Comparison of different coordinate measuring devices for part geometry control

Transcription

1 Digital Industrial Radiology and Computed Tomography (DIR 2015) June 2015, Belgium, Ghent - More Info at Open Access Database Comparison of different coordinate measuring devices for part geometry control Bartosz GAPINSKI 1, Ireneusz ZACHWIEJ 2, Andrzej KOŁODZIEJ 3 1 Poznan University of Technology, Institute of Mechanical Technology Poznan, POLAND; bartosz.gapinski@put.poznan.pl 2 ITA Poznan, POLAND iz@ita-polska.com.pl 3 The President St. Wojciechowski Higher Vocation State School in Kalisz, Faculty of Technology Kalisz, POLAND; a.kolodziej@ip.pwsz.kalisz.pl Abstract Never-ending evolution of market expectations leads to new solutions in a quality control. Nowadays, in most cases control of the geometry in industry is carried out using devices that apply coordinate measuring technique. Since couple of years metrological computed tomography X-ray 3D has become on a market. It is the latest field of coordinate measuring technique what makes many aspects of its accuracy and possible applications still open. The paper presents information on the functioning and measurement using computed tomography X-ray 3D. The results of comparative tests that have been carried out on a different coordinate measuring devices e.g. coordinate measuring machine (CMM), formtester and X-ray 3D computed tomography are presented. The results allow us to conclude that the computed tomography makes it possible to obtain comparable results with the other measuring devices. The conducted investigations show the areas where the application of the devices is appropriate and those in which the device is not adequate. In case of the X-ray 3D CT considerable versatility is presented which allows to describe the measured part with a cloud of points. Obtained information has an equal density of the part exterior and interior. This allows to reliably assess the test characteristics. The differences in presented results do not exceed 0.05 mm. One of the reasons for this situation is the method of measurement, e.g. for CMM the tactile measurement causes information gap between the data points, whereas for the optical scanner it is difficult to collect the data points in holes of the small diameter what causes errors in the assessment of the position and diameter. For the CT, a significant reduction of the absorption of X-rays exists which translates into an actual measurement volume and the occurrence of artefacts. The paper shows the effect of different research methods on the obtained results and their application areas. This information can help a potential user to select the appropriate measuring device. Keywords: Dimensional control, coordinate measuring technique, measurement accuracy, 1. Introduction Inseparable part of each manufacturing process is its control. The assessment of manufacturing correctness allows us to define the compatibility between the manufactured product and design requirements. It also allows to verify the correctness of operation of particular stages of the manufacturing process. Depending on expectations one can choose the measuring devices which can give metrologically correct results for required time limits. These requirements apply to all control processes connected with the measurement of mass, time or geometrical quantities [1, 2]. The most popular method of geometrical dimensions control is coordinate measuring technique. It allows to do contact and non-contact measurements, macro and nano measurements and the measurements with the application of specialized devices e.g. for gear wheels. A common feature of these methods is the assurance of visual observations of the measured surfaces or physical contact between the gauging point and investigated object. The application of computed tomography allows to observe and measure closed interior spaces, pores and cracks etc. [3, 4]. Nowadays X-ray tomography is one of the most modern tool in the coordinate measuring technique which has the features of NDT device and allows to control the dimensions of both surfaces that can be measured with other methods and closed spaces that must be destroyed during measuring with classical methods [5].

2 The measuring accuracy is one of still developing problems in the area of the computed tomography [6, 7]. The paper presents the investigation results for a special stepped shaft made of steel. This cylinder was measured with the measuring coordinate machine, specialized device for the measurement of form and position deviations, contour measuring device, optical measuring device for axisymmetric elements and computed tomography. The obtained results can be useful to define the possibilities of application of computed tomography in relation to other known measuring devices and can be also useful to present the areas of applications of particular devices. 2. Coordinate measuring technique Coordinate measuring technique is one of dynamically developing field of metrology of geometrical quantities. The measurement is based on localization of coordinates of measuring points which describe the investigated object. The efficiency and accuracy of coordinate measuring devices is still developing and new constructional solutions of these devices are appearing on the market. Depending on the requirements for the specific measuring task one can choose the device which allows to do the task with defined accuracy in defined time. The measurement can be done with contact method e.g. with the application of coordinate measuring machines or arms (Fig. 1). The coordinate measuring machines allow to obtain high accuracy decimal digit of micrometer, and get full automatization of measuring process. During the measurement with contact method the interaction exists between the probing tip and measured element. This fact has advantages and drawbacks especially in case of the measurement of soft and flexible elements. Fig. 1. Coordinate measuring machine from Coord3 company and measuring arm from Faro company [8, 9] Non-contact methods are alternative solutions for the measurement of flexible elements. These methods can be done with the application of measuring microscopes, profile projectors, optical machines, structured-light scanners or CMM with optical probe head (Fig. 2). Noncontact measurement has many advantages, but the optical measurement is sensitive

3 to variations of illumination conditions, reflections of light and contaminations of the measured surfaces. Fig. 2. Profile projector Baty, optical measuring machine OGP, structured-light scanner GOM [10, 11, 12] The latest field of coordinate measuring technique is computed tomography (Fig. 3). The tomographs have been known in medicine since 1970s. However, in the metrology of geometrical quantities the tomographs have been used for 10 years. Computed tomography allows us to analyse the external and internal geometry of the investigated part, the distribution and size of pores and cracks etc. Measuring tomography allows to conduct the research for inaccessible areas. It is necessary to choose the methodology and parameters of the measurement in order to obtain the metrologically correct results. Fig. 3. Measuring tomograph exact XS from Wenzel company and tomograph connected with coordinate measuring machine from Werth company [13, 14] 3. Comparison of coordinate research devices A multi-stepped shaft made of steel have been investigated. The least measured diameter was equal 10 mm, and the greatest measured diameter was equal 26 mm. The values of diameter, length, chamfer angle, roundness deviation, cylindricity deviation and run-out deviation have been estimated. The measurements have been done with the following five measuring devices: optical measuring device Opticline from Jenoptic company, tactile roundness measuring device Roundscan from Jenoptic company, contour measuring device

4 T8000 from Jenoptic company, coordinate measuring machine Global Image from Hexagon Metrology company and computer tomograph v tome x s from GE-Phoenix company. 3.1 Optical measuring device Opticline Optical device such as Opticline from Jenoptic company works on the basis of recording the boundary between the light and shadow. To do the measurements, the investigated object should be placed between the light source and reading sensor (Fig. 4). In this way we can observe the shadow on detector which is the reason of covering a part of illumination by the investigated object. Telecentric optical systems are applied to provide high accuracy of the measurement these lens give the optimal distribution of the light beam. Fig. 4. Principle of light-shadow effect during the measurement [15] During the measurement the object is rotated and illuminator-detector system is moved along the object axis. This allows to find the extrema of the object profile and register the real shape of the object profile. In case of the presented shaft it was possible to obtain the values for all investigated measuring features. The Opticline device is a very good solution for the measurement of axisymmetric objects. 3.2 Formtester Roundscan Formtesters are specialized devices for the measurements of form and position deviations. Measured object is placed on the rotational table and the sensor allows to register the variations of the radius value in function of angle of rotation e.g. for the measurement of roundness, or the variations of the radius value in function of linear displacement e.g. for straightness (Fig. 5). The measurement with the application of this method is assigned to the group of radial measurements and allows to obtain very small values of measuring uncertainty. A proper positioning of the object (i.e. alignment) is very important action during the measurements the object rotation axis must be coaxial with the table rotation axis.

5 Fig. 5. Measurement with the specialized device Roundscan 3.3 Contour measuring device T8000 Contour measuring device is a specialized device for the measurements of the object profile in 2D system. It is equipped with a special measuring needle with small value of radius which allows to reconstruct the profile with small radius and acute angles. In case of T8000 device one can measure the profile from the top only (Fig. 6) this does not allow to measure the diameter. However, it was possible to reconstruct the length of particular steps of the shaft and fillet radii. Contour measuring device T8000 from Jenoptic company is a versatile device and the drive unit of the module for the profile measurement is added to the device for roughness measurement which uses its measuring column. Fig. 6. The measurement with contour measuring device T8000

6 3.4 Coordinate measuring machine Global Image Coordinate measuring machine (CMM) is the most universal measuring device among all applied measuring devices during our investigations. It was equipped with the probe head SP25 from Renishaw company it allowed to conduct both impulse and scanning measurements. This device is applied for the measuring the diameter, roundness, cylindricity and run-out, and the application of small gauging point allows to estimate small radii and chamfers (Fig. 7). One of the main actions during the measurement with CMM is a proper determination of measuring datum features which are defined in the documentation. In case of the measured shaft these datum features were defined on the journals it allowed to determine precisely the element axis. The elaboration of measuring software allows for full automatic measurements. Fig. 7. The measurement with coordinate measuring machine DEA Global 3.5 Computer tomograph GE phoenix v tome x s Tomograph is a very universal measuring device which allows to estimate both external surfaces and closed internal spaces. Material density and total thickness of the wall are the restrictions for the measurements with this device, because these ones limit the possibility of the object penetration by X-rays. In case of our investigations we applied the reflection tube which allowed to get the power for X-raying of the largest diameter of the stepped shaft. Obtaining a proper image in tomograph depends on many factors it is possible to adjust the lamp power by changing the voltage and/or current intensity, change the time of single measuring exposure or apply different types of filters etc. The tomograph allows to measure small dimensions e.g. chamfers or radii, but one should provide a proper geometrical magnification and resolution which depends on the size of voxel.

7 Fig. 8. Computer tomograph GE phoenix v tome x s 4. Analysis of the investigation results Depending on the specification of particular measuring devices it was possible to measure all or some measuring features. A list of measuring features is presented in Table 1. As we can see Opticline, CMM and CT allow to measure all features. Formtester and contour measuring device allow to measure just a part of measuring features, but the measurement specifications of these devices guarantee the higher measuring accuracy. Table 1. List of measured features with different devices Opticline Roundscan T8000 CMM DEA CT Diameter Roundness Cylindricity Run-out Chamfer Radius Length

8 According to the data presented in Table 1 the diameter was measured with three measuring devices, i.e. Opticline, CMM and CT. The measurement results are presented in Table 2. Nominal value of diameter [mm] Table 2. List of diameter values measured with different types of devices Measured values [mm] Opticline CMM DEA CT Ø 9,999 9,996 9,999 9,997 Ø 13,999 13,998 14,001 13,995 Ø 17,999 17,996 17,996 17,989 Ø 21,999 21,998 22,000 21,988 Ø 26,000 26,000 25,999 25,988 The measurements of roundness, cylindricity and run-out deviations have been done with all measuring devices except the contour measuring device - the results are presented in Table 3. Roundness deviation was measured for the journal with diameters 14 mm and 18 mm. Cylindricity deviation was measured for the journal with diameter Ø 10 mm. Run-out deviation was measured for the journal with diameter Ø 14 mm in relation to the datum defined on the journal with diameter Ø 10 mm. Table 3. List of measurement results of roundness, cylindricity and run-out deviation Roundness for the journal Ø14 Roundness for the journal Ø18 Cylindricity for the journal Ø10 Run-out for the journal Ø14 Measured values [mm] Opticline Roundscan CMM DEA CT 0,0025 0,0016 0,0017 0,0140 0,0022 0,0071 0,0068 0,0131 0,0036 0,0046 0,0052 0,0109 0,0087 0,0077 0,0074 0,0181 The measurements of the length of particular steps of the shaft have been done with four selected devices, but the contour measuring device was applied instead of the roundness measuring device (Table 4). The authors have measured the following distances from shaft end face to shaft base face Ø10 mm (it was equal 18 mm), shaft base face Ø14 mm (it was equal 27 mm) and shaft base face Ø18 mm (it was equal 36 mm). Table 4. List of measurement results of lengths for particular steps of the shaft Measured values [mm] Opticline T8000 CMM DEA CT Length of cylinder Ø10 17, , , ,0096 Length of cylinder Ø14 26, , , ,0743 Length of cylinder Ø18 35, , , ,0720

9 Last group of the investigated features was chamfer angle for the cylinder diameter 10 mm. The nominal value of this angle was equal 45 and the fillet radius R was equal 0,5 mm for the cylinder diameters Ø10, Ø14 and Ø18. In this case the measurements have been done with all measuring devices except the roundness measuring devices. The obtained results are presented in Table 5. Table 5. List of measurement results of chamfer angle and fillet radius Measured values Opticline T8000 CMM DEA CT Chamfer angle Fillet radius for Ø10 0,4563 0,4865 0,4962 0,5076 Fillet radius for Ø14 0,5117 0,5327 0,5427 0,5241 Fillet radius for Ø18 0,4790 0,4626 0,4671 0,4712 The obtained values represent different groups of the results. These groups are the following: diameters, lengths, form deviations roundness and cylindricity, run-out, chamfer angles and fillet radii. In order to compare the measurement results obtained with different measuring devices the authors have resigned from the estimation of internal surfaces. On the basis of the analysis of obtained data one can observe a high convergence of obtained values for each group of the results. The measurements of the diameter are characterized by small differences (max 0,003 mm) for the dimensions up to Ø18 mm. For the diameters Ø22 and Ø26 mm one can notice higher differences (up to 0,012 mm). This fact is connected with difficult penetration of steel shaft with these diameters. However, on the basis of comparison of measuring accuracy for particular measuring devices one can state that these values are placed within uncertainty areas for particular devices. The highest differences can be observed for the measurement of form deviation with tomograph. Higher values of deviations are connected with the measurement of entire cylinder surface in case of the measurement with roundness measuring device we measure the diameter for a single cross-section. Rotation and positioning accuracy of the element during the measurement and reconstruction process of 3D image have an additionally effect on the measurement results. Similar phenomena can be observed for the estimation of the length of the particular steps of the shaft. In case of the measurement with tomograph the entire surface is estimated in case of CMM we measure single points only which are the representation of the surface. 5. Conclusions The conducted investigations confirm the possibility of application of computed tomography for the estimation of geometrical quantities. In case of the estimation of closed spaces this technique is the only one which allows to do the measurement without a prior destruction of the part. The element size and material density are the restrictions for computed tomography. The other measuring devices have not these restrictions. In case of contact measurements it is very important to choose the proper measuring strategy which will eliminate the deformation of flexible and slender elements. Summarizing the results analysis we can conclude that all applied types of coordinate measuring devices allow to achieve the effective measurement of the element. We cannot

10 choose the most suitable device for the measurement of different geometrical features, because each of the applied devices is characterised by a series of advantages and drawbacks depending on a specific measuring application. In connection with this fact none of these groups of devices will be taken out of production and service. Measurement time and uncertainty are also very important parameters during the measurements of the geometrical features. References 1. Bucher L. J. (editor), 'The Metrology Handbook, Second Edition', ASQ 2012, ISBN Howarth P., Redgrave F., 'Metrology in short 3 rd edition', EURAMET project 1011, 2008, ISBN Kruth J-P., Bartscher M., Carmignato S., Schmitt R., De Chiffre L., Weckenmann A., 'Computed tomography for dimensional metrology, keynote paper', CIRP Annals 61/2, Heinzl Ch., Kastner J., Georgi B., Lettenbauer H., 'Comparison of surface detection methods to evaluate cone beam computed tomography data for three dimensional metrology, DIR International Symposium on Digital industrial Radiology and Computed Tomography, June 25-27, 2007, Lyon, France. 5. Gapinski B., Wieczorowski M., Marciniak-Podsadna L., Dybala B., Ziolkowski G., 'Comparison of Different Method of Measurement Geometry Using CMM, Optical Scanner and Computed Tomography 3D, Procedia Engineering, 69, 2014, Bartscher M., Staude A., Ehrig K., Ramsey A. 'The Influence of Data Filtering on Dimensional Measurements with CT', 18th World Conference on Nondestructive Testing, April 2012, Durban, South Africa. 7. Galovskyi B., Flessner M., Loderer A., Hausotte T., 'Systematic form deviations of additive manufactured parts - methods of their identification and correction', 11 th International Symposium on Measurement and Quality Control 2013, , Cracow-Kielce, Poland CMM COORD3 HERA, Measuring Arm FARO Prime Baty R400, SmartScope Flash CNC 300, ATOS Triple Scan, exact XS, Werth TomoScope HV 800, Opticline Optical Shaft Metrology,