micro CMM reserve engineering

Sản xuất ngược là một khái niệm bắt nguồn từ kỹ thuật ngược (Reserve Engineering), là kỹ thuật tái hiện lại một chi tiết hay bộ phận có sẵn không phải qua thiết kế từ đầu mà thông qua một thiết bị số hóa biên dạng bề mặt. Sản xuất ngược ngày nay được ứng dụng rất rộng rãi trên nhiều lĩnh vực, nhiều ngành nghề, đặc biệt là trong công nghệ chế tạo ô tô. Nắm bắt được thị hiếu của người tiêu dùng, nhiều loại xe đã được ra đời một cách nhanh chóng với nhiều kiểu dáng mẫu mã khác nhau. Mỗi lần thay đổi công nghệ như vậy sẽ rất tốn kém, ảnh hưởng rất lớn đến chi phí trong sản xuất. Dó đó nhà sản sản xuất chỉ việc số hóa một chiếc xe, từ đó chỉnh sửa trên các phần mềm CAD thì có thể cho ra đời một mẫu xe mới. Các lĩnh vực ứng dụng chính của thiết kế ngược bao gồm: + Thiết kế chế tạo khuôn mẫu (khuôn nhựa, khuôn đúc , ..) + Gia công CNC (dữ liệu mô hình CAD đầu vào ) + Thiết kế, sản xuất hàng tiêu dùng (điện thoại, đồ gia dụng ) + Công nghiệp ô tô, hàng không, y tế và giáo dục, ... + Sao chép, phục hồi, sản xuất phụ tùng đơn chiếc không còn sản xuất. + Ngoài việc phục vụ thiết kế chế tạo, quy trình thiết kế ngược còn được sử dụng để kiểm tra, đánh giá độ chính xác giữa sản phẩm gia công so với nguyên mẫu. + Tạo các mẫu mã mới so với hình dáng ban đầu.

Precision Engineering 39 (2015) 1–15 Contents lists available at ScienceDirect Precision Engineering journal homepage: www.elsevier.com/locate/precision Review A review of the existing performance verification infrastructure for micro-CMMs J.D Claverley ∗ , R.K Leach Engineering Measurement Division, National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom a r t i c l e i n f o Article history: Received 31 July 2013 Received in revised form April 2014 Accepted 23 June 2014 Available online July 2014 Keywords: Micro-CMM verification Specification standards Calibrated test lengths a b s t r a c t The performance verification of micro-CMMs is now of intense interest because of their capability to perform length measurements in three dimensions to high accuracy with low uncertainties Currently, verification of micro-CMMs is completed in the spirit of existing specification standards, because strict adherence to these standards is often difficult This review aims to present and discuss verification techniques available for micro-CMMs: specification standards, existing calibrated test lengths and traceability routes that can be associated with micro-CMMs Three specification standards used in the testing of CMMs will be considered In addition, a wide range of calibrated test lengths are reported, and any advantages and disadvantages associated with their use are discussed It is concluded that micro-CMMs cannot yet be verified in accordance with existing specification standards Suggestions are made for future standardisation work required to rectify these issues Crown Copyright © 2014 Published by Elsevier Inc All rights reserved Contents Introduction Verification of CMMs 2.1 ISO 10360 2.2 ASME B89.4 2.3 VDI/VDE 2617 Part 12.1 2.4 Discussion Calibrated test lengths suitable for micro-CMMs 3.1 1D artefacts and ball bars 3.1.1 Gauge blocks 3.1.2 METAS miniature ball bars 3.1.3 A*STAR mini-sphere beam 3.2 Ball plates and 2D artefacts 3.2.1 Carl Zeiss miniature ball plate for the F25 3.2.2 METAS ball plate 3.2.3 Kruger plates – column, ball and hole 3.2.4 PTB micro-ball plates – smooth and roughened 3.2.5 Silicon micro-machined dimensional calibration artefact 3.3 Other artefacts 3.3.1 Aztec standard 3.3.2 Micro-hole standard 3.3.3 Polytec step height standard 3.3.4 Calotte cube, calotte plate and other XCT artefacts 11 3.3.5 Micro-contour standard 12 ∗ Corresponding author Tel.: +44 2089436242 E-mail address: james.claverley@npl.co.uk (J.D Claverley) http://dx.doi.org/10.1016/j.precisioneng.2014.06.006 0141-6359/Crown Copyright © 2014 Published by Elsevier Inc All rights reserved 2 J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 3.4 Discussion Conclusions Acknowledgements References Introduction A co-ordinate measuring machine (CMM) is defined as a measuring device with the means to move a probing system and the capability to determine spatial co-ordinates on a workpiece surface [1] CMMs have become essential for industrial dimensional metrology, and it is, therefore, also essential that their accuracy can be estimated and their traceability can be confirmed The verification of the performance of CMMs is well understood, with extensive specification standards available for users and manufacturers to ensure conformity A recently developed generation of CMMs are those specially designed for miniature geometries ranging from between a few micrometres to approximately mm [2] The first recognisable micro-CMM was developed and constructed at the National Physical Laboratory, NPL, in 1999 [3,4] A concurrent project at Eindhoven University of Technology designed and built a micro-CMM that would act as the prototype for a commercial machine [5] Several other micro-CMMs were developed by National Measurement Institutes over the following years [6–9] Two custom built microCMMs, also known as ␮CMMs, precision CMMs, or miniature CMMs, are shown in Fig Since their initial development, micro-CMMs have been subject to research in the area of verification, calibration and standardisation [10–13] This research has become ever more essential as micro-CMMs have become commercially available and are now used in industrial environments Two commercially available micro-CMMs are shown in Fig 2, the F25 from Carl Zeiss AG [14] and the Isara 400 from IBS Precision Engineering BV [15] Several other micro-CMMs are commercially available, including (but not limited to): the VideoCheck UA from Werth Meßtechnik GmbH [16], the NMM from SIOS Meßtechnik GmbH [17], and the UMAP vision system from Mitutoyo Corporation [18] For the purpose of this review, a micro-CMM is defined as a contacting CMM (of various types and makes) which is used to measure geometries whose dimensions range from a few micrometres to approximately mm [19] The verification of micro-CMMs is now of intense interest because of their capability to perform length measurements in 2.5 dimensions (2.5D) to high accuracy with low uncertainties Although it is widely reported that most micro-CMMs are 3D instruments, limitations on the orientation of probe styli often results in an inability to measure undercuts Therefore, although the microCMM platforms themselves are able to measure 3D co-ordinates to high accuracy with low uncertainties, certain 3D geometries cannot be measured A typical maximum permissible error of length measurement of a micro-CMM for size measurement (EL,MPE ) is ±250 nm with an additional length dependent term, which tends to be around nm for every millimetre measured With such precise measurements being commonplace in the area of micro-co-ordinate metrology, verification of the instrument to agreed international standards and availability of traceable calibrated test lengths is essential Currently, EL,MPE for micro-CMMs is determined according to ISO 10360-2:2009, Geometric Product Specification (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMMs) – Part 2: CMMs used for measuring linear dimensions [20], however, significant sections of this standard are not applicable to micro-CMMs due to their size and design 12 13 13 13 Similarly, when completing a micro-CMM calibration, or determination of the micro-CMM’s error map, certain procedures are difficult to complete This review will present and discuss verification techniques currently available for micro-CMMs These discussions will include a description of the existing and pertinent specification standards, existing calibrated test lengths and traceability routes that can be associated with micro-CMMs Verification of CMMs The verification of CMMs is governed by specification standards and good practice associated with dimensional measurement An example of an international specification standard for the verification of CMMs would be the series ISO 10360 – Geometric Product Specification (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMMs) Good practice in dimensional measurement is reliant on experience and know-how, and therefore difficult to report A great deal of effort has been spent in disseminating CMM good practice in NPL Good Practice Guides 41, 42 and 43 (CMM measurement strategies, CMM verification and CMM probing, respectively) [21–23] An extensive review of classical CMM verification techniques and calibrated test lengths can be found in [24] For clarity, ISO 10360-1, Geometric Product Specification (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMMs) – Part 1: Vocabulary [1], defines acceptance tests and reverification tests as follows • Acceptance test – a set of operations agreed upon by the CMM manufacturer and the user to verify that the performance of a CMM is as stated by the manufacturer, performed when the CMM is installed or after any major modification • Reverification test – a test to verify that the performance of a CMM is as stated by the user and executed according to the same procedures as those of the acceptance test, performed periodically as required Also, it is important to note that a third test, the interim check, is also available for the user of a CMM • Interim check – a test specified by the user and executed between reverifications to maintain the level of confidence is the measurements taken on the CMM, performed by the user at any time Other terms used to describe testing of a CMM include qualification, verification and calibration The term qualification usually refers to the day-to-day determination of the effective radius of the stylus tip The terms verification and reverification are essentially interchangeable and describe tests completed to verify that the performance of a CMM is as stated by the manufacturer It should be noted that verification procedures are used to test the performance of the CMM, whereas calibration procedures (often referred to as error mapping) are used to determine the magnitude of all twenty-one kinematic error sources of the CMM The technical procedure for the acceptance tests of a CMM are detailed in ISO 10360-2:2009 [20] There are problems when trying to apply existing acceptance tests, as defined in ISO 10360-2, to micro-CMMs due to several factors Firstly, the nominal artefact J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 Fig The NPL Small CMM developed in 1999 (left) and the METAS Ultraprecision CMM developed in 2001 (right – courtesy of METAS, CH) sizes suggested by the standards are usually larger than the maximum size of artefact that would fit in the measurement volume of the micro-CMM In some cases, this is not a major issue, as the suggested measurement lengths are only a recommendation and can be altered to fit the needs of the user However, this issue does highlight the fact that the standards were not written with micro-CMMs in mind Secondly, there are several requirements of the verification test that, due to kinematic designs of micro-CMMs, are impossible to complete The designs of many micro-CMMs are such that certain orientations of calibrated artefacts are inaccessible to a micro-CMM probing system The overall stylus length and the effective working length of the stylus system are short to increase accuracy with the downside that many measurement features become inaccessible Finally, there is limited availability of calibrated test lengths suitable for micro-CMMs which are also calibrated to uncertainties comparable to, or indeed significantly better than, the capability of the micro-CMM to measure linear dimensions In fact, it is possible that only gauge blocks measured according to the measuring principle of light interference [25], offer sufficiently small measurement uncertainty to verify the length measurement capability of micro-CMMs All of these points combine to produce a situation where microCMMs are covered by very few specification standards, and are therefore verified according to tests agreed between the instrument manufacturer and the user To properly describe this situation, three specification standards used in the testing of CMMs have been selected for discussion as to their applicability to micro-CMMs 2.1 ISO 10360 The ISO 10360 series of standards is prepared by Working Group 10 of Technical Committee ISO/TC 213, the committee concerned with the dimensional and geometrical specification and verification of products The complete specification standard consists of several parts (seven at the time of writing, with three further draft parts in draft form), that each detail aspects of CMM performance verification, depending on the number and type of axes the CMM has, and which probing technology is used Some limitations of ISO 10360 with respect to micro-CMMs will now be briefly highlighted, referenced specifically to sections within the ISO 10360 series A major part of the acceptance and reverification test is selection and positioning of suitable calibrated test lengths within the measurement volume of the CMM ISO 10360-2 requires the measurement of five different calibrated test lengths placed along seven orientations within the measurement volume of the CMM, four of which must be the space diagonals Although the remaining three orientations are left to the discretion of the user, it is usual that Fig The Zeiss F25 micro-CMM (left) and the IBSPE Isara 400 micro-CMM (right – courtesy of IBS Precision Engineering) 4 J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 they are parallel to the CMM axes A further requirement is that the longest test length should be at least 66% of the maximum travel of the CMM along a line through the calibrated test length The suitability of the calibrated test length to perform adequately depends on several parameters other than its length Firstly, the uncertainty of the calibrated lengths should be at least ten times smaller than the expected EL,MPE of the CMM The suitability of the coefficient of thermal expansion of the bulk material of the calibrated test length is also important Finally, the measurement features that define the calibrated lengths should be accessible to the CMM being tested This accessibility requirement can be difficult for micro-CMMs, especially when measuring across the space diagonals and positions parallel to the z-axis A short note regarding ‘very small CMMs’ is found in section 6.5.2.2 of ISO 10360-2, where it is stated that it is acceptable to translate the calibrated test length, while keeping it oriented along the diagonal being tested This allowance is designed to provide sufficient clearance for measurement and in some cases may allow certain body diagonal measurements to be made; as long as the minimum length requirements for the longest test length are adhered to A further limitation comes about in ISO 10360-5:2010, Geometric Product Specification (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMMs) – Part 5: CMMs using single and multiple stylus contact probing systems [26] In section 6.2.3, the material standard of size for the probing acceptance and reverification test (single-stylus probing error test) is defined as a sphere with diameter between 10 mm and 50 mm Given that the working stylus length of micro-CMM probes is often less than 10 mm, even as low as mm [27], the requirements on the diameter of the test sphere can be problematic Also section 6.2.4.1 limits the stylus length during the probe acceptance test to 20 mm, 30 mm, 50 mm or 100 mm Considering common styli lengths for micro-CMMs, this is another problematic constraint Small or micro-CMMs are infrequently mentioned within the ISO 10360 series, but a common theme within the definitions of scope for each part of the series is the allowance for a mutual agreement between the manufacturer and the user for the verification of instruments with unusual designs or probing systems It is conceivable that similar agreements can be, and indeed are, made between manufacturers and users of micro-CMMs so that the verification and calibration of these instruments can be closely linked to international standards 2.2 ASME B89.4 Another standard available for defining performance evaluation methods for CMMs is ASME B89.4.1b-2001 – Methods for performance evaluation of co-ordinate measuring machines [28] This standard does not differ significantly from ISO 10360-2 in content; however, certain changes and additions are made in areas that may be pertinent to the calibration of micro-CMMs A great deal of the standard is dedicated to the investigation of the environmental conditions in which the CMM is operated Micro-CMMs are able to take measurements to an uncertainty often comparable to the resolution of macro-scale CMMs Hence, environmental factors could have a significant effect on their operation Similar to ISO 10360-2, the positions of a calibrated test length are defined for both the determination of the linear displacement accuracy and the volumetric performance Any measurements taken purely in the z-axis of a micro-CMM are, again, limited by the orientation of the probe Several of the locations suggested for measurement of the face diagonals and the space diagonals will also be difficult to measure using some micro-CMMs due to the access limitations afforded by the short stylus length Certain differences exist between ASME B89.4 and the ISO 10360 series that are very relevant to the compliance of micro-CMMs For example, ASME B89.4.1b section 6.1.1 describes the test sphere with a diameter of mm, which is more suitable for use with microCMMs than the 10 mm diameter test sphere defined in ISO 10360-5 [26] However, the probing analysis procedure includes the requirement for a probing point to be taken 10◦ below the equator of the test sphere This probing may not be possible with many microCMMs because, due to the stylus geometry, the shaft, rather than the tip, could contact the test sphere (called shanking) It is suggested that, in the case of ISO 10360-5, the required size of the test sphere is not ideal, and in the case of ASME B89.4, the size of the test sphere is more suitable, but the required probing strategy is not ideal Like ISO 10360-2, ASME B89.4.1b covers most aspects of the acceptance testing and reverification of CMMs, and should be applicable to micro-CMMs However, these specification standards were prepared when few, if any, micro-CMMs existed Therefore, there are considerable limitations on the application of these methods to the verification of micro-CMMs These limitations occur mostly due to the lack of flexibility when considering small styli, both in effective length and tip diameter, and the kinematic design of most micro-CMMs 2.3 VDI/VDE 2617 Part 12.1 The Association of German Engineers (VDI) develops guidelines which cover a wide range of applications These guidelines are often less detailed than those produced by ISO or ASME, but are released more often, and also tend to adopt novel techniques earlier Hence, the VDI guideline VDI/VDE 2617 Part 12.1 – Accuracy of coordinate measuring machines – characteristics and their checking – Acceptance and reverification tests for contacting CMM measuring microgeometries [19], is the only guideline published that addresses the need for an approach to testing micro-CMMs The guideline clearly defines the scope of micro-CMM use to geometries of a few micrometres up to about mm The guideline also defines the stylus system of a micro-CMM to be shorter than mm in length and to have a probe tip diameter below 300 ␮m It is important to note that the guideline refers specifically to ‘contacting CMMs measuring micro-geometries’ while measuring using single point probing Therefore the guideline also covers macro-scale CMMs equipped with small styli used for measuring micro-geometries The guideline refers to the issues associated with using small styli with low probing forces and short over travel Several miniature probing systems are highlighted within the guideline including piezo-resistive probes, opto-contacting probes and vibrating probes To complete the acceptance and reverification tests for microCMMs, the guideline describes a similar set of measurements to those described in ISO 10360-2 and ASME B89.4 VDI/VDE 2617 Part 12.1 requires a minimum of five test lengths along seven orientations within the measurement volume Four of the seven orientations must be the space diagonals of the micro-CMM A suitable calibrated test length should be used, which should have a coefficient of thermal linear expansion below 13 × 10−6 K−1 Inconel, Stellite, ferritic type stainless steel and most ceramics meet this requirement A similar set of tests is prescribed as acceptance and reverification tests of the probing system This procedure is similar to that described in ISO 10360-5, however all previously described limitations, such as stylus length, stylus tip diameter and test sphere diameter are removed The guideline finishes with several annexes, which describe common issues associated with the measurement J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 of microgeometries with micro-CMMs These include contact pressure exerted by miniature stylus tips, material pairing, contamination effects, form errors of calibrated test lengths, especially on the reference sphere, and feasibility of the calibration uncertainties of the calibrated test lengths The guideline also suggests some useful calibrated test lengths, all of which are described in this paper 2.4 Discussion The content of the three described documents, the international standard ISO 10360 series, the American national standard AMSE B89.4 and a German national guideline VDI/VDE 2617 Part 12, all cover the verification of CMMs using calibrated test lengths As no international standard exists specifically describing the full verification of micro-CMMs, aspects of ISO 10360 and ASME B89.4 must be applied to complete the task of verification, usually under a mutual agreement between the instrument manufacturer and the user VDI/VDE 2617 Part 12 goes some way to identify the parts of the two described standards which need specific attention when dealing with micro-CMMs, however, this guideline is not exhaustive for all aspects of the operation of micro-CMMs One major issue, highlighted in the VDI/VDE document, is the difficulty of achieving the required calibration uncertainties on calibrated test lengths To address this issue of uncertainties, the remainder of this review will highlight existing calibrated test lengths suitable for micro-CMM verification Calibrated test lengths suitable for micro-CMMs All of the previously mentioned standards and guidelines depend heavily on the use of calibrated reference artefacts to ensure traceability during acceptance and reverification testing ISO 10360-2:2009 refers to the use of calibrated test lengths, as opposed to material standard and material standard of size which are defined in ISO 10360-1:2001 The term ‘calibrated test length’ is intended to include the use of laser interferometers while still being equivalent to ‘material standard of size’ (defined according to International vocabulary of metrology – basic and general concepts and associated terms (VIM) [29]) The calibrated test lengths described in this section will be considered for their suitability for use during acceptance testing of micro-CMMs In some cases, the calibrated test lengths may not be suitable for either acceptance testing or reverification In this case, their suitability for interim testing will be considered It is assumed that, as the following review is concentrated on contacting systems, a calibrated reference sphere is used for the purposes of probe tip diameter qualification Common tip diameter qualification procedures resulting in a single result for the effective stylus tip diameter are often insufficient for determining the particulars of the stylus tip of a probe for micro-CMMs Several expanded mapping techniques have been developed [30], and the problem of sphere tip diameter evaluation is the subject of several national and European Metrology Research Programme projects at the time of writing [31] The use of high precision optical distance sensors (rather than simple high precision 2D optical or video, sensors) is not yet covered by any part of the ISO 10360 series, although at the time of writing, a draft of ISO 10360-8.2 [32] is due for publication Although the use of optical distance sensors on micro-CMMs is not specifically considered in this review, some of the calibrated test lengths in this review are suitable for use for optical distance sensors Several criteria need to be met if an existing artefact were to perform well as calibrated test lengths, as described in ISO 10360-2 Firstly, the material that defines the dimensional quantity to be measured must be dimensionally stable over time Secondly, it is common that the artefact should be made up of geometric features This requirement is essential so that a correlation can be drawn between the measured points and the evaluated features The use of geometric features allows the measured points to be different to the evaluated features It is also essential that these features be accessible by a micro-CMM’s contacting micro-probe The artefact should also exhibit good temperature stability, both when the artefact is stored in a controlled environment, with small temperature deviations and low temperature gradients, and also when the artefact is transported, i.e., when the artefact is subject to large temperature deviations and gradients After any environmental excursions, the artefact should return to the same shape and size after soaking in a controlled environment for several hours Good temperature stability is often gained by using a material with a low coefficient of thermal expansion This coefficient should be well known so that a correction can be applied to the reference temperature of 20 ◦ C [33] Finally, given the specialised design of all commercially available micro-CMMs, it is important that the artefact be suitably sized to test the full measurement volume of the instrument A typical measurement volume for a micro-CMM is a cube with sides 100 mm long According to ISO 10360-2, a suitable calibrated test length is at least 66% of the longest measurable length within that micro-CMM measurement volume For this common case, a 66 mm calibrated length along the machine axes, or 93 mm calibrated length along a face diagonal, or a 115 mm calibrated length along the space diagonal, will be sufficient A literature review has been carried out to identify existing reference artefacts for use as calibrated test length for micro-CMM acceptance and verification testing The review mainly focusses on artefacts that fulfil the requirements of the three previously mentioned specification standards It is immediately apparent that all the calibrated test lengths found in the literature fit into three main categories: gauge blocks; ball-bars and other 1D calibrated test lengths; ball-plates and other 2D calibrated test lengths; and other calibrated test lengths (those that are not necessarily useful for micro-CMM verification, but serve a purpose in the field of micro-CMM testing) One main omission from this review is the use of laser interferometry as a means of producing a calibrated test length Although several research papers exist on this subject [5,8,12,34,35], and also the application of laser interferometers as the measurement scales of micro-CMMs is now a common occurrence [17,36–38], this technique will not be reviewed as this technology is costly, difficult to use and interpret, and presents several further technical challenges 3.1 1D artefacts and ball bars It is an essential part of any specification standard that few limitations are put on the nature of the calibrated test length used in testing a CMM This requirement is, initially, a practicality requirement, but also an economic one pertaining to availability and cost Likewise, the nature of the calibration of the calibrated test length, especially the geometrical definition of the calibrated test length, can be flexible, provided that the definition used in calibrating the test length is replicated in the CMM measurement strategy Often, this flexibility refers to the nature of the measurement, either bidirectional or unidirectional measurements However, as ISO10360-2 is seen as a ‘black box test’, an overall check of the CMM, the measurement of calibrated test lengths is required to be made bi-directionally Often, both uni- and bidirectional measurements are possible on many artefacts and it is therefore essential that the probing strategy for the calibrated length is well defined As it is common that the calibrated test length is defined using geometric features, the J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 simplest geometric features, such as planes and spheres, are usually used 3.1.1 Gauge blocks Gauge blocks are one of the most common artefacts for disseminating the unit of length [25] Their commonality mostly stems from the wide range of available lengths and materials Also, gauge blocks are ideally placed to be used for the calibration of microCMMs because of the ability to define the central length to a measurement uncertainty of below 40 nm (k = 2) for lengths up to 100 mm [39] With the MPE(EL ) (or EL,MPE ) of most commercially available micro-CMMs being below 300 nm, only gauge blocks offer calibration uncertainties suitable to verify linear measurements made on a micro-CMM However, due to the shape of a gauge block, chosen to ease the transfer of the standard of length to meso-scale and macro-scale instruments, there are limitations on their use with micro-CMMs The use of gauge blocks directly addresses the provision of test lengths suitable for z-axis verification, although this is also, to an extent, dependant on the length of the stylus To perform a test in this orientation, a calibrated gauge block is wrung to a reference platen and placed in the measurement volume of the micro-CMM such that the reference length is aligned with the z-axis The microCMM can be used to measure both the planar reference surface of the platen and the measuring face of the gauge block This strategy coincides directly with the definition of the length of a gauge block [40] This strategy is only valid provided the length of the gauge block is sufficiently small to not hinder the probing system The maximum length for which this procedure is successful is similar to the stylus length of the probing system being used, which could be as little as mm Longer gauge blocks can still be measured, however the measurable area of the reference platen becomes severely limited, resulting in measurements that are not directly comparable to the calibrated length of a gauge block During the measurement of 2D lengths, requiring lateral probing, micro-CMMs are unable to fully probe the gauging faces of the blocks, due to the reduced effective length of the micro-probe stylus and the edge chamfer on typical gauge block measuring faces This limitation could introduce errors in the measurements, as the calibrated length of the gauge block is defined as being measured at the centre of a gauging face, and also because the parallelism of the gauging faces cannot be fully determined There are several solutions to this problem, including the calibration of the gauge blocks at a more suitable position, i.e., at a position that is accessible by the micro-probe, or the manufacture and calibration of non-standard gauge blocks designed for easy use on micro-CMMs A set of non-standard gauges have been manufactured at the Physikalisch-Technische Bundesanstalt (PTB), the National Metrology Institute of Germany, whereby a gauge block bridge made of Zerodur has been manufactured [41] The calibrated test length is suitable for the verification of test lengths in 1D and 2D, and also for the determination of the straightness deviations of the horizontal axes of a micro-CMM An image of the Zerodur gauge block bridge is shown in Fig The distances between the internal and external bridge faces were calibrated by PTB with an expanded uncertainty of 20 nm, using the principle of light interference The calibration position is less than 1.5 mm from the top edge, allowing the definition of the calibrated test length to be replicated by the probing strategy of a micro-CMM The results of measurements made on this calibrated test length have indicated that the investigated micro-CMM exhibited straightness errors of under 50 nm over a 43 mm length, and also that reference distances were measured to better than 100 nm [42] However, it should be noted that this arrangement only allows for one measured distance per setup Fig A non-standard set of Zerodur gauge blocks used as a calibrated test length for the calibration of micro-CMMs Image courtesy of PTB, DE When any face diagonals including z-axis travel and any space diagonals are tested, access problems affect the measurement protocol Currently there are no commercially available angled probes available for micro-CMMs, and hence a second auxiliary gauging surface is required to complete the measurement A physical setup of this procedure, suggested in VDI/VDE 2617 Part 12, is shown in Fig 4, where the length L is inferred from length of the reference gauge block This represents a method for unidirectional measurement of a gauge block This arrangement of gauge blocks could potentially introduce errors due to the wringing of the two gauge blocks and due to the flatness and parallelism errors of the gauging faces Even though the length of a gauge block includes the effect of one-face wringing [40], this wringing effect is to a reference platen and not a second gauge block A reasonable estimate of the resulting contribution to the expanded uncertainty due to these errors could be of the order of 50 nm, based on the tolerance for the variation in length of a grade K gauge block (calibration grade) [40] Even though this increased error is acceptable in terms of the manufacturer’s specified value of EL,MPE of most commercial microCMMs, the logistical complications of using this setup results in completion of a verification test taking many hours, or indeed several days Even though the completion of an ISO 10360-2 like acceptance test is possible using gauge blocks, and a devised procedure, it would be time consuming and not adherent to the requirement of bi-directional measurement A good deal of time will be spent in setting up the various length gauge blocks in the measurement volume, and also on waiting for the system to thermally stabilise after interference from the operator The test would also require a great amount of skill and very precise operation of the CMM, and is hence unsuitable for non-expert users Fig A suggested physical setup for testing a length, L, along any face diagonal including z-axis travel or any space diagonal of a micro-CMM J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 Fig METAS miniature ball bars Image courtesy of METAS, CH 3.1.2 METAS miniature ball bars A previously stated criterion for the design of a calibrated test length is that it should comprise several geometric features, such that a correlation can be made between the measured points and the evaluated co-ordinates In the case of gauge blocks and auxiliary gauging faces, the geometric feature is a plane, and the evaluated features are perpendicular distances between a plane and a point Therefore, the uncertainty in the length is dependent on the form and the parallelism of the planes These geometries become inaccessible when any tilt is introduced on the calibrated test length to include measurement in the z-axis of the micro-CMM An obvious solution for this inaccessibility of geometrical features at an angle is to use a feature whose nominal geometry is invariant with rotation, i.e., a sphere Spheres and sphere beams are common reference standards in co-ordinate metrology, and the development of a miniature ball bar artefact is an ideal starting point for the development of calibrated test lengths suitable for micro-CMMs A set of miniature ball bars have been manufactured by METAS using Zerodur bars and synthetic ruby spheres [43] The ball bars range in length from 20 mm to 100 mm, and have been used to verify the global measurement precision in the whole volume of a micro-CMM to be 15 nm + L/1400 ␮m/mm These miniature ball bars are shown in Fig This design of ball bar can be used parallel to the x and y measurement axes, along face diagonals and along space diagonals The probing of the end spheres is relatively fast, and it is suggested that due to short measurement time and the materials used, this method is almost unaffected by thermal drift [43] However, as with gauge blocks, any procedure to fully determine EL of the micro-CMM will be very time consuming due to the need to manually change and rearrange all of the different length ball bars and wait for thermal stability To reduce this disruption, an artefact that includes several measurable lengths (at least five, according to ISO 10360-2) would allow each orientation of the artefact to be measured at once 3.1.3 A*STAR mini-sphere beam A mini-sphere beam has been developed at the National Metrology Centre in Singapore that consists of ten evenly spaced Grade [44] ruby spheres, each of diameter mm whose centres are spaced at 10 mm intervals [45] The spheres are mounted onto a carbon fibre rod A specific type of carbon fibre was chosen because of its low co-efficient of thermal expansion of −0.8 × 10−6 /◦ C The spheres have a specified deviation from spherical form of 0.08 ␮m and an arithmetic average surface roughness specification of 0.10 ␮m An image of the mini-sphere beam is shown in Fig Calibration of the mini-sphere beam was undertaken by Eidgenössisches Institut für Metrologie METAS (the Federal Institute for Metrology, Switzerland), the National Metrology Institute of Switzerland, on the micro-CMM shown in Fig An expanded Fig The mini-sphere beam, as developed by A*STAR, Singapore measurement uncertainty of 63 nm was quoted for the 90 mm maximum length of the sphere beam The beam was subsequently used to re-verify a micro-CMM at the National Metrology Centre, Singapore Results from these measurements revealed a possible squareness error on the machine [45] The mini-sphere beam was proposed as an artefact suitable for evaluating the volumetric measurement error, specifically as an alternative to gauge blocks It was designed to reduce the problems which occur when measuring gauge blocks across any diagonal that includes vertical inclination As such, it can be positioned within the measurement volume in many of the orientations required by ISO 10360-2 The only geometrical limitation of this calibrated test length is its unsuitability for use to verify the z-axis of a micro-CMM Even though the co-efficient of thermal expansion for this calibrated test length is low, it should be noted that some types of carbon fibre can have coefficients of thermal expansion up to 2.0 × 10−6 /◦ C Also, the susceptibility of carbon fibre and epoxy to changes in humidity, which have non-zero coefficients of moisture expansion [46], and also possible low dimensional stability over time, suggests that a low expansion metal structure may be more suitable A similar mini-sphere beam concept has been developed by Trapet Precision Engineering [47], however no literature could be found that described its performance 3.2 Ball plates and 2D artefacts When extending the calibrated lest lengths offered by a test artefact into two dimensions, the distinction between bi-directional and unidirectional measurement becomes unnecessary Due to their design, all 2D artefacts are unidirectional Their design therefore begins to limit the calibration methods for such artefacts, as length measurement using the method of light interference becomes more difficult in two dimensions 3.2.1 Carl Zeiss miniature ball plate for the F25 A miniature ball plate has been developed by Carl Zeiss that consists of a Zerodur base with nine silicon nitride hemispheres wrung to its surface The design of the plate results in a total of thirty-six individual ball-to-ball distances ranging from 13 mm to 100 mm This artefact is shown in Fig The design of the miniature ball plate covers several requirements set by ISO 10360-2, including its ability to be used to J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 Fig Miniature ball plate manufactured by Carl Zeiss (left) in its inclined position (right) complete measurements in 1D, 2D and 3D The design of the plate is such that the hemispheres can be probed from above and from below, and is shown schematically in Fig The result of this feature is that a reversal method, incorporating classical rotation reversals, can be applied that largely eliminates the systematic deviations of the CMM However, because the reference surfaces are hemispherical, measurement of a full hemisphere to the equator is impossible Any measurement taken on the hemispheres will only include points to 88◦ from the pole, resulting in 96.5% coverage of the total available hemispherical surface area When probing the sphere from below, this amount of coverage is reduced to about 5% The miniature ball plate has been calibrated at PTB using a Zeiss F25 micro-CMM [48] The traceability of the measurements taken on the micro-CMM is ensured using gauge blocks The final centre to centre distances of the nine balls on the plate are quoted with an uncertainty of 110 nm (k = 2) A comparison between the current results from PTB and a micro-CMM capable of interferometric length measurement was completed at the end of 2011, and confirmed comparable measurements of the ball plate to within 100 nm [49] It has been noted that this artefact is not suitable for some reversal techniques because the hemispheres are not symmetrically placed on the plate [50,51] One limitation on the use of this hemisphere plate arises from the kinematic design of many micro-CMMs A number of face diagonals and space diagonals can be tested using this artefact by tilting it, as shown in Fig However, due to the hemispherical nature of the reference spheres, in its inclined position, the definition of the sphere centres becomes less accurate, as the measurement area is further reduced by up to 25% With the calibration of this ball plate being performed using a micro-CMM, there is little scope for direct comparison to other primary measurement techniques, such as those used for gauge block or step gauge calibration Also, it could be problematic that ball plate is intended for use as a verification artefact for microCMMs when it is itself calibrated using a micro-CMM Fig The geometry of the hemispherical plate allows the hemisphere to be measured from both directions Fig A ball plate manufactured from Invar with 25 precision ruby spheres Image courtesy of METAS, CH 3.2.2 METAS ball plate A ball plate for measurement on various contacting microCMMs has been developed by METAS [43] It consists of twenty-five precision ruby spheres, nominally mm in diameter that push-fit into an Invar base plate, 85 mm × 85 mm in size An image of this ball plate is shown in Fig The ball plate is calibrated on a precision micro-CMM at METAS by using an error separation technique This technique calls for measurements to be taken from both sides of the plate, so the design of the ball plate allows it to be reversible This method of calibration also allows extra information about the micro-CMM completing the measurement to be computed, such as the orthogonality and straightness of the axes and any other angular distortions The plate is manufactured from Invar, a low expansion material To remove residual thermal drift, all the spheres on the plate are measured twice, with the second sequence being measured in reverse order The design is similar to classical macro-scale ball plates available commercially However, the push fit design, coupled with styli of short length, results in little or no tilt angle being possible during measurement This artefact is one of several used during the recent Euramet project 1088, ‘Towards truly 3D metrology for advanced micro-parts’ J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 [52] The results of this project are not yet published at the time of writing 3.2.3 Kruger plates – column, ball and hole A set of 2D plate artefacts have been designed and manufactured at the National Metrology Institute of South Africa (NMISA, ZA) [51] The specific aim of the development was to be able to undertake a complete performance evaluation of several CMMs, both mesoscale and miniature, using contacting and optical probes A classical ball plate, similar to that produced commercially [53] was manufactured on a small scale This ball plate can be used to verify micro-CMMs using a reversal method, because the precision spheres can be accessed from both sides and are also arranged in a symmetrical pattern Two similar Kruger plates were developed to address the need for cross calibration of contacting and vision CMMs The previously mentioned small scale ball plate is unsuitable for measurement using a vision CMM because the optical sensor is unable to perform automatic edge detection of the precision spheres To overcome this problem, two further plates were developed that incorporated columns and holes The sharp edges of these geometrical features are easily detected by vision CMMs, and the plates can be calibrated on a micro-CMM using a contacting micro-probe Images of all three plates are shown in Fig 10 During calibration of the hole plate and ball plate, they are rotated and inverted to facilitate reversal calculation techniques The thickness of these plates is approximately mm, allowing contacting micro-probes to access the measurement features from both sides 3.2.4 PTB micro-ball plates – smooth and roughened A ball plate concept has been developed by PTB resulting in two separate artefacts that act as transfer standards between CMMs with contacting and optical sensors The first artefact consists of a set of thirty-six stainless steel spheres, mm in diameter, arranged in a × array The full array covers an area of only 20 mm × 20 mm No indication of measurement uncertainty has been found in the literature, however, it is estimated that this measurement standard could be calibrated to an expanded uncertainty of 500 nm A second micro-ball plate has been manufactured using thirtysix roughened balls, 500 ␮m in diameter, covering an area of 6.5 mm × 6.5 mm [54] This artefact is a good example of a ball plate suitable for cross-checking of micro-CMMs using contacting probing systems and optical distance sensors, especially contrast detection systems such as variable focus techniques [55] An image of the two artefacts is shown in Fig 11 The magnitude of the measurement uncertainty for both plates is unsuitable for micro-CMMs This high uncertainty is mostly due to the high form deviations of the spheres, especially the roughened spheres, which may be several hundred nanometres The need for roughened spheres with low roundness deviation shall be essential as the use of optical micro-CMMs becomes prevalent This need is being addressed by Keferstein et al [56] through the development of precision reference spheres designed for multi-sensor microCMMs 3.2.5 Silicon micro-machined dimensional calibration artefact Sandia National Laboratories, USA, have developed a meso-scale dimensional artefact manufactured using silicon bulk micromachining [57,58] The artefact was designed to be used to evaluate the performance of micro-CMMs using either contacting or optical sensors This hybrid functionality was achieved by using a feature geometry, flanked walls and edges, which can be probed by both contacting and optical probes Images of tested designs for these artefacts are shown in Fig 12 Calibration of these artefacts, to an expanded uncertainty of 400 nm (k = 2), is undertaken using a macro-scale CMM, a microCMM, a high accuracy optical CMM and also a profile stylus instrument [59,60] The seemingly high uncertainty associated with these artefacts arises from their use as calibrated test lengths specifically for high accuracy optical (or video) CMMs Future development of the concept is aimed at calibration of the artefact to an expanded uncertainty of 100 nm Any improvement is directly dependant on improved silicon bulk machining techniques A 1D artefact is also available in this product family, acting as a low cost option to the 2D artefact An example standard is shown in Fig 13 3.3 Other artefacts Several miniature geometrical standards exist that although unsuitable as calibrated test lengths for acceptance tests, serve a purpose in the field of micro-CMM calibration, either for interim checks or as task specific reference artefacts Several of these standards will be described, but this is not an exhaustive list, as many more are known to exist, and are reviewed elsewhere [61] 3.3.1 Aztec standard A 3D dimensional pyramidal artefact has been designed at PTB, which is manufactured in silicon using micromachining fabrication techniques [62] This pyramidal artefact, or Aztec standard, has several measurement points in the xy-plane and also at different z-heights The overall dimension of a × array is 13 mm × 13 mm × 1.4 mm The Aztec standard consists of three structured wafers and one non-structured silicon wafer resulting in four reference z-planes and various other tilted planes (angled at 54.7◦ ) With a contacting probing system it is possible to determine the positions of the inclined planes, and subsequently calculate their intersection points, resulting in 3D reference points A SEM image and a photograph of the Aztec standard are shown in Fig 14 3.3.2 Micro-hole standard The PTB micro-hole standard is a task specific artefact focusing on micro-holes, such as injection nozzle measurement [61] The calibration of this micro-hole is difficult because its high aspect ratio is beyond the capability of most micro-probes The main type of probe this artefact is designed for is opto-mechanical probes, or fibre probes, which, although provide high aspect ratio probing, are subject to many geometrical effects [63] A further issue is that the measurement uncertainty of these opto-mechanical probes increases as they descend into high aspect ratio holes To solve the issue of calibration, the micro-hole standard is manufactured from several sheets, each containing a well-positioned hole These individual holes are then calibrated separately and assembled to create a continuous and well calibrated microhole The alignment of the individual sheets is defined by two vee-grooves, one for rotational positioning and one for height positioning A schema of the concept and an image of the realised standard are shown in Fig 15 3.3.3 Polytec step height standard Testing the z-axis of a micro-CMM is difficult to perform due to the lack of probe/stylus combinations with orientations other than vertical, and because tests using gauge blocks can be very time consuming A suitable solution for this problem could be a step artefact covering a certain range of the z-travel of the machine A step artefact has been developed by Polytec GmbH [64], and is shown in Fig 16 This fine step height artefact covers a range of 2.25 mm in nineteen steps of height 0.125 mm Although initially designed for the 10 J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 Fig 10 The three Kruger plates: ball plate (left), cylinder plate (middle) and hole plate with ceramic inserts (right) Image courtesy of the NMISA, ZA Fig 11 Two ball plate artefacts suitable for measurement on a contacting micro-CMM and various optical systems Image courtesy of PTB, DE Fig 12 Two tested designs of the silicon micro-machined dimensional calibration artefact, left [59] and right [60] Image courtesy of Sandia National Laboratories, USA Fig 13 A 1D artefact for cross-calibration of optical and contacting micro-CMMs Image courtesy of Sandia National Laboratories, USA Fig 14 The Aztec standard, fabricated through the micromachining of silicon Image courtesy of PTB, DE J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 11 Fig 15 Schema of the micro-hole standard concept (left) and the realised standard (right – courtesy of PTB, DE) the estimated flatness error on the steps of ␮m, means that the standard is not yet suitable for use as a calibrated test length for the testing of a micro-CMM Calibrations with lower uncertainties may be possible using a long scale interferometer, such as those used for length bar measurement The need for calibrated test lengths that contain several heights is apparent in many areas of dimensional metrology, and several other artefacts are being designed for 3D optical microscopy [65] Fig 16 The fine step height standard, covering a range of 2.25 mm (right) Image courtesy of Polytec GmbH, AT verification of large-scale white light interferometers, this artefact could be used to test the z-axis of a micro-CMM The advantage with these standards is that they include multiple steps, significantly reducing setup and measurement time However, one major disadvantage is the large area over which the z heights are defined on the artefact This results in inclusion of considerable x-axis and y-axis travel, therefore reducing the validity of the z-axis testing Currently, these artefact are calibrated on a precision CMM, resulting in an expanded uncertainty on the step height distance of 0.4 ␮m + (0.6D × 10−6 ) ␮m, where D is the nominal distance to the reference plane of the artefact in metres, which results in a maximum expanded uncertainty over 70 mm of approximately 0.45 ␮m This relatively high uncertainty, coupled with 3.3.4 Calotte cube, calotte plate and other XCT artefacts Several of the miniature dimensional standards encountered during the literature search were calibrated test lengths designed to test micro-scale X-ray computed tomography (micro-XCT) A more thorough review of these artefacts can be found in [66] and [67], however several examples will be highlighted here due to their ideal size for micro-CMMs A set of artefacts based on calotte (skull-cap) shaped depressions have been developed at PTB with an intended application in micro-scale X-ray computed tomography (micro-XCT) A calotte cube has been manufactured in titanium and has 150 calottes The measurement uncertainty associated with the distances between the calottes is about ␮m and therefore unsuitable for use with most micro-CMMs A calotte plate consisting of a Zerodur plate with sixteen spherical calottes with diameters of mm, in one face, has also been developed The material used is a glass–ceramic with a low thermal expansion coefficient making the plate be suitable for verification of micro-CMMs However, due to the lack of precision during manufacture, a form error of up to 2.5 ␮m can be observed on the calottes Similar to the calotte cube, the distances between the calculated centres on the calotte plate are calibrated with an uncertainty of around 1.5 ␮m The calotte cube and calotte plate are shown in Fig 17 Fig 17 The PTB calotte cube (left) and calotte plate (right) Image courtesy of PTB, DE 12 J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 Fig 19 The PTB micro-contour standard caused by different lateral scale calibrations in the x- and y-axes and vertical scale calibration If used to verify a stylus contour based measuring instrument, the artefact will also indicate tip radius and the ability of the system to handle thresholding and correction of roughness and material influences 3.4 Discussion Fig 18 A miniature tetrahedron suitable for precision measurements on a microCMM and for measurements on a micro-XCT system The full height of the tetrahedron is approximately mm Another high precision micro-scale artefact suitable for both XCT and micro-CMMs is a tetrahedron of 0.5 mm diameter ruby spheres This artefact was designed and manufactured by PTB specifically to address the need for transfer artefacts from microCMMs to micro-XCT [68], and therefore covers a significantly smaller volume than may be otherwise expected for a micro-CMM artefact The high quality spheres and precision assembly enables it to perform very well in tests involving micro-CMMs, indeed this artefact was used during the Euramet TC Project 1088 [52] A picture of the miniature tetrahedron is shown in Fig 18 3.3.5 Micro-contour standard A micro-contour standard, also developed by PTB [69], has been manufactured using electro-discharge machining (EDM) The resulting surface roughness enables the artefact to be measured by micro-CMMs using both contacting and optical distance sensors An image of this artefact is shown in Fig 19 A range of details and features present on the artefact allow several parameters of the measuring instrument to be investigated, including: lateral scale calibration in the x- and y-axes, distortions The previously described calibrated test lengths, of which some may be suitable for the verification of micro-CMMs, can be organised according to several features: their fitness as a calibrated test length with 2D or 1D calibrated distances, their suitability for use with other systems beyond contacting micro-CMMs (such as video and optical CMMs), and their geometry, which determines their ease of use, their limitation and their suitability for use with reversal algorithms The calibration uncertainty of each calibrated test length can also be considered, although for many of the described artefacts, this value is not reported These categories and results are listed in Table Subsequently, the main flaws of each material measure can be considered The most common flaw is that certain orientations of the calibrated test lengths within the micro-CMM volume for completion of the length measurement error tests are very difficult, or in the worst cases, impossible to achieve and measure These limitations are usually due to the design of the rigid elements of the micro-CMM, especially the probing system and the metrology frame The design of these calibrated test lengths tend to be specialised towards a specific micro-CMM design, making it difficult to complete performance comparisons between machines Also, most micro-CMMs measure only in 2.5D because of kinematic and probing system limitations Therefore, as these micro-CMMs would not be able to measure true 3D structures, perhaps it is suitable to only complete verification tests in 2.5D Table A table listing the artefacts described in this report Data as available at the time of writing 2D standards 1D standards Gauge blocks METAS miniature ball bars A*STAR mini sphere beam Sandia silicon 1D standard 2D standards Zeiss miniature ball plate PTB micro-ball plate – smooth PTB micro-ball plate – rough METAS ball plate Kruger ball plate Kruger cylinder plate Kruger hole plate Sandia silicon 2D standard Other 3D Calotte cube PTB micro-hole standard Polytec step height standard PTB micro-tetrahedron Calotte plate PTB micro-contour standard Aztec standard 1D standards Other × × × × × × × × × × × × × × × × × × × × × × × × × × × × Tactile × × × × × × × × × × × × × × Video Optical Reversal × × Calibration uncertainty / nm 30 50 65 400 [25,42] [43] [45] [60] 110 [49] [54] [54] [43] [51] [51] [51] [57,59] × × × × × × × × × 400 1000 × 450 1500 × References [67] [61] [64] [68] [70] [69] [62] J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15 Another way to distinguish between test lengths for microCMMs is the uncertainty to which they can be calibrated With current micro-CMMs exhibiting an EL,MPE of 250 nm or less, it is essential that any test lengths used for verification should be calibrated to an uncertainty better than 50 nm Often, these artefacts are not calibrated to an uncertainty five times lower, or even the suggested ten times lower than the expected uncertainty of measurements taken on the micro-CMM [71] Although this value is a recommendation, the calibration uncertainty of the artefact defines the lower limit of the EL,MPE calculation for the micro-CMM This limit will have implications when applying the contents of other ISO documents in the GPS range to either confirm conformance or non-conformance of measured workpieces with specifications [71], or estimate measurement uncertainty [72,73] The method of calibration is also important to note, as several test lengths reviewed in this paper were calibrated using a micro-CMM (i.e the mini-sphere beam and the Carl Zeiss miniature ball plate), which could lead to some confusion with regards to the traceability of measurements However, this is often mitigated by calibrating the artefact using reversal methods, or by using a micro-CMM with associated virtual CMM software Not every existing reference artefact available for micro-CMMs has been described here Several artefacts were investigated that were only suitable for direct comparison measurements These included gear standards [74], several contour standards [54], and other calibrated test lengths [75] Finally, it can be noted that, to the best of the authors’ knowledge, of the five regional metrology organisations, only EURAMET has completed and recorded any projects pertaining to the use, verification or testing of micro-CMMs [49,52,76] 13 distance of their probes, owing to the short stylus working lengths, and therefore it is suggested that any additional standardisation focus on that main common feature The likelihood of the relevant ISO committee taking on this work is dependent on the need of the community, the industrial take up of this technology, and the expected market size in the future Currently, the needs of the community are high, as is evident from the extensive research being completed in this field, although the community is small Industrial take up is relatively low, with few micro-CMMs available on the market However, trends in the area of micro-manufacturing suggest that applications for micro-CMMs will increase, and therefore a sound set of standards and calibrated test lengths will be required Secondly, it is suggested that several of the calibrated test lengths described here are already suitable, in either design or calibrated uncertainty, for use during acceptance testing and reverification For example, gauge blocks provide suitable calibrated test lengths in any single axis direction (x, y or z) and in face diagonals in lateral directions (xy) Also, a few designs of ball plates and ball bars provide good capability in terms of probe access and calibrated length definition Unfortunately, the calibration of these standards to uncertainties below 100 nm is difficult, and tends to rely on other micro-CMMs with contacting probes, which does suggests a possible ‘circular’ traceability route Finally, until viable alternatives for calibration of ball plates and ball bars becomes possible, to a considerably better uncertainty than is possible on a micro-CMM, it is likely that acceptance tests and reverification tests will have to rely on time consuming reference measurements using gauge blocks This may result in EL values higher than are actually possible using these high precision micro-CMMs Conclusions Following an extensive review into the existing traceability environment for micro-CMMs, focusing on suitable specification standards and reference artefacts, several conclusions can be drawn Although extensive information is available for the acceptance testing and reverification of CMMs in existing specification standards, major parts of the described methods are unsuitable for use with micro-CMMs – or indeed CMMs with small stylus systems Several reference artefacts have been designed such that the geometrical definition of the calibrated length is easily probed by a micro-CMM, and also that the maximum number of test positions within the measurement volume can be measured These complex artefacts are usually very difficult to calibrate to the required accuracy However, less suitable geometric artefacts, such as gauge blocks, can be calibrated to the required accuracy Therefore, several suggestions can be made with regard to the acceptance testing and reverification of micro-CMMs Firstly, a dedicated international standard, or an appendix to an existing standard, could be developed to detail the solutions to the described problems It is debateable whether new standard development is the best use of resources of the committee given the small number of micro-CMMs used in industry, and therefore an amendment to existing standards would be the preferred route These solutions could include adding various methods for carrying out acceptance tests on micro-CMMs, as well as definition of suitable reference artefacts and traceability routes A simple transposition of ISO 10360-2 and ISO 10360-5, or their associated equivalents, to cope with the typical linear measurement dimensions of micro-CMMs will not be suitable to fully address all the issues of verification of micro-CMMs Instead, it is likely that a new set of procedures, treating micro-CMMs as inherently 2D or 2.5D measuring machines, will have to be considered The limitation of micro-CMMs to 2.5D is primarily due to the limits to the working Acknowledgements This work is funded by the UK National Measurement Office Engineering and Flow Metrology Programme 2011–2014 This work is also supported by the European Commission within the project “Minimizing Defects in Micro-Manufacturing Applications (MIDEMMA)” (FP7-2011-NMP-ICT-FoF-285614) Several images have been used with the kind permission of their owners, who are referenced within the relevant figure captions The authors would like to extend special thanks to Mr David Flack for his extensive review of this work Thanks should also go to Dr Alan Wilson and Dr Andrew Lewis for their help References [1] ISO 10360-1:2001 – Geometrical Product Specifications (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMM) – Part 1: Vocabulary Geneva, CH: International Organisation for Standardisation; 2001 [2] Cao S, Brand U, Kleine-Besten T, Hoffmann W, Schwenke H, Bütefisch S, et al Recent developments in dimensional metrology for microsystem components Microsyst Technol 2002;8(March (1)):3–6 [3] Peggs GN, Lewis AJ, Oldfield S Design for a compact high-accuracy CMM CIRP Ann – Manuf Technol 1999;48(1):417–20 [4] Leach RK, Haycocks J, Jackson K, Lewis AJ, Oldfield S, Yacoot A Advances in traceable 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[76] Thalmann R EUROMET TC Project 807 – Workshop on micro coordinate measuring machines (␮CMM); 2005  ... following years [6–9] Two custom built microCMMs, also known as ␮CMMs, precision CMMs, or miniature CMMs, are shown in Fig Since their initial development, micro- CMMs have been subject to research... with micro- CMMs in mind Secondly, there are several requirements of the verification test that, due to kinematic designs of micro- CMMs, are impossible to complete The designs of many micro- CMMs... usual that Fig The Zeiss F25 micro- CMM (left) and the IBSPE Isara 400 micro- CMM (right – courtesy of IBS Precision Engineering) 4 J.D Claverley, R.K Leach / Precision Engineering 39 (2015) 1–15