The Best Tools and Techniques for Milling Aerospace Materials

By Scott Causey, Aerospace and Power Generation Segment Specialist

Composites, titanium alloys and Inconel are popular materials in aircraft part design because they offer exceptional strength-to-weight ratio and corrosion resistance, enabling planes to fly faster, further and carry bigger loads using less fuel. These advanced materials, however, present unique machining challenges, especially in milling, that can negatively impact part accuracy.

Because part quality takes precedence over cost per part and productivity in the aerospace industry, it’s important to strive for process control and consistency through predictable performance of machines and tooling.

Specialized solid rotary mills offer excellent process control and efficiency. Through the incorporation of various innovative coatings and geometries used in tandem with the right machining techniques, these tools not only provide process security, but also increased production speed and output.  

Milling Composites

Solid-carbide milling cutters that are hard, sharp and uncoated as well as those with special CVD surface coatings are highly recommend for machining composites because they offer high wear resistance and help prevent delamination.  

From a geometry standpoint, effective cutters for composites incorporate low helix angles to reduce axial forces on the laminate layers of the material to prevent delamination. Cutters with both a left and right helix are also effective geometries. Often known as compression routers, they direct and compress cutting forces toward the centers of workpiece thicknesses – in the case of side milling – to keep the laminate layers intact. Plus, these cutter geometries make for much freer cutting of composites.

While compression cutters are a common approach, some cutting tool companies, such as Seco, have developed compression cutters with new different geometries, such as a double helix. Seco, for instance, developed two such double-helix routers. One is a multi-flute tool with smooth cutting edges. The other has fewer flutes, providing more chip clearance, and chip breakers on its cutting edges. The latter is more for roughing operations, while the former multi-flute option without chip breakers offers ideal performance for finishing operations.

Regarding machining techniques, cutting parameters for composites are often dependent on the various materials themselves. Typical speeds for solid-carbide cutters for composites are about 150 m/min, and feedrates are around .07 mm. But it should be noted that within this group of materials, there are a variety of different types of binders used, each requiring their own speeds and feeds. The melting points of these binders are often what determine speeds and feeds when cutting composites.  Also fiber content and fiber orientation have a significant influence on the machining process, governing cutting speeds and feeds and the optimum tool path.

Milling Titanium Alloys

While titanium can be machined with general-purpose solid-carbide cutters, those cutters designed specifically for the machinability characteristics of titanium will nearly always provide superior results. These special cutters provide extremely high levels of performance, but they can be less versatile when it comes to the number of different materials to which they apply.

For example, Seco has a high-speed steel (HSS) cutter designed for both titanium and Stainless steels. Cutters that are part of the Jabro HPM are specifically designed for certain material designations like titanium. These cutters incorporate special geometries and design qualities that have been optimized for titanium.

The geometries and design features include high helix angles between 40 and 50 degrees; internal coolant channels to quickly evacuate chips as well as cool the cutting zone; uneven tooth pitches for reducing vibrations during high depths-of-cut; and a combination of carbide with aluminum chromium nitride coating. No titanium nitride is used to prevent a chemical reaction between the cutter and material.

Cutter diameter dictates when to use a solid-carbide tool or an HSS tool. Solid-carbide tools should be used on applications requiring smaller diameter cutters or those involving complex work piece geometries. They should also be used when incorporating HSM strategies and if L/D ratios pose a problem. 

HSS cutters are recommended for less-complex workpieces in high-volume applications and when both large AEs (width of cut) and heavy APs (depths of cut) are the goal. The tools should also be considered when older conventional machine tools with high torque and high horsepower are being used.

Milling Inconel

Inconels (nickel-based superalloys) are the most difficult materials to machine. They have very low thermal conductivity and very high levels of strain hardening – higher even than those of titanium. Inconel also has high adhesion, so cutting speeds can rarely exceed 25 or 30 m/min when applied in a conventional machining method.

Cutter geometries for machining Inconel differ greatly from those used for titanium. Inconel geometries are angular relieved with very steep angles. Such geometry reduces contact between the cutter and material as much as possible. This is critical because Inconel is flexible and has a high memory, meaning it will “give” somewhat when subjected to the forces of a cutting tool. So the longer the contact time between the cutter relief and material, the higher the abrasive wear on the tool and the shorter its working life. To further reduce the friction between cutter and Inconel, Seco incorporates a coating of aluminum titanium nitride that is polished to an extremely smooth and fine surface finish.  

Conclusion

Part quality and process security require the best possible tool designed for the particular application at hand, whether it be composites, titanium or Inconel. But that tooling must come from a supplier able and willing to provide guidance as to the proper way to apply it for optimum performance. Training is key to getting the most benefit out of today’s advanced tooling designed for tough aerospace materials. You’ll find all of this and more when you work with Seco!

About the Author

As Seco’s Aerospace and Power Generation Market Specialist, Scott is responsible for supporting the company’s aerospace and power generation segment customers, which includes optimizing current processes and defining new technologies. In his spare time, he enjoys spending time with his family and working with horses. Contact Scott at scausey@secotools.com.

Turn More Profit With Directed High-Pressure Coolant Tooling

By Don Halas, Product Manager – Threading & MDT

Seco Jetstream Tooling System

If you’re turning parts out of materials that are poor conductors of heat, such as titanium or superalloys, ask yourself this: Is your method for removing high temperatures from the cutting zone generating increased productivity and profitability?

Flood-type coolant systems that drench the cutting tool and part help minimize temperatures, but do little to maximize operational efficiencies. To make your process as effective as possible, you need to get your coolant exactly where it needs to be as quickly as possible, which is achievable via a direct high-pressure coolant delivery tooling system.

Such a tooling system hits closer to the cutting zone and directs itself towards the workpiece/cutting tool interference, achieving both cooling and optimized chip control. As a result, you can eliminate downtime and gain problem-free, lights-out machining capabilities. But just as beneficial, you can increase turning speeds and feeds, extend cutting tool life and improve part surface finishes – all because of advanced chip control. In some cases, you can double, even triple, your speeds and feeds, while still extending your tool life by 25%.

When choosing a direct high-pressure coolant delivery tooling system, it’s important to understand the differences between those that are available. The most common 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 have coolant outlets situated further away from the cutting zone must use higher pressures to compensate for the increased distance.

Seco Jetstream Tooling System

Plus, if a system’s coolant outlets are too far from the cutting zone, you may need additional pumps. 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 our 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.

Solutions such as our Jetstream Tooling System incorporate strategically placed coolant exit holes machined into swiveling top clamps (inducers) on insert holders. The small diameter apertures of the inducers are what generate the acute, high-velocity stream of coolant that easily penetrates and lubricates the high friction zone between the workpiece and tool’s cutting edge.

Additionally, before you consider a high-pressure coolant delivery tooling system, you should evaluate the system not only based on its performance, but also its versatility and simplicity covering whole ranges of available coolant pressure levels in your machine. Systems should be easy to assemble and install into your machine.

Ideal systems will also offer you the choice of coolant being fed to a turning or grooving toolholder externally or internally. For feeding externally, systems such as our Jetstream Tooling System use hoses attached at the sides or underneath holders. For feeding internally, the system has channels within holders, as is the case for Capto-style holders.

You can obtain different hose lengths to connect the coolant supply at almost any position on a machine turret or tool block. If you no longer wish to run the system, it can easily be removed and the machine restored back to its original coolant setup.

In the end, no matter what type of coolant system you incorporate, the key to effective chip control, tool life optimization and increased productivity is first getting the coolant jetstream as close to the cutting zone as possible, then directing it to the right place within the cutting zone.

About the Author

As Product manager for Threading & Grooving, Don is responsible for threading, threadmilling, cut-off, grooving and oil field chasers. In his spare time, he enjoys restoring old motorcycles. Contact Don at dhalas@secotools.com.