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Major Breakthrough in Multi Material Drilling, Using Low Frequency Axial Vibration Assistance
Abstract:
Aircraft design has mainly changed in the past years, introducing new materials such as CFRP at a large scale. Even if this great change brought many advantages: weight, sustainability, reliability… it upset the way to produce basic parts and to assembly. A special difficulty lies in drilling multiple stacks made of different materials, i.e. metal (aluminum, titanium, stainless steel) + CFPR. Indeed, as the process has to drill through those stacks during a unique operation, cutting conditions and tool technologies are a compromise. As a consequence, tool life cycle, productivity and above all, hole quality are no longer optimum.
Some materials, such as titanium requires low cutting speed and high feed rate whereas CFRP requires opposite trends. Moreover, heat generated by cutting metal (especially titanium) can damage CFRP when overheated metallic chips pass through. Finally, the compromise for cutting conditions and tool technology can generate long metallic chips that may congest drill flutes and entail hole damages or tool breakage.
For ten years, a new technology has been developed consisting in reducing and overcoming chip size. The principle lies in adding a periodic low frequency motion to the constant feed motion of the tool. With optimized parameters (amplitude and frequency) this combination allows to create weak points on the chips that make them break when twisted by the cutting edge. Unlike over processes, micro-peck or micro-shock drilling, the cutter never goes out from the material which avoids shocks and tool breakages.
This technology has been applied to ADE in aerospace field where benefits are significant. Drilling process is perfectly steady during the whole operation: no thrust, torque or heat increases or peaks. Thus, hole quality and tool life cycle are more predictable. This solution has been wide spread in aircraft assembly operations even for mono material drilling where constant small chips prevent ADE congestion and ease part cleaning.