Grant M. Robinson

Laser dressing of alumina grinding wheels

High-power lasers are being explored as a non-contact-type dressing tool for alumina grinding wheels. The alumina grinding wheel surface underwent melting and/or vaporization on the surface when laser-dressed, forming a modified layer on the surface. Refinement of the grain size took place. The individual particles that formed on the surface had well-defined faceted structures. Microcutting edges were generated on the individual grains and particles, which can act as cutting edges for efficient grinding. The results of x-ray diffraction and pole figure analysis suggested that the formation of these faceted structures was due to the preferential orientation of the grains after dressing.

Machining Cancellous Bone Prior to Prosthetic Implantation

The structure of cancellous bone can be described as heterogeneous, and as such, is difficult to shape by cutting tools during clinical surgical practices. The structure of bone can have a devastating effect on the performance of the cutting tool unless it is coated with a hard-wearing, thin solid film. Here, the use of diamond-coated cutting tools to prepare bone for biomedical implants are investigated. This paper describes developments in the use of coated cutting tools for machining of cancellous bone and to prepare a nanostructured surface.

Femtosecond laser micromachining of aluminum surfaces under controlled gas atmospheres

The interaction of 180 femtosecond (fs), 775 nm laser pulses with the surface of aluminum under controlled gas atmospheres at ambient pressure has been investigated to study material redeposition, residual surface roughness, and ablation rate. The effect of using various gases to protect the surface of the material appears to interfere with the effects of the plasma and can change the resulting microstructure of the machined surface. By varying the combinations of fluence and laser-scanning speed during ultrafast ablation at high repetition rates, an optimum micromachining condition can be reached, depending on the type of gas used during machining. The debris produced under certain laser-machining conditions tends to produce pure aluminum nanoparticles that are deposited very close to the machined feature by the gas used to protect the surface of the aluminum.

Fundamentals of Machining

During chip formation there is a substantial increase in the specific energy as chip size is reduced during machining. It is believed this is due to the fact that all metals contain defects such as grain boundaries, missing and impurity atoms, and when the size of the material removed decreases the probability of encountering a stress-reducing defect decreases. Since the shear stress and strain in metal cutting is unusually high, discontinuous microcracks usually form on the primary shear plane. If the material is very brittle, or the compressive stress on the shear plane is relatively low, microcracks will grow into larger cracks giving rise to discontinuous chip formation. When discontinuous microcracks form on the shear plane they will weld and reform as strain proceeds, thus joining the transport of dislocations in accounting for the total slip of the shear plane. This chapter focuses on machining at the micro- and nanoscale and attempts to explain the dominant features of machining as the size effect becomes significant.

The measurement of ultrafine particles: A pilot study using a portable particle counting technique to measure generated particles during a micromachining process

The accurate measurement of airborne particles in the nanometer range is a challenging task. Because several studies have linked exposures to airborne ultrafine particles to elevated human health risks, the need to assess the concentrations of particles in the workplace that are below 100 nm in diameter is imperative. Several different techniques for monitoring nanoparticles are now available, and others are currently being tested for their merit. Laboratory condensation particle counters (CPC), field-portable CPC, nanometer differential mobility analyzers, electron microscopy, and other novel and experimental approaches to measuring nanoparticles have been recently used in investigations. The first part of this article gives an overview of these techniques, and provides the advantages and disadvantages for each. The second part of this article introduces a portable technique, coupling two particle measurement devices that are capable of characterizing microscale and nanoscale particles in the field environment. Specifically, this pilot study involved the use of a direct-reading CPC and a laser particle counter to measure airborne concentrations of ultrafine particles during a laboratory machining process. The measurements were evaluated in real time, and subsequently, decisions regarding human exposure could be made in an efficient and effective manner. Along with the results from this study, further research efforts in related areas are discussed.

Laser surface preparation of vitrified grinding wheels

A new method for surface cleaning loaded grinding wheels is introduced by applying CO2 laser irradiation onto the grinding wheel surface. It was demonstrated that effective cleaning can be achieved by selection of the laser power flux and the duration of the irradiation. Fusion and evaporation of clogged metal chips play important roles in the laser cleaning process. It is suggested that high laser power irradiance and short irradiation duration are essential for effective grinding wheel cleaning.

Time-modulated chemical vapor deposition of diamond films

This article investigates the role of substrate temperature in the deposition of diamond films using a newly developed time-modulated chemical vapor deposition (TMCVD) process. TMCVD was used to deposit polycrystalline diamond coatings onto silicon substrates using hot-filament chemical vapor deposition system. In this investigation, the effect of (a) substrate temperature and (b) methane (CH4) content in the reactor on diamond film deposition was studied. The distinctive feature of the TMCVD process is that it time-modulates CH4 flow into the reactor during the complete growth process. It was noted that the substrate temperature fluctuated during the CH4 modulations, and this significantly affected some key properties of the deposited films. Two sets of samples have been prepared, in each of which there was one sample that was prepared while the substrate temperature fluctuated and the other sample, which was deposited while maintaining the substrate temperature, was fixed. To keep the substrate temperature constant, the filament power was varied accordingly. In this article, the findings are discussed in terms of the CH4 content in the reactor and the substrate temperature. It was found that secondary nucleation occurred during the high timed CH4 modulations. The as-deposited films were characterized for morphology, diamond-C phase purity, hardness, and surface roughness using scanning electron microscopy, Raman spectroscopy, Vickers hardness testing, and surface profilometry, respectively.

Design and manufacture of high-speed spindles for dry micromachining applications

The efficiency of microcutting tools that are used in high-speed air turbine spindles depends on the rotational speed of the rotor. A high-pressure variation on the surface of the rotor causes the rotor to retard and this severely limits the reliability and durability of High-Speed Spindles (HSSs) to support new developments in the developing area of micromachining. A variety of spindle designs were proposed and numerical simulations were carried out for each design using CFX software. The results revealed that changes in the rotor, inlet and outlet geometries affect the pressure distribution on the rotor significantly. The optimum design was identified based on the lowest pressure variation on the rotor surface obtained from the CFX results. Spinning the rotor at very high speeds provides a new direction in the development of mechanical micromachining.

Dynamic response of a tetrahedral nanomachining machine tool structure

The dynamic characteristics of a revolutionary machine tool structure used for machining engineering components at the micro and nanoscales are of paramount importance. Minimising the effects of vibrations at the micro and nanoscale is vital because if a machined workpiece oscillates during the machining process, then an increase in the depth of cut will occur that will reduce the quality of surface finish and the dimensional accuracy of the machined component. The stacking of atoms inspired the design of a new platform structure in order to improve the existing technology, since the goal of the machine is to allow the manipulation of molecules at the nanoscale. The idea for the structure comes from the structural stability afforded by the tetrahedron. The tetrahedral structure is an extremely stable structure and it is hypothesised that the shape could minimise vibrations better than conventional machine tool structures. The reason lies in the way that the structure is kinematically balanced. To explore the hypothesis, an experimental modal analysis established the vibration characteristics of the structure. The modal analysis consisted of measuring the Frequency Response Functions (FRFs) from impact tests to determine the natural frequencies, damping and mode shapes of the structure. A finite element model was compared with the experimental data, thereby validating the model so that it may be used for modelling future changes to the design of next generation machine tools.

Surface grinding of space materials using specially formulated vitrified grinding wheels

The quantum leap that is expected in the reliability and safety of machined engineering components over the next 20 years, especially in the space industries, will require improvements in the quality of cutting tools if science-based manufacturing is the goal for manufacturing by 2020. Significant improvements have been made in the past 10 years by understanding the properties of vitrified bonding systems used to bond conventional and superabrasive materials in grinding tools. The nature of the bonding system is of paramount importance if next-generation cutting tools are to be used for aerospace materials, especially if they are dressed using laser beams.