Attacking the bends

The drive to find stronger and lighter materials affects every element of modern aircraft design, but it also puts pressure on suppliers who have to process these materials. Ed Hill visited tube assembly specialist SL Engineering to see how it is meeting OEM demands. The use of advanced alloys and composites to achieve operating savings on aircraft is obvious for aerostructures, but the same advances are being applied to elements such as the engines and actuation systems.   Engine manufacturers are increasingly using lightweighting materials on components such as fan blades and turbines, but also for the external elements such as tube assemblies. Traditional stainless steel pipework is being replaced by assemblies manufactured from exotic materials such as titanium, Inconel and Nimonic alloys. However, these materials present complex and difficult processing challenges for suppliers of these parts. SL Engineering is at the forefront of rigid tube manufacturing, particularly in aerospace applications. The Sleaford, UK company's in-house capabilities include CNC tube manipulation, CMM contact and non-contact tube inspection, CNC multi-axis precision machining, for the manufacture of bespoke end fittings, computerised orbital and hand TIG welding, brazing, fabrication and assembly. In-house inspection capabilities include Nadcap-approved NDT dye penetrant and x-ray testing and pressure testing of assemblies in air, nitrogen gas or hydraulic media. The company is a tier 1 supplier of components for engines used on Airbus and Boeing passenger aircraft as well as military aircraft and helicopters. It also supplies tube assemblies for a number of airframe applications for the conveyance of fluid around the aircraft, as well as servicing the gas turbine, marine, oil and gas and medical markets.  Shaun Stevenson, director and general manager of SL Engineering, comments: “Pipes made from exotic materials would only have been used on a very limited number of critical applications in the past but with advances in new technology they are becoming the norm rather than the exception.” <Tight corners> The bending process of these metals can be particularly challenging. Materials such as titanium are less forgiving than stainless steel which can be manipulated more easily. Titanium requires greater torque forces and the correct shape must be achieved with the first bend run as little manual adjustment can be made afterwards. These factors, combined with ever tighter tolerances demanded by the customers and the need to avoid scrap of this costly tube, present significant challenges to manufacturers. One project for a major aero engine manufacturer required SL to supply newly designed fuel pipes manufactured in titanium for an existing engine design. The parts consisted of large diameter pipes made from thin walled titanium and bent to tight radiuses. In another example, an actuation assembly required thin walled titanium tube to be bent to a tight centre line radius (2D) with very little straight tube between the bends. Ovality (out-of-roundness) of the tube had to be kept to less than 5%, twice the requirement of the industry norm. The positional tolerance after adding a welded component had to be within 0.005”. Stevenson continues: “Traditionally that tolerance would be ± 0.025”. These are challenging specifications and to produce that consistently on a cost effective production basis is a difficult task.” <Electric approach> To achieve improved production of such high specification parts the company turned to Unison, manufacturers of electric tube bending machines. The company purchased a 30mm Unison Breeze CNC machine with Multi Stack tooling. The specification also included a higher torque setting than the standard machine particularly to deal with complex alloys.  Unison says its electric mechanical bending machines offer more reliable results than older hydraulic machines. Steve Haddrell from the Scarborough, UK company advised SL Engineering on the purchase of the machine. “They had very specific mechanical configuration requirements and a need for a high level of bend torque to cope with exotic aerospace materials,” he states. “Before we finalised the design of the machine we analysed all these variables to ensure we didn't compromise the rigidity and stiffness characteristics of the machine which are fundamental in this application sector. And of course, the all-electric control of all of the motion axes on the machine ensure that the tooling and therefore the bend programs replicate perfectly from batch to batch.” Manufacturers of tubular assemblies also have to be flexible in the aerospace market. These parts are often one of the last details to be designed by SL's customers. The lead-time and expense for hard tooling often cannot take place until trial sets have been fitted. To quicken this process SL has developed a modular fixturing system for initial ‘off-tool' small batch development production. This is combined with a closed loop CAD/CAM system that can transmit data to the Unison bending machine and a CMM to produce a model pipe to a set of master co-ordinate data. The modular fixture system can then be set up using the model pipe to produce the remainder of the batch without the need to purchase expensive hard tooling. Unison's simulation software also means use of actual machine time on trials is greatly reduced. SL has to be able to manufacture its diverse range of parts from its records. The company estimates it has 3,000 assemblies on its books and around 500 to 1,000 bent parts can be considered ‘live' at any time. Additionally to the tube there are typically around four machined parts connected to the pipe and batch sizes can range from one-offs to 500 per month. Tube bending of many of these existing designs has been transferred to the Unison machine to increase production.  Steve Eggleton, technical director at SL Engineering explains: “With the repeatability and ease of setting up the Unison machine we can now take a more considered view of our batch loading. With this increased flexibility and throughput it now means the man operating the machine can be re-deployed more productively in other parts of the business.”  The investment in the Unison machine is just part of SL's all-round approach to improving performance. The results of which are clear given the company's SC21 Bronze Award status and being named a single source supplier by a number of its customers. It has also been identified as a centre of excellence by one of its biggest OEM aero engine customers. In 2013 the company managed to increase turnover by 22% with plans to increase productivity and explore new markets in the future.  <Backup support>  And to continue this impressive performance the company knows it can draw on the expertise from its supplier Unison which sells machines to many companies in the aerospace sector both in the UK and abroad. In fact due to its performance SL has now ordered a second machine from the company. Haddrell affirms: “Our engineers really understand the process of bending, which makes an enormous difference to the performance of the machine. It's very easy to produce scrap on a tube bending machine so the control and software is designed for these kinds of situations at SL. It can handle low volumes, small batch sizes and quick changeovers. This makes a huge difference in the dead time between jobs.” Stevenson notes: “There will be more and more engine content using these advanced materials because they mean ever more efficient and environmentally friendly engines. This means we have to invest in the future and give the customer what they want, whether that's in machinery, technology, or training our employees to push the boundaries in terms of ever more complex formed shapes, so that we are prepared for the challenges ahead.” “Our customers expect us to invest so we can be at the forefront of what they are developing. This enables us to remain agile and able to adapt quickly to their requirements and Unison has given us a machine that successfully mirrors those expectations,” Eggleton concludes. www.sl-engineering.co.uk www.unisonltd.com