Browsing by Author "Gee, A. E."
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Item Open Access Dynamic metrology of error motions in precision spindles using optical metrology(Cranfield University, 1998-10) Idowu, Ade; Gee, A. E.Knowledge of the accuracies of air bearing spindles in the sub-micrometre to nanometre range is required for the design, commissioning and operation of ultra-precise machine tools, measurement systems and other machines employing high precision rotational motion. In order to verify the dynamic performance of a spindle, measurement is required of its error motions in the unwanted five degrees of freedom (one axial, two tilts and two radial motions). Presentation of these error motions (eg in the form of polar charts) can then be used to provide critical spindle metrology data including total, asynchronous and average error motion rosette profiles and their average and peak values. This thesis describes a metrology system based on optical interferometry for measuring such unwanted error motions in three degrees of freedom involving motion along the spindle axis (axial rectilinear displacement and tilts about orthogonal axes), incurred with rotation of a precision air spindle over its specified speed-range. The system is not sensitive to orthoaxial translations which may be measured using alternative methods. Possible alternative techniques for measuring any of the degrees of freedom include an array of proximity sensors, (one for each translational degree of freedom and a further one for each of the other rotational degrees of freedom), to measure the run-out of an artefact. Proximity sensors based upon capacitive or optical fibre back-scatter techniques each offer the required single degree-of-freedom non-contacting capability and bandwidth. In the current work, a Fizeau interferometer is used to monitor the motion of the spindle of a vertical axis ultra-precision facing machine using a test-artefact. This is a mirror with less than one fringe departure from planarity from which interferogram. fringe-patterns are captured, digitised and analysed synchronously as the spindle rotates. The issue of the prediction of the dynamic form and motions of the observed interferograrn arises and the earlier theory is extended to optimise the set-up, including provision of automatic servo- alignment of the optical axis with the axis of the spindle. Measurement interferograrn data is sampled at selected angular incremental positions of spindle-rotation and image processing techniques used to filter the fringe pattern, enabling measurement of spindle tilt and axial displacement. Issues of sampling with respect to the anticipated spatial angular frequency of the spindle run-out are considered with respect to the speed/frequency capability of data-acquisition and processing arrangements. Essentially, with a spindle rotating at typical machining speeds of 300- 3000 rev/min, for consistent error motions, the resolution of an error plot is principally a function of observational time. It is foreseen that the system will be applicable in research and production-support in ultra-precision machining production processes and in rotational metrology.Item Open Access Investigations of the machining of glasses and other normally brittle materials in the ductile regime(1991-06) Chao, C. L.; Gee, A. E.; Spragg, R. C.Increased demands for precision components made of brittle materials such as glasses and advanced ceramics, are such that conventional (free abrasive) grinding and polishing techniques can no longer meet the requirements of today’s precision manufacturing engineering. Both fast production rates and high quality surfaces of complex shapes are required in addition to the spherical or planar surfaces produced which are most readily produced by conventional free abrasive techniques. The work investigates the feasibility of using ductile-mode single-point diamond turning both as an alternative machining technique in its own right and as a model for certain parameters involved in (rigid-wheel) grinding. Indentation and ruling/scribing were used to study the underlying material properties, mechanical stress fields, the ductile-brittle transition and material removal mechanisms. Several material removal mechanisms were identified and discussed; these were ploughing, cutting, delamination and brittle fracture. The results of indentation and scribing experiments show that, with penetration depth of less than a critical value (the critical cut depth) brittle materials can be machined in a ductile manner and with chips very similar to those obtained from the classical ductile cutting of metal, save that, in this case it is at a much smaller scale. The influence of tool shape has shown to be important in determining the material removal mechanism. The experiments on single-point diamond turning (facing) machine were carried out on a highly stiff diamond facing machine. During the present project continuous machining of a number of materials to Ra values of nanometres order has been achieved, these include soda-lime glass, fused silica, Zerodur and single crystal silicon. Ductile crack-free machining has been demostrated at spindle speeds up to 4500 rpm. The technical feasibility of ultra-fine single-point machining of optical, electronic and ceramic materials has thus been established. Investigations were undertaken into methods of measuring the nature and extent of sub-surface damage (SSD) using scanning acoustic microscope (SAM), Rutherford back-scattering technique (RBS) and X-ray topography. The results of SSD studies suggested that coarse machining marks could still be detected in the sub-surface region even though the surface has been subsequently machined to a condition of no (optically) visible damage.Item Open Access Micro-tilt controlled rotating face-plate stage for sigle-point diamond turning(Cranfield University, 1993-09) Mizuno, H.; Gee, A. E.; Spragg, R. C.The machining of brittle materials such as glasses and ceramics is an area of rising interest in the 'Precision Engineering' field due to the advantageous characteristics of ceramics and demands for glass machining from optical component manufacturers. In general the 'ductile mode' machining of brittle materials requires cut-geometry to be sub-micrometre. In order to improve machining accuracy of single-point diamond turning for brittle materials in the ductile mode, a controlled micro-tilt stage system was proposed for improvement of the motion accuracy and dynamic stiffness of an aerostatic spindle. Mechanical arrangements for the proposed controlled micro-tilt stage system including slip rings for transferring voltage signals to a rotating body were developed together with a strategy for spindle metrology using three optical fibre sensors. Algorithms for averaging and spacial filtering were applied to remove random noise caused by the variation of surface texture. The micro-tilt stage was designed to satisfy specifications in respect of travel range, resolution, stiffness, and resonant frequency. Efforts were also made to minimize static and dynamic cross-coupling-interference between the required three degrees of freedom. The micro-tilt stage showed satisfactory performance, and the effectiveness of non-crosscoupling design was seen. After considering various control strategies, hardware and software were arranged with PID and repetitive controllers. The diagonal dominance of the micro-tilt stage control system permitted 'SISO' system design. The performance of the controlled micro-tilt stage was investigated both stationary and during rotation. The stationary controlled micro-tilt stage worked satisfactorily; the controlled rotating micro-tilt stage demonstrated its error-correcting capability with some speed limitations, primarily due to the spacial filtering and time averaging required to reduce the surface texture noise.Item Open Access Modelling and evaluation of time-varying thermal errors in machine tool elements(Cranfield University, 1997-04) Gim, Taeweon; Corbett, John; Gee, A. E.This thesis addresses a comprehensive approach to understanding the time-varying thermal errors in machine tools. Errors in machine tools are generally classified as being time or spatial dependent. Thermal errors are strongly dependent on the continuously changing operating conditions of a machine and its surrounding environment. Uniform temperature rises or stable temperature gradients, which produce time-invariant thermal errors, are considered to be rare in ordinary shop floor environments. Difficulties in analysing time-varying thermal errors are that, first of all, the temperature distribution within the components of a machine should be evaluated, and secondly, the distribution is continuously changing with time. These difficulties can be overcome by introducing a point-wise description method with three thermal parameters. From the theoretical analysis of simple machine elements such as bars, beams and cylinders, and extensive finite-element simulation data for a straightedge subject to room temperature variations, three thermal parameters, i. e. time-delay, time-constant and gain, were identified to obtain a precise description of the thermal deformation of a point of a machine body. Time-delay is dependent largely on thermal diffusivity, and the heat transfer mechanism. The time-constant is governed by heat capacity, heat transfer mechanism and body size. Gain, on the other hand, is determined by the thermal expansion coefficient, heat transfer mechanism and mechanical constraint. The three thermal parameters, in turn, imply that thermal deformation of a point in a body can be described by a simple first- order differential equation. Regarding their dependence on the heat transfer mechanism, a more refined description requires a time-varying linear first-order differential equation. Such an equation can be applied to each point of interest of a machine body. The final form of modelling, using the parameters, is a state-space equation gathering the governing equations for the points of interest. By adopting the point-wise discrete modelling method, we can overcome the difficulty of the spatial distribution of the temperature. Indeed, the calibration of a machine tool is usually performed at discrete points. The completion of this approach was made by presenting the methods by which the three thermal parameters can be evaluated. The first method employs analytical tools based on simplifying assumptions about the shape and boundary conditions of machine components. The second method was to apply numerical techniques to complex machine components. Because there are many drawbacks in theoretical approaches, experimental techniques are essential to complement them. The three thermal parameters can be easily identified using popular parameter identification techniques which can be applied to time-varying cases by their recursive forms. The techniques described were applied to modelling the thermal errors in a single-point diamond turning research machine. It was found that the dominant error component was spindle axial growth. The predictive model for the time-constant was shown to be in agreement with both the machine and with the scaled physical model rig.