Step Gauges Research Articles

A non-contact calibration system for step gauges using automatic collimation techniques

To calibrate transfer standards for length calibration, a non-contact calibration system for step gauges and gauge blocks has been developed, in which a laser interferometer was equipped to obtain the length displacement and dual-stage driving-position techniques were used to carry step gauges to approach the measuring location. A non-contact collimation technique combined with an optoelectronic microscope was proposed for achieving identification. The optoelectronic microscope eliminates both the contact deformation resulting from the mechanical force and motion deviations. The combination of dual-stage driving-position techniques and the non-contact collimation technique can ensure that the collimation repeatability is less than 20 nm. The experimental tests and comparison results indicated that the non-contact strategy and dual-stage position techniques can provide a reliable method for building a length calibration system for gauges, industrial line scales, and end bars.

Analysis of thermal contribution to the measurement uncertainty in step gauge calibration

Thermal expansion errors represent an important source of measurement uncertainty in precision length measurements. The authors present research performed on thermal error minimisation in calibration of step gauges, with the goal to develop a new high precision procedure for step gauge calibration in the Slovenian National Dimensional Metrology Laboratory. Thermal conditions during step gauge calibration were analyzed by using a combination of experimental measurements and Finite Element Method (FEM) simulations. Good final agreement between the experimental measurements and the simulation proves that the FEM simulation is a useful tool for studying and compensating thermal errors. The final outcome of the research is a non-linear model for compensating thermal expansion of a step gauge during calibration in a microclimatic chamber, where the temperature is controlled within the limits of (20 ± 0.1) °C. Different types of step gauges were examined and modelled which are used commonly for testing the performance of Coordinate Measuring Machines (CMMs). The results show that the developed error compensation method is effective enough for our metrology purposes as regards measurement uncertainty.

Overlap method for the performance evaluation of coordinate measurement systems and the calibration of one-dimensional artifacts

This article describes a method called ‘overlap’ to perform two main tasks. The first is the performance evaluation of a coordinate measurement system (CMS) with a measuring range larger than the length of artifacts. The second is the calibration of one-dimensional artifacts using a CMS with a measuring range smaller than the length of the artifacts. Examples of one-dimensional artifacts are ball bars, step gauges, or scale bars with lines or circle marks. For the first task, the method uses the translation between errors in a line inside the volume of the CMS. For the second task, it uses the rotation and translation between 3D coordinates of elements belonging to the artifact. Validation of the method for the first task is done by evaluating the performance of a coordinate measuring machine (CMM) in parallel lines to two of its axes. For this, we use a step gauge and a ball bar covering the range of the CMM’s lines under evaluation not using overlap. Later, we evaluate the same lines using a segment of the same step gauge and ball bar but this time using overlap. For the second task, the validation is similar; we use a ball bar whose length falls inside the measurement range of a high-accuracy CMM, then the ball bar can be measured in its entire length with the CMM not using overlap and later in segments using overlap.

Use of Miniature Step Gauges to Assess the Performance of 3D Optical Scanners and to Evaluate the Accuracy of a Novel Additive Manufacture Process.

In this work, we show how miniature step gauges featuring unidirectional and bidirectional lengths can be used to assess the performance of 3D optical scanners as well as the accuracy of novel Additive Manufacturing (AM) processes. A miniature step gauge made of black polyphenylene sulfide (PPS) was used for the performance verification of three different optical scanners: a structured light scanner (SLS), a laser line scanner (LLS), and a photogrammetry-based scanner (PSSRT), having comparable resolutions and working volumes. Results have shown a good agreement between the involved scanners, with errors below 5 μm and expanded uncertainties below 10 μm. The step gauge geometry due to the bidirectional lengths, highlights that there is a different interaction between the optical properties of the step gauge under measurement and each optical instrument involved and this aspect has to be considered in the uncertainty budget. The same geometry, due to its great significance in the detection of systematic errors, was used, as a novelty, to evaluate the accuracy of Lithography-based Ceramics Manufacturing (LCM), a proprietary additive manufacturing technology used for the fabrication of medical implants. In particular, two miniature step gauges made of Tricalcium Phosphate (TCP) were produced. Measurements conducted with the SLS scanner were characterized by a negligible error and by an uncertainty of about 5 μm. Deviations of the manufactured step gauges with respect to the Computer Aided Designed (CAD) model were comprised between ±50 μm, with positive deviations in the order of 100 μm on vertical sides. Differences in the order of 50 μm between the two step gauges were registered.

Measurement of polymers with 3D optical scanners: evaluation of the subsurface scattering effect through five miniature step gauges

In this work, the interaction between 3D optical scanners and objects realized with different materials and colours was investigated. The interaction is mainly related to the translucency of some materials, such as polymers, which is a category of materials in ever-increasing demand by industry. At the same time, the increase of the geometrical complexity of manufactured parts raises the need for the use of 3D optical scanners thanks to their rapidity and effectiveness. Translucency causes a subsurface scattering effect that has been modelled for certain cases and affects the dimensional accuracy of measurements, as well as the quality of reconstruction. Recently, an investigation considered its effect in dimensional verification using a structured light scanner with fringe projection. In this work, the investigation is extended to other 3D optical scanners exploiting different measuring principles: laser, structured light based on phase shifting and gray code, and photogrammetry. This was done in order to study how the subsurface scattering effect can vary with the materials, colours, and also the measuring principle used. To this end, five miniature step gauges made of different polymers and colours were used, and the effects due to their different optical properties investigated. The analysis focused on dimensional verification of bidirectional lengths, first calibrated with a traceable coordinate measuring machine. The quality of the reconstruction was also evaluated through the flatness values.According to the results obtained, translucency of materials has a dual and significant effect: a measurement bias and an increase of the uncertainty due to the different quality of reconstruction retrieved. Thus, it has to be considered properly for each case.

Interferometric step gauge for CMM verification

The verification of the measurement capability of coordinate measuring machines (CMM) is usually performed using gauge blocks or step gauges as reference standards. Gauge blocks and step gauges are robust and easy to use, but have some limitations such as finite lengths and uncertainty of thermal expansion. This paper describes the development, testing and uncertainty evaluation of an interferometric step gauge (ISG) for CMM verification. The idea of the ISG is to move a carriage bearing a gauge block along a rail and to measure the position with an interferometer. For a displacement of 1 m the standard uncertainty of the position of the gauge block is 0.2 µm. A short range periodic error of CMM can also be detected.

High accuracy step gauge interferometer

Step gauges are convenient transfer standards for the calibration of coordinate measuring machines. A novel interferometer for step gauge calibrations implemented at VTT MIKES is described. The four-pass interferometer follows Abbe’s principle and measures the position of the inductive probe attached to a measuring head. The measuring head of the instrument is connected to a balanced boom above the carriage by a piezo translation stage. A key part of the measuring head is an invar structure on which the inductive probe and the corner cubes of the measuring arm of the interferometer are attached. The invar structure can be elevated so that the probe is raised without breaking the laser beam. During probing, the bending of the probe and the interferometer readings are recorded and the measurement face position is extrapolated to zero force. The measurement process is fully automated and the face positions of the steps can be measured up to a length of 2 m. Ambient conditions are measured continuously and the refractive index of air is compensated for. Before measurements the step gauge is aligned with an integrated 2D coordinate measuring system. The expanded uncertainty of step gauge calibration is .

Bi-lateral comparison APMP.L-K5.2006.1—calibration of step gauge

This report concerns an international bilateral key comparison involving two laboratories using three facilities in the subject of calibration of step gauges, which are artefacts used for performance verification of coordinate measuring machines. A third laboratory was selected as an independent adjudicator. The comparison ran from August 2012 to May 2013 during which time there was no significant deterioration in the circulated artefact but the pilot laboratory observed slight change of about 0.15 μm in the measurement result between two measurements, at opposite ends of the circulation. The artefact was a step gauge containing 102 nominally parallel planes half facing each direction (forwards, reverse) with the measurands being the distances between a reference plane and the other planes, measured in the centres. Both participants were able to demonstrate their CMC as listed on the KCDB.Main textTo reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Optimizing step gauge measurements and uncertainties estimation

According to the standard ISO 10360-2 (2001 Geometrical product specifications (GPS)—acceptance and reverification tests for coordinate measuring machines (CMM)—part 2: CMMs used for measuring size (ISO 10360-2:2001)), we verify the coordinate measuring machine (CMM) performance against the manufacturer specification. There are many types of gauges used for the calibration and verification of CMMs. The step gauges with parallel faces (KOBA, MITUTOYO) are well known gauges to perform this test. Often with these gauges, only the unidirectional measurements are considered which avoids having to deal with a residual error that affects the tip radius compensation. However the ISO 10360-2 standard imposes the use of a bidirectional measurement. Thus, the bidirectional measures must be corrected by the residual constant offset probe.In this paper, we optimize the step gauge measurement and a method is given to mathematically avoid the problem of the constant offset of the tip radius. This method involves measuring the step gauge once and to measure it a second time with a shift of one slot in order to obtain a new set of equations. Uncertainties are also presented.

Experimental investigation of surface determination process on multi-material components for dimensional computed tomography

The possibility of measuring multi-material components, while assessing inner and outer features simultaneously makes X-ray computed tomography (CT) the latest evolution in the field of coordinate measurement systems (CMSs).However, the difficulty in selecting suitable scanning parameters and suitable surface determination settings, limits a better acceptance of CT as a CMS. Moreover, standard CT users are subject to the algorithms and boundary conditions implied by the use of commercial analysis software.In this context, this paper is concerned with the experimental evaluation of the influence of surface determination process on multi-material measurements, using functions available in the commercial CT data analysis software Volume Graphics VGStudio Max 2.2.6.Calibrated step gauges made of different materials, i.e. PEEK, PPS, and Al were used as reference standards. The step gauges were assembled in such a way as to have different multi-material X-ray absorption ratios. Comparative measurements of mono-material assemblies were performed as well. Different segmentation processes were considered (e.g. ISO-50%, local threshold, region growing, etc.), patch-based bidirectional length analyses were carried out to perform in-material measurements on the assemblies.This work discusses the different approaches based on real CT scans, and aims to provide advice on the segmentation process for multi-material measurements.

Performance evaluation of CT measurements made on step gauges using statistical methodologies

In this paper, a study is presented in which statistical methodologies were applied to evaluate the measurement of step gauges on an X-ray computed tomography (CT) system. In particular, the effects of step gauge material density and orientation were investigated. The step gauges consist of uni- and bidirectional lengths. By confirming the repeatability of measurements made on the test system, the number of required scans in the design of experiment (DOE) was reduced. The statistical model was checked using model adequacy principles; model adequacy checking is an important step in validating the applicability of a model to fitting experimental results. If the residuals after fitting the model are normally distributed (normality test), then the residuals represent random errors in the data. If the normality test is not satisfied, the model is said to fit the data poorly. If the model fit to the data were correct, the residuals would approximate the random errors (also called normality). The most common significance level is α=0.05; for normality to be satisfied, the P value for the residuals must not be smaller than 0.05. The initial results show that the residuals failed the normality test due to a small P value (P<0.005). Some remedies in achieving a larger P value include (1) fitting a different model, (2) evaluating the data differently and (3) excluding outliers. The authors determined that there were too few data to delete outliers. Instead, discussions and conclusions were carried out based on main effect and interaction plots. It means that the conclusions were not based on the analysis of variance (ANOVA). The following conclusions can be drawn: X-ray scattering is more problematic for dense metal parts. Thus, the statement of “influence of orientation” should be constrained by the material type. But it is important to have in mention that probably the repeatability of the vertical case is much worse. This can be documented experimentally, by tracing back to the fitting quality of the planes. The fitting point deviation of these planes are typically larger when dense metal step gauges are scanned vertically; this can also be done with simulation, as it is “assumed” that the large errors related to “vertical direction” might be caused by X-ray scattering noise, this theory can be tested by simulating CT scans of metal and plastic step gauge without X-ray scattering and see whether “vertical direction” is still worse for aluminium step gauge.

A Model for Geometry-Dependent Errors in Length Artifacts.

We present a detailed model of dimensional changes in long length artifacts, such as step gauges and ball bars, due to bending under gravity. The comprehensive model is based on evaluation of the gauge points relative to the neutral bending surface. It yields the errors observed when the gauge points are located off the neutral bending surface of a bar or rod but also reveals the significant error associated with out-of-straightness of a bar or rod even if the gauge points are located in the neutral bending surface. For example, one experimental result shows a length change of greater than 1.5 µm on a 1 m ball bar with an out-of-straightness of 0.4 mm. This and other results are in agreement with the model presented in this paper.

Final report on inter-RMO Key Comparison EUROMET.L-K5.2004: Calibration of a step gauge

The results of the inter-RMO key comparison EUROMET.L-K5.2004 on the calibration of a step gauge are reported. Eighteen National Metrology Institutes and one Designated Institute from four different metrological regions all over the world participated in this comparison which lasted three years, from December 2004 to December 2007.A lack of stability was observed through the shifting of some of the inserted gauges. In order to save the comparison and get valuable and useful conclusions, it was agreed to exclude four gauges from calculation and assume that only seven gauges were reasonably stable so as to get the corresponding reference values. It was also agreed to divide the participants into two groups, analyze separately their results and, taking the pilot as the linking laboratory, refer the results to common reference values.The inverse-variance weighted mean was taken as reference value. Due to the significant instability of the step it was also considered an artefact uncertainty. The reported uncertainties ranged from 0.045 µm to 1.2 µm (k = 1). The uncertainty of the artefact ranged from 0.018 µm (for the 20 mm face) to 0.176 µm (for the 400 mm face).The compatibility of all participants for measuring step gauges was demonstrated with the only exception of a participant showing very high systematic (both positive and negative) errors. Five participants communicated higher uncertainties than the corresponding approved CMCs. A set of Recommendations and Actions were agreed therefore.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Quantitative evaluation of powder spray effects on stereovision measurements

The effects of powder spray on surface topography and gauge repeatability and reproducibility (R&R) of a stereovision measurement system are investigated for specular machined components. Results obtained in the micro-scale laser profilometer and macro-scale stereovision measurement of powder sprayed surfaces are correlated. Three artifacts, two step gauges and a gauge ball, all with specular surfaces, are utilized to evaluate this optical, non-contact measurement method. While the use of powder spray has demonstrated the reduction of the measurement noise of the stereovision system, as shown in this study, the surface topography of powder spray is not suitable for µm-level precision measurements with the powder droplets as high as 98.4 µm. The accuracy and gauge R&R of the system for a 32 µm step height are 9.4 and 4.6 µm, respectively. A case study of flatness measurement of an automotive engine head combustion deck surface is conducted.

Design analysis and applications of a 3D laser ball bar for accuracy calibration of multiaxis machines

Conventional techniques for measuring the volumetric errors of Cartesian coordinate machine tools are time consuming using a laser interferometer or step gauges. For multiaxis machines, where the spindle can swing, volumetric error calibration is even more difficult. In this study, a novel 3D laser ball bar (3D-LBB) has been developed for the easy setup and rapid measurement of the tool position relative to the worktable at any working point of multiaxis machines. The instrument makes use of one laser ball bar and two rotary laser encoders to detect the target path in the spherical coordinate system. The design of the instrument is discussed, and the error attributes are analyzed to enhance the instrument's accuracy. Applications to the volumetric error measurement of a robot and two types of machine tools have demonstrated the high-precision capability of this 3D laser ball bar.

High-accuracy interferometric measurements of flatness and parallelism of a step gauge

The most commonly used method in the calibration of step gauges is the coordinate measuring machine (CMM), equipped with a laser interferometer for the highest accuracy. This paper describes a modification to a length-bar measuring machine for the calibration of step gauges to a high accuracy. A system was also developed for interferometric measurements of the flatness and parallelism of gauge block faces for use in uncertainty calculations.

A precision length measuring system for a variety of linearartefacts

To calibrate precision length standards and length measuringtransducers, the Korea Research Institute of Standards and Science (KRISS) hasbuilt a new linear measuring machine. The main requirement on the machine isto achieve sufficient flexibility to be able to calibrate a variety ofstandards such as step gauges, long-range linear transducers, industrial linescales and end bars with high accuracy. This system is not designed forcalibrating primary standards such as a gauge block or a primary standard linescale, which are usually measured by specially designed preciseinterferometers. The main parts of the system are a precision stage and alaser interferometer. For smooth motion and precise positioning, we have builtdouble stages, which consist of a global stage with an air bearing and amicro-stage with a flexure structure. Furthermore, two tilting stages are usedfor compensating for the pitch motion and the yaw motion of the global stage.Two double-pass plane mirror interferometers based on the heterodyne techniquemeasure the displacement of the stage. The Abbe offset error is minimized bytwo interferometers with double-pass arrangements and two servo-controlledtilting stages. The measuring range of the system is 2500 mm and the expandeduncertainty for 1000 mm end bar calibration is 0.8 µm at the 95%confidence level.

Performance verification for large volume metrology systems.

A method for the verification of the performance of conventional co-ordinate measuring machines is defined in the ISO 10360-2 standard. This specifies how measurements of a set of traceable lengths (for example, step gauges, length bars, etc.) can be used to verify whether the length measuring capabilities of a given instrument are within the manufacturer’s specification. Large volume measurement systems such as the laser tracker, photogrammetry, portable arm CMM’s and theodolite systems are not specifically addressed by the ISO 103602 standard. However, these systems are increasingly being used in high value processes and the requirement for verification procedures for such systems is clear. Extending the principles of ISO 10360-2, this paper describes a consistent and comprehensive method for the verification of large volume measurement systems.

Development of computer integrated system for error diagnosis of a CMM using calibrated mechanical artefacts

Error calibration and frequent reverification of coordinate measuring machines (CMMs) and machine tools are currently acknowledged as essential processes in order to maintain high performance of the machines and high quality of products. This paper shows a micro-computer integrated error calibration system, which is a fast and efficient error calibration system: a micro-computer stores calibration data of mechanical artefacts, such as ring and step gauges that have been calibrated with respect to more precise equipment, then the machine probes the gauges. The gauge probing data are compared with the gauge calibration data. The micro-computer plans measurement paths thus generating CNC codes for measurement of the mechanical artefacts located in a working volume of a commercial CMM. Then measurement operations are performed according to the planned paths for practical measurement of the gauges. For ring gauge measurement, the lobing error of the probe stylus has been efficiently measured then used for the probe compensation. The circular errors are successfully calculated from the comparison between the ring gauge geometry and the ring gauge probing data after the probe stylus compensation in terms of software. Then from the error pattern of circular error, machine errors such as scale errors and orthogonality error are successfully calculated, showing good approximation to the evaluated circular error. In the case of the step gauge measurement, the gauge probing data are compared with those of calibration data; thus complete linear displacement error analysis is successfully evaluated along any direction in a working volume. The error evaluations are then processes in real time, and presented in computer graphic environments. Practical application to a commercial CMM of CNC-type shows the validity and efficiency of the developed system. Thus a rigorous computer integrated error calibration system is proposed with the full potential of practical application to most commercial CMMs and machine tools of a CNC-type.

In situ measurement of the pressure drop in a Langmuir-Blodgett film during fast deposition

The fact that 22-tricosenoic acid can be deposited in different thicknesses when the film pressure during deposition is varied is used to measure the pressure drop in the monolayer during fast deposition. It is found that at speeds of 1 cm s −1 the pressure can drop several mN m −1. The thicknesses are estimated by comparing the interference colors of thick films with “step gauges” fabricated from the same film, deposited under well-controlled conditions.