02 January 2012
X-ray vision and more
The application of computer tomography is still limited in engineering, but one German mould maker is benefiting. Elsewhere, probing is paying dividends. Andrew Allcock reports (includes links to product information and videos)
Product information and video links
Injection moulder Vema GmbH, Stuttgart, Germany, has employed Werth Messtechnik (01788 522060) computer tomography (CT - X-ray inspection) technology to support the precise moulding of LED lenses for the automotive industry.
CT is widely used in the medical field, but is still relatively new in the industrial world. Vema was recommended the technology as an alternative to traditional measurement methods, and a nearby service provider, HeMa-CT, located in Schönaich Germany, about 100 km from Vema, was able to support Vema's application, using a TomoScope 200 from Werth Messtechnik. (UK bureau service, visit www.mantechgeometrics.co.uk)
Werner Veser, Vema's managing director reports: "Even the first results were absolutely promising. After we communicated and adapted our requirements, we had measurement and correction results that I had never dreamed of."
Mr Veser explains the company's LED speciality and the challenge: "We have developed very specialised expertise, so that many light designers come to us to have their ideas implemented in practice. Because we can provide mouldmaking and injection moulding from a single source, and our experience in both areas complements each other nicely, we are the ideal partner for mould development and initial series production."
Image: LEDs require accurate mould tools, if their efficiency is to be high
The production of lenses and reflectors, however, is no simple task. The products not only have to be as clear as glass, but the surfaces are also critical. They must be absolutely flat and at a defined angle to each other. Deviations of just over 0.020 mm result in a loss in transmitted light of about 30%. High precision is therefore required for the functional optical surfaces in the final product, with this challenge mostly involving the moulding, which must be designed for process reliability.
The mould tool is manufactured using CAD data to generate the required mould tool design, taking account of shrinkage, and NC programmes. Following manufacture of the mould tool, sample runs on an injection moulding machine are undertaken, as are simulations. Together, these result in a reliable production process; critical dimensions stay within a tolerance of 0.1 mm.
Image: Vema makes mould tools such as this
But then fine-tuning must be undertaken, which requires that the injection moulded products be measured. If deviations from the target data are found, the mould must be corrected. Thick-walled parts, in particular, which include the LED lenses, are subject to significant shrinkage. They require several correction loops when using typical tactile measurements, due to the relatively low resolution of the measurement data.
Besides the limited number of measurement points, measuring with probes or other methods has limitations, due to reasons of access to surfaces and undercuts. Typical methods also do not have any way to measure the assemblies, as a whole. This means that the only option is to integrate dimensions obtained from several measurement runs, which means further work and an additional source of inaccuracy.
Using computer tomography, it is now possible to measure first article parts precisely and meaningfully. The necessary corrections can then be transferred to the mould in just one step.
Image: CT technology highlights out of tolerance areas - red......
Image: ...while green areas are correct, with the more the better
Staying in Germany and Ernst & Landerer GmbH, Balzhausen, is a family-owned company serving leading automotive gearbox manufacturers. It has discovered the benfits of Blum-Novotest probes in its machining operations (01543 257111).
Image: Ernst & Landerer GmbH machines parts such as this
Established as a foundry more than 40 years ago, it now offers machining, which has become the largest part of the company's business. It runs 22 Haas machining centres and between 30 and 40% of parts are supplied to gearbox manufacturers, where tolerances are j6, h6 for boring, and 0.0100 mm for rectangularity and parallelism.
Indeed, tolerances are so tight that almost all parts can only be clamped once for the entire machining process. For years, the parts were transferred to a 3D measuring machine after machining – essential, but time consuming – and, if finished components exhibited tolerance deviation, rework was only possible at high cost. It was a logical solution to measure the components in the machining centre itself. For example, the last component produced was measured with a tripod type measuring instrument that was used to manually measure a particular diameter. If it still complied with the relevant tolerance, all previously bored diameters also had to be within the tolerances.
But this, too, was time consuming, as the machine had to be stopped for the measuring process, while, on the other hand, it was susceptible to errors, as measurement was carried out manually and the measuring method required the surfaces to be completely clean.
Two machining centres installed came with a probe from a measuring technology manufacturer and this allowed measuring during the machining process for the first time. Today, more than 25 systems from Blum are in use at the company.
"All gearbox parts produced with strict tolerances are now measured using the Blum probes," Ernst & Landerer director Thomas Ernst explains. "We have discovered that it is a fast and reliable process. There were few complaints, even before the introduction of measuring technology from Blum-Novotest; however, since the probes have been in use, this figure has decreased even further. From a product range made up of high precision gearbox parts with around 20,000 manufactured every year, fewer than 10 workpieces were returned with complaints in 2010."
And he adds: "Using Blum's probes has significantly increased the availability of the machining centres. Inspection of workpieces was extremely stringent in the past, so the number of rejects has not fallen significantly overall. However, it used to take considerably longer and, with Blum's measuring technology, the processing times are lower and process reliability has improved significantly."
Image: Part measurement in action
In addition to part measurement, the company also uses Z-Nano tool probes, such that missing threads, for example, no longer get missed until the end of the process, because a broken tool has not been noticed.
Each individual tool is moved to the Z-Nano tool probe after machining the component and checked in a matter of seconds. The Z-Nano is also used to perform length measurement checks on tools, with a repeatability of 0.002 mm.
Image: The Z-Nano is used for tool measurement
As well as the Z-Nano, the TC52 probe is used for zero point detection, verifying clearances, and for monitoring the position and orientation of workpieces prior to repositioning, using an in-house system, and then machining.
Summing it all up, Mr Ernst says: "Previously, we were able to produce excellent parts with precise dimensions, but it involved much more measuring time and work."
More information and video links
Tech Soft 3D toolkit – Geomagic inspection with CAD data importers
Blum-Novotest – 25 mm digital/analogue touch-probe
Doosan machine tools have Renishaw GUI for probing (video)
ZeGage – cost-conscious shopfloor form/roughness measurement
Renishaw XR20-W rotary axis calibrator offers lightweight ease of use and portability
Hommel Etamic Roundscan form/roughness/twist measurement (video) )
First published in Machinery, January 2012
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