An integrated measurement method for complex micro-scale geometries

Micro-scale geometries are becoming commonplace in many high-precision manufacturing applications. Micro-drilling processes, for example, are being employed for producing holes in demanding applications involving fluid transfer, atomisers and micro-mechanics. This paper explores the measurement and characterisation of a high aspect-ratio micro-hole (nominal diameter approximately 1000 pm, aspect-ratio 1:10), produced using abrasive waterjet in Ti6AI4V. X-ray computed tomography, contact micro-coordinate metrology and focus-variation microscopy are used for measuring the hole surfaces, and dedicated computational geometry algorithms are applied to obtain critical hole dimensions, such as radius as a function of depth. The comparison of the measurement and characterisation results obtained by means of the different solutions explored hints at new approaches for multisensor data fusion that can help reduce bias in the measurement of high aspect-ratio micro-scale features

ISO compliant reference artefacts for the verification of focus varation-based optical micro-coordinate measuring machines

Demand form micro-coordinate measuring machines (micro-CMMs) within industry is increasing due to the need for accurate measurement of the geometry of small-scale objects. Optical micro-CMMs have the advantage over traditional stylus-based CMMs of being non-contact instruments, and have the ability to acquire large amounts of data, with high resolution, in a relatively short period of time. The focus variation (FV) technique is typically used in the field of surface topography measurement, but has the potential to be implemented as a sensor technology for optical micro-CMMs. Exploring the possibility of the FV technique as an optical micro-CMM requires that the instrument and measuring procedure can be performance verified. Consequently, a prototype FV based optical micro-CMM should have a verification route that is traceable to the definition of the metre. The ISO 10360 specification standard for acceptance testing and verification of CMMs has several parts, all specific to different groups of instruments and configurations. Each section of ISO 10360 identifies methods and artefacts best suited for the acceptance testing and verification of each group and configuration. ISO/DIS 10360-8.2 (due for ISO/FDIS publication in 2013), is a verification standard written for CMMs with optical distance sensors. There are four main parts to the acceptance and re-verification tests: length measurement error, probing form error measurement, probing size error measurement and flat form error measurement. The probing form and size error tests require a calibrated reference sphere that has a diameter of at least 10 mm. FV instruments are potentially covered by this standard but the recommended minimum size of the calibration sphere is too large to fully fit within one field of view of a typical FV instrument.. Optical distance measuring instruments similar in operation to FV systems, such as confocal microscopes and coherent scanning interferometers are, therefore, also currently excluded from the application of this standard by default, unless the user, and the instrument manufacturer, can potentially agree to use a smaller calibrated reference sphere for the assessment of the probing size and form error. A prototype FV based optical micro-CMM should, therefore, be verified with calibrated reference spheres of similar size to objects for which the technique has been designed to measure. The research reported here considers the use of 0.5 mm, 1 mm and 2 mm diameter reference spheres as suitable components for a reference artefact, with the surfaces of the spheres roughened using bespoke micro-roughening techniques. The research presents a novel verification artefact composed of multiple small-scale spheres, specifically designed to evaluate probing size error, probe form error, and dimensional accuracy of a prototype FV based optical micro-CMM, compliant to ISO/DIS 10360-8. The results suggest that the artefacts and procedures detailed in ISO/DIS 10360-8 can, and should, be applied to optical micro-CMMs

areal topography measurement of metal additive surfaces using focus variation microscopy

Abstract In this work, the performance of a focus variation instrument for measurement of areal topography of metal additive surfaces was investigated. Samples were produced using both laser and electron beam powder bed fusion processes with some of the most common additive materials: Al-Si-10Mg, Inconel 718 and Ti-6Al-4V. Surfaces parallel and orthogonal to the build direction were investigated. Measurement performance was qualified by visually inspecting the topographic models obtained from measurement and quantified by computing the number of non-measured data points, by estimating local repeatability error in topography height determination and by computing the value of the areal field texture parameter Sa. Variations captured through such indicators were investigated as focus variation-specific measurement control parameters were varied. Changes in magnification, illumination type, vertical resolution and lateral resolution were investigated. The experimental campaign was created through full factorial design of experiments, and regression models were used to link the selected measurement process control parameters to the measured performance indicators. The results indicate that focus variation microscopy measurement of metal additive surfaces is robust to changes of the measurement control parameters when the Sa texture parameter is considered, with variations confined to sub-micrometre scales and within 5% of the average parameter value for the same surface and objective. The number of non-measured points and the local repeatability error were more affected by the choice of measurement control parameters. However, such changes could be predicted by the regression models, and proved consistent once material, type of additive process and orientation of the measured surface are set

Areal texture and angle measurements of tilted surfaces using focus variation methods

Optical instruments for areal surface topography measurement have seen significant commercial development in the last five years, along with the ISO 25178 areal standard. Providing the user with confidence in new instruments depends on understanding instrument behavior and sources of error. Focus variation techniques rely on the inherent micro- or nano-scale roughness of a surface to allow acquisition of topography data. The work reported here has been examining the sensitivity of the focus variation technique to surface slope, using areal parameters to characterize surface roughness at extended slope values. The results illustrate links between instrument variables and slope characterization

On-machine focus variation measurement for micro-scale hybrid surface texture machining

Fast and accurate in-line areal surface topography measuring instruments are required to control the quality of microscale manufactured components, without significantly slowing down the production process. Full-field areal optical surface topography measurement instruments are promising for in-line or on-machine measurement applications due to their ability to measure quickly, to access small features and to avoid surface damage. This paper presents the development and integration of a compact optical focus variation sensor for on-machine surface topography measurement mounted on to a hybrid ultraprecision machine tool. The sensor development is described and a case study involving the on-machine dimensional measurement of the depth of hydrophobic microscale features, including microchannels and micro-dimples, is presented. Comparisons of results between the on-machine measurements obtained by the developed sensor and a desktop focus variation microscope are presented and discussed. The comparison results show that the developed focus variation sensor is able to perform on-machine dimensional measurement of microscale features within sub-micrometre accuracy

The assessment of residual flatness errors in focus variation areal measuring instruments

Optical instruments for areal surface topography measurement rely on high-precision lenses that guide the light from the object surface to the image plane. Lens aberrations may cause distortion of the transmitted image and consequently a residual flatness error in the measurement data. Previous work at NPL suggests using an averaging method for residual flatness error assessment for optical surface topography instruments. However, the averaging method does not apply to the focus variation technique, which relies on the nano-scale roughness of a surface to allow acquisition of topography data. This paper presents alternative methods for measuring residual flatness for focus variation instruments

An international comparison of surface texture parameters quantification on polymer artefacts using optical instruments

An international comparison of optical instruments measuring polymer surfaces with arithmetic mean height values in the sub-micrometre range has been carried out. The comparison involved sixteen optical surface texture instruments (focus variation instruments, confocal microscopes and coherent scanning interferometers) from thirteen research laboratories worldwide. Results demonstrated that: (i) Agreement among different instruments could be achieved to a limited extent; (ii) standardised guidelines for uncertainty evaluation of areal surface parameters are needed for users; (iii) it is essential that the performance characteristics (and especially the spatial frequency response) of an instrument is understood prior to a measurement

Combined inkjet printing and infrared sintering of silver nanoparticles using a swathe-by-swathe and layer-by-layer approach for 3-dimensional structures

Despite the advancement of additive manufacturing (AM)/3-dimensional (3D) printing, single-step fabrication of multifunctional parts using AM is limited. With the view of enabling multifunctional AM (MFAM), in this study, sintering of metal nanoparticles was performed to obtain conductivity for continuous line inkjet printing of electronics. This was achieved using a bespoke three dimensional (3D) inkjet-printing machine, JETx®, capable of printing a range of materials and utilizing different post processing procedures to print multi-layered 3D structures in a single manufacturing step. Multiple layers of silver were printed from an ink containing silver nanoparticles (AgNPs) and infra-red sintered using a swathe-by-swathe (SS) and layer-by-layer sintering (LS) regime. The differences in the heat profile for the SS and LS was observed to influence the coalescence of the AgNPs. Void percentage of both SS and LS samples was higher towards the top layer than the bottom layer due to relatively less IR exposure in the top than the bottom. The results depicted a homogeneous microstructure for LS of AgNPs and showed less deformation compared to the SS. Electrical resistivity of the LS tracks (13.6 ± 1μΩ cm) was lower than the SS tracks (22.5 ± 1 μΩ cm). This study recommends the use of LS method to sinter the AgNPs to obtain a conductive track in 25% less time than SS method for MFAM

Optimisation of surface measurement for metal additive manufacturing using coherence scanning interferometry

Surface topography measurement for metal additive manufacturing (AM) is a challenging task for contact and non-contact methods. In this paper, we present an experimental investigation of the use of coherence scanning interferometry (CSI) for measurement of AM surfaces. Our approach takes advantage of recent technical enhancements in CSI, including high dynamic range for light level and adjustable data acquisition rates for noise reduction. The investigation covers several typical metal AM surfaces made from different materials and AM processes. Recommendations for measurement optimisation balance three aspects: data coverage, measurement area and measurement time. This study also presents insight into areas of interest for future rigorous examination, such as measurement noise and further development of guidelines for the measurement of metal AM surfaces

Topography of selectively laser melted surfaces: A comparison of different measurement methods

Selective laser melting (SLM) of metals produces surface topographies that are challenging to measure. Multiple areal surface topography measurement technologies are available, which allow reconstruction of information rich, three-dimensional digital surface models. However, the capability of such technologies to capture intricate topographic details of SLM parts has not yet been investigated. This work explores the topography of a SLM Ti6Al4V part, as reconstructed from measurements by various optical and non-optical technologies. Discrepancies in the reconstruction of local topographic features are investigated through alignment and quantitative assessment of local differences. ISO 25178-2 areal texture parameters are computed as further comparison indicators