A direct-drive spindle motor and linear guideways on a machining centre enable high-speed metal-cutting, but this configuration does not lend itself to the efficient machining of highly alloyed nickels like Inconel or duplex and super duplex stainless steels; a gearbox drive and box-ways are more appropriate, but they are difficult to find on smaller C-frame vertical machining centres (VMCs).

However, Stroud-based LB Bentley (part of the Severn Glocon Group) has identified “an ideal platform” for producing its sub-sea valves (plus its filter and dryer equipment) from tough materials.

It is the AA1165 VMC, which has an 1,100 x 650 x 600mm working envelope and is built by Toyoda partner WELE in Taiwan.

One of these machines was installed in early January, and a second will be delivered soon — by Kenilworth-based Whitehouse Machine Tools (www.wmtcnc.com), WELE’s sales and service agent in the UK and Ireland.

Although LB Bentley’s existing production equipment has served it well, the company wanted initially to replace two ageing VMCs with new machines capable of meeting the tight positional tolerances and surface finish required when producing components from a range of difficult materials, including 625, 718, 725, 825 and 925 nickel alloys and duplex/super duplex stainless steels.

One of the difficulties with the ageing VMCs was the inability to machine by circular interpolation a sealing surface without four quadrants being visible and lines apparent at each change of axis direction, with the attendant risk of parts being rejected.

Quality is paramount, with ±1.5µm dimensional accuracy and a 0.4mm Ra surface finish required.

The AA1165, fitted with a high-pressure (70 bar) coolant system, has proved itself easily capable of achieving the drawing tolerances when machining nickel and steel alloys.

Features that provide a combination of heavy cutting performance and precision finishing include the two-speed auto-change gearbox and the 18.5/15kW BIG Plus 8,000rev/min spindle delivering up to 469 Nm of torque, plus hand-scraped square-section guideways, cutting feed rates up to 10m/min and 36m/min rapid rates in X and Y (and 20m/min in Z).

The machine’s Fanuc 0i-MF CNC is already prepared for controlling a fourth axis, which is an optional extra offered by Whitehouse on the AA1165 (the 1,300 x 650mm table has plenty of room at one end to accommodate it).

LB Bentley’s operations manager, Andrew Kethro, said: “We have been extremely pleased with the performance of our new machine.

“It is very rigid, and the load monitor shows that it can easily cope with the materials and depths of cut specified, so our two WELE VMCs should be well set to take us into the future, regardless of what the industry throws our way.

“Our machinists have commented how quiet the machine is when cutting at high torques and low speeds, 1,500rev/min being a maximum here when machining our difficult alloys.

“With limited factory space here, the small footprint of the AA1165 supports our 5S initiative and the continuing improvements we are making on our shopfloor.

“We look forward to receiving a second identical machine shortly.”

Rigorous fly cutting

Machine shop supervisor Tracey Harnden was part of the team tasked with sourcing the new capacity.

He said: “We shortlisted two alternatives to the WELE machine and asked both potential suppliers to run a trial using one of our existing programs for cutting an Inconel part.

“The spindle stalled on one machine, while the other company was unable to point to a single user installation where its equipment cuts our type of alloys.

“Other CNC mills we looked at were light high-speed models that were obviously unsuitable — and the original supplier of the VMCs that are being replaced did not even call us back.”

Mr Harnden explained that Whitehouse carried out the cutting trial on LB Bentley’s material using its program and tooling for the designated part, which initially needs rigorous fly cutting across the top of the billet.

He witnessed the demonstration at the Whitehouse technical centre in Kenilworth and noted the quietness of the process, due to the machine’s rigidity and hence low levels of vibration.

The latter also contributed to the required accuracy of machining and surface finish being achieved.

A further point in WELE’s favour was the low spindle load when cutting the part.

Operators in the Stroud factory constantly monitor the load meter on all machining centres, so they can see when a tool is beginning to wear and compromise accuracy.

Mr Harnden added: “When the second WELE arrives and the two VMCs they are replacing have gone, there will only be Fanuc CNCs on the shopfloor, making it easier for operators to move between machines and giving us a greater degree of flexibility to allocate jobs to the available resource.

“The four-axis WELE machines will work round the clock five days a week and at weekends, alongside seven other machining centres and a similar number of lathes.”