Browsing by Author "Stephenson, David J."

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    Abrasive machining with MQSL.
    (Cranfield University, 2011-07) Morris, Tom; Stephenson, David J.; Walton, I.; Nicholls, J. R.
    Grinding and polishing of engineered components are critical aspects of the precision manufacturing of high performance, quality assured products. Elevated process temperatures, however, are a common and for the most part undesirable feature of the grinding process. High process temperatures increase the likelihood of microstructural change within the immediate subsurface layer and are detrimental to the strength and performance of the manufactured products. Increasing processing costs and tighter environmental legislation are encouraging industry to seek innovative fluid application techniques as significant savings in production can be achieved. In this context, and with sponsorship from three industrial partners, namely; Fives Cinetic, Fuchs Lubricants plc and Southside Thermal Sciences Ltd, and also from the Engineering and Physical Science Research Council (EPSRC), this research aimed to develop an understanding of Minimum Quantity Solid Lubrication (MQSL) as a method for abrasive machining, with particular reference to the control of surface temperatures. Improving the lubricity of Minimum Quantity Lubrication (MQL) fluids reduces the frictional source of process heat and controls the finish surface temperature. The application of effective solid lubricants is known as Minimum Quantity Solid Lubrication (MQSL). Molybdenum Disulphide (MoS2), Calcium Fluoride (CaF2), and hexagonal Boron Nitride (hBN) were compared against a semi-synthetic water soluble machining fluid (Fuchs EcoCool). A series of Taguchi factorial experimental trials assessed their performances through ANOVA (ANalysis Of VAriance) statistical method. The hBN produced the lowest grinding temperatures of the solid lubricants tested, although they still remained higher than those achieved using the EcoCool control. The reduction of the machining fluid enabled a Charged Coupled Device (CCD) sensor to be fitted into the grinding machine. The recorded movement in the emitted spectrum from the grinding chips was compared to experimental and modelled process temperatures. This showed that the wavelengths of the chip light correlated to the temperature of the finish grinding surface. This greatly contributed to determining the feasibility of constructing a non-destructive, non-invasive, thermally-adaptive control system for controlling grinding surface temperatures.
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    The application of electrolytic in-process dressing to precision grinding processes
    (2007-06) Zervos, Charilaos; Stephenson, David J.; Robinson, M. J.
    Electrolytic In-Process Dressing (ELID) has attracted a lot of attention in recent years as it is a promising method of grinding a wide range of materials, including ones that are difficult to machine, to high quality finishes. The EC funded project Nanogrind undertook the construction of a high precision grinding machine which will incorporate an ELID system, applying the technique on a spherical wheel for the production of components with intricate geometries (e.g. freeforms, aspherics). In this project, the main objective was to understand the basic features of ELID, such as the electrochemical properties of the oxide layer and its effect on the final finish of the produced components (glass, a hard and brittle material was the main focus). Acoustic emission studies, a promising in-process monitoring tool, verified the effectiveness of ELID grinding when the contact area between the wheel and workpiece is increased. Compared to conventional resin bond wheel grinding, ELID ground components were of higher quality with less damage introduced in them. ELID current monitoring also gave excellent results when it was correlated to surface finish achieved across the surface of the testpieces ground. Electrochemical Impedance Spectroscopy was an innovative technique that was applied in order to acquire basic information about the metal bond wheel, such as the charge transfer resistance. Its correlation to the oxide layer thickness gave an indication of the way corrosion rates change with the presence or not of an oxide film on the wheel surface. Finally, sub-surface damage introduced into glass components was evaluated and was compared to the depths of damage predicted by theoretical models, giving further insight into the way the overall processing time of a component should be optimised in order to acquire high quality, damage-free finished components in a time efficient, cost effective way.
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    Application of response surface methodology for the optimisation of micro friction surfacing process
    (Elsevier Science B.V., Amsterdam., 2010-08-15T00:00:00Z) Vitanov, V. I.; Javaid, N.; Stephenson, David J.
    The aim of this work is to identify relationships between the input variables and the process response and to develop predictive models that can be used in the design of new friction surfacing applications. Moreover to investigate the use of standard CNC machines for friction surfacing. The experimental design techniques and response surface methodology were used to investigate and select the combination of factor levels that produced the optimal response. The main effect of the velocity ratio obtained by dividing the feed rate of mechtrode with traverse rate of substrate was observed to be the most significant factor on the process response. Based on the results of optimisation it was observed that the lower to intermediate levels of rotational speed and intermediate to higher levels of velocity ratio produced good coating quality. (C) 2010 Elsevier B.V. All rights reserved.
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    Burn threshold prediction for high efficiency deep grinding
    (2011-06-01T00:00:00Z) Bell, Andrew John; Jin, T.; Stephenson, David J.
    Burn threshold diagrams are useful for the prediction of thermally induced grinding damage and were originally developed to describe the conventional shallow cut grinding regime. With the development of new high stock removal grinding processes such as High Efficiency Deep Grinding (HEDG), the prevention of thermal damage to the workpiece is of particular concern. The principle of HEDG is based around the change in thermal characteristics of the grinding process at high Peclet numbers, whereby less heat is partitioned to the workpiece. Conventional burn threshold diagrams are valid for Peclet numbers below 50, well below the values expected in HEDG. This study presents a modified approach to the construction of burn threshold diagrams which takes account of the change in thermal partitioning with Peclet number. The approach has been validated through grinding trials over a range of specific material removal rates.
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    Burn threshold prediction for high efficiency deep grinding
    (Elsevier, 2006) Bell, Andrew; Jin, T.; Stephenson, David J.
    Burn threshold diagrams are useful for the prediction of thermally induced grinding damage and were originally developed to describe the conventional shallow cut grinding regime. With the development of new high stock removal grinding processes such as High Efficiency Deep Grinding (HEDG), the prevention of thermal damage to the workpiece is of particular concern. The principle of HEDG is based around the change in thermal characteristics of the grinding process at high Peclet numbers, whereby less heat is partitioned to the workpiece. Conventional burn threshold diagrams are valid for Peclet numbers below 50, well below the values expected in HEDG. This study presents a modified approach to the construction of burn threshold diagrams which takes account of the change in thermal partitioning with Peclet number. The approach has been validated through grinding trials over a range of specific material removal rates.
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    Controlled porosity alumina structures for ultra-precision hydrostatic journal bearings
    (Blackwell Publishing Ltd., 2010-11-04T00:00:00Z) Durazo-Cardenas, Isidro; Stephenson, David J.; Corbett, John
    A series of fine-grade alumina powders has been used in combination with maize starch granules to produce porous structures for porous hydrostatic journal- bearing applications. A comprehensive series of tests was conducted to characterize porosity in terms of density, pore size, and permeability. Successful processing of quality journal-bearing components has been demonstrated for preferred combinations of alumina size and starch content, using fixed-processing parameters. The new porous ceramic bearings showed consistent and reproducible properties and are suitable for a wide range of higher precision engineering applications. The porous ceramic-bearing processing route has also proved to be low cost and environmentally sound.
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    Critical study of high efficiency deep grinding
    (Cranfield University, 2002) Johnstone, lain; Stephenson, David J.; Corbett, John
    The recent years, the aerospace industry in particular has embraced and actively pursued the development of stronger high performance materials, namely nickel based superalloys and hardwearing steels. This has resulted in a need for a more efficient method of machining, and this need was answered with the advent of High Efficiency Deep Grinding (HEDG). This relatively new process using Cubic Boron Nitride (CBN) electroplated grinding wheels has been investigated through experimental and theoretical means applied to two widely used materials, M50 bearing steel and IN718 nickel based superalloy. It has been shown that this grinding method using a stiff grinding centre such as the Edgetek 5-axis machine is a viable process. Using a number of experimental designs, produced results which were analysed using a variety of methods including visual assessment, sub-surface microscopy and surface analysis using a Scanning Electron Microscope (SEM), residual stress measurement using X-Ray Diffraction (XRD) techniques, Barkhausen Noise Amplitude (BNA) measurements, surface roughness and Vickers micro-hardness appraisal. It has been shown that the fundamentals of the HEDG process have been understood through experimental as well as theoretical means and that through the various thermal models used, grinding temperatures can be predicted to give more control over this dynamic process. The main contributions to knowledge are made up of a number of elements within the grinding environment, the most important being the demonstration of the HEDG effect, explanation of the phenomenon and the ability to model the process. It has also been shown that grinding is a dynamic process and factors such as wheel wear will result in a continuous change in the optimum grinding conditions for a given material and wheel combination. With the significance of these factors recognised, they can be accounted for within an industrial adaptive control scenario with the process engineer confident of a more efficient use of time and materials to produce a higher quality product at lower cost.
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    The design and manufacture of biomedical surfaces
    (Technische Rundschau, Hallwag Publishers; 1999, 2007-08-01T00:00:00Z) Ramsden, Jeremy J.; Allen, David M.; Stephenson, David J.; Alcock, Jeffrey R.; Peggs, G. N.; Fuller, G. D.; Goch, G.
    Surfaces are the primary place of contact between a biomaterial and its host organism. Typically, prostheses have to fulfil demanding structural and mechanical requirements, yet the material best for those functions may be bio- incompatible. Surface treatment or coating provides a means to overcome that problem, which means both integration within the host physiology and stabilization with respect to corrosion and wear. The adsorption of biomacromolecules is pivotal for biocompatibility. The impossibility of keeping proteins away from most implants means that very careful consideration has to be given to this aspect, and both prevention (for bloodstream implants) and promotion (for bone replacement and repair) occur with equal importance. This paper also considers the metrology of relevant physical and chemical aspects of surfaces.
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    Determining the onset of grinding burn using Magnetic Barkhausen Noise
    (Cranfield University, 2005) Opoku, Kevin S; Stephenson, David J.; Walton, I.
    A study of the quick non-destructive technique of Magnetic Barkhausen Noise (MBN) in detecting the onset of grinding burn in the absence of any microstructural changes during grinding has been undertaken. 51CrV4 Steel is a used in industry for manufacturing automotive parts (Nissan). Uniaxial tensile tests using a coarse surface whilst taking in situ MBN measurements were used to calibrate MBN to residual stress for hard and soft samples of 51CrV4. X-ray diffraction tests of some ground samples of hard and soft 51CrV4 samples were used to correlate the obtained calibration curves. A depth profile analysis was also carried out from results of MBN and XRD measurements of 51CrV4 soft and hard samples ground at different specific material removal rates. The MBN Vrms and Vpeak parameters in the elastic deformation region were shown to behave linearly with total stress. A predictive model to calculate residual stress values showed good correlation in terms of the direction but the magnitude of the error was poor within its working limits. The Vrms and Vpeak and FWHM also showed a sharp change in the trend during the change from compressive to tensile stress. The application of the MBN technique in detecting grinding burn before it happened meant that waste could be limited and grinding processes optimised without compromising the surface integrity of components.
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    Development of porous ceramic air bearings
    (Cranfield University, 2001-02) Roach, Christopher James; Stephenson, David J.; Corbett, John
    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.
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    Development of porous-ceramic hydrostatic bearings
    (Cranfield University, 2003-10) Durazo-Cardenas, Isidro; Corbett, John; Stephenson, David J.
    Porous-ceramic hydrostatic bearings have been recently developed. These bearings have demonstrated an exceptional overall performance when compared with conventional technology bearings. However, despite all the benefits, porous-ceramic hydrostatic bearings have yet to find widespread acceptance due to the problems found in tailoring the bearings geometry and size to suit precision engineering applications, while producing porous-structures with consistent and reproducible permeability. Using a series of fine grades of alumina powders in combination with maize starch granules, a new method for the manufacture of porous-ceramic bearings has been developed, based on the starch consolidation technique. By employing this method, it has been demonstrated that is possible to manufacture bearings of different geometries and shapes, with consistent and reproducible properties. The new method also proved to be low cost and environmentally sound. The performance of the new journal bearings has been investigated in a highly instrumented test-rig, and a comparable performance to that of previous porous- ceramic journal research has been observed. In a direct performance comparison between a porous-ceramic hydrostatic journal bearing and a conventional hydrostatic bearing of the same size, the porous-ceramic bearing demonstrated a significant performance improvement in terms of stiffness, power consumption and thermal performance. In previous research, water lubrication proved to significantly improve the spindle thermal performance. However, water lubrication is feared to promote corrosion within the spindle components. In the present research, the effects of water lubrication in porous-ceramic bearing systems were investigated. As a result, it has been demonstrated that corrosion in typical machine-tool materials can be effectively controlled by using inhibitors and low cost surface coatings. On the other hand, it has been also demonstrated that undesirable foaming, air entrainment and microbial growth can potentially develop in water/inhibitors lubrication systems. In this sense, the use of low viscosity oils proved to offer a comparable performance.
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    Electrolytic in-process dressing superfinishing of spherical bearings using metal-resin bond ultra-fine CBN wheels
    (Professional Engineering Publishing, 2011-01-01T00:00:00Z) Raffles, Mark H.; Stephenson, David J.; Shore, Paul; Jin, T.
    The use of electrolytic in-process dressing (ELID) superfinishing has been investigated with the aim of substantially improving surface finish on spherical bearing balls as well as reducing process times. Using ELID in a superfinishing configuration is substantially different from the more conventional precision grinding set-up. With this ELID superfinishing system, metal-resin bonded (MRB) wheels containing very small superabrasives (30 to 0.12 μm) were employed. Surface finishes of 2 nm Ra were achieved with a #12 000 wheel, an order of magnitude better than balls produced using the conventional production techniques of barrelling or polishing. Consistently sub-10 nm Ra finishes were achieved with a #2000 wheel. Different ways of using the ELID system, including ELID 1, ELID 2, and ELID 3, were studied to examine how the different types control the cutting condition at the wheel's surface. It is the ability to control easily the cutting condition of superabrasives of this size that allows mirror surface finishes to be efficiently produced. Monitoring of wheel spindle and ELID power usage was found to provide useful information in assessing the wheel c
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    Elid superfinishing of spherical bearings
    (Cranfield University, 2007) Raffles, Mark H.; Stephenson, David J.; Shore, Paul
    Driven by a requirement to extend the lifespan of self-aligning lined spherical bearings, this research investigates the use of Elid (electrolytic in-process dressing) as a method of improving ball surface finish. Elid is a continuous and self-regulating electrochemical dressing process that modifies the surface of a grinding, lapping, or superfinishing wheel. It provides improved grit protrusion, impedes wheel loading / glazing and promotes effective cutting. The characteristics of the newly-developed Elid superfinishing process are in many ways fundamentally different to conventional superfinishing. The main difference is that the use of super-abrasives prevents the wheel from self-sharpening; the normal mechanism by which dulled conventional abrasives are removed and a wheel’s surface refreshed. Because the wheel’s performance and condition is continually maintained inprocess by the Elid system, metal resin bonded (MRB) wheels containing very small super-abrasives can be used. It is the utilization of these fine abrasives (30 to 0.12 μm) that enables surface roughness values below 5 nm Ra to be consistently produced on the spherical surface of corrosion-resistant steel balls. This research provides an in-depth understanding of the Elid spherical superfinishing process; investigating the most effective use of the Elid system, wheel dressing requirements and process performance. Optimisation is provided in terms of evaluating the critical operating parameters, the most effective superfinishing cycle and the implications to the complete ball production chain. A range of techniques are used to evaluate processing performance and ball output quality. These include in-process monitoring of Elid and wheel spindle power levels, analysis of wheel condition, rates of ball surface generation and material removal, ball finish and form. Although predominantly concentrated on corrosion-resistant steel, testing is also conducted on titanium and various ball coatings. In investigating various ways of using the Elid system, this work considers electrodischarge truing, pre-process dressing, Elid 1, Elid 2, Elid 3, and Elid combined with electrolytically assisted superfinishing. The initial process solution of Elid 3 (electrodeless) superfinishing provides the capability of working on all standard size balls, however the dressing system lacks stability. The development of a fixturing system that has a small separate electrode enables Elid 1 (conventional) to be used on the majority of ball sizes. Elid 1 allows more aggressive and consistent dressing, a faster rate of ball material removal and thus a substantially reduced processing time. Results with a #12,000 wheel show that surface quality is vastly improved through the use of Elid whilst maintaining current production standards of form accuracy. Surface finishes of 2nm Ra are achieved, which is an order of magnitude better than balls currently produced using barrelling / polishing. Processing times are equivalent or faster when using Elid 1. Alternatively, consistently sub 10 nm Ra finishes can be reached with a #2,000 wheel using Elid 2 (interval dressing). Generally MRB-CBN wheels provide a more effective carbide cutting action than conventional superfinishing wheels. Controlling wheel condition and achieving full and even ball to wheel conformity are the two most significant contributory factors to the success of Elid spherical superfinishing. Insufficient control of these factors results in poor output quality. Monitoring of wheel spindle and Elid power usage provides useful information in assessing the condition of the wheel and identifying potential problems. High spindle power correlates with fast material removal and is a result of high loads and a free cutting action. Elid processing can be employed for improving surface finish after the conventional honing stage, or after cylindrical grinding for improving both ball form and finish.
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    Erosion - corrosion modelling of materials used in petroleum production
    (Cranfield University, 1994-09) Hamzah, Razali Bin; Strutt, J. E.; Stephenson, David J.
    Experimental erosion/corrosion modelling was performed on C-Mn steels and corrosion resistant alloys commonly used in petroleum production. A graded commercial sand of 50-30b pm size range, similar to sands produced from typical oil/gas fields was used to simulate the erosive medium. The experimental conditions such as the sand flux, particle velocity, pressure and temperature, were chosen based on typical field operating conditions for corrosion, erosion and erosion-corrosion tests imposed on the materials. The corrosion environment was created by feeding in CO2 gas with atomised water into the test chamber. A centrifugal erosion rig (45) with necessary modifications to suit wet and dry test conditions was used in the experiments. The rig was capable of generating over 150 m/s particle velocity and could operate at high temperatures to generate consistent results. Results show that (a) there was a soft thin layer of corrosion products formed on the C-Mn specimens even at 20°C which can easily be removed by the impacting particles , (b) the scale growth rate kinetics were found to be parabolic and vary between 8x10-3 pun'/h to 50x10-3 iim2/h depending on the material and temperature, (c) the process of metal recession consists of the removal and regrowth of soft corrosion scale at low flux and low velocity, and erosion of substrate at high flux and high velocity, (d) the erosion-corrosion rates were found to vary between 2- 30 times over the pure erosion rates under the same conditions, depending on the sand flux, particle velocity and temperature. An existing computer simulation model (46) was used to simulate the conditions observed from the experiments. Some improvements were incorporated into the programme and the final model is capable of predicting the erosion and erosion-corrosion rates applicable to petroleum production.
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    Grinding processes and their effects on surface integrity
    (Cranfield University, 2005-03) Comley, Paul; Corbett, John; Stephenson, David J.
    The introduction of high performance grinding machines in combination with the latest superabrasive technology has the potential to impact significantly on existing process chains. The aim of the research was to look at both the high and low rate removal grinding processes and their effects on the surface integrity, as a means to exploit the above technologies. A major objective was to determine the feasibility of High Efficiency Deep Grinding (HEDG) in cylindrical plunge grinding. HEDG is a high speed removal process which differs from conventional forms of grinding in that it uses large depths of cut together with high feedrates. Together, these changes affect the thermal energy partitioning within the work zone. Through this work an understanding of the process conditions enabled the development of this process, such that prevention of thermal damage to the finished workpiece surface is achievable. At the opposite extreme to the high material removal rates of HEDG, kvdrk was carried out in the high precision finish grinding regime. Developments *ere undertaken to look at the implementation of a modified path into the normal cylindrical plunge grinding action, in a process referred to as Superfinish Grinding. The aim of this process being to demonstrate an improvement to the surface texture primarily through a reduction in grinding directionality. Surface integrity is an important consideration in the development of any grinding process. Damage as a result of grinding is predominately of a thermal nature and results in changes to the material properties in the near surface region. One such change is the residual stress, which was measured using Barkhausen Noise intensity instrumentation, which provided a reliable early indication to a build up in thermal energy. Developments in thermal modelling supported by temperature measurements provided a better understanding of the HEDG regime. The model employed new energy partitioning theories together with circular arc modelling of conditions along the contact length. A model was derived to predict the surface finish produced with the Superfinish Grinding approach, this again provided an increased understanding of the grinding process. Industrial trials have shown how HEDG can be implemented on standard production machine tools for the cylindrical plunge grinding of crankshaft components. The process demonstrated the potential for improved surface integrity, whilst maintaining surface finish and form accuracy. The same grinding machine was also used to generate high quality surfaces using a Superfinish Grinding process. Roughness values of the order of 0.11um RQ were routinely obtained exhibiting reduced levels of grinding directionality. Thus, using a single machine tool and a single set-up, exceptionally high stock removal rates are achievable in a roughing cycle followed by superfinishing to generate the required surface characteristics and profile.
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    The impact of tool performance on micromachining capability
    (Cranfield University, 2012-06) Zdebski, Daniel; Stephenson, David J.; Allen, David
    Micro-milling represents a versatile and fast manufacturing process suitable for production of fully 3D micro-components. Such components are demanded for a vast number of industrial applications including safety systems, environmental sensors, personalized medical devices or micro-lenses and mirrors. The ability of micro-milling to process a wide range of materials makes it one of the best candidates to take a leading position in micromanufacturing. However, so far it does not seem to happen. By discussion with various industrialists, low predictability of micro-milling process was identified as the major limiting factor. This is mainly because of strong effects of the tool tolerances and process uncertainties on machining performance. Although, these issues are well known, they are not reflected by the current modelling methods used in micro-milling. Therefore, the research presented in this thesis mainly concentrates on development of a method allowing a prediction of the tool life in manner of tool breakage probability. Another important criterion which must be fulfilled is the method applicability to industrial applications. This means that the method must give sufficiently accurate prediction in reasonable time with minimum effort and interactions with day-to-day manufacturing process. The criteria listed above led to development of a new method based on analytically/numerical modelling techniques combined with an analysis of real tool variations and process uncertainty. Although, the method is presented in a relatively basic form, without considering some of the important factors, it shows high potential for industrial applications. Possibility of further implementation of additional factors is also discussed in this thesis. Additionally, some of the modelling techniques presented in this thesis are assumed to be suitable for application during designing of micro end-mills. Therefore, in the last part of this thesis is presented a systematic methodology for designing of micro end-mills. This method is based on knowledge and experience gained during this research.
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    Investigation into the grinding of titanium alloys
    (Cranfield University, 2000-07) Wang, S. H.; Corbett, John; Stephenson, David J.
    Titanium alloys are used extensively in the aerospace industry due to their high specific strength and excellent corrosion resistance. However, their poor thermal conductivity and high chemical reactivity with tool materials make the machining difficult, especially when grinding. During grinding, the excessive heat generated at the wheel-work piece interface may result in poor surface finisil,1_, a transformed surface layer, excessive plastic deformation, thermallyinduced residual tensile stress, burn and micro-cracking on the ground surface. The poor surface integrity and metallurgical changes of the surface and sub-surface may impair the surface sensitive properties such as fatigue life during service. ln order to overcome the thermal problems when grinding titanium alloys, a new cooling strategy, cryogenic grinding, was studied which involves the supply of liquid nitrogen into the grinding zone using a nozzle jet system. lt was found that cryo-cooling with conventional grinding wheel decreased surface roughness values, burn and plastic deformation of Ti-6Al-4V alloys and produced better a cutting mechanism than when using water-based coolant, especially at higher depths of cut. However, the thermal problems still introduced high residual tensile stress which degraded the fatigue life of ground specimens. Another approach to reducing the grinding temperature was to use a superabrasive wheel (diamond), because of its superior thermal conductivity, the integrity of the ground surface was improved and the fatigue life properties of the specimen were maintained at higher values than for conventional grinding wheels. An ultra stiff machine tool, Tetraform C, was also used to grind Ti-6Al-4V alloys and under selected conditions the ground surface reached a good surface finish and the fatigue properties were also retained at lower depth of cut. An ELID system on the Tetraform C was also investigated. It produced a rougher surface finish in this study. However, it is believed that reducing the wheel loading problem when grinding titanium alloys may contribute to maintaining the fatigue properties.
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    Machining of Optical Surfaces in Brittle Materials using an Ultra-Precision Machine Tool
    (Cranfield University, 1995-03) Shore, Paul; Stephenson, David J.
    Investigations of machining optical surfaces into brittle materials using an ultra precision machine tool are presented. The newly developed ultra precision NION machine is evaluated to gain a good appreciation of its operating performance. The machining accuracy capability of this machine is established by careful measurement of its; motion accuracy, thermal and dimensional stability and loop stiffness. Corroboration of these measurements are provided by assessment of surfaces which were produced in soft "easily machined" metal materials. It was found that surfaces smooth to -1 nm Ra could be produced on the NION machine and with a form error of less than 100 nm P-V. The main source of figure error, approximately 80 nm, was found to be caused by the synchronous axial error motion of the workhead spindle. Other elements of the machine, including thermal effects, incurred less than 25 nm of additional figure error. Assessment of the diamond turning process for the producing optical surfaces made in a number of important optical materials, which are ostensibly brittle, were undertaken. Turning tests were carried out to establish the relative difficulty for machining optical surfaces in these materials and to define the most important parameters which affect the attained surface quality. Assessment of the produced surfaces was based on their roughness quality, surface morphology and residual stress condition. It was found that diamond tool edge quality degraded with total cut distance. Tool cut distance was found to be a major influence on achievable material removal rate before micro- fractures became present at the surface. Surface quality and residual stress condition were also greatly influenced by the overall tool cut distance. Diamond grinding trials were also carried out using the NION machine tool. These grinding trials were carried out using a mode of grinding which permits complex shape optical surfaces to be produced. Various grinding technologies were employed to establish the optimum methods. Selected grinding trials were carried out to establish the dominate parameters affecting the optical quality. Assessment of the machined surfaces was in regard of their surface roughness, residual stress and severity of sub-surface micro cracking. It was found that grinding wheel specification was a major influence on surface quality and sub-surface damage. The level of residual stress associated with "ductile" mode grinding was not found to prohibit its application toward the direct manufacture of optical elements. Selection of grinding parameters which ensured the grain depth of cut, GDOC, ., allowed glass surfaces to be parameter did not exceed the materials critical depth, d, ground to 1-2 nm Ra. These ground glass surfaces appeared free of any surface fractures. Sub-surface assessments did however reveal small levels of micro-fractures hidden below the surface. Discussion of both machining processes is provided. Available material removal rates for each process is given when cutting a number of important optical materials. Conclusions regarding the production of both Infrared and visible wavelength optics using the NION machine tool are provided. Recommendations for future work to improve both; the understanding of the processes and the effectiveness of applying the processes are suggested.
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    Machining surfaces of optical quality by hard turning
    (Cranfield University, 2003-11) Knuefermann, Markus M. W.; Corbett, John; Shore, Paul; Stephenson, David J.
    The main aim of this work was the machining by hard turning of surfaces with optical surface quality. A numerical target had been set as a surface roughness Ra = 10nm. It has been shown that achieving roughness of that magnitude by hard turning is possible. Individual work pieces exhibited the desired surface properties for short lengths at a time, but it proved to be very difficult to achieve these surfaces consistently and over longer cuts. The factors influencing the surface roughness were identified as tool defects and machine vibration in addition to the standard cutting parameters and choice of cutting tool. A model of surface generation in hard turning has been developed and good correlation between simulated and experimentally determined surface roughnesses was achieved. By introducing a material partition equation which determines the proportional contribution of material removal mechanisms in the undeformed chip a comprehensive method for assessing the contributing factors in material removal was developed. While it has been shown that surfaces in hard turning are almost exclusively generated by chip removal and plastic deformation the developed model is versatile enough to include elastic deformation of the work piece. With the help of the model of surface generation in hard turning it has been possible to attribute magnitudes of the influencing factors with respect to the cutting parameters such as feed rate and tool corner radius, and the main disturbances - tool defects and machine vibration. From this conclusions were drawn on the requirements for machine tools and cutting tools, which will need to be realised to make ultra-precision hard turning of surfaces of optical quality a feasible manufacturing process.
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    The Mechanism of Dross Formation on Aluminium and Aluminium Magnesium Alloys
    (Cranfield University, 1989-12) Impey, S. A.; Stephenson, David J.; Nicholls, J. R.
    Metal loss is an unavoidable consequence of the large scale melting of aluminium and its alloys. The objective of such processing must be to minimise losses, both from an economic viewpoint and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting are primarily due to the formation of dross, a mixture of oxide and melt. Many of the commercially important aluminium alloys contain appreciable levels of magnesium (up to 5%) which can result in enhanced oxidation rates that give rise to particular problems in recycling. Results are presented from a study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the early stages in oxide scale growth, a study of growth kinetics and subsequent breakdown of these initial scales to form dross. In humid atmospheres, the amorphous oxide covering both aluminium and aluminium-magnesium at 750°C provides a highly effective barrier protecting the molten metal. In the absence of water vapour, oxide crystal development in aluminium-magnesium alloys is dominated by magnesium, and is extremely rapid in comparison with pure aluminium. Despite the different oxides formed, the manner of crystal formation at the 'amorphous' oxide-melt interface at 750°C on both aluminium and aluminium-magnesium is comparable. Nucleation and growth of crystals in the 'amorphous' film generates high stresses which result in failure of the surface oxide. Scanning electron microscopy has shown that the localised failure of this protective oxide film results in exudations forming on the melt surface, the size and number of which increase with exposure time. These exudations would appear to be the onset of dross formation. Parallel studies of the wetting characteristics of aluminium to alumina have shown that the reported non-wetting is due to the presence of the thin alumina film on the melt surface. Once broken, wetting of the alumina takes place and accounts for the exudation of molten metal through the surface oxide and hence dross formation.