This chapter presented, in details, the development of a numerical (FEM) model for the simulation of plunge cutting process. Numerical simulations were carried out considering silicon and silicon carbide as work material and diamond as a cutting tool having 0º, –25º and –30º rake angle. FEM based numerical model was developed by incorporating simultaneous speeds along x and y-axis directions to achieve a depth of cut from 0 to 600 nm. Various zones of material removal, viz. ductile zone, transition zone, and brittle zone were studied. To identify the critical depth of cut, i.e., ductile to brittle transition, various output parameters such as machined surface, cutting force and specific cutting energy were thoroughly analyzed.
Pressure sensitive Drucker-Prager material model was used to define the material behavior of
Ductile
zone Transition zone Brittle zone
Force (N)
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silicon and silicon carbide. Material damage law and friction law were used to account for chip separation and contact interaction.
During the study, initially, the surface profiles were studied using the visual method.
The criterion of initiation of first brittle fracture was used to determine the critical depth of cut. For 0º rake angle tool with 1000 mm/s speed in the y-direction, the initiation of first brittle fracture was noted to occur at a depth of cut of 211.4 nm, which was considered to be the critical depth of cut. Similarly, for –25º and –30º rake angles, critical depths of cut were found to be 225.41 nm and 243.16 nm respectively. These results were found consistent with the fact that with the increase in the rake angle, there will be larger plastic deformation and hydrostatic pressure and ultimately increase in the critical depth of cut.
In the second part, the variation of machining forces as a function of depth of cut was analyzed for the determination of critical depth of cut. It was seen that, with the increase in depth of cut, the fluctuation/frequency of the force becomes more prominent in the brittle region than that in the ductile region. The ductile to brittle transition thicknesses were found to be 78 nm, 84 nm and 108 nm for 0º, –25º and –30º rake angle tools respectively. The transition points or thicknesses are also verified by studying the coefficient of friction (the ratio of cutting force to thrust force). At the transition point, the cutting force becomes higher than the thrust force and the coefficient of friction becomes higher than unity.
In the third section, the criterion of specific cutting energy was used to identify the transition depth. The intersection point of the ductile zone and the brittle zone were obtained and accordingly the values of transition depths were obtained. The curves were fitted using 9th order polynomial curve fitting algorithm and intersection points were calculated by fitting the high slope and flat slope linearly. The estimated CDCs were found to be in the range of 24–34 nm.
Finally, a comparative analysis of the three methods was carried out. From the comparison, it was observed that the CDCs obtained from the visual inspection were the highest and CDCs from the SCE method were the lowest. However, the CDCs obtained from the force analysis were found close to the experimental CDCs obtained from the available literature and earlier nanoindentation simulations described in Chapter 3. However, based on the above discussion, it can be concluded that the visual inspection method and the SCE method have inherent limitations and the force analysis method is found to be realistic.
Further, the work was extended by carrying out a simulation for silicon carbide and the critical depth of cut was determined. For this simulation, only the force analysis was TH-2306_10610325
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It is envisaged that this work provides a quicker prediction of transition depths; which would help in generation of desired surfaces during SPDT operation. After completion of the study on DRM in SPDT, the machining forces generated during SPDT operation were studied. The details about the same are presented in the next chapter.
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115 CHAPTER 5 NUMERICAL MODELING AND SIMULATION OF CUTTING FORCES DURING
SINGLE POINT DIAMOND TURNING PROCESS 5.0 Scope
This chapter presents the development of two-dimensional non-linear plane strain FEM based numerical model of single point diamond turning (SPDT) to simulate the cutting phenomenon and to predict the cutting forces. Initially, the need to carry out the present numerical analysis of SPDT process is defined. An overview of the proposed approach for modeling of SPDT using FEM is presented. Details of the development of a submicron level orthogonal cutting process that captures ductile deformation leading to material separation has been elaborated. A comparative study on the effect of material models for silicon and silicon carbide has also been carried out. The developed models were validated using the measured cutting and thrust forces during the experiments. A systematic study has been presented on the influence of process parameters on the process performance measures (machining forces) using response surface methodology and full factorial analysis. Finally, the predictive models for the process performance were developed.