Simplified structure produces low cost machining centre

An innovative cube design promises to be half the price of conventional five-axis machines

Just 17 months is all the time it took for Swiss machine tool maker Rigid to develop a simultaneous, five-axis machining centre from concept design to full blown prototype machine.

And if the speed of development were not enough, the machine’s highly unusual design has already caught the eye of two high-tech manufacturing outfits for which performance and cost are major considerations.

In the US, turbine impeller blade manufacturer Williams has ordered two of these machines off the drawing board, while another has been ordered by BMW/Rolls-Royce in Germany.

From the outset, Peter Meier, Rigid’s managing director and Walter Hagspiel, chief designer, set their minds on developing a machine tool that was low cost but capable of producing highly complex precision components from a wide range of materials.

Initial concept designs sketched out during quiet moments at Mach 96, the machine tool exhibition held in Birmingham in March, had turned into fully-dimensioned drawings by November.

At last month’s EMO international machine tool show in Hanover, the prototype ZS Series machining centre made its public debut.

The novel construction is based on a highly rigid, load-bearing cube producing a compact machine structure. All axes movements and machining take place inside the cube. But all the axes drives and main spindle unit are located outside the cube.

The simplified structure has enabled Rigid to slash building costs and pass the benefit on to the user for a capital outlay of around half that of a conventional five-axis machine.

The design provides an uncluttered work access while the cutting performance has also improved with contour machining speeds going up between three and sixfold, depending on the metal being machined.

With all the axes motion drives placed outside the cube and isolated from contaminants including cutting fluids and swarf, the ZS Series could make use of lower-cost linear motor powered slides instead of ground ballscrews.

Studies at the University of Darmstadt are looking into the possibility, although there is some concern over the magnetising effect on the workpiece of the surrounding electric motor fields, since any post-processing such as demagnetisation would only add to machining costs.

The prototype ZS 500/130 has a working envelope of 750mm diameter by 550mm in the Y axis. A work table capable of swivelling/rotating/traversing in the A, B, Z axes is worked on by a traversing (X and Y axes) StepTec 12,000rpm 40kW spindle producing 250Nm torque.

A higher speed unit from StepTec with a 20,000rpm/30kW/80Nm spindle can also be fitted.

Traverse rates are a fast 40m/min with simultaneous five-axis contouring at 12m/min depending on the material being cut.

The control system used on the demonstration machine is a Siemens Sinumerik 840D CNC which seems to be the preferred choice of controller for another novel design of five-axis machining centre in evidence at EMO – the Hexapod (Six-legged revolution, page 17).

Rigid has developed the software for the controller. The machine has a positioning accuracy of 9microns and repeatability of 5microns in X, Y and Z. Rotational axis positioning accuracy is 9 seconds with a repeatability of 5 seconds of arc.

Rigid’s Meier foresees a wide range of aerospace component applications in aluminium alloys, alloyed steels, titanium and its alloys as well as other exotic materials. Applications in the medical and electronic industries are also being investigated.

Meier was not prepared to disclose the price, but says it is around half the price of a Rigid ZT 800 five-axis machining centre on sale for the past five years.