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.
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.
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.
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 firstname.lastname@example.org.