A lightweight machining challenge

Lightweight materials used in aerospace can, for the most part, be divided into five categories: thermoplastics, carbon fibre reinforced thermoplastics, fibre-composites, honeycombs, and composite stacks that incorporate layers of both plastic and metal. All of these materials are challenging to machine, especially with the precision and repeatability that's needed in aerospace applications. The machining challenges and their solutions are, however, specific to each material type.

Thermoplastics, such as PEEK, POM and PA6, are relatively inexpensive and are easily moulded into complex shapes. However, compared with most other aerospace materials, they have a low melting point and low thermal conductivity. When carrying out machining operations on these materials, therefore, it is essential to minimise the amount of heat produced by the cutting tool, otherwise the material may locally exceed its melting point, leading to distortion of the workpiece and smearing of the machined surface.

To minimise these problems, the use of solid carbide cutting tools is recommended. These must have very sharp cutting edges so that the material is cut cleanly with a minimum of drag and friction. In addition, very specific tool geometry is required to ensure that chips, which carry away most of the heat generated by machining, are discharged easily and cleanly. When using drills that meet these requirements, bore tolerances up to IT7 can easily be achieved, even in the most difficult of materials.

Thermoplastics reinforced with carbon fibre present all of the same problems as plain thermoplastics with the added challenge that the inclusion of very abrasive carbon fibres greatly exacerbates tool wear. To ensure consistent performance with these materials and good tool life, tools with diamond-coated cutting edges are recommended. Correctly designed, these will minimise the risk of heating and distortion of the thermoplastic component of the material, and will also cut the fibres cleanly without burr formation.

Dealing with delamination

Fibre composites such as CFRP differ from carbon fibre reinforced thermoplastics because the matrix that carries the carbon fibre is usually made from resin which does not soften or melt at high temperatures. While this reduces some of the machining challenges, there remains a significant risk of delamination, particularly with the thin parts that are often made from these materials.

For working with fibre composites, diamond-coated tools are strongly recommended as, provided they are properly selected, these will give a consistently clean cut combined with good tool life. Milling cutters for use with fibre composites should also be designed so that they give a simultaneous pulling and pushing cut which produces a compression effect in the part. This minimises the risk of delamination and fibre projection.

Honeycomb materials are not only lightweight but also have great flexural strength. Their principal aerospace application is to provide support and reinforcement between the inner and outer skin of aircraft. The relaxed inhomogeneous structure of these materials makes them hard to machine, but for end milling at least, an effective solution has now been developed.
This takes the form of an eight-blade end cutter with extremely sharp cutting edges, a helix angle of 15° and very fine serration. In exhaustive tests, this new cutter has been proved to reliably machine even those honeycombs that have a variety of outer layers and fillers.

Multilayer composite stacks are a popular choice for aerospace applications where composites alone cannot deliver the required mechanical properties. Examples are skin assemblies round cabin doors and in the wings where the loads from the engines are applied. The stacks most often used are made up of layers of CFRP and aluminium, layers of CFRP and titanium or, in some applications, all three.

Cutting tools designed for use with composite stacks must meet the specific machining requirements of each of the materials used in each of the layers, something that is difficult to achieve as these requirements often conflict to a greater or lesser extent. The machining characteristics of the composite layers are inevitably completely different from those of the metal layers.

The hole picture

When drilling composite stacks, one way of addressing the difficulties is to drill the holes in several stages, starting with a small diameter bore and then increasing this in stages with further drilling operations. This approach is capable of producing good results but it adds significantly to production times and costs. This is particularly important in airframe components that may have thousands of holes.

Suppliers of cutting tools, including Mapal, have been working for many decades to provide a better solution and this has culminated in the introduction of replaceable head drills and reamers that have been shown to deliver excellent results on composite stacks. The replaceable head format means that these tools are considerably more cost-effective than monoblock tools.

Both the coating and the geometry of the tools varies according to the composition of the stack and the direction of machining but, with the latest optimally designed tools and CFRP-aluminium stacks, bores can be drilled in a single operation from solid and produce H8 quality in all of the hole sizes normally associated with airframe construction.
Development work on tools that will deliver similar results from one-shot operations with CFRP-titanium stacks is at an advanced stage.

The use of lightweight materials in the aerospace sector will undoubtedly continue to grow. While these materials are by no means easy to work with, many of the challenges associated with machining them have been solved. The key to achieving the best results is in reality very simple: work with an expert tool supplier that has proven experience in this challenging field.

www.mapal.com/en