The benefits of using coolant to remove heat and lubricate (reduce friction) are well known; with coolant usually being applied by simply flooding the machining area. Although, for coolant to be really effective, it needs to remove heat quickly from the cutting zone, and a directed high-pressure coolant flow that puts coolant precisely where it is required is much more efficient.
Challenges in cooling the metal cutting process
There are several examples of challenges found at the cross-roads of coolant application and productivity demands in machining, but one of the more striking can be taken from the aerospace materials e.g. titanium alloy Ti6Al-4V. Ti6Al-4V alloy has low thermal conductivity and low modulus of elasticity, making it a suitable material for the high-strength, heat-resistant and lightweight parts in jet engines. However, it is notoriously costly to machine because required cutting speeds are typically quite low, chips are impossible to control and cutting tool life relatively short. Unbroken, long chips can result in chip jamming, subsequent tool failure and, in the worst case, damage costly parts. They can also scratch surface finishes and cause a valuable component to be scrapped.
Bird-nest-like tangles of chips generated by the metal cutting process cause significant productivity losses in attended production, making it almost impossible for machines to run unattended in lights-out operations. This particularly holds true when machining with toolholders or toolblocks with conventional coolant nozzles that are not located close to the cutting edge.
While significant advancements have been made in high-pressure systems, most manufacturers still rely on flood-type coolant systems that drench coolant over a cutting tool and the component being machined to reduce heat. In addition, more flexible coolant nozzles often move, making them inaccurate when it comes to directing coolant to the cutting zone. Such systems also lack adequate control for sufficient pressure.
As flood coolant just washes over the cutting zone, it can also heat up locally to a point where a steam vapour barrier forms. This vapour then actually insulates the cutting zone and keeps heat from dissipating. To combat this situation, high-pressure cooling systems can remove heat fast enough and with enough pressure to prevent such vapour barriers from developing.
Manufacturers must also keep in mind that there are differences between high-pressure coolant delivery tooling systems. The most common of those differences involve distance from the cutting zone, or how far away a system’s coolant outlet is from the workpiece/cutting tool interface. Some system outlets may not be close enough to effectively and accurately reach the optimum point within the cutting zone for the most benefit. Systems that incorporate coolant outlets situated further away from the cutting zone must use higher pressures to compensate for the increased distance.
If a system’s coolant outlets are too far from the cutting zone, additional pumps may be needed. Comparatively, this results in higher costs to achieve the same level of results provided by a system that has outlets closer to the cutting zone. When coolant is channeled through holders then through inducers, as with Seco Tools’ Jetstream Tooling® system, coolant outlets can be arranged in very close proximity to the cutting zone, achieving better results with pressure generated from a machine’s standard coolant pump. The need for a second high-pressure pump is thus eliminated.
Benefits of directed high-pressure coolant
It is to overcome the drawbacks of existing coolant delivery systems that developers of coolant delivery tooling systems, such as Seco, have worked to optimise system performance by boosting the pressure and precision with which coolant streams are directed into cutting zones.
Systems such as Seco’s Jetstream Tooling® incorporate strategically placed coolant exit holes machined into swiveling top clamps (inducers) on insert holders. Coolant pressure, flow and the small diameter holes are what enable the acute, high-velocity stream of coolant to easily penetrate and lubricate the primary heat zone just behind the cutting edge.
Seco has discovered that providing a coolant “wedge”, very close to the cutting edge, proves most effective. This means that the system’s exit outlet positions the jetstream of coolant between the insert rake cutting zone and chip, contributing to the lifting and breaking off of the chip.
The latest generation of Jetstream Tooling® for turning, grooving and parting-off provides holders with coolant outlets directed towards rake faces, but also towards insert clearances – the secondary heat zone. The coolant jet underneath provides optimal cooling just below the cutting edge. This extra coolant jet directed towards insert clearance increase tool life by another 10 percent and improves surface finish.
When coolant delivery tooling systems provide both cooling and optimised chip control, manufacturers also eliminate downtime and gain problem-free, lights-out machining capability. But just as beneficial, they are able to increase turning speeds and feeds, extend cutting tool life and improve the surface finish of the component – all because of advanced chip control. In some cases, speeds and feeds can be doubled or even tripled, and tool life can also often be doubled. This increases productivity and profitability by machining more parts faster.
Laboratory tests done at Seco with a titanium workpiece at a cutting speed of 40 m/min (130 ft/min), feed rate of 0.25 mm/rev (0.01 ipr) and 2 mm (0.08 in) depth of cut with flood coolant gave a five-minute cycle time. Researchers then applied a high-pressure coolant delivery tooling system to the operation and were able to increase the cutting speed to 80 m/min (260 ft/min) and reduce cycle time by half, as well as increase tool life by 100%.
Today’s machine tool coolant pumps generally provide between 20 bar (290 psi) and 70 bar pressure (1000 psi), which is a range more than adequate for high-pressure coolant delivery tooling systems such as Seco´s Jetstream Tooling®. Seco’s system for instance, offers the versatility to operate from low pressures with some productivity impact at as low as 5 bar (70 psi) to high pressures at 70 bar (1000 psi), and to ultra-high pressures 350 bar (5 000 psi) with the same system.
At between 20 (290 psi) and 40 bar (580 psi), significant productivity boosts with Seco Jetstream Tooling® can be seen as well as improved chip control in most applications and different workpiece materials. The performance increases with the amount of pressure and flow (l/min). And, 70 bar (1 000 psi) is quite adequate to break the most demanding chips formed from sticky, long chipping commercial materials.
Applications and industries
Turning, parting off and grooving operations benefit the most from high-pressure coolant. In these operations, the contact time between the cutting edge and the part is often a few seconds or longer and defined as “continuous cuts.” The continuous cuts generate high temperature in the cutting zone, and therefore, the cutting parameters need to be adapted to prevent rapid flank wear and plastic deformation. The combination of a continuous cut and a gummy, ductile material can produce a very long, uncontrollable chip that is highly undesirable.
Most industries can benefit from high-pressure coolant, but traditionally power generation, aerospace and medical areas have adapted to this trend the most with their many exotic materials where intense chip formation management is required.
For example, titanium, often used in aerospace and medical applications, is a poor conductor of heat, which causes high temperatures to remain in the cutting zone. This can lead to welding, galling and smearing, all of which can quickly destroy a tool’s cutting edges. To make matters worse, machining titanium produces thin, high velocity chips that are difficult to break into manageable lengths. These chips will also carry critical coolant away from the cutting zone, causing thermal damage to component surfaces.
Conventional tools with high positive rakes and sharp edges can minimise the detrimental effects caused by machining titanium, but high-pressure coolant delivery tooling is needed to control chips and optimise overall machining operations. University experiments and studies confirm that a rapid temperature reduction in a chip as it passes over the cutting edge invokes a hardening effect, much the same way as quenching a hot piece of metal very quickly. The hardness of the titanium chip is increased. The combination of the direction of flow, the wedge effect and pressure of the coolant against the chip with its increased hardness force the chip to break into small, easily managed pieces.
In general, it is better to run coolant when cutting any of the more challenging types of workpiece materials. Again, some of the best machining improvements documented resulting from high-pressure coolant delivery tooling systems have been with applications involving today’s exotic materials, such as titanium and nickel-based and cobalt-based alloys such as Nimonic C263, Inconel 718, Udimet 720 and Waspaloy. These materials are very gummy and ductile and thus require high levels of chip control attained by high-pressure coolant delivery tooling systems.
However the exotic alloys are not the only materials to show improvements when using high-pressure coolant delivery tooling systems. The automotive, marine, nuclear and food & beverage industries involve the use of ductile, long-chipping materials with rather low machinability. Austenitic and duplex stainless steel, low-carbon steels, and aluminium alloys, all show vast increases in metal removal rates, improvements to cutting data, chip control and surface finish, as well as reductions in production time.
As Seco Jetstream Tooling® eliminates chip evacuation issues, there is no need for operator intervention and therefore no disruption in production. Chip removal time is no longer part of the floor-to-floor time calculation. Additionally, the coolant inducer pivots of the Seco system allow machine operators to index a new cutting edge very quickly, guaranteeing that the coolant is where it was before – in exactly the right place.
To cool, or not to cool
It should be noted that some turning operations are better run dry without flood or directed high-pressure coolant. For instance, applications with intermittent or interrupted cutting conditions often generate thermal shock and benefit from running dry, as coolant can further magnify the negative effects of that thermal shock. In these cases, a high-pressure coolant tooling solution may circumvent the negative effects of coolant application.
Generally, continuous cutting conditions are typically best run with coolant. But where there are environmental issues, open-structure lathes or with certain types of steels, turning operations can be performed dry and with good results. Seco´s Duratomic® CVD-coated inserts with their effective heat resistance capabilities are well suited for this. Operations such grooving and parting-off, however, become more difficult when run dry mainly because of chip build up. In these operations, coolant – more specifically directed high-pressure coolant – can significantly improve performance.
Finishing operations often also should be run with coolant. Otherwise, shops can experience issues with component accuracy, as well as with surface finish quality. While most shops use coolant in up to 80% of their machining applications, some choose to run all of their operations dry e.g. for environmental reasons. Doing so requires closely monitoring and adjusting for heat generation within their machine tools, then keeping it in check to ensure component accuracy in terms of size and surface finishes. It is without doubt challenging and worth another tooling discussion, but fully possible today. Productivity and tool life reductions are significant when machining dry compared to machining with coolant.
The ideal system
When considering high-pressure coolant delivery tooling systems, manufacturers should evaluate systems not only based on their performance, but also on versatility and simplicity covering whole ranges of available coolant pressure levels in the machines. Systems should be easy to assemble and install into turning machines.
Ideal systems will also offer the choice of coolant being fed to a turning or grooving toolholder externally or internally. For feeding externally, systems such as Seco’s for shank toolholders, use hoses attached at the sides or underneath holders. Manufacturers can obtain a wide range of hose kits to easily connect the coolant supply at almost any position on a machine turret or toolblock. If the user no longer wishes to run the system, it can easily be removed and the machine restored back to its original coolant setup. For feeding internally, the system has channels within holders, as is the case when feeding coolant through the taper interface for Seco-Capto-style holders.
Manufacturers can obtain different hose lengths to connect the coolant supply at almost any position on a machine turret or toolblock. If the user no longer wishes to run the system, it can easily be removed and the machine restored back to its original coolant setup.
No matter what type of coolant system is incorporated, the key to effective chip control, tool life optimisation and increased production is first getting the coolant stream as close to the cutting zone as possible then directing it to the right place within the cutting zone. Jetstream Tooling® toolholders are a very minimal part of a machine investment in relation to the savings they provide in reduced cost per part. Any costs saved by purchasing a slightly lower priced low-pressure coolant system and tooling will be insignificant when compared to poor tool life and production losses due to machine downtime caused by chip build up.
*The terms “cutting fluid,” “coolant delivery tooling system” or “coolant” used in this article refer to the actual metalcutting process only, and no other machine tool lubrication functions.
Tommy Pitkänen, Product Manager Turning, Seco Tools
Seco Tools is a leading manufacturer of high performance metal cutting tools. Seco’s product range includes a complete programme of tools and inserts for turning, milling, drilling, reaming and boring as well as complementary tool holding systems. With more than 25,000 standard products, Seco is a complete solutions provider for the metal cutting industry and equips machine tools from the spindle down to the cutting edge.
The company is headquartered in Fagersta, Sweden and represented in more than 50 countries worldwide with 40 subsidiaries, distributors and channel partners.