Freeform best practice

5 min read

Steed Webzell joined more than 100 delegates at the launch of NPL's National Freeform Centre in Teddington, Middlesex, last month

Efficiency of aero engines, drag reduction for automotive bodies and increased life span of prosthetics are just some examples of the gains potentially achievable with suitable advances in freeform (described simply as "non-prismatic") manufacture. However, such advances are partly limited by poor metrology infrastructure, lack of measurement traceability, and the absence of specialised facilities and knowledge base. "This is why we recommended to government that we establish a national centre for freeform metrology," stated Dr Michael McCarthy, head of the National Physical Laboratory's new freeform centre, during his introduction to an event that attracted delegates from many industry sectors, including those from blue chip OEMs such as BAE Systems, Airbus UK, Jaguar Land Rover and Rolls-Royce. The new centre is part of a three-year project, funded by government and the European Metrology Research Programme. Dr McCarthy said that "over £1 million has been spent on new freeform metrology equipment, making it the highest concentration of such technology in the world". Image: The new NPL facility will have a particular CMM focus to its activities IMPRESSIVE PORTFOLIO Among the impressive portfolio of new freeform metrology is: a Mitutoyo co-ordinate measuring machine with laser probe; a Phase Vision SMS800 fringe projection system; a Breuckmann StereoScan white light scanning system from UK agent CDG; a GOM ATOS II 3D digitising unit; a Konica Minolta Range 7 non-contact digitising system; a Faro Laser ScanArm V3; and a Hexagon Romer arm with laser probe. To mark the opening of the centre, the launch event featured a number of keynote speakers highlighting their experiences of freeform measurement, along with some of the solutions applied to date. LEADING-EDGE TECHNOLOGY First up was Nick Orchard, senior metrologist and leader of the Measurement Technology Team at Rolls-Royce, who spoke informatively about the choice of contact or non-contact techniques for the measurement of freeform components used on the Trent 900 jet engine. Image: A Trent 900 jet engine Today, Rolls-Royce uses a number of non-contact freeform metrology systems, depending on the application. For instance, a GOM ATOS structured light system is used for large aerofoil sections, while, for even higher resolution, conoscopic holography rapid single-point systems are sometimes deployed. In terms of prime contact method, Renishaw's much publisised Revo scanning probe is preferred, largely because its five axes of motion allow complete aerofoil measurement in a single set-up, with useful modes such as 'sweep' and 'curve'. It has also helped overcome the previous out-of-plane Z-height errors. "An aerofoil used to take 22 hours and 14 minutes to measure, whereas the same part can now be processed in 2 hours and 11 minutes, using Revo on a Mitutoyo CMM," said Mr Orchard. "We also apply PolyWorks point cloud processing software from 3D Scanners to improve even further the accuracy of the sweep scan." The leading edge of any aerofoil is, of course, a critical feature and here Rolls-Royce now has a number of software patents capable of providing automated blade edge assessment, including leading-edge curvature measurement. Here, the software can assess percentage changes in curvature, so that it is possible to determine the presence of unwanted features, such as flats, spikes or bias. Next to address the delegates was Andrew Brownhill, a research student with University College London, who Has been working with the UK Atomic Energy Authority to evaluate suitable non-contact metrology processes for the remote measurement of heat protective tiles mounted inside the EFDA-JET experimental fusion device located in Oxfordshire. The device is having its carbon fibre tiles replaced with beryllium counterparts, as these feature fewer impurities. The downside is that beryllium is both less heat resistant (hence positioning becomes critical) and is a carcinogen and thus creates handling issues. EDGES AND STEPS "A non-contact remote measuring system is required for the tiles, of which there are 5-7 per assembly, with over 1,800 assemblies located within the device," explained Mr Brownhill. "Complicating the task, many of the tiles are castellated, so there are a large number of edges and steps, many featuring tight tolerances." Various test pieces have been manufactured in order to help evaluate a number of potentially suitable measurement technologies. With trials ongoing, Mr Brownhill has already reached a number of conclusions, such as: an area-based measurement system is preferred, although orientation is critical; tiles need to be removed from the device chamber; edge data is not yet good enough; and an automated process is required. David Bambrough has enjoyed a distinguished career with Nissan Motor Manufacturing UK, spanning 18 years. Today, he has overall responsibility for CMMs and metrology equipment across the entire Sunderland plant, and in 2004 was tasked with introducing freeform measurement to NMUK. Speaking at the freeform centre launch event, he explained that the move to freeform was driven by Nissan headquarters in Japan, where the need was identified to reduce the cost and complexity of traditional panel gauges. Initially, NMUK invested in a Faro Gold Arm to perform laser scanning before comparing data to CAD, while subsequent investment led to the installation of a VTS Vectoron 3D system – technology that was developed in collaboration with engineering group Tokyo Boeki. The VTS Vectoron has improved the range of freeform surfaces that can be scanned successfully, in terms of textures and colours. Now the global strategy is to have one such system in every Nissan plant around the world." The advantage is that it is based on spot, not line laser technology," explained Mr Bambrough. "This means we can set the optimum power for each colour and adjust on the fly." NMUK also boasts many other types of freeform metrology equipment at Sunderland, including a Faro Laser ScanArm V2 for body shop fit checks, as well as a V3 version in the press shop for panel checks. A recent case study saw NMUK solve a body assembly problem regarding the outer fit of the rear pillar to wheelhouse. Using laser scanning techniques, the problem was identified quickly, with pressing dies subsequently modified and production resumed with minimal delay. "A normal turnaround for an investigation of this nature, using a CMM, is two shifts," said Mr Bambrough, "but we can do it in one hour, using laser scanning." ON CLOUD NINE For the past two years, NPL's Andy Robinson has worked on freeform projects specialising in laser scanning and point cloud processing software. At the launch event, he discussed a number of current projects, including one to measure heat shield tiles taken from an npower gas turbine engine. Here, tiles have warped and cracked after little more than six months in service. The task for NPL is to investigate the cause of failure – something that freeform measurement can help establish, using a Faro Platinum Arm and V3 laser scanner, in tandem with PolyWorks IMAlign module. Another current project sees Mr Robinson perform laser scanner performance analysis, with some very interesting observations already coming to light, such as the influence on outcome offered by different operators, laser power, scanning distance, surface finish, ambient lighting and working environment. For instance, one test demonstrated that three different operators, scanning the same part under identical conditions, recorded differences of up to 40 micron. Mr Robinson is also involved heavily in freeform metrology software validation, concentrating on alignment, comparison and shape fitting. Image: A part exhibiting freeform features Box Item Freeform facilities The industrially-focused new National Freeform Centre consists of measuring facilities, traceability routes, and support for diverse measuring techniques and data handling processes. NPL's dimensional metrology laboratories at Teddington, where the centre is housed, will also enable customer-owned commercial instrumentation and equipment to be evaluated and compared against NPL freeform reference artefacts and facilities. The prime focus is to support UK manufacturers and academics in freeform measurement, and also co-operate with other measurement institutes and ISO standards. Particular focus will be on evaluation and traceability for CMMs fitted with tactile and non-contact probes; laser scanners, articulated arms and Moiré and fringe projection systems. First published in Machinery, December 2009