Presentation on theme: "Submitted by: Raja Vardhan Movva 2008AMD3103 Submitted to: Asst. Prof. M.P.Gururajan Dept. of Applied Mechanics, I.I.T Delhi."— Presentation transcript:
Submitted by: Raja Vardhan Movva 2008AMD3103 Submitted to: Asst. Prof. M.P.Gururajan Dept. of Applied Mechanics, I.I.T Delhi
Introduction: Manufacturing of a component must provide its basic size and shape with desired surface finish and tolerances. Grinding is one most widely used process to achieve material removal and desired surface finish with acceptable surface integrity, dimensional tolerance and form tolerance. In grinding, an abrasive material rubs against the metal part and removes tiny pieces of material. On a microscopic scale, the chip formation in grinding is the same as that found in other machining processes. The abrasive action of grinding generates excessive heat so that flooding of the cutting area with fluid is necessary. The grinding technology for advanced materials such as optical glass, WC, ceramics and silicon has substantially grown with the widespread use of precision components made of such materials in various applications like micro lenses, semiconductor components, automobile diesel injectors, and magnetic heads for computers etc.
Ductile regime grinding The ductile regime grinding hypothesis of Bifano et al.  states that, for any material, if the dimensional scale of material removal is small enough, then plastic flow of the material will take place without fracture. Under certain controlled conditions, it is possible to machine brittle materials like ceramics, glasses etc using single- or multipoint diamond tools so that material is removed by plastic flow, leaving a crack-free surface. This process is called ductile regime machining. Mechanism of ductile/ shear mode grinding of brittle materials The ductile surface is the result of material removal due to shearing, where the material is planed off at a micro scale level.
In the ductile mode, the energy is transferred into a permanent deformation of the work piece material; in contrast, in the brittle mode, more machining energy is transferred to a fracture process. Another way of viewing the ductile regime machining is described by Miyashita as,The material removal rates for grinding and polishing are compared and there is a gap in which neither technique has been utilized. This region can be termed the micro-grinding gap since the region lies in between grinding and polishing. This gap is important because it represents the threshold between ductile and brittle grinding regimes for a wide range of materials like ceramics, glasses and semiconductors.
Principle of ductile regime machining The transition from brittle to ductile mode during machining of brittle materials was described in terms of the energy balance between strain energy and surface energy. The critical penetration depth dc for fracture initiation is described as follows Where Kc is the fracture toughness, H is the hardness, E is the elastic modulus and b is a constant which depends on tool geometry. Fig: projection of the tool perpendicular to the cutting direction. According to the energy balance concept, fracture damage will initiate at the effective cutting depth and will propagate to an average depth yc. If the damage does not continue below the cut surface plane, ductile regime conditions are achieved
Another interpretation of ductile transition phenomena is based on cleavage fracture due to the presence of defects. The critical values of a cleavage and plastic deformation are affected by the density of defects or dislocations in the material. Since the density of defects is not so large in brittle materials, the critical value of fracture depends on the size of the stress field. a. small stress field b. large stress field When the uncut chip thickness is small, the size of the critical stress field is small to avoid cleavage. Consequently a transition in the chip removal process from brittle to ductile may take place depending on the uncut chip thickness. The negative rake angle provides the required hydrostatic pressure for enabling plastic deformation of the work material beneath the tool radius. The energy required to propagate cracks is believed to be greater than the energy required for plastic deformation below the critical depth of cut region.
Yoshikawa  classified the stress field into four domains as shown in Fig 1. Domain I — material removal takes place not only by mechanical action but also by chemical/temperature effects. Only a very small quantity of material is removed. 2. Domain II — here no dislocation is present and the material is assumed as an ideal crystal. Dislocations are created prior to brittle fracture. After the creation of dislocations, the crystal is assumed to behave as in Domain III. 3. Domain III — plastic deformation occurs at this domain followed by crack initiation at the deformation zone. 4. Domain IV — material removal takes place only due to cracks. with decrease in the un deformed chip thickness, the distribution of movable dislocations in micro-structures approaches zero and cutting forces have to overcome the very large atomic bonding forces within the micro-structures.
CHIP FORMATION: The lattice of the work piece is noted to become deformed, or buckled, due to the ploughing effect of the cutting edge. When the strain energy of deformed lattice exceeds a specified level, the atoms begin to rearrange, so that the lattice strain is relaxed. But it is usually not sufficient to provide for complete or flawless rearrangement, some dislocations are generated in the work piece lattice. As the cutting edge advances, many dislocations are successively generated at the interface with the tool and Some will move into the shear zone, ‘‘disappearing’’ from the free surface as a chip is formed for the length corresponding to an atomic layer. This effect corresponds to the elemental process of morphological chip formation. Figure shows an example of the deformation behavior of the work piece atoms at the leading edge of the cutting tool. As a result of successive generation and disappearance of the dislocations, a stable chip removal process can be envisaged.
The other dislocations penetrate into the work piece under the cutting edge. After the cutting edge has passed, these dislocations begin to move back, before finally disappearing from the work piece surface, as a consequence of the relaxation of the lattice, as the work piece ‘‘springs back.’’ Fig :Model of chip removal with size effect Fig: deformed top layer As a result of this relaxation, atomic-size steps are formed on the work piece surface The height of these steps on the work surface can be considered to be the ultimate surface roughness attainable in micro cutting. A ductile mode micro machined surface consists of an upper compacted layer that sits above the bulk material. A very little subsurface sub surface damage occurs. The efficiency of ductile removal is strongly influenced by environmental effects such as lubrication and heat generated during the machining, which may alter the fracture toughness of the work piece surface. A higher degree of permanent deformation arises in this process. The magnitude and depth of the deformation are dependent on the grit size.
Selection of tool material for various work piece materials Although ductile mode cutting can be achieved and nano metric surface finishes can be obtained, the tool life is still a major obstacle in the use of diamond cutting technology. One important reason for this could be that with decrease in the un deformed chip thickness, the distribution of movable dislocations approaches zero and cutting forces have to overcome the very large atomic bonding forces within the micro-structures Ultra precision cutting tools need to be hard and sharp and to have enhanced thermal properties in order to maintain their size and shape while cutting. Advantages offered by diamond include: 1.Crystalline structure, which enables very sharp cutting edges to be produced, 2.High thermal conductivity, the highest of any materials at room temperature, 3.Ability to retain high strength at high temperatures, 4. high elastic and shear modules, which reduce deformation during machining. S.No Work piece materialTool material 1For brittle materials ( silicon wafers, ferrite crystals, ceramics, and glasses) Diamond abrasive 2For hardened tool steels and some aerospace alloysCBN abrasive 3For normal steel and most cast ironsAluminum oxide 4For most nonferrous metalsSilicon carbide
Surface Quality Control: Work piece surface quality includes the aspects of texture and integrity. Surface texture refers to the micro geometry or topography, which is usually characterized by surface roughness, although other characteristics such as waviness lay, and flaws may also be of interest. The surface generated with ductile grinding consists of mostly overlapping scratches produced by the interaction of abrasive cutting points with the work piece. Surface integrity is associated more with mechanical and metallurgical alterations to the work piece surface layer induced by machining. The variations, ranging from clearly observable cracks in the surface to subtle transformations such as hardness change, recrystallization, fatigue strength, or residual stress in the underlying metallic structure, These are caused mainly by the following: 1. Plastic deformation resulting from the point work of abrasive grits or 2. Heat generation through cutting, changes of temperature, and its non uniform distribution in the surface layer.
Advantages: This technique are used in machining a variety of engineering materials for electronic, automotive, and optical applications, as well as others which provides damage-free surface quality, tighter dimensional and tolerance control, and higher geometrical form accuracy. This process can be distinguish from conventional grinding, lapping,polishing for following reasons: 1. Provision of controlled, predictable machining for difficult brittle materials, 2. Ability to impart desirable compressive stresses in finished surfaces, 3. Provision of predictable surface finish patterns to meet specific design criteria for wear, sealing, or lubrication, and 4. Broadening the range of machineable materials. Drawbacks: The main drawbacks are rapid tool wear and lack of proper modeling of ductile mode for all the brittle materials. one approach to prolong tool life is to apply ultrasonic vibration to the diamond cutting tool with vibration during cutting, the lubricant can easily penetrate the cutting zone. Also, the shatter contact time between the cutting tool and work material improves the tool life. In addition to increasing the tool life, another advantage of the use of ultrasonic vibration assisted cutting is that the critical depth of cut can be increased.
Electrolytic in process dressing Electrolytic in-process dressing (ELID) grinding is one new and efficient method that uses a metal-bonded diamond grinding wheel in order to achieve a mirror surface finish especially on hard and brittle materials. Truing and dressing of the wheels are major problems and they tend to glaze because of wheel loading. Electrolytic in process dressing (ELID) is the most suitable process for dressing metal-bonded grinding wheels during the grinding process and wheel loading can be avoided. Principle of ELID grinding: The basic ELID system consists of a metal-bonded diamond grinding wheel, an electrode, a power supply and an electrolyte. The metal-bonded grinding wheel is made into the positive pole through the application of a brush smoothly contacting the wheel shaft and the electrode is made into the negative pole. In the small clearance of approximately 0.1 to 0.3 mm between the positive and negative poles, electrolysis occurs through supply of the grinding fluid and an electrical current.
Mechanism of ELID grinding: After truing Fig. (a), the grains and bonding material of the wheel surface are flattened. Predressing must be done electrically to protrude the grains on the wheel surface. When predressing starts Fig. (b), the bonding material flows out from the grinding wheel and an insulating layer of oxidized bonding material is formed on the wheel surface Fig. (c). This insulating layer reduces the electrical conductivity of the wheel surface and prevents excessive flow-out of the bonding material from the wheel. As grinding begins Fig. (d), diamond grains wear out and the layer also becomes worn out Fig. (e). As a result, the electrical conductivity of the wheel surface increases and the electrolytic dressing restarts with the flow- out of bonding material from grinding wheel. The protrusion of diamond grains from the grinding wheel therefore remains constant. This cycle is repeated during the grinding process to achieve stable grinding.
Applications of ductile regime grinding and ELID Fig: Micro Lens machining at the tip of a 5-mm glass rod. Fig: Optic fibre connector fabricated by micro grinding Fig: Topography comparison Fig: Micro mould insert with diameter of 200 μm fabricated by micro grinding
References: 1. Z. W. Zhong, V. C. Venkatesh,” Recent developments in grinding of advanced materials “ Int J Adv Manuf Technol 2001 2. T. G. Bifano, T. G. Dow and R. O. Scattergood, “Ductile regime grinding – A new technology for machining brittle materials”, Journal of Engineering for Industry, 113, pp. 184–189, 1991. 3. B. K. A. Ngoi and P. S. Sreejith,“Ductile Regime Finish Machining – A Review” 4. H. Yoshikawa, Brittle–ductile behavior of crystal surface in finishing, Journal of JSPE 35 (1967) 662–667 5. Nakamura, Tsunetaka Sumomogi and Takayuki Goto, “Study on Ductile Mode Grinding of Brittle Materials using Single Abrasive Grain” 6. F Z Fang, X D Liu and L C Lee,”Micro-machining of optical glasses – A review of diamond cutting glasses” 7. Micro Machining of Engineering Materials by Joseph Mc Geough, Marcel Dekker, Inc. 8. M Rahman, A Senthil Kumar, H S Lim and K Fatima, “Nano finish grinding of brittle materials using electrolytic in-process dressing (ELID) technique” 9. H.S. Lim, K. Fathima, A. Senthil Kumar, M. Rahman,“A Fundamental study on the mechanism of electrolytic in-process dressing (ELID) grinding”