Modern micro cutting technology (2)

1. Micromachining technology

In addition to the above-mentioned 1-3 types of micro-manufacturing technology, most of them can be attributed to such processing technology, and micro-machining technology can be divided into three categories: micro-machining technology, non-cutting processing and special processing (see Figure 3). This article mainly introduces micro-cutting technology.

Figure 3 Micromachining Technology Classification

2. Micromachining technology

Micro-cutting is a fast and low-cost machining method for small parts, and is not limited by materials. CNC machining centers can be used to realize 2D and 2.5D simple features to micro-machining of complex 3D curved parts (see Figure 4). The small molds processed by this method can achieve the purpose of mass production. The following mainly introduces micro-cutting equipment, tools, and cutting mechanisms.

Figure 4 Micro-cut parts

(1) Micro cutting equipment

The size and quality of the part (machining accuracy, surface roughness, repeatability) are closely related to the performance of the machine tool (such as accuracy, dynamics, etc.). The performance of the machine tool is mainly related to the spindle, the table and the control system. The diameter of the tool used for micro-cutting is very small. In order to improve the machining efficiency, the spindle speed of the micro-cutting machine is very fast. In order to meet the torque requirements, electric spindles and hybrid angular contact bearings are usually used. These bearings have thermal expansion due to frictional heat generation, and the maximum speed generally does not exceed 60,000 r/min. When the speed is higher, the air bearing should be used, but the air bearing provides less torque. At present, the maximum speed of the air bearing spindle can reach 200,000 r/min. In order to achieve a higher cutting speed, the taper of the spindle coincides with the taper of the high speed cutting shank. The table of the micro-cutting precision machine tool is generally driven by a linear motor. Compared with the ordinary drive such as the ball screw, the linear motor drive system has no cumulative error caused by friction and electromagnetic coupling, and there is no loss of precision due to wear and tear. The gap, and can provide a large acceleration, the accuracy of the linear motor drive system can reach ±1μm. Micro-cutting precision machine tools have good stiffness, low vibration, and most come with a variety of sensors and actuators. However, due to its large size, the control requirements for the surrounding environment are relatively strict, making the cost of processing small parts higher. The machining centers and CNC lathes currently available for micromachining on the market are shown in Figure 5.

Figure 5 Machining Center for Micro-Cutting and CNC Lathe

Figure 6 Small machine tool

Due to the small size of the machining features of tiny mechanical products, researchers are trying to develop tiny machine tools to machine small parts. Small machine tools are very small in size and can save a lot of raw materials, so they can be made from materials with better performance. In addition, due to the small mass, the natural frequency of the tiny machine tool is higher than that of the ordinary machine tool, which enables the small machine tool to stably use it over a wide range of spindle speed without flutter. Even if vibration occurs, the amplitude of the minute machine tool is small under the same load. The positioning accuracy of the tiny machine tool can reach the nanometer scale, and the machining precision is submicron.

The development of tiny machine tools has introduced a new concept called “miniature plants”. The miniaturization plant has a very small footprint and can be placed anywhere in any building, even on the battlefield or in a space station, which is almost impossible for ordinary machine tools. Miniaturization plants consume very little energy and greatly save energy use. There are different production units in the mini-chemical plant, such as mini lathes and micro-milling machines.

The development of tiny machine tools currently faces a number of challenges, such as the need to develop sensors and actuators of sufficiently small size to be installed in tiny machine tools. The rigidity of small machine tools is not as good as that of micro-cutting precision machine tools. In addition, in order to prevent external interference, small machine tools need to add vibration isolation devices to meet the processing accuracy requirements. Reducing the processing costs of miniaturization plants and developing multi-functional composite micro-machine tools are the future development trend of micro-cutting equipment.

Figure 7 micro chemical plant

(2) Micro cutting tool

In the field of micro-machining technology, how to refine the tool material grain and miniaturize the tool to process the micro-workpiece has always been the focus of research.

The cutting depth and feed rate of the micro-cutting are very small, so the cutting force per unit cutting area is large, and at the same time, a large amount of heat is generated, so that the temperature of the local portion of the cutting edge tip is raised, so the performance of the micro-cutting tool material is improved. The requirements are high, and it is necessary to use tool materials with high wear resistance, heat resistance, high temperature hardness and high temperature strength. With the miniaturization of the minimum diameter of the rotation, the bending strength, rigidity and fracture toughness of the rotary cutter are required to be high. Micro-cutting tool materials are mainly made of hard alloy (tungsten carbide), PCBN (stereo boron nitride) and diamond. Non-ferrous metal processing such as aluminum alloys below the micron size mainly uses single crystal diamond tools, and single crystal diamond tools can be used for cutting probes or probes with nanometer precision. In order to improve the performance of cemented carbides, tool manufacturers are currently researching to make carbide grains more fine, and have achieved gratifying results. Ultrafine grained carbides with a particle size of 90 nm have been developed and the particle size has been experimentally produced. It is a 60 nm advanced superfine grained carbide.

In addition to tool materials, the geometry of the tool is critical to achieving micromachining. Under micro-cutting conditions, precise cutting of very thin materials requires an extremely sharp cutting edge, which is a very small cutting edge radius. Not only that, the sharpness of the cutting edge is also related to the quality of the cutting surface, the microstructure and the lattice dislocation. Accurate measurement of the tool edge contour is a prerequisite for the tool edge grinding and quality analysis of the fine cutting process. Micro-drill or micro-end milling cutters have high hardness and difficult processing. The commonly used grinding wheel grinding method has low processing efficiency, and FIB (Focused Ion Beam), WEDGE (Wire Electro Discharge Grinding) The method of making carbide micro drills or micro-milling cutters is very convenient and easy to meet the accuracy requirements. End mills with two-tooth, trapezoidal, semi-circular, inline, square and other shapes can be used for milling. Carbide shank milling cutters suitable for micro-machining have been widely used in the industry. High-precision fabrication of micro-milling cutters and drill bits requires high technical requirements. The smaller the diameter, the more difficult the production, the milling cutter with a minimum diameter of 0.1 mm and The drill bit has been able to produce. Among the hard alloy micro drills currently available on the market, the drilled drill has a minimum diameter of 0.03 mm and a flat drill of 0.01 mm. According to reports, in the laboratory using electrolytic grinding, a very small diameter drill bit of 0.005mm can be produced.

The micro-tools currently available on the market are extremely uneven in size and shape. For example, 31 drills with a diameter of 0.02 mm supplied by the same supplier were tested. The test results showed an average diameter of 0.021 mm and a standard deviation of 0.0015 mm. The average thickness of the core was 0.0063 mm and the standard deviation was 0.0017 mm. This accuracy is obviously poor. Therefore, improving the manufacturing precision of micro-tools is one of the problems that micro-cutting needs to solve.

Fourth, the micro-cutting mechanism

The study of micro-cutting mechanism is of great significance for rational selection of cutting parameters, guaranteeing the quality of micro-cutting, reducing production costs and increasing productivity. In micro-cutting, due to the small size of the workpiece, it is not allowed to use a large depth of cut and feed in terms of strength and rigidity. At the same time, in order to ensure the dimensional accuracy of the workpiece, the thickness of the surface finish layer of the final finishing must be less than its accuracy. Value, so the amount of cutting must be small, such as the depth of cut is sometimes smaller than the grain diameter of the material, so that the cutting can only be carried out in the grain, then the cutting is equivalent to cutting one by one, the physical essence of cutting is Cutting the molecules and atoms between materials to remove atoms or molecules, the traditional cutting theory based on continuum mechanics is not suitable for micro-cutting. Therefore, the study of micro-cutting mechanism needs to adopt a different method from traditional plastic theory. research. The strain gradient plasticity theory is the extension and improvement of the traditional plasticity theory, and it is the necessary bridge between the classical plastic mechanics theory and the atomic simulation. In recent years, a variety of strain gradient plasticity theories have been developed. The typical CS (couple stress) strain gradient plasticity theory, SG (stretch and rotation gradients) strain gradient plasticity theory and MSG (mechanism-based strain gradient) strain gradient plasticity theory . The most basic relationship of strain gradient theory—the equivalent strain can be described as

Where εe is the traditional equivalent strain; ηijk and xijk are the strain gradient and deformation rate respectively; lg and lR are the parameters that characterize the strength of the material stretching gradient and the strength of the rotation gradient, respectively; μ (between 1 and 2) To characterize geometric dislocations and statistical dislocations.

Using strain gradient theory, scale effect and dislocation effect can be predicted, and the results are consistent with the test. Microindentation, crack tip field, interface crack, filament twist and meager have been successfully analyzed in the field of micromachines and micro-components. Problems such as beam bending have begun to be applied in micro-molding research. Using strain gradient plasticity theory to study micro-cutting deformation will be the direction of micro-cutting mechanism research. In addition, the spindle rotation speed during micro-cutting is generally very high, and the machining precision is very precise. Therefore, micro-cutting has the characteristics of high-speed precision cutting. Applying the research results of high-speed precision cutting mechanism to the micro-machining field is also a trend of micro-machining research.

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