Innovations have changed the use of coolants
Coolants have played a role in metal-cutting for many decades, mainly as a means for evacuating heat away from the cutting zone and the component. In the past, cutting tools broke down rapidly if not cooled but today machining in general is not dependent upon coolant supply thanks to the high hot-hardness of modern tool-materials. Dry machining is often a better alternative.
But coolants still play a vital role for lowering temperatures when machining some demanding materials, evacuating chips from internal turning and stabilizing heat-levels in some operations. Coolant at high pressure has now also been developed into a means with which to boost productivity in a broad range of applications in the form of a variable chip-former.
The growth of cemented carbide tools have provided considerably higher hardness in tools for coping with higher temperatures, leading to larger capacities in metal removal rates and longer tool-lives. Coolants have also become seen a being more detrimental than advantageous, because flooding the cutting zone was not enough as in the hotter zones the coolant evaporated and did not have much effect. In many milling operations, coolants were beginning to be seen as negative because of the boost to heat variations because of the intermittent insert-engagement. This caused temperature fluctuations and stress in the cutting edge material, with premature wear. Coolants came more to play the role of keeping heat accumulation down in components during and after machining, which meant a more stable level in temperature and dimensions.
Sandvik Coromant had at an early stage been involved with making use of coolants in deep-hole drilling. Having invented the Ejector drill by 1970, with twin tubes for coolant supply and chip evacuation, using the venturi-effect. This is a jet effect where the velocity of the coolant increases as the cross-section area decreases. Coolant is pumped in between the outer and inner tube and the negative pressure which is then generated by the venturi-effect, in the front section of the inner tube, leads to that the coolant is partly sucked out through the inner tube, along with chips. So, innovatively using coolants at pressure was nothing new.
More development work was done during the seventies and in the early eighties, jets of high-pressure coolant (HPC) were applied for turning demanding materials for manufacturing oil-industry parts in 1984 with the focus on breaking chips in tough materials. In 1985 and thereafter, jets of high-pressure coolant were applied to some success in several operations in the aerospace industry. Although, very much at a development stage, important experience was gained and when modular tooling was introduced in the form of Coromant Capto in 1990, it led to set-ups with quick-change tooling having internal coolant-supply and thus a natural part of HPC. With this tool-system, machine-tool builders were persuaded to get on board and make available HPC in their machines. External tubes and conections could be eliminated.
During the nineties, R&D in high-pressure jet-assisted turning was carried out on an increasing scale, some of it in collaboration with academic research. One of the first objectives was to break aluminium chips in turning. One finding with HPC-assisted turning was that the temperature in the cutting zone was considerably lowered – more than anticipated. This led to the conclusion that HPC would be useful to develop for other much more demanding materials and to use the lower zone-temperature to increase cutting data, and thus productivity.
In 1999, at the Third International Conference on Industrial Tooling, a paper was presented jointly where developments based on sponsored research using coolants at pressures between 400 to 3000 bar were detailed. The emphasis was the effect on chip formation, temperature and tool-life. Results were revealed showing that the technology had a potential for improving productivity in operations through both cutting speed and feed. Toward the end of the nineties, some machine tool makers also began taking an interest in HPC machining. One example was an Italian machine company who built special-purpose machines for the automotive industry, where performance in turning un-alloyed steel was improved through HPC at 80 to 450 bars using Coromant Capto.
In parallel with this, development was ongoing to optimize the role of HPC-jets in breaking chips and extending tool-life in turning titanium and metals such as Inconel 901. This was again based on Coromant Capto and involved pressures ranging from 100 to 1000 bar, with coolant volumes of two to fifteen litres per minute. This was in comparison to that of conventional coolant pressures of half-a-bar up to 80 bar, with coolant volumes of thirty to forty litres per minute. Coromant Capto was the only machine interface that had the possibilities to include seals needed for ultra-high pressures. Although the all-important nozzle-technology for perfecting jets, and the targeting at the cutting edge, had not been perfected, considerable headway was made at this stage in achieving longer tool-life at higher cutting speeds and with shorter chips produced.
When a coolant-jet penetrates the cutting zone, a hydraulic wedge is formed between the cutting edge and the chip.The pressure of the wedge results in a force that acts as a lever on the chip, changing its formation and the contact-length. Varying the position and pressure of the jet along the width of the chip alters the force positions, and adding more than one jet results in an accumulative effect on the chip and heat management. It became clear that the importance of understanding the coolant-wedge shape, position and effects were crucial to achieving the right performance in HPC machining. Thus, nozzle technology became a priority development objective because it was concluded that the key to success was the combination of impact position and the type of jet applied. These findings were to found the basis of coming developments into future standard and engineered HPC tooling for both turning and milling using low as well as high pressure.
The Sandvik Coromant developments in ultra-HPC (80 to 1000 bar coolant pressure from the jets) were introduced as the Jetbreak concept. It was based on Coromant Capto because this was the only machine interface with which it was technically possible to seal coolants at such pressures without risking the function and stability of the coupling in the machine. It was also the only system where it was possible to apply four channels and jets. And the foundation of quick change of tools.
Researched since the 1980s, the perfected HPC-system matured to be widely marketed at the start of the new century. Jetbreak was particularily directed at aerospace and energy industry-applications on vertical lathes with a ram as a problem solver as well as a productivity booster. It provided temperatue and chip control as well as improved component-quality. Difficult-to-machine materials and operations with chip-control problems were the main targets in various machines where turning was performed.
Nozzle technology played a vital role in optimizing performance and a range of nozzle diameters were introduzed for jet pressure, flow, velocity and momentum to be balanced. In this way, chips could be formed through a “variable chip-former”, where chips could be guided in desired directions. Super-alloys, titanium and very ductile materials were materials where Jetbreak really improved machining. Quick change of tools did also become a vital part of this concept, and that it was secure enough for unmanned production. In addition, engineering and application support was an important, contributing factor. Typically, Jetbreak can today make it possible to double cutting speed (say, from 50 to 100 m/min when turning Inco 718) thanks partly to the cooling effect in the cutting zone. Jetbreak, however, was limited to vertical turning lathes on static applications leaving an untapped potential for turning centers and Multi-Task machines.
The situation as regards pressurized coolant-supply in machinery is very different today. About a quarter of all lathes have coolant pressures of 70 bar while nearly all multi-task machines have HPC and there is a clear trend for coolant-pressure levels to rise in machines. With HPC technology being available also in shank-type tools, practical obstacles to using HP-technolgy have all but disappeared.
With this backgound it has been a logical development for HPC-tooling to be made available for broader application and more machines and as part of a standard tool programme – complimentary to the range and applications of Jetbreak. Sandvik Coromant has developed and introduced a programme of tools, easily applied, aimed for turning in machines that can supply up to 80 bar of coolant pressure. But the CoroTurn HP standard concept for external and internal turning is also advantageous in machines that can only supply lower pressures. The HP-tools have fixed-nozzle technolgy that give parallel, laminar jets with high velocity, accurately directed at the right place on the insert in the tool. The precision and properties of the resulting jets make the difference and no setting is required.
When machines have coolant pressure of 70 bar or more, the advantages of HPC-tooling can be made use of more fully. Lower coolant-pressure still has a positive effect with the standard-range tools but the full potential is not reached. Today further innovation has boosted HPC-possibilities in that insert geometry has been studied to further enhance the effects.
Completely new innovations are the dedicated indexable inserts that have been developed to make the most of turning with HP-coolant. Conventional inserts do not make full use of the coolant-jet technology so HPC-inserts, to be used with the toolholders, were motivated. The new CoroTurn HP inserts get the most out of the pressurized coolant in helping jets to penetrate the cutting zone. They also overcome some of the negative effects that HPC can have when using some conventional insert-geometries. Working with coolant pressures of 30 bar and above - with an optimum effect at 70 bar and above – HPC-turning, never experienced before, takes place, adding advantages, when using the combination of HP-nozzle technology and dedicated inserts.
The chipbreaker-geometry on the specially-developed inserts has been designed with an edge rounding that gives equally good performance for medium and finish turning. The geometry has been developed to “collect” the coolant-jets to form the most effective hydraulic wedge. This has extended tool-life as well as added control of chip-formation on a unique level in applications involving not only turning very ductile metal, stainless steel and superalloys but also to provide adavantages when turning more broadly used in most alloyed steels.
Successful titanium milling was, during development and practically in industrial applications, found to be dependant on coolant being used – the more qualified the application of coolant, the better the performance. HPC-application at 70 to 80 bar pressure provided optimum advantages for milling. Consequently, as pressure-coolant is a standard feature on many of today`s machines with rotating tools, it is is a potential resource to optimize especially the many radial-milling operations on titanium components.
In the development of CoroMill 690, an advanced indexable-insert, long-edge milling cutter dedicated for titanium milling, coolant channels and nozzels were designed to provide HPC-effect at each insert. With several inserts making up each radial edge along the cutter, coolant nozzles were positioned so to provide optimum benefits. In cases where the whole axial depth capability of the cutter is not used, plugs can be used instead of nozzles, in that way avoiding unwanted coolant jets. Supplying the right volume of coolant is a factor in HPC, especially for the numerous nozzles along the long-edge cutter. For this reason, a dedicated calculator for flow and pressure was developed to provide the optimum data for jets at the selected nozzles based on machine, cutting data, axial cutting depth and tool specifics.
Verified tests and applications show that cutting speeds in demanding materials can be often be increased considerbly and tool-life can in many applications be increased by a half or more. Chip control improvement is the other major benefit that eliminates a lot problems in machining zones and on chip conveyors. Decades of Sandvik Coromant innovations and experience in machining assisted by high-pressure coolant jets have come to benefit numerous applications today and above all, not limited to demanding-to-machine materials.
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Sandvik Coromant
Sandvik Coromant is a world-leading supplier of cutting tools and tooling systems for the metalworking industry and is represented in 130 countries. 27 state-of-the art Productivity Centers located around the world provide customers and staff with continuous training in tooling solutions and methods to increase productivity. Sandvik Coromant is part of the Tooling business area of the Sandvik Group.
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