Learning the hard way

It's no industry secret that, to enhance product performance, designers in sectors such as aerospace, oil and gas, and medical are increasingly looking towards the latest generation of hard metal alloys. These materials are intended to retain toughness and strength at elevated temperatures and in demanding performance conditions. The knock-on effect for the machining process, of course, is reduced machineability. To address this issue, the National Metals Technology Centre (NAMTEC – 01709 724990) staged a seminar at the Knowledge Transfer Centre on the Advanced Manufacturing Park in Rotherham, Yorkshire. First up to the microphone was Dr Peter Osborne, a programme manager within the process technology group at the University of Sheffield's Advanced Manufacturing Research Centre (AMRC – 0114 222 1747). Among the many trends witnessed in this area, Dr Osborne cited increasing raw material costs, greater demand for near-net-shape components, tighter tolerances, increasing design complexity, shorter lead-times and growing demand for machining technology innovation. "There is always a need to first understand the process constraints of cutting hard-to-machine materials," he says. "This might require analysis of dynamics, volumetric accuracy, process modelling and tooling requirements. At the AMRC, we adopt a critical path approach, examining the bottlenecks and limitations that prevent cycle time reductions – these could include vibration, tool wear, geometric accuracy, toolpath strategy or part movement. Machineability depends on three principal factors: tool life, surface finish and the power demands/cutting forces of the process." To determine what Dr Osborne calls the "optimum parameter window under ideal conditions", the AMRC frequently undertakes trials on new hard-to-machine materials brought to market, working extensively with its tooling, workholding and software partners to deliver solutions that offer higher metal removal rates and increased productivity. LESS THAN STRAIGHTFORWARD But it's far from straightforward – there are multiple issues with machining a material such as titanium, for instance. First, high hot hardness can lead to cutting tool deformation, while high dynamic shear strength localises stresses that are responsible for producing abrasive edges and tool notching. With heat-resistant super alloys (HRSAs), the low thermal diffusivity of these materials causes heat concentration in the cutting zone, leading to built-up edge and tool burn-out. And there are complications for the material, too. According to Dr Osborne: "The impact on micro-structures (see box next page) is becoming more of a concern in aero engine applications, where OEMs are increasingly limiting the amount of 'damage' allowed." Near-net-shape parts are also problematic, as forgings are susceptible to residual stresses caused by differential cooling rates, particularly with aerospace alloys. This presents difficulties, as manufacturers are likely to be machining through these areas. "With this in mind, we have to 'tune' the sequence of metal removal operations to minimise distortion," states Dr Osborne. "High pressure coolant is also sensible. Ultimately, we are looking to optimise the process by striking a balance between all the variables." Next up was Ian Lain, UK business development manager for advanced machining at Sandvik Coromant (0121 504 5400). According to Mr Lain, customers using HRSAs such as Inconel, Waspalloy, Udimet and Stellite are demanding cost-effective, yet high quality and robust solutions, ready for what he terms "green light manufacturing" – hitting 'start' and walking away. Sandvik Coromant deploys a planning process that typically starts with breaking the component down and offering a feature-by-feature solution, encompassing machining strategy, cutter selection, cutting data and CAM programming. For turning processes involving HRSAs, first, intermediate and final cutting operations are recommended, with respective depths of cut of 10, 5 and 0.25-1.0 mm, respectively. "The shape of the insert is also critical," Mr Lain explains. "Many machine shops will opt for an 80° diamond CNMG shape, because it is easy to program, it machines into square corners and is used extensively elsewhere, but this isn't typically the best shape for hard-to-machine alloys." MINUTES SAVED IN THE ROUND To cite an example, cutting Inconel 718 at 2 mm depth of cut, 50 m/min cutting speed and 0.25 mm/rev feed, Sandvik Coromant trials revealed that switching from CNMG to rhombic-shaped CNMX inserts offered more even wear patterns and doubled metal removal rates. Switching again to a round RCMT insert produced even greater gains – removing the same amount of material in 4.2 minutes that the CNMX insert had managed in 6 minutes. "Other important factors include insert grade requirements," Mr Lain adds. "We need high hot hardness and a low percentage of cobalt [used as binder]. Furthermore, good edge toughness is required, typically demanding the use of fine grain carbide, while coating adhesion is also vital. From a geometry perspective, positive rake is preferred, as these materials prefer to be cut, rather than pushed, off the workpiece, while small ER (edge rounding) and a small land for roughing are other important considerations. Finally, the angle of entry can also play a big part in successful machining. Reducing the approach angle achieves a chip thinning effect to give longer and more predictable tool life. It also allows the deployment of more wear-resistant grades that permit higher speeds." TROCHOIDAL TECHNIQUE Regarding the milling of materials such as titanium, Sandvik Coromant promotes the use of trochoidal techniques, using round shank tooling. "We often get asked if we are going to produce a full-slot cutter, but, due to the forces involved and the problems of chip evacuation, we currently have no plans in this area. Instead, we recommend a trochoidal milling strategy, which is essentially an edging process with small engagement that repeats several times. This process has advantages that include: chip thinning to permit increased feed; the pocket does not close in behind; large depth of cut potential [2xD typical]; good swarf evacuation; less heat in the cutting zone; and more predictable tool life." Mr Lain adds that arguably the biggest area for future gains is in the use of ceramic/Sialon inserts, where cutting speeds up to 400 m/min are possible. And while tool life is often not more than 2.5 minutes in some applications, the amount of material removed in this time more than compensates. One Sandivk Coromant customer able to testify to such benefits also gave a presentation. Otley-based Craftsman Tools (01943 466788) is not only a supplier of toolholding and workholding technology, but also offers extensive subcontract machining services. "For many years, we've been making oil field stem shafts made from Inconel 718 for Cameron," says the company's product manager, John Telford. "To retain this contract, we've had to find ways to continuously improve the machining process for these shafts, which contain turned diameters, grooves and threaded sections. Using technology and support from Sandvik Coromant has proved pivotal: we switched to using RCMT button inserts for roughing and VNMG carbide tips for finishing and, as a result, we could increase our speeds and feeds and cut hours from the cycle time. "For another part, we increased speeds from 29 to 50-60 m/min, and feeds from 0.15-0.25 to 0.4-0.6 mm/rev, using Sandvik Coromant solutions," he continued. "We are now looking at ceramic inserts, which, to be honest, cut through Inconel like a knife through butter. This is where our next leap in competitive gain will come from." Mark Kirby, technical director at Technicut (0114 256 0036), and Steve Eckersall, senior technical engineer at Nikken Kosakusho Europe (01709 366306), highlight how the two companies have developed a solution for the high productivity milling of titanium, a material that Technicut predicts will witness a 40% increase in consumption by 2015. "Our TiTAN Rippa carbide cutter is designed to slot at depths of 2xD, in comparison with conventional alternatives, which typically achieve 0.5-1xD," Mr Kirby explains. "It also provides the potential to produce four times greater metal removal rates, thus giving a significant reduction in tooling costs per kilogram of material removed, while good tool life is achieved, due to using the entire flute length. The same benefits apply to shoulder milling." EXTREME DEVELOPMENT During the development of TiTAN Rippa, the main problem Technicut encountered was cutter extraction from the chuck, due to the presence of high cutting forces. This led to the involvement of Nikken and the development of X-Treme; a new generation of the company's Multi-Lock milling chuck range. "In a traditional chucking configuration, you have a cylinder gripping on a cylinder by friction only," Mr Eckersall offers. "During heavy cutting and increased tool loads, such a set-up will witness micro-radial movement of the part in the chuck. This exaggerates to produce a corkscrew effect, leading to pull-out." Nikken's X-Treme provides the answer, incorporating a number of new innovations, such as the company's Sure-Lock quad clamp technology, which provides positive locking and restraint of the tool shank during cutting. The combination of X-Treme milling chuck and TiTAN Rippa cutter has provided impressive results. Using a 25 mm diameter TiTAN Rippa at 50 mm depth of cut on titanium Ti-64, a metal removal rate of 267 cm³/min has been witnessed. The solution will be available in a range of cutters and chucks from either source as of September 2012. Box item Simulation offers early design feedback James Farrar, principal engineer - process modelling at Wilde Analysis (0161 474 7479), the UK distributor of DEFORM analysis software for machining processes, spoke passionately about how cutting hard-to-machine materials results in a change of micro-structure. Reducing this effect means finding a way to limit distortion during machining and one of the best ways to understand such a phenomenon is to use simulation software. "Why use DEFORM software, instead of knowledge? Well, because expertise can take years to acquire, real-life tests cost money and time to market is slow," said Mr Farrar. "Using DEFORM, it is possible to get design feedback much earlier in the process, as it analyses [in 2D or 3D] variables that include cutting force, stress, temperature, tool wear, chip behaviour, surface integrity, tool geometry design and coating effects." For very fine detail, 2D simulation provides the best results, although 3D is preferred for full tooling geometry or long run applications. The simulation can model effects that include chip forming, serration, exit burr formation and chip breaking, while, among a host of other features, finite element simulation is useful for identifying the amount of 'white layer' likely to be produced. "DEFORM was first released in 1989 and we have endless examples that show extremely good correlation between simulated predictions of cutting zone behaviour and actual test results," said Mr Farrar. First published in Machinery, August 2012