Stuart T. Smith

Elliptical flexure hinges

This paper presents closed form equations based on a modification of those originally derived by Paros and Weisbord in 1965, for the mechanical compliance of a simple monolithic flexure hinge of elliptic cross section, the geometry of which is determined by the ratio e of the major and minor axes. It is shown that these equations converge at e=1 to the Paros and Weisbord equations for a hinge of circular section and at e ⇒∞ to the equations predicted from simple beam bending theory for the compliance of a cantilever beam. These equations are then assessed by comparison with results from finite element analysis over a range of geometries typical of many hinge designs. Based on the finite element analysis, stress concentration factors for the elliptical hinge are also presented. As a further verification of these equations, a number of elliptical hinges were manufactured on a CNC milling machine. Experimental data were produced by applying a bending moment using dead weight loading and measuring subsequent ...

Giant magnetoresistance-based eddy-current sensor

The purpose of this paper is to introduce a new eddy-current testing technique for surface or near-surface defect detection in nonmagnetic metals using giant magnetoresistive (GMR) sensors. It is shown that GMR-based eddy-current probes are able to accurately detect short surface-breaking cracks in conductive materials. The self-rectifying property of the GMR sensor used in this study leads to a simplified signal conditioning circuit, which can be fully integrated on a silicon chip with the GMR sensor. The ability to manufacture probes having small dimensions and high sensitivity (220 mV/mT) to low magnetic fields over a broad frequency range (from dc up to 1 MHz) enhances the spatial resolution of such an eddy-current testing probe. Experimental results obtained by scanning two different probes over a slotted aluminum specimen are presented. General performance characteristics are demonstrated by measurements of surface and subsurface defects of different sizes and geometries. Dependence of the sensor output on orientation, liftoff distance, and excitation intensity is also investigated.

Development of a virtual probe tip with an application to high aspect ratio microscale features

Nondestructive measurement of microscale features remains a challenging metrology problem. For example, to assess a high aspect ratio small hole it is currently common to cut a cross section and measure the features of interest using an atomic force microscope, scanning probe microscope, or scanning electron microscope. Typically, these metrology tools may be suitable for surface finish measurement but often lack the capability for dimensional metrology. The aim of this article is to discuss the development of a high aspect-ratio microscale probe for measurement of microscale features. A 700:1 high aspect ratio probe shank is fabricated with a 7μm diameter, and attached at one end to an oscillator. The oscillator produces a standing wave in the oscillating probe shank as opposed to conventional probes that use a microscale sphere on the end of a comparatively rigid shank. As a result of the standing wave formed in steady state vibration, the free end of the shank generates an amplitude of oscillation grea...

The Effect of Tool Length on Stable Metal Removal Rate in High Speed Milling

Abstract This paper describes the effect of the tool length on achievable stable metal removal rate in high speed milling. Through finite element computation and experiment, it is shown that the tool length most strongly affects the natural frequency of the most flexible mode. The change in frequency changes the most stable speed in the stability lobe diagram. The highest metal removal rates occur when the tool length is selected to match the stable zone to the highest spindle speed. Sometimes longer tools can provide higher metal removal rates than shorter tools because they utilize the stability lobe phenomenon more effectively.

Sub-Nanometre Surface Texture and Profile Measurement with NANOSURF 2

Abstract A surface texture and profile-measuring stylus instrument, developed at NPL and called ‘NANOSURF 2’, which has a continuous dynamic horizontal range of 50 nm to 50 mm, is being used for sub-nanometre surface texture and profile measurement on a wide range of engineering, optical and semi-conductor surfaces. The paper presents a selection of results on such specimens. Built almost entirely of near-zero thermal expansivity materials, with a tight measurement loop and minimum interfaces, the instrument exhibits very low susceptibility to thermal and mechanical disturbances. Some features of the instrument are incorporated in a novel, ultra-high precision machine tool under development, which is briefly mentioned in the paper.

A controlled-force stylus displacement probe

A novel design of displacement profiler with a controllable stylus force is presented. It provides highly controlled conditions for contact measurement of, for example, small step heights or surface roughness. Incorporating an electromagnetic force actuator and force feedback control, the profiler provides electronically selectable contact force in the range of 0.01–10 mN and gives a constant static and dynamic loading. In a typical configuration, it has a range of a few micrometers with a discrimination to better than 1 nm at a bandwidth higher than that of a conventional stylus instrument.

The design and operation of monolithic X-ray interferometers for super-precision metrology

A review of the techniques of X-ray interferometry relevant to micro-displacement metrology is given, with particular emphasis on its role in calibration. Monolithic interferometer designs are advocated because of their favourable metrology loop structures. New developments are reported of techniques that should widen the range of practical situations to which X-ray interferometric calibrations can be applied. One is the use of image processing directly from an X-ray camera to analyse X-ray moire images. The other uses capacitive sensors and precision elastic drives to effect rapid transfer calibrations.

Deep Crack Detection around Fastener Holes in Airplane Multi‐Layered Structures Using GMR‐Based Eddy Current Probes

Rotational and linearly scanned eddy current probes based on giant magnetoresistive (GMR) sensors have been developed for detecting buried cracks and flaws emanating from fastener holes. The use of shaped excitation coils, the original positioning and orientation of the magnetic sensors enhance the sensitivity of these probes to buried defects while reducing the influence of fastener holes edge. Corner cracks of 2.5 mm in length were detected in the second layer of a 13 mm thick two‐layer structure.

Improvement of the fidelity of surface measurement by active damping control

This paper reports on the modification of a stylus instrument for studying dynamic effects in surface finish measurements. A feedback system allows control of the force from freely suspended to 10 mN and of the damping ratio from 0.02 to larger than one. Specimens of mild steel and copper have been traced repeatedly with damping ratios of 0.02 and 0.59 over the traverse speeds of 0.05-1 mm s-1. From these profiles it is observed that more consistent profile shapes and surface finish parameters are obtained within an optimal damping range. The performance of the system is investigated over a large range of damping ratios by freely suspending the system while applying a force proportional to a previously measured profile. The closest correspondence between the stylus output and the original profile, taken to represent optimal fidelity, occurs with damping ratios in the range of 0.5-0.7. Dynamic forces at the stylus are evaluated by direct measurement using a load cell and by indirect estimation through friction measurements. To compensate the additional mass due to the force actuator and damping sensor, it is demonstrated that the state feedback control can be used to vary the natural resonant frequency and the damping ratio independently.

A six-degree-of-freedom precision motion stage

This article presents the design and performance evaluation of a six-degree-of-freedom piezoelectrically actuated fine motion stage that will be used for three dimensional error compensation of a long-range translation mechanism. Development of a single element, piezoelectric linear displacement actuator capable of translations of 1.67 μm with 900 V potential across the electrodes and under a 27.4 N axial load and 0.5 mm lateral distortion is presented. Finite element methods have been developed and used to evaluate resonant frequencies of the stage platform and the complete assembly with and without a platform payload. In general, an error of approximately 10.0% between the finite element results and the experimentally measured values were observed. The complete fine motion stage provided approximately ±0.93 μm of translation and ±38.0 μrad of rotation in all three planes of motion using an excitation range of 1000 V. An impulse response indicating a fundamental mode resonance at 162 Hz was measured with...