Takahiro Shirakashi

Finite element simulation of plane strain plastic-elastic indentation on single-crystal silicon

Meso-plasticity FEM technique was applied to simulate the dislocation generation and propagation during indentation of a single-crystal silicon. Dislocations were generated and concentrated under the indenter and propagated into the interior of the workmaterial as the indentation progresses. Similarly, the hydrostatic stress and the principal stress were concentrated directly underneath the indenter. The magnitudes of these stresses are found to increase with increase in the depth of indentation. It is proposed that pre-existing microcracks are not necessary for the defect generation in the workmaterial. Instead, a concentration of dislocations generated by plastic deformation under light loads and high hydrostatic pressures can play a similar role. The role of hydrostatic pressure in suppressing fracture was investigated. Based on these studies, it appears feasible to generate crack-free, smooth surfaces below a critical load or cut depth in ultraprecision machining of silicon.

A Study on Cutting Force in the Milling Process of Glass

The paper discusses the milling process for machining glass without brittle fracture. Although glass is a brittle material, it can be deformed in a ductile mode if the undeformed chip thickness is less than a micrometer. The undeformed chip thickness starts at zero in the up-cut milling process. Glass, therefore, can be machined without brittle fracture at the beginning of the cut, and then brittle fracture occurs at a certain undeformed chip thickness. The area of brittle fracture can be finally removed with the cutter feed. As a result, the machined surface can be made in a ductile mode of the cutting process. The cutting process in the up-cut milling of glass is discussed and the cutting force measured. The cutting force gradually increases with the cutter rotation at the beginning of the cut, and oscillates about a constant mean value after a certain undeformed chip thickness. The transition from ductile to brittle mode is specified in the change of cutting force. The effect of cutting conditions on the transition is investigated with the mechanism of the milling process. It is proved that the machined surface without brittle cracks can be obtained in a large radial depth of cut at a high feed rate.

Some experiments on the scratching of silicon:: In situ scratching inside an SEM and scratching under high external hydrostatic pressures

Abstract To elucidate the mechanisms of material removal in ultra precision machining of silicon involving deformation and fracture, in situ scratching of silicon with a diamond stylus inside a scanning electron microscope (SEM) using a specially designed tribometer and scratching under zero and high (400 MPa) external hydrostatic pressures using a specially designed high-pressure machining apparatus were conducted. The resulting scratches were examined in an SEM to evaluate the influence of depth of cut and hydrostatic pressure on the nature of scratches produced (smooth versus fractured surfaces) and the possible brittle–ductile transition. Experimental results indicate that hydrostatic pressure plays an important role in minimizing fracture and producing smooth surfaces. Reasonable agreement of the experimental results with the meso-plasticity FEM simulation of indentation was obtained.

Scratching Test of Hard-Brittle Materials Under High Hydrostatic Pressure

This paper proposes machining under high hydrostatic pressure as a new damage-free machining method for hard-brittle materials. Experiments for this study utilized a specially designed scratching test device, and the pin-on-disc scratching tests were conducted with three (3) hard-brittle materials (i.e., silicon, glass, and quartz) under pressure of 400 MPa and zero MPa. Traces of scratches on these specimens were examined with microscopes to evaluate the effects of hydrostatic pressure on machining defects. The results of the experiments show that hydrostatic pressure is efficient in minimizing machining defects and optimizing the productivity of the hard-brittle materials used in these experiments. Based on these findings, the paper concludes that the origin of a machining crack must exist in the subsurface of the workmaterial.

What is expected from numerical simulation on machining process

An outline for numerical simulation on cutting process by Finite Element Method (FEM) and required data are discussed. The recent reports concerned on the simulation and their targets are referred. An importance of material characteristics, friction one and fracture or separation criterion on practical machining phenomena is also discussed. The simulation methods on both continuous chip formation and discontinuous one are shown. The points at issue on these simulation results and the future trend of them are also discussed. Finally, an availability of the simulation and the expected fruits are discussed.

Deterioration Process of Sintered Material by Impact Repetition

For prediction of time dependent tool breakage of sintered carbide tool in interrupted turning operation, the special impact stressing set‐up is prepared. A change of fracture stress—deterioration process—of a sintered carbide tool material with both tensile and compressive impact stressing repetition is discussed and the process is evaluated through the fracture stress criterion superposed by Weibull’s distribution. The reliability of fracture stress is decreased with the repetition, the maximum fracture stress, however, is not decreased. The equivalency between compressive and tensile stresses on the process is also discussed and the process is shown as change of probabilistic fracture locus with impact repetition times. Finally a deterioration state of sintered carbide tool under interrupted turning operation with the so called parallel entry and a very soft exit condition is estimated based on the deterioration process and the probability map of breakage occurrence on tool surface is shown under given...

Simulation of Ball End Milling Process with Cutter Axis Inclination

A force model based on the minimum cutting energy is applied to the simulation of the milling processes with the cutter axis inclination. The cutting model during the cutter rotation is made with coordinate transformation rotated at the tilt angle and the lead angle. Based on the orthogonal cutting data, the cutting force in the ball end milling process can be simulated with the chip flow model. The comparison of the analyzed forces with the measured ones verifies the simulation based on the presented force model.

Development of Reaction-Controllable Milling Head and Its Cutting Performance.

In order to control cutting force and its direction, a combined milling head was newly developed. This head has two milling cutters which rotate in opposite directions, and a meshing phase of cutters is also variable. The cutting performance of the head was discussed based on the cutting mechanism of a single milling cutter, and a method for simulating the performance was proposed. Cutting force, its change and direction are greatly affected by the change of the ratio of depth of cut for each cutter and the change of the meshing phase. Using the numerical simulation of cutting performance of the head, the optimal cutting conditions, such as the ratio of the up/down cutters depth of cut and the phase, can be determined. The simulated performance and the experimental one show good agreement.

Crater Wear Prediction of Coated Tools.

No matter what wear mechanism controls tool wear, tool wear rate can be expressed as functions of stress and temperature on the tool face. Thus stress and temperature distributions on the rake face were measured with split tools and with thermocouples respectively to obtain wear equations. Specific crater wear rate of coated tools was linear with normal stress and increased exponentially with temperature. Using the wear equations, crater wear of coated tools was predicted through cutting temperature analysis by the newly proposed finite difference method. The calculated results for the tools with single and double coating layers were in good agreement with experimental results. The analysis for predicting crater wear of coated tools showed that thermal conductivity of substrate has a great influence on tool wear rate.