Designers learn to love hexapods

Once the preserve of elite research, the revolutionary six-legged machine tool is winning over firms

Hexapods are a new kind of machine tool that look set to replace conventional machines in many applications – from milling a complex surface to simple polishing.

They are light, dextrous, flexible and should, ultimately, become cheaper than conventional machine tools that do a similar job. But as yet, only a handful of companies are building them, mostly for research purposes.

Machine tool builders are interested in hexapod technology, but many are nervous of taking the plunge.

The reason, according to some component suppliers, is a lack of standardised components developed exclusively for hexapods. Without standard components such as bearings, companies will lack the confidence to build hexapods and so the technology will stay where it is now, locked in to elitist research programmes.

Rolling element bearings maker Ina Waelzlager Schaeffler has been talking to all the big machine tool builders about hexapods since 1993.At EMO ’97 in Hanover, the company emerged as the first to announce a range of high-precision joint units and telescopic arms developed specifically for hexapods.

‘I see a very big future in hexapod technology,’ says Helmut Bode, Ina head of production machines and systems.

Using Ina’s technology, machine tool designers can create their own ball-joints and struts, or specify the design which Ina will supply, ready-configured, from Germany.

The move should boost confidence among designers. To prove its designs in a machine environment, Ina has been working with the University of Stuttgart on a hexapod model called Hexact.

Like all hexapods, the Hexact works on a system of parallel kinematics. But this one looks different, as it is noticeably lighter and resembles a pair of large, airy bellows.

Two features of parallel systems distinguish them from the linear system of conventional machine tools. One is the lack of axes that are associated with orthogonal linear systems in which the axes are perpendicular.

That can give rise to a problem as there is no fixed datum point for the machine controller to relate to when calculating where the spindle is.

As a result, maintaining volumetric accuracy is a big challenge for machine designers.

But the other feature of hexapods is a positive benefit, helping to stiffen the machine structure which is so important to accuracy.

It also serves to highlight the key role of components in hexapod design. Unlike hefty machine tools made from cast-iron, in hexapods the structure is made up of the ball-joints and struts themselves.

As the name suggests, hexapods have six legs or struts. Arranged in pairs, they form a system based on opposing triangles, always in tension or compression.

Another important benefit of hexapod design is its flexibility. Machines can be set up in a variety of configurations to suit the changing needs of production.

A gantry-mounted hexapod can be set up as a portal machine for milling the spars of aircraft wings. Several hexapods can form a cell to machine a car engine block.

Once the technology takes off and research and development costs are absorbed, hexapod prices are expected to tumble to considerably less than the cost of a conventional machining centre.

Crawley-based Geodetic Technology has already put a £110,000 price tag on its entry-level G500 Evolution hexapod launched at the Hanover show. Volumetric accuracy on the G500 is a healthy +/-25 Micro m repeatable to 10 Micro m.

Following in Ina’s footsteps at Hanover, Geodetic is to extend its partnership programme started with Siemens on the Sinumerik 840D controller. Suppliers are shortly to receive letters inviting them to develop a broad range of components specifically for hexapods.

Aggressive marketing on the part of Geodetic’s owners, Ghassan and Amer Matar, seems to be starting to pay off.

And while Geodetic will not say how many hexapods have been sold, it is understood that at least two machines have been shipped to commercial buyers – one packager, the other a vehicle manufacturer.

Market milestones

Milestones in a drive to get the market for hexapods moving:

* Siemens, working with hexapod builders Geodetic and Ingersoll, hots up its Sinumerik 840D controller to handle the new challenge of the extraordinary, six-legged, machine.

* Metrology company Renishaw perfects a very fast laser interferometer measuring system fitted to the leg struts of the Giddings & Lewis Variax hexapod, now part of a £3m research project at Nottingham University.

* Bearings manufacturer Ina pledges to supply ball-joints and leg struts to hexapod designers as standard components.

* Geodetic produces a customer price list, a crucial element of getting hexapod technology accepted in the marketplace.