Search

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, parameter did not exceed the materials critical depth, dc, allowed glass surfaces to be 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.

Freeform surfaces find wide application, particularly in optics, from unique single-surface science programmes to mobile phone lenses manufactured in billions. This thesis presents research into the mathematical and algorithmic basis for the generation and measurement of smooth freeform surfaces. Two globally significant cases are reported: 1) research in this thesis created prototype segments for the world’s largest telescope; 2) research in this thesis made surfaces underpinning the redefinition of one of the seven SI base units – the kelvin - and also what will be the newly (and permanently) defined value for the Boltzmann constant. Theresearchdemonstratestwounderlyingphilosophiesofprecisionengineering, the critical roles of determinism and of precision measurement in precise manufacturing. The thesis presents methods, and reports their implementation, for the manufacture of freeform surfaces through a comprehensive strategy for tool path generation using minimum axis-count ultra-precision machine tools. In the context of freeform surface machining, the advantages of deterministic motion performance of three-axis machines are brought to bear through a novel treatment of the mathematics of variable contact point geometry. This is applied to ultra-precision diamond turning and ultra-precision large optics grinding with the Cranfield Box machine. New techniques in freeform surface representation, tool path generation, freeform tool shape representation and error compensation are presented. A comprehensive technique for very high spatial resolution CMM areal scanning of freeform surfaces is presented, with a new treatment of contact error removal, achieving interferometer-equivalent surface representation, with 1,000,000+ points and sub-200 nm rms noise without the use of any low-pass filtering.

In 2012, plasma figuring was proven to be an alternative solution for the fabrication of large scale ultra-precise optical surfaces. Indeed, plasma figuring was successfully demonstrated on a metre class glass surface. The process was exceptionally rapid but residual errors were observed. This thesis addresses this issue by proposing an enhanced tool that provides a highly collimated plasma jet. The enhanced tool is characterized by a targeted material removal footprint in the range 1 to 5 mm FWHM. The energy beam is provided by an Inductively Coupled Plasma (ICP) torch equipped with a De-Laval nozzle. This thesis focuses on characterization and optimisation of the bespoke plasma torch and its plasma jet. Two research investigations were carried out using both numerical and experimental approaches. A novel CFD model was created to analyse and understand the behaviour of high temperature gas in the De-Laval nozzle. The numerical approach, that was based on appropriate profiles of temperature and velocity applied to the nozzle inlet, led to a significant reduction of computational resources. This model enabled to investigate the aerodynamic phenomena observed from the nozzle inlet up to the processed surface. Design rules and the effect of changing nozzle parameters were identified. Sensitivity analysis highlighted that the throat diameter is the most critical parameter. A challenging power dissipation analysis of the plasma torch was carried out. Temperature and flow rate in key components of the torch were measured. Experimental results enabled to calculate the power dissipation values for RF power up to 800 W and for the entire series of designed nozzles. This work enabled to scientifically understand the power dissipation mechanism in the bespoke ICP torches. In addition, by comparing the intensity of the power dissipation values, one nozzle was clearly identified as being more capable to provide a highly efficient plasma jet.

Achievable cutting distance of a diamond tool during turning is finite and is a limiting factor in the size of component that can be turned. This limit is particularly problematic when attempting to turn brittle materials, such as those used in infra-red optics. Natural diamond tools have been used for this application. However natural diamond introduces problems: the gems can contain possible contamination with a range of impurities and strong residual stresses from formation. Cutting distance is therefore inconsistent when using natural diamond. Industry is keen to increase possible cutting distance and to increase the consistency of cutting distance. One possible solution is synthetic diamond materials. New CVD single crystal synthetic diamonds possess high purity and consistent growth conditions and therefore have the potential to be a superior tool-material that provides longer achievable cutting distance and extremely consistent cutting behaviour. This new material is compared against natural and HPHT synthetic diamonds in machining tests against silicon workpieces in a selection of tool-orientations. Aluminium workpieces are machined with MCC and natural diamond tools to assess the performance of the new material against this commonly diamond turned material. While analysing the results from these cutting trials the failure modes of diamond tools were examined closely, resulting in discovering the existence of two separate failure modes and the development of a new wear-model. Natural diamond tools were carefully tested using a range of techniques hoping to find a root cause of the wide variability seen. FTIR offered a strong clue as to the defect within natural diamond tools that leads to occasional high cutting life.

In this thesis, the research and development of a novel rapid figuring procedure for large ultra-precise optics by Reactive Atom Plasma technology is reported. The hypothesis proved in this research is that a metre scale surface with a form accuracy of ~1 μm PV can be figure corrected to 20 – 30 nm RMS in ten hours. This reduces the processing time by a factor ten with respect to state-of-the-art techniques like Ion Beam Figuring. The need for large scale ultra-precise optics has seen enormous growth in the last decade due to large scale international research programmes. A bottleneck in production is seen in the final figure correction stage. State-of-the-art processes capable of compliance with requisites of form accuracy of one part in 108 (CNC polishing, Magneto-Rheological Finishing and Ion Beam Figuring) have failed to meet the time and cost frame targets of the new optics market. Reactive Atom Plasma (RAP) is a means of plasma chemical etching that makes use of a Radio Frequency Inductively Coupled Plasma (ICP) torch operating at atmospheric pressure. It constitutes an ideal figuring alternative, combining the advantages of a non-contact tool with very high material removal rates and nanometre level repeatability. Despite the rapid figuring potential of this process, research preceding the work presented in this manuscript had made little progress towards design and implementation of a procedure for metre-class optics. The experimental work performed in this PhD project was conducted on Helios 1200, a unique large-scale RAP figuring facility at Cranfield University. Characterisation experiments were carried out on ULE and fused silica surfaces to determine optimum process parameters. Here, the influence of power, surface distance, tool speed and surface temperature was investigated. Subsequently, raster-scanning tests were performed to build an understanding on spaced multiple passes ... [cont.].

The Reactive Atom Plasma® (RAP) process is a plasma chemical etching process. RAP was developed at RAPT Industries as a process for removing subsurface damage from silicon carbide optics. The process is being investigated at Cranfield University as a novel method for the fine surface correction of large optics, with the aim of shortening the manufacturing period of the next generation of large telescopes. RAP offers material removal rates that are up to 10 times higher than those of ion beam figuring, the current state-of-the-art technique and the convenience in that it can be operated at atmospheric pressure. Cont/d.

A dynamically optimal design is essential for a motion system to perform high speed operation without compromising its accuracy, settling time and vibration specification. Good design practice which accounts for dynamic characteristics in the modelling of a motion system warrants higher performance precision machines and cuts down redevelopment effort to ‘patch’ inherent shortcoming of the machine dynamics. This research aimed to accurately describe the non-linear dynamics of a non-mechanical contact aerostatic guideway system in order to achieve an accurate FEA model of the design stage. The single axis aerostatic guideway is comprised of several machine in¬terfaces that impact the dynamic behaviour of the guideway. Modelling each air bear¬ing pad by a single stiffness element is not adequate to predict the guideway modal behaviour accurately. The aerostatic guideway has been broken down into several key machine interface elements. In-depth investigation of the air film and the air bearing mounting mechanism was carried out. A dedicated air film test rig was designed and built to acquire insight of the air film dynamic characteristics. It is observed that the mounting mechanism of the air bearing constitutes to a signifi-cant dynamic effect to the entire air bearing setup. Based on the findings of the mount-ing mechanism’s stiffness properties, a method was developed to estimate ‘true’ air gap heights which cannot be easily assessed and measured directly in most aerostatic guideway carriages. The estimation method enables a more rigorous FE model of the aerostatic guideway system. The comprehensive dynamic analysis methodology pro-posed in this research greatly increases the confidence and accuracy of the aerostatic guideway’s FE model.

Large scale nuclear fusion and astronomy scientific programmes have increased the demand for large freeform mirrors and lenses. Thousands of one metre class, high quality aspherical optical components are required within the next five to ten years. Current manufacturing process chains production time need to be reduced from hundred hours to ten hours. As part of a new process chain for making large optics, an efficient low damage precision grinding process has been proposed. This grinding process aims to shorten the subsequent manufacturing operations. The BoX R grinding machine, built by Cranfield University, provides a rapid and economic solution for grinding large off-axis aspherical and free-form optical components. This thesis reports the development of a precision grinding process for rapid manufacturing of large optics using this grinding mode. Grinding process targets were; form accuracy of 1 m over 1 metre, surface roughness 150 nm (Ra) and subsurface damage below 5 m. Process time target aims to remove 1 mm thickness of material over a metre in ten hours. Grinding experiments were conducted on a 5 axes Edgetek high speed grinding machine and BoX R grinding machine. The surface characteristics obtained on optical materials (ULE, SiC and Zerodur) are investigated. Grinding machine influence on surface roughness, surface profile, subsurface damage, grinding forces and grinding power are discussed. This precision grinding process was validated on large spherical parts, 400 mm ULE and SiC parts and a 1 m Zerodur hexagonal part. A process time of ten hours was achieved using maximum removal rate of 187.5 mm 3 /s on ULE and Zerodur and 112.5 mm 3 /s on SiC. The subsurface damage distribution is shown to be "process" related and "machine dynamics" related. The research proves that a stiffer grinding machine, BoX, induces low subsurface damage depth in glass and glass ceramic.

Machine tools are made up of many different parts connected together. In order to understand and create more accurate thermal and deformation numerical models the heat transfer characteristics of machine tool interfaces need to be understood. Previously heat transfer across interfaces has not been incorporated in finite element models in thermal and deformation analysis of machine tools. To a varying degree the issue of interfaces will have an effect on the overall performance of machine tools, and clearly if not considered and resolved at the strategic design stage they may be difficult and expensive to correct, once machine tools are operational. An inability to adequately understand and model the interfaces in machine tools is therefore a major limitation to current knowledge. In this research programme heat transfer and corresponding thermal deformation of metal blocks in contact having pre-defined interface geometries, were experimentally and numerically investigated. Factors such as the contact pressure, the contact conformance, and the interface material were investigated. It was found that the most influential factors were contact pressure and contact conformance. Interface material was also found to have a bigger influence if the contact area was non-conforming. When interface material was thicker than the surface roughness, the heat flow across the interface was reduced as fewer contact asperities came into contact. In order to optimise modelling it was necessary to optimise the convection coefficient - this was found to have an effect on overall heat transfer across the interface. It was also found that the overall contact area is important and not the spread of contact regions for the same overall contact. This was investigated using two different contact geometry arrangements. The numerical prediction of heat transfer across an interface is accurate if the contact is conforming. However, if the contact is non-conforming an accurate prediction is not possible. This is due to the fact that direct modelling of non-conformance is not achievable. It was found that the most influential factor on thermal deformation is contact pressure, on the other hand factors such as interface material and interface geometry showed very little effect.

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.

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.