Tools of the trade

To satisfy environmental demands for less CO2, aero engine manufacturers are being pushed to make parts that fly higher to provide a leaner fuel burn. However, this means components get much hotter and, consequently, the increasing adoption of materials such as heat resistant super alloys (HRSAs) and advanced titanium alloys.

For engine parts that require turning, ceramic inserts have been capturing the attention of machine shops in recent times, and among the latest developments is the Bidemic series from NTK (www.ntkcuttingtools.co.uk) which feature the company's new JP2 turning grade. According to NTK, JP2 is capable of machining at 15 times the speed of high end carbide insert grades.

This series of coated, multi-tipped brazed inserts can run at a surface speed in excess of 500m/min, and are said to overcome previous concerns about the edge chipping of ceramic grades when finish-cutting Inconel. This is credit to a TiN coated insert that has a strong brazing to withstand high temperatures during machining. It is suitable for machining at cutting depths of 0.1 to 3mm on materials that include Inconel, Rene, Waspaloy and Hastelloy.

Of course, each engine component and material presents its own turning challenge. A case in point is a turbine disc made from Inconel 718, Waspalloy or Udimet 720. Here, Sandvik Coromant (www.sandvik.coromant.com) says that the selection of trochoidal turning for roughing operations over the more conventional ramping method allows productivity to be doubled (due to fewer passes) with the added benefit of enhanced security and tool life. Here, another ceramic insert grade is recommended, the company's GC6060.

Hot stuff, cool jets

Due to high temperatures in the cutting zone when turning HRSAs, the successful deployment of coolant is linked to its accurate delivery. According to Sandvik Coromant, the secret to using high precision jet nozzles is to position them directly at the cutting edge. This allows the operator to create a parallel laminar flow which helps lift the chip, reduce contact length and create a hydraulic wedge to actually break the chip.

Seco Tools (www.secotools.com) concurs with this thinking, stating that HRSAs are difficult chipping materials.
“In addition, the heat-resistant materials themselves are poor conductors of heat,” says Spencer Adams, technician at Seco's UK technical centre. “Temperatures in the cutting zones can often reach 1100-1300˚C, which if not dissipated quickly can cause reduced tool life and even workpiece deformation.

“In addition to deploying sharp-edge cutting tools, the use of high pressure direct coolant can help increase productivity. If the cutting speed for an HRSA material is 50m/min, this type of coolant system can permit cutting speeds as high as 200m/min, and thereby quadruple output.”

Seco's latest Jetstream Tooling Duo technology is designed specifically for turning titanium and HSRAs, and features the introduction of additional and specifically-positioned coolant jets aimed at the cutting zone. The upper jet directs coolant to the optimal point of the rake face, while additional jets flush the clearance surface.

Elsewhere, US-based cutting tool specialist, Korloy, is reporting good results using its VP chipbreaker turning insert with PC5300 grade substrate. The inserts, which are available in the UK from Cutwel (www.cutwel.co.uk), are said to offer oxidation resistance and hardness at high cutting temperatures to prevent common failures such as wear, chipping and built-up edge.

Using a CNMG-style insert for the internal and external turning of a turbine ring supporter made from Inconel 628, a customer trial showed tool life could be extended by 25% when using PC5300 against the incumbent competitor grade. This was achieved with a cutting speed of 50-80m/min, 0.25mm/rev feed rate and 0.2-0.7mm depth of cut.

Thrill of the mill

So, what of milling? Well, many of the same principles and technologies apply. For instance, NTK says its SX9 ceramic insert is the company's toughest milling grade to date, offering machining beyond 800m/min. It has been created specifically for milling Inconel 706, 713 and 718.

At Sandvik Coromant, a specific application is offered for using ceramics: consider rough milling around an external boss on an HRSA turbine casing using a mill-turn machine. Here, the company recommends its CoroMill 300C ceramic cutting tool, which again, like in turning applications, is able to offer much higher resistance to heat than carbide.

WNT (www.wnt.com) is another tooling specialist reporting good milling results on aero engine materials. The company's Dragonskin-coated HCN 5235 and HCF 5240 inserts, for example, offer geometries that present a positive rake angle to the material, which is crucial when machining materials with high chrome, nickel or titanium content in order to achieve accuracy and surface quality.

In a customer trial, HCN 5235 inserts mounted in an 80mm diameter A2700 face mill showed impressive results when cutting heat-resistant X15CrNiSi20-12 without coolant. With 1mm depth of cut, 210m/min surface speed, 0.15mm/tooth feed rate, and 60mm cut width, the machining time was reduced by 40% while tool life increased by 50% to 11.7m of cut length.

Over at Walter (www.walter-tools.com), recent focus has been on titanium milling, where the company's M3255 tangential porcupine cutter is able to offer rapid removal rates thanks to its monobloc design coupled with full effective teeth. With a flute design on a 27⁰ helix that features adjustable radial coolant outlets, the M3255 can both perform both shoulder milling and full slotting. Walter's latest four-edge Tiger·tec Silver inserts in WSP45S grade are recommended.

Another titanium-specific insert to arrive on the market lately is the KCSM30 grade from Kennametal (www.kennametal.com). Featuring a fine-grain carbide substrate and TiAlN PVD coating, the grade is said to deliver cutting speeds up to 70m/min. This is thanks in part to Kennametal's proprietary Beyond Blast composition, where integral coolant channels deliver coolant through the insert to the cutting edge (under the chip).

Moving to solid carbide milling cutters for HRSAs, Seco's Jabro 780 range is designed purposely for materials such as Inconel, Nimonic and Waspaloy. Among its novel design features is differential flute spacing, which causes tooth impact to be non-uniform, helping reduce vibration and chatter.

When machining the latest aero engine materials, any tool wear increases both the cutting forces and work hardening of the component surface, which could lead to possible cracks during operation. With this in mind, recent focus at Guhring (www.guhring.co.uk) has been on finding the optimum combination of carbide grade and micro/macro geometry, as well cutting strategies to avoid vibration.

The latest result from this research is the Guhring Diver Milling programme. As a consequence of optimised flute geometry, ramping is now possible up to 45°, as are hole depths of 2xD. The company says that the drilling of these materials is difficult as the cutting and guiding areas are in constant contact with the hole surface. However milling cutters enter and leave the material with each revolution and can therefore cool down. It's a salutary point, and one that's indicative of technology area determined to provide genuine competitive gain for engine part machine shops everywhere.