Gauge Block Research Articles

Prototype of Depth Standard for Elastomer Hardness Tester

National Institute of Metrology (Thailand) has fabricated the prototype of depth standard to calibrate the depth of indentation for elastomer hardness tester according to ISO 18898, ASTM D2240 and ASTM D1415. It was designed as a displacement device consisting a linear scale inside which can measure the indentation-depth covered the measuring range from 0-2.5 mm. The depth standard can acquire the value of indentation depth in accordance with the scale of elastomer hardness tester indicated from 100 shore to 0 shore in steps. The calibration for the prototype of depth standard was carried out by a set of gauge blocks. The accuracy of the prototype was within ±1 µm which met the requirement according to ISO and ASTM standard. Then, the depth standard was used as a part of the indentation-depth measuring system. The calibration stand which was the other part of the indentation-depth measuring system was investigated as a problem issue due to the stand deformation during the indentation depth calibration. NIMT solved the problem by installing another laser measuring device to compensate the movement of elastomer hardness tester due to the deformation of stand. This paper also pointed out the use with a caution of depth calibration procedure by gauge blocks according to ASTM D2240.

The measuring method for actual total magnification of metallographic microscope — digital image method

In this paper, taking MM6 metallographic microscope 160 × as an example, one kind of new method is studied on measuring actual total magnification of metallographic microscope by measuring digital images, the digital image is virtual image viewed through eyepiece of microscope and acquired by digital camera instead of eyes. Results show: 1) The digital images of 5/100 ocular micrometer and 1/100 stage micrometer in the eyepieces of microscope are acquired. The digital images of the 5 grade gauge block, which is 5mm length and is from camera 250 mm, are acquired in the same acquiring conditions. The lengths of the ocular micrometer and gauge block in the digital image are measured, and then the magnification of eyepieces is calculated. The lengths of the ocular micrometer and stage micrometer in the same digital image are measured, and then the magnification of objectives and tube factor is calculated. The actual total magnification of metallographic microscope is the product of magnification of eyepieces and magnification of objectives and tube factor; 2) For 160 × magnification of MM6 metallographic microscope, the measurement result of actual total magnification are stated as: 154.8 ×, U = 2.8 ×, k = 2, and the actual relative deviation is - 3.2%. 3)The digital image measuring method for actual total magnification of metallographic microscope can be a standardized method.

Real-time correction of self-heating effect of capsule-type SPRTs used in gauge block calibration at KRISS

Standard Platinum Resistance Thermometer (SPRT) is the most accurate temperature sensor directly calibrated by the fix point cells given in the International Temperature Scale 1990 (ITS-90). However, when it is used for temperature measurement of an object in high precision dimensional metrology, self-heating effect may cause error in the measured temperature, and thus also in the measured length. The two-current method, which is the well-known method to compensate the self-heating effect of SPRTs is not practical for dimensional metrology because it takes several tens of minutes to measure temperature. A new method has been p6roposed by the authors which can enable a real time correction of the self-heating effect of SPRTs. The method utilizes the fact that the resistance of SPRT is linear with respect to its temperature within the short range around 20 °C. The correction value for the AC bridge resistance ratio corresponding to the self-heating effect is measured in advance by using the two currents of 1 mA and 1.414 mA. This correction value can be directly added to the AC bridge readout in real time to compensate for the self-heating value in length measurement applications. Since no current change is required during actual length or temperature measurements, there is no need to wait se4veral tens of miutes like in the case of using the two-current method during the length measurement. In this paper, we present the measurement results of the self-heating effect of Minco’s capsule type SPRTs used in the gauge block interferometer of KRISS.

Measurement of Low Carbon Concentration in Polycrystalline Silicon by Second Generation Infrared Absorption Spectroscopy

Measurement of carbon concentration in semiconductor grade polycrystalline silicon by infrared absorption spectroscopy was examined. Synthetic reference removing substitutional carbon (Cs) by electron irradiation was used. Removal ratio dependence on initial [Cs] and electron dose was systematically examined. As a result, production of synthetic reference with [Cs] of about 1013 atoms/cm3 and of block gauge with known [Cs] was successfully accomplished. For estimation of [Cs] in polysilicon after growing float zone crystal, low temperature measurement was examined in addition to the previously established RT measurement. Interfering fractional virtual phonon bands were examined and removed similarly with that at RT. To escape from the contamination during the FZ process, direct measurement of polysilicon was examined. The difference between the two - phonon absorption background spectra at room temperature, low temperature and in polysilicon was clarified. Based on the result, spectrum processing for measurement at low temperature and in polysilicon was established. Measurement procedures for [Cs] down to 1 x 1014 atoms/cm3 was established.

A novel method of diameter measurement of pistons used in pressure standards using scanning principle and fusion technique

The requirement of improved certainties in the measurement of the diameter and the form of cylindrical artifacts is increasing day-by-day. Such requirements become indispensable in typical applications, like the inspection of the fuel injection systems and calibration of pressure balances. A new experiment is set to measure the diameter of a cylindrical artifact, particularly the piston of a pressure-measuring device. Three displacement sensors are used to scan the cross-section of a cylinder and a reference gauge block. At the same time, the scanning process is simulated for ideal conditions. The coordinate data thus obtained in the scanning is fused on to a circle. The misalignments in the experimental setup are refined by iterative scanning the artifacts under test until the experimentally obtained surface profiles match with the simulated ones. Then, the fused circle represents the diameter of the cylindrical object. According to substitution technique, the deviation of the diameter of the artifact under test is determined from the size of the gauge block. Finally, the expanded measurement uncertainty is estimated.

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.

Control chart analysis for interferometric length measurement

INTI is the national metrology institute of Argentina and the responsible for maintenance and the dissemination of the national length standard. The practical realization of this reference is performed according to the optical wavelength standards established and published by the International Committee of Weights and Measures (CIPM). The length unit is materialized by artefacts known as gauge blocks which length is measured by optical interferometric methods. For this purpose INTI has a commercial interferometer based on a Twyman-Green configuration. By this system INTI provides length traceability to national calibration laboratories and others national metrology institutes of the region. Even though the equipment that forms the interferometric system is calibrated periodically, it is convenient to establish quality control procedures for monitoring not only the system as a whole but also the calibration process. We report on a 5-year study of two control steel gauge block sets, the statistical techniques applied on control chart and its selection criteria. The pool of data allows long term stability analysis of these standards as well as the operating examination of the interferometer.

Evaluation of long-term stability of low thermal expansion coefficient materials using gauge block interferometers

The long-term stability of NEXCERA™ ceramics having a low coefficient of thermal expansion was evaluated over a period of eight years. Several gauge blocks of differing lengths were prepared, using two types of NEXCERA. Each gauge block was kept wrung to a platen and its absolute length was periodically measured by gauge block interferometer during the eight years. Relative uncertainties of measurement of changes in gauge block length were estimated as 4.1 × 10−8 and 2.9 × 10−8 for 200 mm and 800 mm gauge blocks, respectively. The experimental results show the trend of expansion and a relative change of less than 0.1 × 10−6/year for every gauge block.

3D artifact for calibrating kinematic parameters of articulated arm coordinate measuring machines

In this paper, a 3D artifact is proposed to calibrate the kinematic parameters of articulated arm coordinate measuring machines (AACMMs). The artifact is composed of 14 reference points with three different heights, which provides 91 different reference lengths, and a method is proposed to calibrate the artifact with laser tracker multi-stations. Therefore, the kinematic parameters of an AACMM can be calibrated in one setup of the proposed artifact, instead of having to adjust the 1D or 2D artifacts to different positions and orientations in the existing methods. As a result, it saves time to calibrate the AACMM with the proposed artifact in comparison with the traditional 1D or 2D artifacts. The performance of the AACMM calibrated with the proposed artifact is verified with a 600.003 mm gauge block. The result shows that the measurement accuracy of the AACMM is improved effectively through calibration with the proposed artifact.

Comparison of 10 cm and 50 cm Long Iodine Cells in Iodine-Stabilized Diode Laser-Pumped Nd:YAG Laser at 532 nm

An iodine-stabilized diode laser-pumped Nd:YAG laser system at 532 nm was developed at the National Institute of Metrology of Thailand (NIMT) for the first time in Thailand. Two iodine cells with the lengths of 10 and 50 cm were used for the comparison of frequency stability of Nd:YAG laser. The experiment was set for phase modulation spectroscopy. The Nd:YAG laser was locked to the a10 component of the R(56)32-0 transition of iodine molecule. The variation of wavelength with time and the frequency stability were carried out at various iodine cell temperatures of 20, 10, 5, 1, -1, -3 and -5°C. The results from the frequency stability measurements showed that the lowest Allan standard deviation value of 6.643×10-10 was obtained at -5°C for 50 cm long iodine cell. From this Allan standard deviation value, it can be concluded that the iodine-stabilized diode laser-pumped Nd:YAG laser system at 532 nm developed in this study is acceptable to be used with a gauge block interferometer for the length calibration of the gauge block.

Experimental study of phase correction determination in steel by stacking method experiment

The phase correction determination by stacking method experimentation is carried out by comparing the interferometric length of a set of gauge blocks wrung to each other, with the individual interferometric lengths of each gauge block. In this work the repeatability of the phase correction in steel and the uncertainties associated with this correction by using the stacking method are analyzed. Some variables considered in the analysis are the number of gauge blocks that form the stack, the nominal length of the set and the difference of the interferometric lengths obtained for both sides of each gauge block.

Three-step interferometric method with blind phase shifts by use of interframe correlation between interferograms

A new three-step interferometric method with blind phase shifts to retrieve phase maps (PMs) of smooth and low-roughness engineering surfaces is proposed. Evaluating of two unknown phase shifts is fulfilled by using the interframe correlation between interferograms. The method consists of two stages. The first stage provides recording of three interferograms of a test object and their processing including calculation of unknown phase shifts, and retrieval of a coarse PM. The second stage implements firstly separation of high-frequency and low-frequency PMs and secondly producing of a fine PM consisting of areal surface roughness and waviness PMs. Extraction of the areal surface roughness and waviness PMs is fulfilled by using a linear low-pass filter. The computer simulation and experiments fulfilled to retrieve a gauge block surface area and its areal surface roughness and waviness have confirmed the reliability of the proposed three-step method.

Morphology Evaluation of Microelectronic Packaging Substrates Using Shadow Moiré Technique

The surface quality and long-term reliability of packaging substrates in microelectronic packaging processes have become subjects of increasing concern as circuit board component density increases while the pitches of these components decrease. As a result, packaging substrate morphology inspection is becoming more crucial than ever before. This paper presents a noncontact measurement technology based on a shadow moire technique to evaluate the surface quality and morphology of microelectronic packaging substrates. Fringe pattern images of packaging substrates are acquired through the noncontact measurement technology, and the resulting images are then analyzed using the phase-shifting technique to determine the morphology and surface quality (specifically, the flatness) of the microelectronic packaging substrates. A full-field morphology measurement system with adjustable sensitivity and measurement range was developed. The measurement system was tested using three typical types of microelectronic packaging substrate, a coin and a step made from two standard gauge blocks. The measurement system proved to be both feasible and effective by the test results.

Fusion learning for inter-laboratory comparisons

In this paper we propose a Generalized Fiducial Inference inspired method for finding a robust consensus of several independently derived collection of confidence distributions (CDs) for a quantity of interest. The resulting fused CD is robust to the existence of potentially discrepant CDs in the collection. The method uses computationally efficient fiducial model averaging to obtain a robust consensus distribution without the need to eliminate discrepant CDs from the analysis. This work is motivated by a commonly occurring problem in inter-laboratory trials, where different national laboratories all measure the same unknown true value of a quantity and report their CDs. These CDs need to be fused to obtain a consensus CD for the quantity of interest. When some of the CDs appear to be discrepant, simply eliminating them from the analysis is often not an acceptable approach, particularly so in view of the fact that the true value being measured is not known and a discrepant result from a lab may be closer to the true value than the rest of the results. Additionally, eliminating one or more labs from the analysis can lead to political complications since all labs are regarded as equally competent. These considerations make the proposed method well suited for the task since no laboratory is explicitly eliminated from consideration. We report results of three simulation experiments showing that the proposed fiducial approach has better small sample properties than the currently used naive approaches. Finally, we apply the proposed method to obtain consensus CDs for gauge block calibration inter-laboratory trials and measurements of Newton’s constant of gravitation (G) by several laboratories.

Characteristics of Linear Variable Differential Transformer(LVDT) Probe for Gauge Blocks Calibration

The LVDT probe is a very important component used in gauge blocks calibration via a mechanical comparative method. The probe is used to determine the central length difference between a reference gauge block and gauge block under-tested (UTC). Typically, an UTC and a reference gauge block have the same nominal length. However, some gauge blocks UTC are specially made for specific purpose where the reference gauge blocks with the same nominal length are not commercially available. Various reference gauge blocks are wrung in order to provide nominal length the same as that of the UTC. Wringing process is the troublesome step and causes larger measurement uncertainty. The lower accuracy is higher number of gauge block used to create reference gauge block, in order to improve accuracy of measurement, the LVDT probe was used at the longer range where the reference gauge block and the UTC don’t need to be the same nominal length. In this paper, characteristics of LVDT probe was investigated as it is related to the accuracy of the measurement result. Errors of LVDT probe came from non-linearity, calibration factor, retrace error, repeatability and maximum difference in length. A pair of gauge block, calculated by the Twyman-Green interferometer, length different range 5 μm to 230 μm was used in the study. Non-linearity of LVDT is evaluated by a simple linear regression model. The non-linearity of LVDT probe, calibration factor, retrace error, repeatability and maximum difference in length are 25 nm, 1.0003, 3 nm, 6 nm and 80 nm, respectively. Therefore, by using this technique, central length difference between the 2 gauge blocks up to 80 nm can be calibrated with the uncertainty due to non-linearity of 15 nm The experiment shows the large error of retrace closes to 0.02 μm at 0.09 μm. This can be determined the maximum difference in length to calibrate gauge blocks at difference nominal lengths. The measurement uncertainty of non-linearity is evaluated and it is close to 15 nm.

Development of measurement system for gauge block interferometer

We developed a measurement system for collecting and analyzing the fringe pattern images from a gauge block interferometer. The system was based on Raspberry Pi which is an open source system with python programming and opencv image manipulation library. The images were recorded by the Raspberry Pi camera with five-megapixel capacity. The noise of images was suppressed for the best result in analyses. The low noise images were processed to find the edge of fringe patterns using the contour technique for the phase shift analyses. We tested our system with the phase shift patterns between a gauge block and a reference plate. The phase shift patterns were measured by a Twyman-Green type of interferometer using the He-Ne laser with the temperature controlled at 20.0 °C. The results of the measurement will be presented and discussed.

Uncertainty evaluation associated with versatile automated gauging influenced by process variations through design of experiments approach

Recent advances in versatile automated gauging have enabled accurate geometric tolerance assessment on the shop floor. This paper is concerned with the uncertainty evaluation associated with comparative coordinate measurement using the design of experiments (DOE) approach. It employs the Renishaw Equator which is a software-driven comparative gauge based on the traditional comparison of production parts to a reference master part. The fixturing requirement of each production part to the master part is approximately ±1mm for a comparison process with an uncertainty of ±2μm. Therefore, a number of experimental designs are applied with the main focus on the influence of part misalignment from rotation between master and measure coordinate frames on the comparator measurement uncertainty. Other factors considered include measurement mode mainly in scanning and touch-trigger probing (TTP) and alignment procedure used to establish the coordinate reference frame (CRF) with respect to the number of contact points used for each geometric feature measured. The measurement uncertainty analysis of the comparator technique used by the Equator gauge commences with a simple measurement task using a gauge block to evaluate the three-dimensional (3D) uncertainty of length comparative coordinate measurement influenced by an offset by tilt in one direction (two-dimensional angular misalignment). Then, a specific manufactured measurement object is employed so that the comparator measurement uncertainty can be assessed for numerous measurement tasks within a satisfactory range of the working volume of the versatile gauge. Furthermore, in the second case study, different types of part misalignment including both 2D and 3D angular misalignments are applied. The time required for managing the re-mastering process is also examined. A task specific uncertainty evaluation is completed using DOE. Also, investigating the effects of process variations that might be experienced by such a device in workshop environments. It is shown that the comparator measurement uncertainties obtained by all the experiments agree with system features under specified conditions. It is also demonstrated that when the specified conditions are exceeded, the comparator measurement uncertainty is associated with the measurement task, the measurement strategy used, the feature size, and the magnitude and direction of offset angles in relation to the reference axes of the machine. In particular, departures from the specified part fixturing requirement of Equator have a more significant effect on the uncertainty of length measurement in comparator mode and a less significant effect on the diameter measurement uncertainty for the specific Equator and test conditions.

System for automatic gauge block length measurement optimized for secondary length metrology

This paper presents a contactless system for automatic gauge blocks calibration based on combination of laser interferometry and low-coherence interferometry. In the presented system, the contactless measurement of the absolute gauge block length is done as a single-step operation without any change in optical setup during the measurement. The optical setup is combined with compact gauge block changer with a capacity of 126-ga blocks, which makes the resulting system fully automatic.The paper also presents in detail a set of optimization steps which have been done in order to transform the original experimental setup into the automatic system which meets secondary length metrology requirements. To prove the measurement traceability, we conducted a set of gauge block length measurement comparing data from the optimized system and the established reference systems TESA NPL A.G.I. 300 and TESA–UPC operated in Czech Metrology Institute Laboratory.

Multi-pose system for geometric measurement of large-scale assembled rotational parts

To achieve virtual assembly of large-scale assembled rotational parts based on in-field geometric data, we develop a multi-pose rotative arm measurement system with a gantry and 2D laser sensor (RAMSGL) to measure and provide the geometry of these parts. We mount a 2D laser sensor onto the end of a six-jointed rotative arm to guarantee the accuracy and efficiency, combine the rotative arm with a gantry to measure pairs of assembled rotational parts. By establishing and using the D–H model of the system, the 2D laser data is turned into point clouds and finally geometry is calculated. In addition, we design three experiments to evaluate the performance of the system. Experimental results show that the system’s max length measuring deviation using gauge blocks is 35 µm, max length measuring deviation using ball plates is 50 µm, max single-point repeatability error is 25 µm, and measurement scope is from a radius of 0 mm to 500 mm.

Stylus Tip Center Position Self-Calibration Based on Invariable Distances in Light-Pen Systems.

The light-pen coordinate measuring machine (LPCMM for short) is portable and flexible to measure features including invisible ones in-situ. Since different styluses are needed to measure different features and even during the process of measuring a single workpiece with complicated configurations, to improve the system measurement accuracy it is beneficial to calibrate the stylus tip center position after it is mounted to the light-pen before measurement in an industrial field. A novel and simple method aiming at self-calibrating the position of the tip center based on invariable distances is presented. The distinguishing feature of the proposed method is that the center position of the tip can be calibrated by using a kinematic seat with an inverted cone hole without any external reference and auxiliary devices. Calibration is based on that the distance between the tip center and that of any LED is invariable when the light-pen is swung smoothly with its spherical tip firmly touching the fixed cone seat. To ensure the repeatability of the algorithm some error constraint parameters are given. Based on invariable distances, the tip center position in the light-pen coordinate system can be obtained. Experiment results show that the self-calibration method has the advantage of good repeatability, with standard deviations 0.027, 0.023 and 0.014 mm in U, V and W directions, respectively. Experimental results of measuring a circle and a gauge block indirectly demonstrate the accuracy of the proposed self-calibration method.