Additive advance: measurement helps additive techniques into manufacturing

After years of hype, additive manufacturing techniques that build products layer by layer are starting to become more commonplace in shop-floor production, as well as in the design office. They are already producing a step change in the way certain customised products are made, for example, medical implants and bespoke aerospace components. However, 3D printing remains largely separate from conventional subtractive machining and, as a result, a long way from becoming part of automated assembly lines.

Part of what’s holding additive techniques back is the historical tendency for them to be enclosed in their own 3D-printer boxes, according to Dr Jason Jones, co-founder of Hybrid Manufacturing Technologies (HMT). The best way of creating a product might involve both additive and subtractive manufacturing and this might mean moving it from a milling machine to a 3D printer and perhaps back again to finish the additive surface. But HMT has commercialised a technology that combines the two approaches
in one set-up.

‘Most additive manufacturing is in fact rapid prototyping,’ said Jones, ‘and it’s a bit like computers were before the internet: individually very capable but you don’t get the synergy of their interconnected productivity.’

Another problem with additive processes is that they usually take a long time and provide little or no way to alter manufacturing mid-way to match unexpected changes in operating conditions or the way the product is turning out. This concept of ‘adaptive machining’ is particularly important in repair or remanufacture work, where the product in the machine is likely to have a different geometry from its original CAD model.

In fact, HMT was spun out from a £1m research project co-funded by Innovate UK (formerly the Technology Strategy Board) designed to improve remanufacturing by creating a single machine to carry out adaptive machining with a combination of subtractive and additive techniques. The result of the Reclaim project, which was carried out at Coventry’s Manufacturing Technology Centre and sparked by research from Jones at De Montfort University, was a device housing a range of exchangeable tool heads that enabled automatic transition from inspection to machining to laser cladding.

”HMT sees it starting to close the feedback loop for additive techniques, vital for its entry into mainstream production

The challenge, said Jones, was in the underlying measurement, inspection and control that brought the different technologies together. ‘In terms of manufacturing, the real innovation is in how we can connect and disconnect all these things. In a CNC machine you have a spindle that rotates and at the end of that spindle you don’t usually have facilities such as laser energy or powder deposition.’

These measurement and automation technologies — made possible by work from Reclaim partners Renishaw and Delcam — capture the information about the specific defects on an individual component, make a comparison with the original optimum configuration and then produce a tool path that can achieve this using both additive and subtractive machining, and other processes within the same set-up.

Renishaw came on board, says the firm’s principal machine tool applications engineer Paul Moore, because the Reclaim project identified a specific need to capture the surface condition and geometry of individual pieces. ‘You could use a touch trigger probe to do this,’ he said, ‘but it took ages. You have to take hundreds or thousands of points.’

At the time, Renishaw was developing a different form of probe: a 3D contact scanning system, now the Sprint series of probes, that constantly reports its position via wireless infrared communication. ‘What this can do is to take thousands of points per second and we could use it not only to find its position and orientation, but also return back the exact profile of the edge of the turbine blade,’ said Moore.

The scanned data about the blade surface feeds back into the Delcam CAD system where it is compared with the nominal blade model: ‘Delcam then recreates the model surface and generates a cutting path to machine off that surface or to build it up or change the properties using a laser cladding process.’

The innovation is in the application, Moore added: ‘Contact scanning as such isn’t new, but this is a whole system and it hadn’t really been done before as a toolchanger device where the scanner is mounted in the machine tool’s toolchanger system. Also the software side has moved on a lot so that we can process points and generate surfaces that can be used by third parties for generating models and regenerating a tool path.’

The work involved not just control and output software modifications, but also physical challenges: the inside of a CNC machine tool, with metal chips and cutting fluids flying, is a brutish place to put a sensitive measurement and inspection device.

Reclaim also helped to define system demands for Renishaw: ‘We were unsure before how accurately you needed to measure a part to remachine it,’ said Moore, ‘and we found you have to do it very accurately because with turbocharger blades they want to machine it back to virtually a finished state.’

The research came along at the right time to give Renishaw a specific project for refining and productionising its scanning probe, and the work has opened up a new business area in the repair and remanufacturing market.

But the ambitions for Sprint are broader than that: ‘We’ve created a product that is of more use to the repairwork industry, but that isn’t the main point of the scanning probe; it’s maybe a 10-to-15 per cent opportunity in a new market,’ said Moore. There is a wider business in new areas of adaptive machining to be won, and also a big market in in-process inspection without taking a part off the machine tool.

Meanwhile, HMT sees the technology as starting to close the feedback loop for additive techniques, something that will be vital if this type of manufacturing is to enter mainstream production. The company is now producing a series of automatically changeable tool heads that can allow a single conventional CNC machine to perform everything from cutting and drilling to 3D deposition to surface finishing. On the additive side it can print with up to four different materials, allowing it to do for metals what Objet’s multi-polymer printer does for plastics, said Jones.

This, he hopes, will open up new common uses for additive, enabling companies to experiment with changing the surface composition of their products by depositing extra layers of material, and to create structures that combine the geometric possibilities of 3D printing with the strength and speed of subtractive techniques. ‘Because these tools have not been widely available, there have just been people tinkering with it in the research world,’ he said. ‘By us driving down the cost and accessibility of this technology, I really think this will make its way into the manufacture of real products.’