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초음파가공(ultrasonic machining)

1.초음파가공
    

Ultrasonic machining is a non-traditional mechanical means of uniform stock removal, applicable to both conductive and nonconductive materials. Particularly suited for very hard and/or brittle materials such as graphite, glass, carbide, and ceramics.
It uses medium to high frequency vibrations (in the area of 20KHz) through a designed or formed tool having the shape of the cavity to be machined, combined with a fine abrasive slurry to produce accurate holes (holding tolerance on the order of 0.0005 inch) or cavities of regular or irregular shapes. Vibration of the tool top is obtained through a transducer. It works as an ultrasonic generator converting electrical energy into mechanical energy.
The process is non-thermal, non-chemical, and non-electrical. It does not change the metallurgical, chemical, or physical properties of the material machined.
The majority of the material is removed from the work-piece by micro-chipping or erosion caused by the impact of the accelerated abrasive particles interacting with the surface. The amplitude of the vibration of the transducer is on the order of 0.001 mm and must be amplified to within a range of 0.025 through 0.075 mm to get the desired cutting effect. The larger the amplitude, the greater faster the cutting or material removal rate.
Ultrasonic machining, also known as impact grinding, has been around for since the 50's. Where its initial use was as a surface finishing process for parts made by electro-discharge machining. Today it can be used to cut virtually any material, but it is most effective on very hard materials. Particularly in the area of ceramics where it offers the promise of higher accuracy, better surface finish, a lower degree of surface degradation, and greater complexity of geometry.
The basic machining process uses a tool to push against the work piece with a constant force. A continuous stream of abrasive slurry passes between the work-piece and the tool surface. The slurry medium provides a constant source for the abrasive particles (which are constantly breaking) and a means to carry away the machining debris and fractured particles. The abrasive slurry is circulated by a refrigerated pump system. It removes the heat generated by the cutting process, thus preventing boiling in the 25-40 micrometer gap. This prevents the undesirable cavitation effect due to high temperatures. The majority of the cutting action comes directly from the cyclic accelerated particles being forced against the work piece surface.
The tool, whose shape is translated to the work-piece surface, can be made from an easily machined ductile material. The tool material is subject to wear, necessitating proper care given to the selection of work-to-tool combinations. Tool materials are usually mild steel, carbide, tool steel, or brass. Tool wear will vary depending on the hardness of the tool material. Usually wear ratios are in the range 1:1 to 100:1 (material removed vs tool wear). Ductile tool materials allow tool surface hardening in some processes, thus increasing wear ratios. At the same time it provides ease of manufacture of tools by traditional processes and thus create cost reduction in the creation of tools making the entire process more affordable.
The abrasive grit in the slurry medium are typically of low cost easily available materials like aluminum oxide used for glass, or silicon carbide. Boron carbide is also used for tungsten carbide, die steel, and gems though a little more expensive. In special cases, boron silicarbide or diamond grit may be considered. Typical grit size for the abrasive slurry range from 100 to 800. The proper selection of abrasive particles is dependent on the type of material being machined, hardness of the material, the desired removal rate of material, and the surface finish needed. The larger grit is used for a rougher cut on the piece, and the smaller grit is used to create a finer finished surface.
The most common fluid medium used for ultrasonic machining is water. However, it is not the only fluid medium that can be used. Others are benzene, glycerol, or oils.
The concentration of abrasive grains or grit in a water slurry range from 20% to 60% by volume. However the best average results are achieved around 30% concentration.


2. 초음파가공


High-frequency, low-amplitude energy is transmitted to the tool assembly. A constant stream of abrasive slurry passes between the tool and workpiece. The vibrating tool, combined with the abrasive slurry, uniformly abrades the material, leaving a precise reverse image of the tool shape. The tool does not come in contact with the material; only the abrasive grains contact the workpiece.

In the UM process, a low-frequency electrical signal is applied to a transducer, which converts the electrical energy into high-frequency (~20 KHz) mechanical vibration. This mechanical energy is transmitted to a horn and tool assembly and results in a unidirectional vibration of the tool at the ultrasonic frequency with a known amplitude. The standard amplitude of vibration is typically less than 0.002 in. The power level for this process is in the range of 50 to 3000 watts. Pressure is applied to the tool in the form of static load.
A constant stream of abrasive slurry passes between the tool and the workpiece. Commonly used abrasives include diamond, boron carbide, silicon carbide and alumina, and the abrasive grains are suspended in water or a suitable chemical solution. In addition to providing abrasive grain to the cutting zone, the slurry is used to flush away debris. The vibrating tool, combined with the abrasive slurry, abrades the material uniformly, leaving a precise reverse image of the tool shape. Ultrasonic machining is a loose abrasive machining process that requires a very low force applied to the abrasive grain, which leads to reduced material requirements and minimal to no damage to the surface. Material removal during the UM process can be classified into three mechanisms: mechanical abrasion by the direct hammering of the abrasive particles into the workpiece (major), micro-chipping through the impact of the free-moving abrasives (minor), and cavitation-induced erosion and chemical effect (minor).2
Material removal rates and the surface roughness generated on the machined surface depend on the material properties and process parameters, including the type and size of abrasive grain employed and the amplitude of vibration, as well as material porosity, hardness and toughness. In general, the material removal rate will be lower for materials with high material hardness (H) and fracture toughness (KIC).
UM effectively machines precise features in hard, brittle materials such as glass, engineered ceramics, CVD SiC, quartz, single crystal materials, PCD, ferrite, graphite, glassy carbon, composites and piezoceramics. A nearly limitless number of feature shapes?including round, square and odd-shaped thru-holes and cavities of varying depths, as well as OD-ID features?can be machined with high quality and consistency (see Figure 3). Features ranging in size from 0.008 in. up to several inches are possible in small workpieces, wafers, larger substrates and material blanks. Aspect ratios as high as 25-to-1 are possible, depending on the material type and feature size.
Unlike conventional machining methods, ultrasonic machining produces little or no sub-surface damage and no heat-affected zone. The quality of an ultrasonic cut provides reduced stress and a lower likelihood of fractures that might lead to device or application failure over the life of the product (see Figure 4). UM is particularly well-suited for high-reliability applications where preservation of the critical material properties and avoidance of the introduction of residual stresses from machining processes are vital to the project’s success.
An added benefit is that parts machined ultrasonically often perform better in downstream machining processes than do parts machined using more conventional machining methods. The improved performance can result in economic advantages from higher yields, lower scrap and operating costs, and improved efficiencies.

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