Future of hole-making

February 25, 2014 7:33 am

With tool costs playing a significant role in making holes in CFRP and metal stacks, a focus on the total cost per hole is all the more important.
Fig. 1: In the manufacturing of composite and metal stack aerospace components, focus is needed on the total cost-per-hole. This takes into account the time needed for tool change, the tool cost and above all productivity, where security, tool-life and performance are decisive. Sandvik Coromant and Precorp provide solutions and support to optimise hole-making.
Today, a third of all hole-making is done with manual units. About a quarter is done in machine shops using CNC machinery or robots whilst around 40 per cent is carried out using power-fed equipment ADU (auto drill units). But through aerospace harmonisation programs, there is a clear, ongoing shift to the more-controlled ways with manual hole-making becoming limited to some parts of assembly.
With the more-controlled, stable and consistent ways of making holes, using power-fed equipment and CNC machineries, the potential of tool-design and material much more suitable for carbon-fibre reinforced plastic (CFRP) and metal-stacked components can be utilised. Precision holes need precision methods and equipment.
The modern cutting tool polycrystalline diamond (PCD) — either as an integral part of the drill or as a coating — is extremely well suited to cutting composites, providing long tool life, high productivity and high hole-quality consistency. Moreover, diamond-coated tools have evolved to achieve an increasingly higher percentage of the tool-life offered by solid PCD bits. Even cemented carbide can offer a preferred solution for more controlled machining ways when a very sharp cutting edge is needed. This can be easily achieved with a ground, fine-grained uncoated carbide edge.
The cost per hole in composites is highly dependent upon working solutions that are consistent in performance and results. To achieve this, one needs to utilise the resources which dedicated, specially designed hole-making tools and methods will provide. Engineered tooling is a substantial success factor in composite hole-making.
Composite-component manufacturers are mostly concerned with the challenge that tool life and delamination present. This is followed by hole roughness and splintering as well as dust management. When CFRP stacked with metals, aluminium and/or titanium, chip control, burr height and hole-tolerance are additional issues. However, there are issues with a number of operations needed to make the hole and the time to change tools. As demands rise in manufacturing, having a competent partner to work with also become an issue.
What are composites?Composites are very abrasive when machined. In addition to cutting various resins, the fibre part of the material is fractured and shattered. Composites are very good at signalling any weaknesses in the machining process — better than metals. Their broad variation in properties, unpredictability and growing importance as a material demands an unconventional approach and continuous improvements to maintain a competitive machining process.

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