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NCMT has supplied an Okuma MA600 HB horizontal machining centre to Epsom subcontractor WECS Precision.

WECS invested in the machine to fulfil a rail industry contract involving safety-related modifications to electric drive motor casings and reduction gearbox housings.

The size of the motor and gearbox assemblies required a larger-capacity machine than the one the subcontractor previously had on the shop floor.

Although a 500mm pallet would have been sufficient to carry out the work, WECS opted for a 630mm pallet machine.

The contract stipulated that each set of 32 motor and gearbox assemblies had to be modified in one week to minimise the period that each train was out of service.

To achieve such a fast turnaround, the machine was equipped with a six-pallet pool to maximise cutting time by enabling offline workpiece fixturing and removal.

On a rolling weekly programme entailing the 24/7 operation of the Okuma until mid-2011, two of the pallets are devoted to machining motors and two to gearboxes, which leaves two spare for other contracts.

The steel motor casing requires extensive reworking to mill lugs and to drill through-holes to tight tolerances so that each assembly mounts to a redesigned bogie frame using bolts that are secured with a nut rather than entering a threaded blind hole.

The dimensional tolerances on the 14mm- and 21mm-diameter holes are +18 microns and +21 microns respectively.

Typical positional tolerances of the motorcase and gearcase are 25 microns accumulative.

What makes these limits so demanding to achieve is the need to use extended cutters to machine features a long distance from the spindle nose.

For example, to machine the motor case lug faces, a 250mm-diameter side and face cutter is mounted on a 470mm gauge line BT50 toolholder, utilising taper and face spindle contact.

To help to achieve the required component tolerances, a Renishaw OMP3 probe is employed to measure the lug faces after rough machining.

The Okuma program software recalculates the final tool path, ensuring that component tolerances are met.

This machining method has shown the process to be capable of holding a tolerance of +/-5 microns around nominal size.

A further potential difficulty in holding the tight limits at the Epsom factory stems from the location of the machining centre, close to a roller door that opens and closes frequently.

The Okuma MA600 is said to be less affected by temperature variation on the shop floor than most machines, owing to the manufacturer’s Thermo Friendly Concept.

Symmetrical design, combined with tool offset compensation using data sent by temperature sensors located around the spindle and main structural elements of the machine, holds dimensional drift to less than 10 microns.

This is the case even if the ambient temperature fluctuates by as much as 8C.

On the sand cast aluminium gearbox casing, two bores of 255mm and 120mm in diameter are machined to within 32 microns and 25 microns respectively.

There used to be a problem deburring the interface between the machined bore circumferences and the casting, as the position of the interface is not known to be better than +/-2mm.

It means that the mill cannot be programmed to remove the burrs consistently and it previously took an operator 10 minutes to finish each part by hand.

The job is now done in 90 seconds within the machining cycle by a Xebec deburring tool, also supplied by NCMT, resulting in labour and cost savings.

The abrasive rods, each comprising of 1,000 aluminium oxide fibre filaments, splay slightly to cover the entire interface area between the machined bores and the casting.

The amount that the rods splay effectively covers the +/-2mm positional inaccuracies of the casting, producing a uniform deburred profile.

Allistair Bowmar, lead engineer at WECS, responsible for the rail project, said: ‘The advantage of the Xebec system is that the whole of the filament is made of aluminium oxide.

‘The self-sharpening rods last significantly longer than abrasive impregnated nylon brushes and surface finish can be controlled by a simple change to the tool length offset,’ he added.

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