Reciprocation grinding of a nickel alloy aerospace part
Driving down the cost of aircraft manufacture
Budget airlines offering fares for a few pounds on certain routes clearly show that flying, once the preserve of the well-heeled, is much more affordable these days, despite all the hidden charges. A relentless reduction in the cost of manufacturing aircraft over the past decades has made this possible. Air travel is now within reach of a wide demographic, not just in the UK but across the world.
To reduce aircraft cost, aerospace primes and their supply chains have long been demanding the same high quality components from subcontractors at ever lower prices. Achieving cost per part reductions year-on-year while still making a profit has forced manufacturers to rely heavily on the ingenuity of machine tool builders, as well as the availability of novel tooling to extract every last bit of productivity from the machines.
Enter Saint-Gobain Abrasives with its new grinding wheels from the group's Norton brand, the world's leading supplier of bonded abrasives offering a multitude of sizes and specifications, most custom-designed and engineered for specific applications.
Its VORTEX™ 2, QUANTUM VET and ALTOS IPX grinding wheels are the latest types to feature strong yet highly porous vitreous bonds in which sharp abrasive grains are retained. The engineered wheel structures allow the grinding of difficult-to-machine aerospace materials, like high-temperature superalloys, faster than ever before. Super-fast machining is achieved without burning the component and causing residual stresses, microcracking and a heat-affected zone near the surface, as any such metallurgical changes are unacceptable to aircraft manufacturers.
Another trend in the aerospace industry in recent years has been a demand for lower emissions of carbon dioxide and other pollutants. So engines must be increasingly fuel efficient, which at the same time has the benefit of reducing operating costs. The International Air Transport Association (IATA) has set a target of an average improvement in fuel efficiency of 1.5% per year from 2009 through to 2020 and a reduction in CO2 emissions of 50% by 2050, relative to 2005 levels.
These are tough targets and require step increases in engine efficiency, which are invariably accompanied by a requirement for ever more complex and accurate turbine blades, vanes and other components. Grinding wheels that hold an accurate edge profile for longer are required to achieve the tighter tolerances, as well as to minimise wheel wear. This is another area where Saint-Gobain Abrasives' new products score in the race to manufacture more engines.
To put the enormous task in perspective, Airbus foresees a need between 2012 and 2031 for some 27,300 new airliners with capacities of 100 seats and above to replace ageing aircraft in the West and to increase the fleets of airline operators in developing countries. This will more than double the world's passenger aircraft inventory and there will also be a requirement for nearly 900 new freighter aircraft.
Improved creep-feed technology
VORTEX™2, Norton's latest version of its VORTEX™ aluminium oxide wheel, is ideal for creep-feed grinding of turbine parts from nickel alloys and other sticky and difficult-to-machine metals at very high removal rates. The structure of the wheel is engineered to increase porosity and maximise coolant diffusion in the grinding zone, while maintaining grinding wheel strength and form holding.
An innovative formula for bonding the fused alumina grain at optimal spacing in the matrix leads to improved chip clearance and reduced friction. High productivity is thus combined with prolonged wheel life.
Experimental data on metal removal using VORTEX™ 2 show a massive 40% increase in longevity compared with previous wheels, cycle time reductions of up to 60% due to increased depths of cut, and much better holding of the dressed form across the width of the wheels. Overall, metal removal rate is 37.5% higher and the profile lasts longer, so frequency of wheel dressing is reduced, resulting in less wear on the dresser. The life of both the wheel and dresser is therefore prolonged.
Field trials conducted on Makino machining centres, grinding Inconel, show wheel life increases of about one-third and cycle times down by between 11 and 24% respectively, translating into a 20% saving in manufacturing cost per part.
In another test on a Blohm Profimat grinding Inconel turbine blades, wheel life was increased by 50% and cycle time was improved by 16%. Similar manufacturing cost savings were achieved.
Reciprocation rivals creep-feed grinding
QUANTUM VET, another new generation of grinding wheel from Saint-Gobain Abrasives' Norton brand, has readily self-sharpening ceramic grains in a vitreous bond of high strength and permeability. The wheels are able to extract maximum advantage from the latest reciprocating grinding machines, which can generate high speed cutting conditions using dynamic linear drives, torque motors for the rotary axes, high wheel head speed and vibration-free structures.
Again, a highly porous structure is evident in the aluminium oxide/ceramic blend wheels, which contain abrasive grains held strongly within the matrix. Core applications in the aerospace industry include the manufacture of turbine blades and vanes. In most cases, metal removal rate (Q') matches or exceeds that of creep-feed grinding.
Reciprocation is less aggressive to the surface of a component, as it uses a small depth of cut combined with fast movement of the workpiece back and forth. However, until now the shallow cuts tended to prevent proper chip formation, rapidly wearing the abrasive grains and causing wheel glazing and the risk of burning the component.
These difficulties have been overcome for production grinding of nickel alloys. Norton's breakthrough has been to formulate wheels with a porous bond that is strong enough to achieve the difficult task of holding ceramic grain securely in place. As the bond material does not form a high proportion of wheel volume, free cutting results, while the high bond strength maintains mechanical integrity of the wheel at peripheral speeds up to 100 m/s.
The ceramic grain itself also plays a crucial part and represents a big stride forward in technology. Unlike Norton's earlier seeded gel ceramic grains, which were hard and tough and would only microfracture under high grinding force, QUANTUM VET grains have built-in fracture planes that allow sub-micron particles to break off at lower mechanical pressure and expose new, sharp grain edges much more readily.
Tests at Saint-Gobain Abrasives' research and development centre have shown that when grinding nickel alloys using high speed rotation of a 300 mm diameter QUANTUM VET wheel combined with a grinding machine table speed of 80 m/min and a stock removal rate of around 0.1 mm per pass, Q' of up to 133 mm3/mm/s was achieved, equivalent to the performance of creep-feed grinding.
Grinding cycle times halved
Elongated, extra-sharp ceramic grain is combined with a highly porous and permeable open structure in the manufacture of ALTOS IPX high-aspect-ratio seeded gel grinding wheels, allowing the coolant maximum access to the grinding zone and providing improved chip removal and reduced friction. Significant advantages result, including high metal removal rates, improved form holding, longer wheel life and reduced dressing with a greatly reduced risk of metallurgical damage.
Qualified for use up to 50 m/s and ideal for creep feed and surface grinding on 'difficult' materials like nickel based alloys, ALTOS IPX can reduce cycle times by 50% and increase the number of pieces ground by three to four times, providing a complete solution for productivity improvements and cost savings in turbine blade grinding.
Anyone involved in grinding of aircraft parts, particularly engine components, knows that Saint-Gobain Abrasives is not only a supplier of top quality wheels for these applications, but it also excels in its ability to deliver the complete solution for any given application and to provide support for the life of a customer's project.
The mere fact that the group supplies three ranges of open-grained wheels tailored to the aerospace industry already sets it apart from the competition and guarantees that the product supplied will most closely match the application. As part of its Process Solutions Program (PSP), a free service to customers, Saint-Gobain Abrasives undertakes to help manufacturers maximise competitive advantage when using its products.
Dedicated engineers visit to analyse users' grinding operations in their entirety with a view to choosing the best grinding process parameters to increase productivity, improve quality and cut production costs. Reducing cycle times significantly lowers manufacturing cost per part, while fewer wheel changes raises productivity and machine utilisation, so it is on these areas that the PSP analysis mainly focuses.
Quality enhancement and scrap reduction impact heavily on the bottom line and are similarly targeted. Other factors addressed include, optimisation of feeds and speeds, accurate alignment of the wheel and the dressing roll or disc on their spindles, vibration control, coolant delivery and dressing frequency. A safety audit also forms part of the overall assessment.
To minimise cost per part, it is essential to optimise dressing parameters and part of the PSP service is to adjust them to suit the application. Saint-Gobain Abrasives is unique in the UK for manufacturing not only grinding wheels at its new, world-class production plant in Eccleshall, near Stafford, but also reverse plated and infiltrated diamond roller dressers and CNC profiling discs at its facility in Staverton, near Cheltenham. As the company produces both diamond dressers and grinding wheels, it is uniquely placed to deploy experienced engineers familiar with both disciplines to achieve the best results from a PSP assessment.
All of the group's grinding wheels are designed and produced to stringent quality standards to guarantee operator safety. They are tested and certified to EU standard EN12413, which stipulates safety requirements for bonded abrasive products. Saint-Gobain Abrasives is an active member of FEPA, the Federation of European Producers of Abrasives and of oSa, the worldwide Organisation for the Safety of Abrasives.
Press release issued by Saint-Gobain Abrasives (Norton) on January 28, 2013
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