Abstract
The dimensional verification of miniaturized components with 3D complex geometries is particularly challenging. Computed Tomography (CT) can represent a suitable alternative solution to micro metrology tools based on optical and tactile techniques. However, the establishment of CT systems’ traceability when measuring 3D complex geometries is still an open issue. In this work, an alternative method for the measurement uncertainty assessment of 3D complex geometries by using CT is presented. The method is based on the micro-CT system Maximum Permissible Error (MPE) estimation, determined experimentally by using several calibrated reference artefacts. The main advantage of the presented method is that a previous calibration of the component by a more accurate Coordinate Measuring System (CMS) is not needed. In fact, such CMS would still hold all the typical limitations of optical and tactile techniques, particularly when measuring miniaturized components with complex 3D geometries and their inability to measure inner parts. To validate the presented method, the most accepted standard currently available for CT sensors, the Verein Deutscher Ingenieure/Verband Deutscher Elektrotechniker (VDI/VDE) guideline 2630-2.1 is applied. Considering the high number of influence factors in CT and their impact on the measuring result, two different techniques for surface extraction are also considered to obtain a realistic determination of the influence of data processing on uncertainty. The uncertainty assessment of a workpiece used for micro mechanical material testing is firstly used to confirm the method, due to its feasible calibration by an optical CMS. Secondly, the measurement of a miniaturized dental file with 3D complex geometry is carried out. The estimated uncertainties are eventually compared with the component’s calibration and the micro manufacturing tolerances to demonstrate the suitability of the presented CT calibration procedure. The 2U/T ratios resulting from the validation workpiece are, respectively, 0.27 (VDI) and 0.35 (MPE), by assuring tolerances in the range of ± 20–30 µm. For the dental file, the EN < 1 value analysis is favorable in the majority of the cases (70.4%) and 2U/T is equal to 0.31 for sub-mm measurands (L < 1 mm and tolerance intervals of ± 40–80 µm).
Highlights
The manufacturing of micro three-dimensional components with complex geometries increasingly requires high accuracy metrological tools for process optimization and product tolerance verification in the 100 –101 μm range
The most accepted procedure based on the assessment of the measurement uncertainty by means of the calibrated workpiece using Computed Tomography (CT) [13] is not suitable in the case of components with 3D complex geometries, as arguably there is a lack of a traceable measuring technology that can be employed for such measuring tasks and that provides reference calibration data
An alternative method is proposed to provide traceability to the 3D measurements obtained by a micro-CT system for the verification provide traceability to the 3D measurements obtained by a micro-CT system for the verification of of dimensions on miniaturized components
Summary
The manufacturing of micro three-dimensional components with complex geometries increasingly requires high accuracy metrological tools for process optimization and product tolerance verification in the 100 –101 μm range. Sub-μm measurement resolution and repeatability, with combined expanded uncertainty in the single-digit micrometer range down to 1 μm, are to be obtained for effective verification of 3D complex micro geometries. For this purpose, several contact and non-contact micro metrological techniques are currently available. Non-contact measuring instruments based on optical techniques [3] (e.g., optical CMM [4], confocal and focus variation microscopes [5], coherence scanning interferometers [6], fringe detection [7], and photogrammetry [8]), are capable of meeting the metrological requirements for metrology of 3D micro components, but have limitations both in measuring vertical walls and high aspect ratio structures, and accessing out-of-sight features. When a 3D geometry is measured by these systems, their measurement uncertainty results are influenced by, e.g., changes in the effective focus of the lens and needed motions of the workpiece
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