Advanced ceramic materials are assuming increasing industrial importance because of their special properties. However, the extreme hardness and brittleness of these materials make their machining efficiency low. The difficulty arises from shaping ceramics whilst conserving their surface integrity, strength and high quality surface finish. Manufacturing processes such as grinding with diamond bonded wheels present one solution. The use of a diamond grinding wheel on the surface of ceramics, creates a complex system of elastic/plastic deformation, and subsequent cracking. For ceramic materials such as silicon nitride, two types of damage can occur; firstly surface damage in the form of machining marks which cause stress concentrations and surface cracks and secondly sub-surface cracking. These problems caused a major delay in the utilisation of these materials in the aeronautics industry. This research was aimed at providing a better understanding of the grinding process and parameters involved in machining ceramic component. An extensive preliminary study was carried out to understand the behaviour of advanced ceramic material during grinding. The effect of very high infeed and grinding wheel parameters were studied as an initial investigation. A system for on line detection of grinding wheel surface condition during grinding was developed during this research. Acoustic emission AE technique to monitor the wheel condition was also investigated during the experimental work. The grinding machine used is a commercially available machine with a stiffness coefficient of 18.5 N/μm. The machine was modified to accommodate a higher feed rate. The purpose of using this machine was to investigate how a commercial machine would grind hard material such as advanced ceramics and what the effect would be on the process of crack initiation and propagation during grinding. The effect of different grinding wheel and machine tool parameters on crack initiation and propagation was investigated and represent the main concern of this research. Experimental design using modified L27 orthogonal array was used. The response of the surface roughness, fracture strength, AE spectral amplitude, grinding forces, grinding energy, crack size, sub-surface cracks distance from the ground surface, depth of surface damage due to different grinding wheel and machine parameters used in the fractional factorial array were all investigated. Three basic types of cracks that were found to occur after the passage of an abrasive grain on the surface of ceramics materials were identified. These are radial, median and lateral cracks. Only radial cracks and machine marks are visible on the surface, median and lateral cracks (parallel to the surface) are formed below the affected zone and thus not visible. The load of an individual grain and its shape on the grinding wheel surface were found to have an influence on the crack initiation and propagation. Higher damage depth on the ground surface was measured when the grain size increased or was extremely sharp. Median cracks were found to be at a deeper location from the ground surface (2-15 μm) compared with lateral cracks. The dimensions and depth of median cracks beneath a ground surface was found to increase when the levels of grinding wheel grit size, wheel bond hardness, depth of cut and table feed were increased. However, locations of lateral cracks were found not to be affected when theseparameters increased and found to occur at 2-4 μm below the surface. The median cracks were located close to the ground surface at low table feed rates compared with deeper location when the table feed rate increased. A correlation between AE, SEM and 3D data and the ground surface condition was carried out. The condition of the ground surface whether smooth or rough was identified and linked with AE signals. The link between these observations show that an increase in percentage of surface fracture is accompanied by low-amplitude long­ duration events which can be assumed as the characteristic of brittle mode grinding. This observation was confirmed by the SEM and the 3D observations. Higher AE spectral amplitude was found to be associated with smooth ground surfaces. However, sub­ surface median cracks were found for this condition. This correlation was the first reported attempt to link different experimental measurements and parameters. Different artificial cracks were initiated on ceramic specimen surfaces in both longitudinal and transverse directions using v-shaped discs. This method of initiating cracks is more realistic than that produced by a single diamond scratch. The very random nature of diamond grit shape, size and location on the surface of the wheel would alter the characteristics of a crack initiated in a real grinding process. This technique used for creating an artificial crack is the first reported attempt hitherto. The groove generation mechanism observed using SEM was almost brittle when the depth of scratch increased. At larger groove depths, a deeper locations of sub-surface median cracks were found. Grinding processes were then carried out on these cracks to investigate their propagation. The AE spectral amplitude was found to be higher for specimens ground with artificial cracks compared with specimens ground without artificial cracks. The sub-surface cracks initiated due to grinding specimens with artificial cracks were found to be at deeper locations than to those for ceramic specimens ground without artificial cracks. The effect of remaining damage depths on the ground surface on the fracture strength were also studied. Increasing the diamond grinding wheel grit sizes, wheel bond hardness and table feed were found to be the most influential factors that increase the propagation distance of the artificial cracks during grinding. A computer model based on finite element analysis package was used to study the behaviour of the ceramics in grinding. This package was used to establish a theoretical model which was validated using experimental results. Normal and tangential grinding forces from experimental work were used as input to the FE model. These forces were chosen to indicate different wheel and machine parameters. The model simulates the process of grinding by applying these forces on the surface of the ceramic specimen to study the crack initiation, propagation and their distance from the ground surface. A complete simulation for the grinding process was made including removal of material during the process.

Published work concerning the performance of coated and uncoated carbide tipped circular saw blades has been searched and reviewed. The search has shown that little similar experiments on the carbide tipped circular saw blades have been conducted. Furthermore the evidence shows that no work has been published on the application of coatings to the carbide tipped circular saw blades. Finite element analysis has been widely applied to the cutting process, but the application of finite element methods to the carbide tipped circular saw blade is still a new area of research. In this investigation the performance and life of the carbide tool both uncoated and coated were quantified by rigorous measurement. The proposed carbide properties ( composition, cobalt distribution, specific gravity, micro-structure and red hardness of carbide) have been experimentally tested. The failure modes for the carbide circular saw blades and carbide tipped saw segments have been established. In this work a novel experimental approach was used, in which TiN and TiAlN coatings were applied to the carbide tipped circular saw. A feature of this approach is that a lower coating temperature is used and yet still retains the good bonding strength between the substrate and the coating. The parameters obtained in this investigation were used as a basis in the development of a finite element model. This model was used to optimise tooth geometry and predict the failure mode with respect to temperatures and stresses. Investigation has shown that TiN and TiAlN coatings have largely enhanced the tool life in the case of cutting proposed workpiece materials. The crater wear, flank wear and large chipping have been arrested or reduced. Computer model and transverse rupture strength figure have been established. The results shows that the model used for qualitative analysis of a problem is possible and the examples conducted in the case of cutting mild steel is completely consistent with the failure modes obtained in the cutting tests.

Porous air bearings enjoy some important advantages over conventional air bearing types such as increased load carrying capacity, higher stiffness and improved damping. However, these types of bearings have yet to find widespread acceptance due to problems with obtaining materials with consistent permeability, instability issues relating to the volume of gas trapped at the bearing surface in the pores, and manufacturing the bearing without altering the permeability. Using a series of fine grades of alumina powder to minimise surface pore volume it has been demonstrated that it is possible to consistently and reproducibly manufacture porous bearings by injection moulding and slip casting. The relationship between powder size, processing conditions, porosity, mechanical properties and fluid flow characteristics were experimentally determined. The temperature of processing and the green density were found to be the controlling parameters in the resulting fluid flow properties for a given powder size, Test bearings were produced from the range of processing conditions investigated. It was found that the fine powder size bearings were stable over the entire range of test conditions irrespective of their initial manufacturing route. The most important consideration for the bearing performance was the quality of manufacture. The bearings were found to be sensitive to the flatness of their working surface and quality of fit in their test holder. The bearings were compared with published theories for load capacity and stiffness. A reasonable agreement was found with load carrying capacity once a correction for surface roughness was incorporated. Stiffness predictions provided a useful tool for the analysis and prediction of properties such as optimum values of permeability for a given geometry, if certain allowances are made.

No description available.

Many modifications have been made in the past to the conventional drill points and references to the better performance of curved lip drills when cutting cast iron can be found. Similar drill points do not seem to be as successful with steel. The objective of this research was set to analyse drill design and to study the effect on drill performance of changing the drill conventional flute form when cutting steel. Changing the conventional flute form has an immediate effect on the shape of the drill lip - it is no longer a straight cutting edge.

For a gear designer, the meshing gear tooth root bending stresses, and contact stresses are of major importance. To be able to obtain accurate values of these stresses, it is essential to determine the actual load distribution along the contact lines of the meshing gear tooth pairs. this load distribution. The objective of this work is to predict In the current gear design standards such as AGMA 2001¹, B5436², DIN3990³ and 150-0156336⁴the contact line load distribution is estimated by using a two-dimensional "thin slice" model of the meshing gear teeth. Clearly, this cannot account accurately for maldistribution of loading across the tooth face width, which is essentially a three-dimensional phenomenon. As a result, the effects of tooth lead, profile and pitch deviations are inadequately modelled. In this work, the elastic compliance of wide-faced helical gears of standard tooth form, zero addendum modification, and between 10 and 100 teeth, was determined using a 3-D finite element elastic model of the whole gear. These results were incorporated into a micro-computer program which calculates the load distribution across the meshing tooth pair faces. The effects of a number of parameters such as U, Z, b, and β* on the load distribution and contact stresses of an error-free gear were also investigated using the micro-computer program and the results were compared with other published data and those obtained from the standards²,³,⁴ Vedmar⁵ and Simon⁴³. The load peaks near the start and end of contact, attributed by some⁶,⁷ to the resistance of the unloaded portion of the tooth beyond the shorter contact lines in those regions, is very clearly demonstrated by Vedmar⁵, others⁶,⁷ and this work, but certainly not by the standards (this effect is usually referred to as the "buttressing" effect). The thin slice model largely over estimates the tooth mesh stiffness cγ since the convective effects of loading are completely ignored. The effects of lead deviations such as helix angle error and face crowning (barrelling), profile deviations such as profile angle error, profile crowning and tip relief, and pitch deviations such as adjacent base pitch error, were also studied. Their effect on the load distribution factors KH(3' KHO! and the overall load distribution factor KH, were obtained from the compared with the results from the standards2,3,4. micro-computer program and As expected, the standards considerably overestimate these factors due to their overestimation of mesh stiffness. Nevertheless, the pattern of variation in the load distribution factors was similar. The theoretical predictions were compared with experimental results measured on wide-faced test gears (specifications given in Table 5.1) with known (measured) mounting and tooth form errors. Measurement of tooth root strains to determine the load distribution along the simultaneous contact lines showed that the experimental and theoretical results agreed on the average to within 3.5% (end of tooth results not included). Also the total applied load upon comparison with theory agreed to within 6%. Experimental absolute values of transmission error "ft" were not available, however, the pattern of variation of 11ft" during meshing showed excellent consistency with the theoretical results (variations were very small anyway and within the error band). A separate test however, which gave the approximate absolute transmission error (tooth misalignments and form errors not included) agreed to within 1 % with theory.

This thesis is concerned with the dynamic behaviour of oil lubricated journal bearings and particularly with the small vibrations about the equilibrium position known as 'oil whirl'. The importance of shaft flexibility and oil film cavitation to this phenomena are investigated. Several authors have shown that by the use of linear techniques it is possible to derive a stability borderline which can be used for design purposes to ascertain whether or not a bearing is stable. These linear techniques are used to examine journal bearings with flexible shafts operating under a range of cavitation boundary conditions. It is demonstrated that these boundary conditions, particularly the behaviour of a lubricant during a vibration, play a crucial role in determining the predicted stability of the bearing. The effect of shaft flexibility is to make the bearing less stable, but the extent of this change is also governed by the oil film behaviour. Nonlinear analytical techniques are used to carry out an investigation into the behaviour of a journal bearing operating with a rigid shaft close to the stability borderline for a particular set of cavitation boundary conditions. It is found that two types of behaviour are possible: (i) supercritical, in which small stable whirl orbits are possible at speeds just above the threshold speed (the speed above which the bearing is unstable according to linear theory). (ii) subcritical, in which small unstable orbits exist at speeds just below the threshold speed. The parameter space is split into two regions, one subcritical and the other supercritical. Several methods are used in the investigation; it is shown that the methods give identical results, but only if they are applied correctly. These results are subsequently confirmed by a numerical integration of the equations of motion. The thesis concludes with an investigation of the application of nonlinear techniques to a variety of cavitation boundary conditions

In recent years, sewing technology has witnessed dramatic increases in machines speeds, new types of materials, new sewing threads and evaluation methods – but the principal type of sewing machine remains the lockstitch type and this is likely to remain the most common and versatile for the foreseeable future, particularly for sewing woven fabrics. Sewing machine speed increases lead to a loss of control of the sewing process due largely to an increase of the dynamic forces and consequently to problems such as seam pucker. In this research computer-based instrumentation and high-speed digital image and signal acquisition systems were developed to study the dynamic effects of the sewing machine, fabric and thread on seam pucker. Needle thread tension, needle bar pressure/tension, presser-foot pressure and displacement signals were acquired simultaneously by 4 strain-gage/piezo-quartz sensors mounted on a Pfaff-563 machine and results were related to pucker measured by a CCO colour-video camera system integrated with the sewing machine. A series of experiments conducted on various types of fabrics provided illustrative examples of the characteristics of each signal acquired (distinctive shape, amplitudes-peaks valleys locations, duration etc), and also enabled their characteristics to be compared and the interactions among the signals to be studied. For example it was found that the increases in magnitude of the dynamic forces in relation to sewing machine speed increases from 200rpm to 5500rpm were lower than expected (needle thread tension 2.6 fold, presser-foot 1.2 fold and needle penetration force 3.2 fold). A comparison of the needle thread tension for standard woven fabrics and micro-fibre fabrics showed a significant difference in the signal shape, location and amplitudes. The instrumentation developed is located at the Institute of Textiles and Clothing of the Hong Kong Polytechnic University in Hong Kong.

No description available.

No description available.