Turning operations, whether performed on vertical turning machines or multi-task machines, depend for results on every link in a technology chain that stretches from machine spindle to cutting edge.
Profile and groove turning in heat resistant super alloys (HRSA) and titanium machining of components for aero engines are among the most difficult machining operations. As such, raising performance has proved very challenging in recent times. Strict aerospace industry specifications, the nature of the materials and component configurations often meant that sticking with the existing solution was the best bet.
Components such as discs, spools, vanes, shafts and casings comprise tricky-to-machine features requiring proprietary cutting edge shapes, extensive tool reach and the right tool paths. Confined spaces with restricted access, often with thin walls, have necessitated the use of specialized tools with little or no flexibility.
With projections of a tripling of aero engine manufacturing in the coming 20 years, the heat will be on for competitiveness in machine shops. A new approach in tackling production challenges is therefore critical, not least for turning feature-intensive components in HRSA and titanium.
The machine-to-edge chain of technology can now be said to be complete, aided by some of the latest developments in tool holding, tool concepts and indexable insert technology. Limitations on productivity, insecure elements in the process, and issues to do with cutting tool availability, delivery and cost issues can now be tackled more successfully and new practices established.
Tool holding at the machine spindle is the first link in the chain. It is not unusual for machining to encounter vibration if there is any instability at this interface. If not before, this will come to light when demanding materials and long tool reaches are involved. Here, it is essential to use an ISO standard coupling that will offer maximum rigidity in turning, the strength to transmit power through the coupling and a high clamping force. With the tapered polygon coupling of Coromant Capto these essentials are fulfilled, providing an option that is universal for all applications. Key performance factors are high bending strength, torque transmission, precision and the availability of coolant at high pressure.
Tool modularity is the next link from spindle to cutting edge where a vital factor is the assortment of integral cutting tools, the facilities for extending tools and varying the tool size to further operational flexibility. Coromant Capto, represents a system suitable for the entire machine shop with six different sizes to suit applications from heavy duty through to high-speed machining.
So, apart from the ultimate in stability and versatility, how does the extension of this modular system further provide the means needed for turning the large variation in features found on aero-engine components? The next link in the chain provides the answer: the SL70 is a compact but extremely strong interface between the modular adaptor and indexable insert toolholder.
An oval, serrated-face type coupling, with extremely high stability, forms the basis for this system, with the concept being built on a modular adaptor and tool blade. In being able to utilize the capabilities of today‘s CNC machinery it provides the link to a standard range of cutting edge solutions for turning any of the standard grooves and profiles found on aero engine components.
An understanding of the typical, common geometrical features that occur in these components have provided the basis for developing the CoroTurn SL70 concept with tool blades presenting the cutting edges at different angles and overhangs. Having built-in dampening for ensuring performance at extended tool reach, these blades turn features in deep grooves often at some 20 per cent higher cutting speeds with tool life extended by some 50 per cent.
The next link – high pressure coolant – is a standard feature of modern tool concepts and a vital means with which to achieve improved performance in HRSA and titanium turning. Many machines are capable of delivering coolant pressures of 70 to 100 bar. Modular tooling with well connected internal coolant channels are the basis of the standard CoroTurn HP as well as the more specialized ultra-high-pressure system, Jetbreak® (up to 1000 bar).
Coolant jets, controlled by nozzles, are directed at the cutting zone creating a hydraulic wedge between insert and chip that reduces the temperature and facilitates chip flow and evacuation. The effects of high pressure coolant provide a potential for increasing cutting speeds in HRSA and titanium machining without any consequential rise in temperature at the edge. Internal cuts, such as those required in many aerospace components, benefit additionally from high-pressure coolant application in ensuring good chip shearing properties, chip formation and evacuation from tight pockets and grooves.
The indexable insert is the next link in the chain. Here also, recent developments have moved performance forward considerably for HRSA and titanium machining. Rapid notch wear is a common destructive factor in HRSA turning and is at its worst when using a tool having the edge perpendicular to the direction of feed – a 90° angle of entry. A round insert cutting edge gives a variable angle from zero to a maximum of 45° (if the depth of cut is smaller than 15 per cent of the insert diameter), and this lends itself well to various pocketing operations and for minimizing pressure on thin walls.
Consequently, the main range of solutions involving the SL70 tool blade and adaptor system use round-edge inserts, either in the form of CoroCut profiling inserts or as round CoroTurn inserts. With cutting edge geometries like SM and RO, profiling and grooving cuts can be optimized as regards chip control and cutting forces.
The tool material is the penultimate link, in the form of the insert grades for the turning tool. Machining aerospace materials requires dedicated tool materials such as cemented carbide and ceramics. Uncoated carbide grades remain important but the latest developments in insert coating technology has created CVD and PVD inserts, which have progressed cutting edge capability.
Three different examples of the latest insert grade developments, which have improved HRSA and titanium turning performance, are cemented carbide grades S05F and GC1115, as well as the ceramic grade CC6065.
The first, S05F, is a new CVD-coated grade with fine-grained substrate to retain high hardness at elevated temperatures. This provides the capability for higher cutting speeds and longer tool life. The second grade, GC1115, is a broad, all-round PVD-coated grade that provides the toughness needed for sharper cutting edges and thus for many operations needed in aero engine components. The third grade, CC6065, is a new, broad Sialon-type ceramic that offers a high degree of notch resistance and is therefore first choice for pocketing and profiling.
Application technology is the last link from machine spindle to cutting edge. Adopting a suitable machining strategy and applying the best programming methods are essential to turning HRSA and titanium components in a way that is effective by today‘s standards. Establishing the cutting edge approach to individual cuts and the most advantageous tool paths will affect several of the machining factors involved and consequently the outcome in cutting time, tool life, security and quality consistency.
In conclusion one must look at all seven links in the modern tool technology chain that have elevated HRSA and titanium turning to new performance and result levels. Ticking the boxes for each of these links as part of a machining strategy, to include the best standard solutions for standard component features, is the surest way to competitiveness in turning aero engine components.
Learn more about Aerospace Component Solutions from Sandvik Coromant.
Learn more about Turning Solutions from Sandvik Coromant.
Article originally published on www.theengineer.co.uk 6.2010.