Making it better

In July 2007, Germany's MTU Aero Engines GmbH filed a patent for a milling strategy in the US - US Patent No: 7,237,989. The subject was so-called trochoidal milling, a machining process of particular relevance to hard-to-cut materials, such as titanium. In this process, a rotating milling cutter is orbited about its central axis, while also being driven forward. In addition, a fourth motion can be added, that of pivoting or tilting the cutter axis to cause what is described as a "tumbling or wobbling" motion, this allowing the cutter to remain engaged with non-vertical walls. Critically, after moving into the material to be cut and achieving "maximum permissible circumferential contact", this attribute is then maintained during cut. However, according to MTU, this new milling strategy, as employed in the machining of a titanium blisk, is only possible because of the rigid toolholding system employed. Says Götz Lebküchner, materials and processing engineer at MTU: "When processing these thin-walled titanium blades, the tool must be completely free from vibrations. Otherwise, it is absolutely impossible to use the new milling strategy safely. Only [Rego-Fix's] high-tech powRgrip toolholder is capable of this." (Brunner Machine Tools, 020 8992 6011). By applying its new strategy and by employing the powRgrip system, the company has succeeded in reducing processing time by 42 per cent and lowering tool costs by a massive 76 per cent, compared to its previous method of production. Image: MTU is only able to use its novel, patented milling process because of powRgrip's performance PowRgrip consists of a toolholder, high precision collet, and a compact machine to insert the collet and tool into the holder. The specially designed collets are inserted with over 6 tons of force, creating gripping forces higher than those achieved with shrink-fit holders and without the small tool size limitations. Torque transmission for a 25 mm dameter tool is claimed as around 1,100 Nm; runout is 0.003 mm at 3xD, while 20,000 cycles can be accommodated by the system. SLIDING-HEAD BENEFITS Back in the UK and a stainless steel shaft for an aerospace fuel system is machined at both ends in one hit from S143 aerospace stainless steel bar in a reduced cycle time on a Star SR-20RIII sliding-head lathe (01332 864455) at Crediton sub-contractor Rotamic Precision Engineering. Previously, the part required three separate operations on a fixed-head lathe and a vertical machining centre (VMC). The reduction in cycle time has enabled Rotamic to achieve its cost reduction target from the customer. Rotamic has AS9100 accreditation and mainly serves the aerospace industry, although work is regularly carried out for the medical, offshore and hydraulics sectors. "We used to have trouble holding 25 microns total tolerance on three stepped outside diameters (OD) turned down to around 4 mm from 9.5 mm bar," explains Gary Squires, Rotamic's workshop supervisor. "Even though the 89 mm long part was supported by a tailstock and running centre in the fixed-head lathe, flexing of the bar used to cause tool push-off and resulted in scrapped components. Now, with the Star lathe, the guide bush supports the component close to the point of turning, so every part is perfect." A more complex, mating part in the same material is also produced in one hit on the multi-axis Star mill-turning centre more quickly and cost effectively than before. The part is manufactured in batches of 800-off in eight varieties and each previously needed four separate operations, as follows. Image: Rotamic Precision is using its Star sliding-head lathes to reduce cycle times, which has, in turn, meant that it has achieved the target cost reduction specified by one of its customers Installed in October 2008 by Star Micronics GB as a turnkey solution for production of two components, the 20 mm capacity lathe, fitted with an FMB 3.2 metre bar magazine, was Rotamic's first sliding-headstock machine. Star GB programmed the shaft, as well as the largest of the mating parts, and successfully machined 30 of each at its Derbyshire technical centre to prove out the processes. At present, over 90 per cent of components put onto the Star are of stainless steel, so Rotamic went for the 100 bar, high-pressure coolant option, which blasts the stringy chips away from the working area. Oil mist extraction and filtration have also been fitted. So, too, has a long parts ejection system that sits at the counter-spindle end of the machine to accept machined shafts, so that they do not drop into the machine and risk damage. SET-UPS NO PROBLEM "We used to think sliding-headstock machines took a long time to set up, perhaps as much as a day, but it is not so," says Mr Squires. "We were amazed to find that we can reset the Star quickly and be straight back in production, provided that the guide bush does not need changing to accommodate a different size of bar." In addition, once again contrary to belief, ease of set-up and programming encouraged Rotamic to transfer 66 parts from its fixed-head lathes to the Star in the first seven months of operation. All of the components are produced faster and more economically. At H+S Aviation, Portsmouth, the introduction of the measurement arm FARO Gage (024 7621 7690) came as a result of the company's ongoing commitment to process improvement and, specifically, the need to provide high levels of customer service through reduced 'turn-times' within the repair and overhaul business. More than 500 engines were processed in 2006 within the company's two sites in Portsmouth and where over 350 are employed. The FARO Gage was the world´s first personal CMM, which boasts accuracies up to ±0.005 mm and has a working volume of 1.2 m. Having previously used conventional CMM technology through the component repair side of the business, the FARO Gage technology provided an alternative solution to production and build areas, without the need or facility for dedicated programming. The simple nature and ease of use of the FARO Gage allowed H+S to focus on a known process bottleneck relating to the inspection of an auxiliary gearbox module for the Rolls-Royce Model 250 engine. Both the cover and main housing require a number of features to be inspected as part of the mandatory process, and the process time for both has now been considerably reduced. Initial results indicate that the inspection time has been reduced from a three-day queue to a process time of 15 minutes with the FARO Gage and the ability to manage the process queue to less than one day. For the future, H+S engineering teams will continue to review the suitability of FARO to the varying requirements of the repair and overhaul business. Due to the flexibility of the system, H+S Aviation plan to use the FARO Gage on components and processes that are turn-time-sensitive and require a consistent measurement methodology. First published in Machinery September 2009