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Turning components on an Okuma MA600-HB horizontal machining centre instead of on a vertical turning lathe has speeded production and saved money at Bank Bottom Engineering (BBE).

Cutting cycles are at least twice as fast, loading times are almost nil, a considerable amount of money is being saved on fixturing costs and accuracies within 10 microns are easily held, according to NCMT, which supplied the machine in 2006.

To achieve these economies, even when producing one-offs, Bank Bottom Engineering is using the so-called ‘turn-cut’ function in the control of the Japanese-built, twin-pallet horizontal machining centre.

The technique is more effective than established ways of creating turned features on a machining centre using a rotary table or facing head.

The turn-cut function in the Okuma OSP-P200M control allows a single-point turning tool clamped in the spindle to turn a static component.

Advantage is taken of the rigidity of the major linear axes and feed rates of up to 60m/min to circular-interpolate the spindle in the X and Y axes rapidly, while feeding the tool forward in Z.

The control software orientates the tool continuously in the spindle, rotating it at precisely the same speed as the interpolated X/Y path, and in synchrony with it.

The tool therefore cuts at the correct rake angle at every point throughout the 360-degrees to ensure efficient and precise metal removal.

Cylindrical bores, ODs and other features are turned by keeping the X/Y travels constant, while tapers, grooves and other profiles can be turned by varying the amplitude of circular interpolation.

The benefit of turning on a machining centre is that components can be finished in a single clamping, avoiding a subsequent setup on a lathe and the consequent handling costs, increased floor-to-floor time and cumulative tolerance error.

One alternative would be to revolve a component on the rotary table of a horizontal machining centre and feed in a static tool mounted in the spindle to machine the diameter OD.

The bore would be impossible to turn to any reasonable accuracy, as a long right-angle head would be needed.

A similar technique on a vertical machining centre would require a bespoke turning tool with considerable clearance to machine either the OD or ID.

Another option is use a facing head whose slide is positioned off the spindle centreline either manually or under CNC.

One limitation is the difficulty in exchanging cutters automatically between the tool magazine and the facing slide.

There is also the possibility of accuracy loss due to play in the spindle bearing, and/or in the relatively lightweight CNC facing slide if this extra axis is employed, rather than clamped.

David Robins, manufacturing director at Bank Bottom, said: ‘One of our components that benefits from turn-cutting is a cast steel axle box for railway carriages.

‘It used to need repeated re-clamping for seven separate operations on a vertical turning lathe and a machining centre, so the required accuracy and surface finish were difficult to hold.

‘The part is now completed in two operations on the Okuma MA600-HB.

‘All the critical features are machined in the first clamping, avoiding tolerance build-up.

‘By turn-cutting the bore, we easily achieve 10 microns dimensional accuracy and circularity as well as 32 CLA (1.6 micron) surface finish.’ Robins finds such results surprising considering the large size of the machine, which has a nominal 1m working cube.

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