool.
8. Surface roughness and cutting stability
The surface topography of the workpiece is the result of the contour of the tool being mapped onto the workpiece. Therefore, the surface roughness of the machining is determined by the accuracy of the relative motion between the tool and the workpiece and the shape of the cutting edge of the tool. In the micro-cutting, if the cutting depth is smaller than the grain diameter of the workpiece material, it is equivalent to cutting one discontinuous body, the microscopic defects of the workpiece material and the unevenness of the material distribution, etc., so that the cutting force of the tool during micro-cutting is changed. Large, the cutting edge is subjected to large impact and vibration. The effect of vibration in micro-cutting on the quality of the machined surface cannot be ignored.
9. Glitch
The burr is a tiny protrusion formed on the surface of the workpiece due to plastic deformation after cutting. The presence of burrs affects the fit of the part and reduces the dimensional accuracy and surface quality of the workpiece. The use of burred parts poses a safety hazard, especially in certain special applications such as aerospace. Therefore, it is necessary to increase the deburring process, and the methods of deburring include mechanical methods, thermal energy methods, chemical methods, electrolysis methods, electrochemical methods, grinding methods, and the like.
10. built-up edge
The influence of built-up edge on micro-cutting can not be ignored. The built-in edge of cold welding on the cutting edge will cause the geometric angle of the tool to change, affecting the cutting force and cutting deformation. The built-up edge will also affect the surface roughness of the machined surface. The production of built-up edge is affected by the microscopic defects of the blade, the cutting speed and the feed rate. In micro-cutting, the lower the cutting speed, the higher the built-up edge, and the smaller the feed amount, the higher the built-up edge.
11. Tool wear
Similar to conventional cutting, there are two forms of failure of micro-cutting tools: wear and chipping damage. The deformation of the three deformation zones, especially the tool of the third deformation zone - the friction between the workpieces and the mechanical recovery of the machined surface due to the elastic recovery of the machined surface. At the beginning of the cutting, the tool has initial micro-wear, and after a certain period of cutting, the tool wear will gradually increase, and sometimes it will suddenly deteriorate. Tool wear occurs mainly on the front and back knives of the tool. Due to oxidation, diffusion, etc., the tool also produces thermochemical wear. The chipping damage occurs when the stress on the cutting edge of the tool exceeds the local bearing capacity of the tool material. It is the most difficult to predict and control damage, and the effect on the quality of the machined surface is greater than the influence of the front and back face wear. Lowering the cutting temperature reduces tool wear.
6 micro cutting CAD / CAM technology
Cimatron E is a commercial CAD/CAM software for micro-cutting, primarily for micro-milling. Since April 2003, the European Financial Community has begun to fund CRAFT, which has been micro-milling research on injection molds for micro-plastic components for 24 months. The project involved the entire process of micromachining technology, including Fraunhofer Institute of Production Technology (IPT), CAD/CAM software supplier Cimatron Gmbh, milling machine manufacturer Kern, tool manufacturer Magafor, and mold maker Promolding BV Structoform And MMT AG). The hardness of the mold material is 53HRC, the precision of micro-die milling is <5μm, and the surface roughness Ra<0.2μm. The cutter manufacturer offers tool diameters of up to 50μm, and the milling machine offers micro-cutting machine spindles up to 160,000 rpm. CAD/CAM software suppliers offer Cimatron E software for micro-cutting. Unlike pure solid modeling, CimatronE's solid-surface hybrid modeling technology uses the CAD function of “design for manufacturing†to repair geometric models, fuse gaps and become solid through various surface functions, and its ACIS core technology provides up to 1 nm. Internal precision to meet the special requirements of micro milling. In order to reduce the risk and prevent discontinuous micro-curved surfaces generated during tool change, Cimatron E offers a variety of micro-milling strategies. Slash or spiral lower knives are supported in the NC strategy to ensure maximum smooth and continuous entry of the tool into the workpiece. A uniform tool path is obtained during the machining process by applying a high-speed cutting (HSC) strategy, and the knowledge of blank residue is used to prevent the knife from being broken to open the micro-cavity. Cimatron E's micro-milling technology ensures efficient and safe tool path by identifying real residual micro-blanks and the same functions of roughing, secondary roughing, finishing micro and cycloidal roughing. High-hardness materials and high-quality curved surfaces require 5-axis simultaneous cutting for very small diameter short-cone tools. In order to meet the requirements of high-speed micro-milling, Cimatron E uses a variety of high-speed milling strategies, such as corner fillet joints, zero overlap cycloidal finishing, S-joint and spiral lower cutters, adaptive Z-layer finishing and streamlines machining. Cimatron E also supports spline approximation machining and streamline milling to reduce machining time and reduce tool wear and tear. Figure 9 shows a micro-mold that is micro-milled using Cimatron E.
Figure 9 micro-milling micro-mold
7 development prospects of micro-machining technology
Micro-mechanical is an important development direction, and its application prospects are very good. At home and abroad, it is very concerned about research in this field. Micro-machining technology is one of the most active research directions in the field of micro-machine manufacturing. At present, the research on micro-machining process and equipment is still in the exploratory stage as a whole, and has not yet formed a complete and mature technical system and technical capabilities for scale manufacturing. It is expected that in the next 15 years or so, small manufacturing processes and related equipment technologies will be rapidly developed, especially in micro-small weapons, micro-medical devices, biomimetic devices, detection devices, and aerospace devices. In the future, we should pay attention to the following topics in micro-cutting to promote the production and application of micro-cutting technology.
Basic research on micro-cutting applications includes research on key technologies of micro-part cutting equipment, mainly researching high-speed spindle systems, positioning, motion and control technology of precision worktables, composite micro-machining equipment and technology; micro-cutting tool materials and tool making techniques Research; micro-cutting tools, rapid clamping of workpieces, testing and monitoring techniques for micro-machining processes.
The research on micro-cutting mechanism mainly studies the micro-cutting uneven deformation field under the thermo-mechanical coupling stress, studies the constitutive equation of the workpiece material at the micro-scale, and analyzes the size effect, uneven strain and dislocation of the micro-cutting deformation zone. The effects of shear deformation stress and shear deformation energy; the effects of minimum cutting thickness on chip shape, formed surface formation, cutting force, cutting temperature, etc. and the influence of microstructure of workpiece material on surface roughness and subsurface damage, Establish micro-machining theory and technical system; study multi-scale micro-cutting simulation technology, and lay the foundation for application of micro-machining technology.
The micro-machining process includes micro-machining processes for various new materials such as steel, titanium alloys, stainless steel, aluminum alloys, ceramics and other non-metallic materials and various composite materials, and micro-cutting CAD/CAM technology.
Research on the economics and reliability of micromachining technology.
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