Making the cut

The need to increase energy efficiency is a major driving factor in the development of new aircraft and is leading to the ever more widespread adoption of materials such as CFRP, titanium and combinations of CFRP and metal. All combine excellent mechanical characteristics, making them ideal for many applications in the aerospace sector where weight is a key concern. Unfortunately, these materials share less desirable characteristics in that they are all difficult to machine accurately and efficiently.

Consequently, some manufacturers of cutting tools have, either through lack of resources and expertise or simply as a commercial choice, decided to concentrate their efforts on other markets they perceive as being easier to address. The result is that the availability of machining solutions developed specifically for these new materials is limited, meaning that the wider adoption of these materials is progressing relatively slowly despite the benefits they offer.

Fortunately, this situation is changing as some of the most progressive suppliers of cutting tools are devoting considerable effort and money to developing efficient machining solutions specifically for the aerospace sector. In addition to the technical challenges involved, there is also another characteristic of the aerospace industry that makes tool development more than usually challenging – the high value of components.

In some cases when a tool is being developed, it's easy to trial it under real production conditions. The tool is temporarily installed on the production line and if it fails to perform to expectations, all is not lost. The aerospace industry is very different as the components generally are so expensive that it would be unacceptable to work on them with an unproven tool.

To address this problem, tool suppliers work closely with their aerospace customers and undertake tests on specimen materials, both in their own facilities and their customers' sites. The effectiveness of building a strong relationship between tooling supplier and aerospace customer is amply confirmed by some of the remarkable results now being obtained. Consider for example, the machining of CFRP and similar materials.

During machining, protruding fibres and those torn out of the material must be avoided along with delamination. In addition, since the machining of these materials is almost always performed dry, the risk of excessive heating must be taken into account as it can adversely affect tool life and damage the material by, for example, burning the resin.

A frequent machining requirement for CFRP is drilling rivet holes, an application where consistent high quality results must be combined with long tool life. This is now being achieved with specially developed solid carbide drilling and counter-boring tools with diamond coatings. Using these tools, chatter marks on the countersink and delamination at the exit are prevented and high accuracies are achieved.

In one application involving bore diameters from 4.1 to 5.47mm, tools supplied by Mapal are achieving a bore tolerance of 8 microns at a cutting speed of 85m/min and a feed of 0.08mm. In other applications, equally impressive results are being produced by solid carbide drills with special geometries.

For trimming CFRP, various solid carbide milling tools have been introduced that are normally used with parallel feed. The highest quality surface finishes can be reliably achieved by using PCD-tipped end milling cutters as an alternative to standard routers. PCD tipped end-mills with multiple blades are particularly resistant to wear.

Two solutions are proving very effective for milling titanium. For finishing, a replaceable head milling cutter with a special type of connection provides optimum torque transfer and high reliability because of low radial runout error. With the particular tool arrangement adopted, the replaceable milling heads are easy to fit, and the torque-transfer connection can be used to compensate for radial runout errors.

For titanium roughing applications, ISO tangential milling cutters are available that achieve high material removal rates. For example, on a fuselage/wing joint in titanium Ti6Al4V with a cutting depth of 75mm and a cutting width of 25mm, the removal rate was 315cm³/min. In this example, the cutting speed was 70m/min, and the feed 0.12mm. Multi-bladed reamers are also available for use with titanium.

The current pressures to increase the energy efficiency of aircraft are never going to disappear, which means there will be increasing emphasis on the novel materials. This in turn means the most successful aerospace manufacturers will be those who develop efficient ways of working with them.

Fortunately, help is at hand in the form of tool manufacturers like Mapal that have invested in developing effective machining solutions for new materials. Maybe now would be a good time to discover what is on offer and start adopting tooling solutions every bit as innovative as the materials they are designed for.

www.mapal.com