Waterjet method cuts complex shapes in very hard materials

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Nottingham engineers are pioneering a waterjet milling technique that can cut at precisely controlled depths and self-correct to produce complex geometries.

The subtractive manufacturing technique is particularly useful for very hard materials such as ceramics and is being used for aerospace applications and medical prosthetics.

While waterjets have been used for decades to slice clean through sheet materials, the ability to mill shapes with jets is relatively new.

The key to enabling this is the ability to control the depth of penetration of the jet into the part being milled.

‘It depends how much time we’re exposing the part to the jet — so we’re moving with different speeds,’ project lead Prof Dragos Axinte of Nottingham University explained.

‘We developed some models and we know how the part will erode at particular feed speeds; that’s the key element behind the work we are doing.’

The team uses water laced with abrasive garnet particles, which is forced out under pressure as a jet 1mm in diameter travelling at around two to three times the speed of sound.

Another key element to the project is the ability of the jet to self-correct in real time as it is moving along — although this presented a particular challenge for the team.

‘If you imagine a jet with a high velocity going into a surface, the reflection is so bad you cannot see anything. It’s wet, misty and full of grit that’s bounced back. You cannot use a vision sensor or put a force sensor in, so we developed a monitoring system based on acoustic emission sensors,’ Axinte said.

The sensors are calibrated beforehand so they can match an acoustic signature to the status of the surface. The self-adaptive module then makes the necessary adjustments.

‘It is a niche technology; it can be economically viable only when you cannot cut with other processes — so with very hard materials such as ceramics, or materials that you do not want to thermally affect such as nickel and titanium alloys, also parts you don’t want to deflect because actually water machining results in very low cutting forces, less than 5N,’ Axinte said.

The €3.8m (£3.17m) EU FP7 ConforM-Jet project is led by Nottingham University in collaboration with researchers in institutions across Europe, including Zeeko (UK), BAE Systems (UK), Finecut (Sweden) and the Royal Institute of Technology (KTH; Sweden).