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Emerald Group Publishing Limited
Copyright © 2006, Emerald Group Publishing Limited
Dependable quality management in the manufacture of small components
Dependable quality management in the manufacture of small components
Keywords: Image sensors
When it comes to the quality of small components manufactured in large batches customers call for high-quality inspection which can be only achieved by efficient image processing
“Go” or “No Go”, good or bad, this can no longer be a subjective decision. Visual inspection carried out manually by human beings has a number of inadequacies which can no longer be tolerated. These inadequacies include, for example, eye strain, fluctuating inspection quality, damage caused by manual handling, bad parts slipping through due to the complexity of defects and so-called pseudo defects – good parts that are classified as defective.
The medium-sized firm Schauerte Präzisionsdrehtechnik GmbH in Lennestadt, Germany has improved its product quality in line with the requirements of its customers and has redesigned its manufacturing processes to become more efficient. As a manufacturer of ready-to-mount precision lathe-work components such as valves and fittings used in plant construction, mechanical engineering and automotive industries, the company realizes that reliable final testing has become an important part of the strategy. The aim was to devise a comprehensive, self-contained system solution that ensured the highest level of reliability with regard to controlling which could be integrated into the company's data communications system and, above all, a solution that allowed great flexibility in adapting to different part variants. The company achieved this aim.
Certainty and confidence are the prime issues
The requirement was to achieve reliable 100 per cent inspection of smaller and larger batches involving a total of about 6 million sleeve/spigot nuts and connecting pieces a year. These are used in screwed joints in hydraulics and in power steering systems for the automotive industry and they can, therefore, be classified as safety- relevant parts. The connecting pieces are welded onto the power steering pipe, this means that the threads must be perfect and they must not be damaged or soiled. The length of the component must also be correct because the connecting weld is obtained by resistance pressure welding. If the length is not correct, this will lead to varying pressures, thereby causing a bad weld seam. The automotive industry demands flawless parts in order to prevent disruption of any kind on the production line.
Besides replacing previous manual inspection at the Schauerte plant by a more sophisticated system, internal process rationalization measures were also another important aspect. Personnel bottlenecks had to be overcome and qualified specialist staff also needed to be relieved from strenuous work and assigned to more appropriate duties.
Schauerte specified its ideas concerning this task more precisely during a seminar on image processing conducted by Cognex, the world market leader in vision systems. Earlier contacts with system integrator IMR (Gesellschaft für Prozess – und Automatisierungstechnik mbH) in Lennestadt were renewed. This system house for turnkey industrial automation solutions – and a partner of Cognex – came up with important ideas on a complete system solution with integrated image processing. The presentation of a rapidly completed comprehensive feasibility study carried out by IMR was “impressive”, as Schauerte put it. A highly detailed in- depth specification of requirements for a solution was worked out during subsequent discussions between the two firms. IMR's strengths are not only to be found in the different areas of image processing, but also in providing turn-key system solutions for automation projects. This includes the complete arrangement and functional optimization of the mechanical components and drives, of control technology and control software, and also the integration into industrial IT-environments.
Plate 1 Several features of the precision lathe-work parts are inspected and documented accurately piece by piece using In-Sight 1000 vision sensors
The vision system is built into a modular sorting station for different types of connecting pieces and sleeve/ spigot nuts. In addition, the construction of the system allows extension of inspection and sorting tasks at any time. In this turnkey system solution, the parts to be checked are conveyed from the storage bin via vibro-conveyer to the rotary table which is equipped with individual receptacles for the parts where several features are checked by image processing (Plate 1). The sorting station is equipped with a PC which acts as a database and communication server and can be directly integrated into Schauerte's production and communication network via integrated custom-designed interfaces, thereby linking up with production planning and documentation. About 100 per cent of the batches are checked in two-shift operation, and in a planned further development stage, in three- shift operation. The sorting station also provides an important stepping stone towards quality documentation and product traceability.
A closer look inside ...
This sorting station with its expandable range of types for different part diameters and lengths inspects more than 16 different types of connecting pieces and sleeve/spigot nuts, coping with maximum inside diameters up to 19mm and outside diameters up to 25mm. These parts have a length of up to 22mm and several of their features are checked at a rate of 30 pieces/ minute.
The internal threads must be reliably inspected for processing defects such as break-offs and scratches, and also for soiling, e.g. from residual chips. Placed in corresponding recesses on the rotary table, the parts are presented to two cameras that are precisely positioned so that they are axially concentric (camera 1) and centered (camera 2). Internal inspection is carried out by the first vision sensor In-Sight 1000 (camera 1) (Plate 2). At a distance of 10-25mm, the image is created using an inflexible endoscopic lens system with integrated illumination in the endoscope head via an optical fiber bundle. Inspection of the length of the parts is carried out by camera 2 which is equipped with a tele-centric objective. Following this, final go/no-go sorting is performed under control of the PC server.
Plate 2 The conveyer carries the individualized and isolated test parts onto the rotary table and to the image processing inspection station. Internal examination is carried out by an endoscopic objective centered on the axis of the part and length control is performed by means of a tele-centric image
In the context of this complete system solution, the hardware and software of the image processing components provided by Cognex performed especially well. As integrated, standalone image processing units suitable for industrial applications, the In-Sight 1000 vision sensors provided ready-to-use results directly to the server of the sorting station on-site via a built-in ethernet port. The extremely sensitive inspection algorithms of the vision software from Cognex and further special customization completed by IMR allowed simple, dependable programming of the inspection characteristics. This applies especially to inspection of internal threads, including further functions such as verifying the presence of lateral drill holes in the threads. The easy-to- handle, custom-designed user interface which was developed for Schauerte by IMR, gives operating personnel a concise overview of the real-time status of batch assessment. The operator can also program further part variants without assistance by simply setting parameters.
More efficient sequence of operations
Labor-intensive manual part inspection has become obsolete. Qualified specialist staff have been relieved from tiring activities and are employed more productively in other areas of the company. The options for reacting to quality requirements of new customers are now much better. Quality documentation data makes it possible to draw conclusions concerning improvements in production-line processes. Having analyzed and considered the key aspects of rationalization effects and improvements, Schauerte confidently expects that its investment in the sorting and inspecting station will pay for itself within two years. This estimate does not even take into account scheduled expansions and increases in efficiency, such as connection to a bunker system with improved packing facilities and switching to three-shift operation.