Machine tool aids repairs in hazardous environments

Professor Dragos Axinte and Dr John Allen at the Rolls-Royce University Technology Centre (UTC) in Manufacturing Technology designed the system to reduce costs and down time.

Their research resulted in a new concept of a miniature six-axis parallel kinematic platform- named a ‘Free-leg Hexapod’, or FreeHex- that can apply complex computerised milling processes to a wide range of in-situ maintenance jobs.

“It could be used in power plant repairs, to repair a diesel engine on a boat, or plant in the offshore oil and gas industry,” said  Axinte. “You could use it remotely in hazardous environments too.”

The idea for the FreeHex came from a request made by Rolls-Royce’s submarine business. It required in-situ repairs to be done in a very confined workspace.

Its own machine was heavy and hard to move — difficult to use in tight working environments — so Rolls-Royce asked for something smaller and more flexible with the same functions.

“Our development of a Free-leg Hexapod takes this kind of robotised in-situ processing machine a reality. This initial development will be continued as a FP7 EU-funded project, MiRoR,” said Axinte.

The machine they developed is a Stewart-platform-type hexapod structure. Six telescopic struts or legs with ball-screw drives work in parallel to position the moving platform. A compact high-speed spindle is mounted in the centre of this platform between the legs.

This hexapod is distinctive because the designers removed the base, or fixed platform, and allowed the positions of the joints at the lower ends of the extensible actuators to be varied — hence ‘Free-leg’ Hexapod. So the FreeHex can be positioned on different workpiece surfaces, or non-flat geometries. 

To more accurately model and calibrate the free feet and legs on different surfaces, in different spaces, the researchers developed a kinematics calculation model for complex parallel structures based on ‘mean of multiple computations’ (MMC).

This somewhat complex modelling process requires input of various constraints, but can theoretically solve the kinematics problem for any structure including flexible systems and walking hexapods. 

The researchers looked at forces likely to act on the structure in different situations and considered how to reduce these. “To drill a large hole you need a lot of torque, or twisting force. For that you need a lot of power and a solid structure,” says Professor Axinte.

“We took a different approach, using a small tool to make lots of shallow cuts very quickly, making it a miniature low force machining system,” he says. This miniature multi-axis machine tool operates at 50,000rpm with a feed speed of 3m per minute. It can produce anything a conventional CNC machine does, such as slots, cut-offs and freeform surfaces.” 

The Nottingham machine weighs less than five kilos, has a small footprint and can be installed where the work needs to be done quickly and effectively. Its computer numerical control ensures highly accurate and repeatable machining which can operate remotely in dangerous working environments.

This device design has now been patented and Rolls-Royce is looking at using the machine in other parts of its business, such as aerospace. It also recently received a Rolls-Royce Submarines Excellence Award for its unique design.