A dual-purpose positioner-fixture for precision six-axis positioning and precision fixturing: Part II. Characterization and calibration

The kinematics, stiffness, and repeatability of a moving groove, dual-purpose positioner-fixture were determined experimentally. A dual-purpose positioner-fixture is an alignment device that may be operated in a fixture mode or a six-axis nanopositioning mode. When operated in fixture mode, experiments show standard deviation in repeatability of 11, 11, and 38 nm in x, y, and z; and 0.7, 0.3, and 0.3 μrad in θ_x, θ_y, and θ_z. The stiffness characteristics were shown to match predictions within 5%. When operated in nanopositioner mode, the device demonstrated 4 nm resolution and a range, of 40 μm × 40 μm × 80 μm in translation and 800 μrad × 800 μrad × 400 μrad in rotation. The fixture possesses a load capacity of 450 N and a natural frequency of 200 Hz when the fixture is preloaded to 225 N.     

Compliant high-precision E-quintet ratcheting (CHEQR) mechanism for safety and arming devices

Ratchet and pawl mechanisms are used in safety applications to provide mechanical isolation between inputs and an output to insure that extreme environmental conditions do not inadvertently allow an unexpected output. These devices have become smaller and are approaching a size regime where traditional precision components, such as precision bearings and springs, are not available. This paper introduces the compliant high-precision E-quintet ratcheting (CHEQR) mechanism as a means of exploiting the advantages of compliant mechanisms to create safety devices that eliminate the need for bearings and springs. The pseudo-rigid-body model was used to design a mechanism with the desired force-deflection characteristics, and the result is a radical departure from traditional ratchet and pawl mechanisms. Large-scale proof-of-concept prototypes were followed by micro-wire EDM fabrication of precipitation hardened stainless steel devices with flexible segment widths of 50 μm. The device was integrated with a 6 mm ratchet wheel and rotary solenoid actuator.     

Photolithographic manufacture of a superconducting levitation coil on a spherical substrate

We have developed a photolithographic method to pattern a superconducting levitation coil made from Pb on a spherical MACOR substrate. The cross section of the windings is approximately 150 × 40 μm, and a typical coil consists of 70 windings with a minimum pitch of 350 μm. The substrate radius is about 4 cm, and the coil extends from 15 to 60° in polar angle. We present the manufacturing process and suggest future improvements.     

Studies of high temperature sliding wear of metallic dissimilar interfaces II: Incoloy MA956 versus Stellite 6

The development of wear surfaces formed during limited debris retention sliding wear of Incoloy MA956 against Stellite 6 between room temperature and 750 °C, and sliding speeds of 0.314 and 0.905 m s⁻¹ (7 N applied load, 4522 m sliding distance) were investigated. At 0.314 m s⁻¹, mild oxidational wear was observed at all temperatures, due to oxidation of Stellite 6-sourced debris and transfer to the Incoloy MA956; this debris separated the Incoloy MA956 and Stellite 6 wear surfaces. Between room temperature and 450 °C, the debris mainly took the form of loose particles with limited compaction, whilst between 510 °C and 750 °C the debris were compacted and sintered together to form a Co–Cr-based, wear protective ‘glaze’ layer. The behaviour was identical to that previously observed on sliding Nimonic 80A versus Stellite 6 at 0.314 m s⁻¹.At 0.905 m s⁻¹, mild oxidational wear was only observed at room temperature and 270 °C and dominated by Incoloy MA956-sourced debris. At 390 and 450 °C, the absence of oxide debris allowed ‘metal-to-metal’ contact and resulted in intermediate temperature severe wear; losses in the form of ejected metallic debris were almost entirely Incoloy MA956-sourced. This severe wear regime was also observed from 510 up to 630 °C, but increasingly restricted to the early stages of wear by development of a wear protective Incoloy MA956-sourced ‘glaze’ layer. This ‘glaze’ layer formed so rapidly at 690 °C and 750 °C, that severe wear was all but eliminated and wear levels were kept low.The behaviour observed for Incoloy MA956 versus Stellite 6 at 0.905 m s⁻¹ contrasts sharply with that previously observed for Nimonic 80A versus Stellite 6, in that the Incoloy MA956-sourced high Fe–Cr debris formed a protective oxide ‘glaze’, whilst the Nimonic 80A-sourced Ni and Cr oxides formed an abrasive oxide that at high sliding speeds assisted wear. The data indicates that the tendency of oxide to form a ‘glaze’ is readily influenced by the chemistry of the oxides generated.     

Erosive wear behaviour of weld hardfacing high chromium cast irons: effect of erodent particles

Five commercial hardfacing high chromium cast iron alloys were deposited by flux cored arc-welding method. The solid particle erosion studies were carried out using air blast type erosion test rig with 125–150 μm cement clinker, 125–150 μm blast furnace sinter, 100–150 μm silica sand and 125–150 μm alumina particles at a velocity of 50 m s⁻¹ and at impingement angles of 15–90°. The observed erosion rates were rationalised in terms of relative hardness of erodent particles and ability of erodent particle to cause gross fracture of the carbides. The dependence of erosion rate on impingement angle was found to be quite weak for hardfacing high chromium cast iron alloys. However, significant differences were observed in the ranking of the alloys when eroded with different erodent particles. The presence of large volume fraction of carbides proved to be beneficial to the erosion resistance when the erodent particle were softer than the carbides. With silica sand particles at normal impact and with alumina particles large volume fraction of carbides proved detrimental to the erosion resistance. The operating erosion mechanisms involved small-scale chipping, edge effect, indentation and fracture and fatigue.     

The high strain rate compression of 7039 aluminium

Aluminium alloy 7039 was compressed at strain rates between 2 × 10³ and 2·5 × 10⁴s⁻¹ using a modified Hopkinson bar. At strain rates between 2 × 10³ and 1·2 × 10⁴s⁻¹ there was a linear relationship between the flow stress and strain-rate with a slope corresponding to a macroscopic viscosity of 2·9 kPa s. At strain rates between 1·2 and 2·5 × 10⁴s⁻¹ there was a levelling out of the flow stress, but the data was too scattered to give a definite trend. Due to the opposing effects of linear work-hardening and adiabatic heating, at strains above 0·15 the specimens work-softened at a rate inversely proportional to the square root of the strain rate. At the higher strains, specimens cracked along the dominant adiabatic shear band formed during the compression.     

Self-centering probes with parallel kinematics to verify machine-tools

In this paper, a new type of self-centering probe is presented to verify the performance of machine-tools in an efficient and rapid way using ball artifacts. A self-centering probe is placed in the spindle of the machine-tool and a ball artifact on the work table. The probe is moved to the calibrated center positions of the balls of the reference artifact. The probe touches these balls and, in a single measurement, it provides the X, Y, Z offset of the actual machine position from the desired (programmed) position. A non-conventional probe design has been chosen: three independently movable probe styli form a miniature coordinate measuring machine with parallel kinematics. The development process of two variants of such a self-centering probe is presented in this paper. The results obtained in laboratory tests show a repeatability of less than 0.5 μm and an error range of less than 2 μm throughout the large measurement range (2 mm × 2 mm × 2 mm) of the probe.     

Erosion—corrosion characteristics of a 2.25 Cr-1 Mo steel exposed at low particle velocities

The erosion-corrosion characteristics of a 2.25 Cr-1 Mo steel at low particle velocities and elevated temperatures were determined using a nozzle type laboratory erosion tester. The tests were performed with 180–360 μm angular alumina particles at 60° angle of impingement at low particle velocities of 2.6–8.2 m/s and in the temperature interval 20–600°C. The steel was tested both in the as-received condition as well as in two preoxidized conditions. The erosion-corrosion rate of the steel, both in the as-received and in the preoxidized conditions, was found to increase with increasing particle velocity. In contrast, the wastage rates were relatively independent of temperature in the temperature range investigated, the only exception being specimens exposed to the lowest particle velocity (2.6 m/s) at the very highest temperature, i.e. 600°C, which displayed a drastic increase in wastage. Specimens preoxidized at 700°C exhibited a somewhat higher erosion rate compared with non-preoxidized specimens and specimens preoxidized at 500°C. Microscopy revealed four different major wastage mechanisms, i.e. (i) plastic deformation, cracking and micro chipping of surface material of a size corresponding to the area impinged by eroding particles, (ii) chipping of somewhat larger oxide fragments (up to 10–15 μm in diameter), (iii) chipping or spalling of relatively large oxide fragments (up to 30–50 μm in diameter), and (iv) spalling along the steel-oxide interface or within an oxide layer due to cohesive failure, of larger (up to 500 μm in diameter) oxide layer fragments. In the present study extensive spalling was only observed for non-preoxidized specimens exposed to the lowest particle velocity (2.6 m/s) and the two highest specimen temperatures (550°C and 600°C).     

Erosion of α-Fe by spherical glass particles

Polycrystalline α-Fe has been eroded at 30° and 90° with glass spheres of average diameters 70 μm and 200 μm in the velocity range 61–122 m s⁻¹. Detailed studies of the influence of the impact variables on the erosion rate as well as scanning electron microscopy studies of the eroded surfaces have been performed. It was observed that “breaking” waves developed on erosion at 30° and hills and valleys at 90°. Several different material loss processes that operate at various positions within waves, hills and valleys have been identified. It was clear that most material loss processes involved extensive localized shear and required the surface to become “conditioned” by a specific number of impacts before material loss began.     

Lattice distortions in GaN on sapphire using the CBED–HOLZ technique

The convergent beam electron diffraction (CBED) methodology was developed to investigate the lattice distortions in wurtzite gallium nitride (GaN) from a single zone-axis pattern. The methodology enabled quantitative measurements of lattice distortions (α, β, γ and c) in transmission electron microscope (TEM) specimens of a GaN film grown on (0, 0, 0, 1) sapphire by metal-organic vapour-phase epitaxy. The CBED patterns were obtained at different distances from the GaN/sapphire interface. The results show that GaN is triclinic above the interface with an increased lattice parameter c. At 0.85 μm from the interface, α=90°, β=8905° and γ=11966°. The GaN lattice relaxes steadily back to hexagonal further away from the sapphire substrate. The GaN distortions are mainly confined to the initial stages of growth involving the growth and the coalescence of 3D GaN islands.     

A method for achieving effective load transfer between the inner and outer layers of a two-layer braided high-pressure hydraulic hose

A method is proposed for accommodating the misfit between two layers of a braided hose. This is achieved by laying the inner and outer braids at angles that lie symmetrically on either side of the equilibrium helix angle of tan⁻¹ 2 for a single-layer hose. The theory of the method is developed and it is shown that a difference of helix angles of 4° or more will accommodate the fractional radial misfit of 3 × 10⁻³ existing in a particular sample of hose. An analysis of the wire tensions in a pressurised hose, assuming constant inter-layer volume, revealed that seriously unequal tensions exist in the two layers if the two helix angles lie asymmetrically about tan⁻¹ 2. The process of taking up misfit, now proposed, generates asymmetry. However this asymmetry and the associated inequality of wire tension decreases as the difference of braid angles increases. Guided by this generalisation, it is demonstrated that an acceptable hose design is achieved if the inner braid is laid at an angle of 59·74° and the outer braid at 49·74°. The geometrical changes that take up the misfit at low pressures produce an extension of the hose of 0·65% and reduce the braid angles by about . The ratio of final tensions in the two layers is 0·93. The analysis in the paper leads to the conclusion that manufacturing uncertainties in the angle at which a braid is laid can generate serious inequalities of wire tension. In the specific illustrative example analysed here an uncertainty of ±1° in braid angle will cause the ratio of wire tensions to vary, but it never falls below a value of 0·63.     

Design evaluation of a single-axis precision controlled positioning stage

This paper presents the design and control of a single-axis positioning stage with a total travel of 50 mm. The single-axis stage is comprised of a long-range slideway, running on ultra-high molecular weight polyethylene (UHMWPE) bearings, and a short-range positioning stage, comprised of a PZT driven flexure. Feedback errors associated with the long-range stage are reduced by the short-range stage, mounted on top of the long-range stage. Within the evaluation of the long-range stage, two alternative drives are assessed; a Roh’lix^® and a feedscrew drive. To determine the effects of dynamic interaction between the two drive systems, a further assessment of the single-axis stage was undertaken with the short-range stage operating at different bandwidths, ranging from 91 Hz to 2188 Hz. At the highest bandwidths, nanometer performance is demonstrated for a sinusoidal displacement demand corresponding to sinusoidal traverses of 1 mm and 5 mm with a maximum velocity of approximately 30 μm s⁻¹ and 150 μm s⁻¹, respectively. Furthermore, a 12 nm RMS controller error over a traverse of 25 mm at a maximum velocity of approximately 330 μm s⁻¹ was observed with the use of the feedscrew drive only.     

The effect of oil films on the performance of bell provers

Domestic gas meters are tested for accuracy of registration using bell provers. Oil has a tendency to form films on the vertical surfaces of the bell, and this paper investigates its effect on the accuracy of the prover. The geometry of the prover and the manner of its operation are both shown to affect the magnitude of the error. The most important physical property of the oil is the kinematic viscosity, ν, and the error is found to be proportional to (ν). Calculations for an oil having a value for ν of 2·5 × 10⁻⁵m²s⁻¹ indicate a maximum error of about 0·2% under normal operating conditions. The magnitude of the maximum error can be reduced by suitable design of the prover.     

Solid particle erosion of CFRP composite with different laminate orientations

The solid particle erosion behavior of epoxy base unidirectional and multidirectional carbon fiber reinforced plastic composites was investigated. The erosion rates of these composites were evaluated at various impingement angles (15–90°) with a particle velocity of 70 m/s. Irregular SiC particles with an average diameter of 80 μm was used. The dependence of impingement angle on the erosive wear resembled the conventional ductile behavior with maximum erosion rate at 15–30° impingement angle. The erosion rate of unidirectional composites at acute impingement angle was higher for [90] than for [45] and [0] while the difference disappeared at normal impingement angle (90°). On the other hand, the erosion rates of multidirectional laminated composites ([0/90], [45/−45], [90/30/−30] and [0/60/−60]) were not much influenced by the fiber orientation except for 15° impingement angle.     

A study of the wear behavior of polymer–matrix composites containing discontinuous nanocrystalline alloy reinforcements

The tribological behavior of bakelite resin–matrix composites reinforced with nanocrystalline Al 6061 T6 particles produced by machining (grain size 70–500 nm) has been studied using block-on-ring and pin-on-disk tests. The polymer–matrix composite reinforced with nanostructured Al 6061 particles aged for 10 h [Al 6061 (3) 10 h] shows a wear reduction of around 60% with respect to the conventional microstructured reinforcement. Also it shows the lowest wear rates when compared with the nanostructured reinforcements aged for 5 h or 1 h, respectively. Friction coefficients and wear rates increased with increasing sliding speed and normal load. Under 10 N and 0.10 m s⁻¹, Al 6061 (3) 10 h showed an initial friction and contact temperature increase and a very severe wear with material transfer to the steel ball surface. Increasing the steel–composite contact temperature to 100 °C (1 N; 0.05 m s⁻¹) produced a one order of magnitude decrease both in friction and wear. Wear mechanisms for the polymer matrix and the aluminum reinforcement are discussed on the basis of SEM and EDS observations.     

Micro-structuring of glass with features less than 100 μm by electrochemical discharge machining

Micro-electrochemical discharge machining (ECDM) was studied in order to improve the machining of 3D micro-structures of glass. To minimize structures and obtain good surface microstructures, the effects of the electrolyte, the pulse on/off-time ratio, the voltage, the feedrate, the rotational speed, and the electrolyte concentration in the drilling and milling processes were studied.In ECDM, voltage is applied to generate a gas film and sparks on a tool electrode; however, high voltage produces poor machining resolution. To obtain a stable gas film over the whole surface of the tool at a low voltage, a new mechanical contact detector, based on a loadcell, was used; the immersion depth of the tool electrode in the electrolyte was reduced as much as possible. In this study, various micro-structures less than 100 μm in size, such as Ø 60 μm micro-holes, a 10 μm-thin wall, and a 3D micro-structure were fabricated to demonstrate the potential for micro-machining of glass by ECDM.     

Studies of high temperature sliding wear of metallic dissimilar interfaces

The evolution of microstructures in the glaze layer formed during limited debris retention sliding wear of Nimonic 80A against Stellite 6 at 750 °C and a sliding speed of 0.314 m s⁻¹ (7 N applied load, 4522 m sliding distance) was investigated using scanning electron microscopy (SEM), energy dispersive analysis by X-ray (EDX), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). The collected data indicate the development of a wear resistant nano-structured glaze layer. The process of ‘fragmentation’ involving deformation, generation of dislocations, formation of sub-grains and their increasing refinement causing increasing misorientation was responsible for the formation of nano-structured grains. The rapid formation of this glaze layer from primarily cobalt–chromium debris transferred from (and also back to) the surface of the Stellite 6, kept wear of both the Nimonic 80A and Stellite 6 to very low levels.However, increasing the sliding speed to 0.905 m s⁻¹ (750 °C) suppressed glaze formation with only a patchy, unstable glaze forming on the Stellite 6 counterface and an absence of glaze development on the Nimonic 80A sample (the Nimonic 80A surface was covered with at most, a very thinly smeared layer of oxide). The high levels of oxide debris generated at 0.905 m s⁻¹ instead acted as a loose abrasive assisting wear of especially the Nimonic 80A. This behaviour was attributed to a change in oxide chemistry (due to the dominance of nickel and chromium oxides generated from the Nimonic 80A) resulting in poor oxide sintering characteristics, in combination with increased mobility and reduced residency of the oxide debris at 0.905 m s⁻¹.     

Long-term relative stability of thermistors

The relative stability of four glass-bead thermistors has been analyzed for 44 months to determine if thermistors have sufficient relative stability to be used as feedback sensors for milliKelvin levels of temperature control. The thermistors were measured at 20 ± 0.1 °C using commercial two-wire instrumentation. The thermistors have a nominal resistance of 12.5 kΩ and sensitivity of −4.5%/°C at 20°C. They are nested together inside several layers of thermal resistance and capacitance, which are held at nominal 20°C with a temperature-controlled, recirculating air chamber. After correction for instrumentation drift with two high-stability reference resistors, the average relative rms drift of the thermistors is 50 ± 15 μK, with an rms fitted drift rate of 0.26 ± 0.16 μK/week.     

Long-term relative stability of thermistors: Part 2

The relative stability of four glass–bead thermistors has been analyzed for 66 months to determine if thermistors have sufficient relative stability to be used as feedback sensors for milliKelvin levels of temperature control. This paper provides additional drift data and new correlations beyond those described in a previous paper [Prec Eng, J Int Soc Prec Eng Nanotechnol 2001;25:24–8]. The thermistors were measured at 20±0.1 °C using commercial two-wire instrumentation. The thermistors have a nominal resistance of 12.5 kΩ and sensitivity of −4.5%/°C at 20 °C. They are nested together inside several layers of thermal resistance and capacitance which are held at nominal 20 °C with a heater controller that maintains the thermistor nest above the 19 °C ambient air temperature. After correction for instrumentation drift with two high-stability reference resistors, the average relative RMS drift of the thermistors is 48±16 μK, with an RMS fitted drift rate of 0.19±0.08 μK per week.     

A 6-degree-of-freedom measurement system for the accuracy of X-Y stages

A precision 6-degree-of-freedom measurement system has been developed for simultaneous on-line measurements of six motion errors of an X-Y stage. The system employs four laser Doppler scales and two quadrant photo detectors to detect the positions and the rotations of an optical reflection device mounted on the top of the X-Y stage. Compared to the HP5528A system, the linear positioning accuracy of the developed measurement system is better than ±0.1 μm to the range of 200 mm and the vertical straightness error is within ±1.5 μm for the measuring range of ±0.1 mm. The yaw and pitch errors are about ±1 arcsec, and the roll error is about ±3 arcsec within the range of ±50 arcsec.