Polishing freeform surfaces is a technical challenge to any manufacturer because the traditional methods used are either inconsistent, inaccurate or inefficient. In light of this, Ir Prof. Benny Chi-fai Cheung, Professor of the Department of Industrial and Systems Engineering and Head of the State Key Laboratory of Ultra-precision Machining Technology, led a research team to develop a multi-jet polishing system that changes the pressure of each jet according to the changing curvature of a workpiece. It is proven to render a surface 10 times more uniform than existing multi-jet polishing systems and is at least 16 times more efficient than a single fluid-jet system.
Freeform surfaces in daily life
Freeform surfaces, defined as surfaces without rotational and translational symmetry, may sound unfamiliar, but they actually exist in abundance in our daily life. Those suffering from presbyopia may be wearing so-called progressive glasses which are essentially freeform multifocal lenses without an awkward bifocal line on them. Similarly, any modern cameras, including the ones on your smartphone, are likely to contain aspherical or freeform lenses because they can bend the light more precisely to reduce aberration and can replace many lenses. High-end Augmented Reality (AR) or Virtual Reality (VR) headsets also include freeform optical elements for the same reason. On the other hand, orthopaedic implants have to imitate the natural organic shapes of human bones and their surfaces also show freeform curvatures. Finally, turbine blades in aircraft engines are also in freeform shapes to optimize the aerodynamics.
Jet pressure adapts to changing curvature
Freeform surfaces were used to be painstakingly polished by hand. However, it took a lot of time while the result was not consistent and the accuracy was rather low. Traditional mechanical polishing is a bit more efficient, but it suffers from size limitation and unavailability of polishing tools that cater to deep and large variations of the surface curvature. Then came the computer-controlled single fluid-jet system where a jet of water with abrasive particles is directed towards a surface. It is more accurate than the aforementioned methods, but not efficient and scalable enough. For this reason, the team invented a multi-jet system using micrometre-scale abrasive particles that are capable of achieving mirror-like smooth surfaces. By using more than one jet, the system can polish a bigger surface or a number of surfaces at one time, while using computational fluid dynamic modelling and simulation to avoid interference among the jets.
“On top of that, the emphasis of the system is placed on the term ‘curvature-adaptive,’” added Prof. Cheung. “The purpose of polishing is to achieve a specific level of smoothness by evenly removing as little material as possible. However, the amount of material removed from the surface depends on not only the pressure of the fluid jet, but also the curvature of the surface. If the same jet pressure is applied on the whole freeform surface, certain parts would be polished more and certain parts will be polished less, resulting in non-uniform surface finish and form error of the workpiece. Our system is capable of factoring in the minute and continuous changes in curvature of the surface, and adjusting the pressure of each jet independently and accordingly to keep its shape unchanged. By using the curvature-adaptive method, the system is proven to render a surface 10 times more uniform than regular multi-jet systems.”
Portable and retrofittable
The curvature-adaptive multi-jet polishing system developed is extremely portable – all components can fit into a small suitcase. Prof. Cheung said, “It can be retro-fitted on robotic arms or 3D printers easily, so that polishing can be done on the spot right after a workpiece is casted, machined or 3D-printed, without the need to transport the piece to another machine.” By replacing the abrasive particles used in the fluid jets, the system can be adopted for polishing various types of materials, ranging from glass to steel and extremely hard biomedical alloys.
In April 2019, the Curvature-adaptive Multi-jet Freeform Polishing System for Precision Manufacturing won a silver medal and two special merit awards at the 47th International Exhibition of Inventions of Geneva.
