In June 2016, the sub-contract machining firm Norjon Precision Engineering upgraded its multi-tasking turn-milling capacity by adding a Japanese-built Okuma Multus U3000 2SW. Equipped with a Y axis on the column and a motorised B-axis tool carrier, as well as a 12-station lower turret, the twin-spindle machine is capable of 3+2-axis machining, although fully interpolative five-axis machining is an option.

At its 14,000ft2 factory in Gosport, the sub-contractor undertakes CNC turning and turn-milling, gear cutting, grinding and five-axis prismatic machining for a customer base that includes the food, marine, printing, yachting and power generation sectors.

Regarding the purchase of his first Okuma — supplied by Thames Ditton-based NCMT Ltd (www.ncmt.co.uk) — managing director Kevin Fox said: “We have a policy of buying highly capable machine tools and then finding work for them, rather than investing to fulfil a specific contract. The first job onto the Multus was a family of 10 high-precision components for the defence industry. Produced in batches of typically 50-off, these parts have to be machined from 316 stainless-steel billets to within a tolerance of 10µm on some dimensions.

“We previously produced the parts in two hits on a lathe and a five-axis machining centre, but they now come off the Okuma complete. It is much easier to hold tolerance without a second clamping and manufacturing cost per part is 30% lower than previously — due to less manual handling.”

It was a similar story when manufacturing mould tools for producing paper cups for the drinks-dispensing industry, where four operations have become two, resulting in production cost savings of about one-third.

Gear hobbing option

Another job that has been earmarked for the multi-tasking turning centre is the production of nitrided EN40 spline tooling for a US manufacturer of automotive test rigs. With this job in mind, Norjon bought Okuma’s optional gear-hobbing package, which will allow the components to be produced in one hit. Again, the result will be substantial economies in production costs compared with the previous production route of turning on a lathe and gear cutting on another machine.

The ability to produce gears economically in any quantity and of any size up to the lathe’s capacity on a single machining platform provides flexibility and enhanced accuracy, as well as lower capital investment. The hobbing option synchronises the C-axis spindle and the motions of the tool in the B axis to produce straight or helical splines.

Mr Fox said: “Over the years, we have gradually raised the quality of the machine tools we buy. The aim has been to maximise single set-up machining, reduce costs, improve accuracy and add further value to the components we produce. We wanted a step up in our multi-tasking turn-milling capability, and we visited Okuma in Japan a couple of years ago to see Multus lathes being built. The weight and rigidity of these machines was evident, as was the quality. For example, they have a box guideway for the lower turret, rather than linear rails; this sort of attention to detail
is reflected throughout the design.

“If a manufacturer is chasing tight tolerances, only top-end production equipment will do. Otherwise the machinist is for ever fighting the machine to achieve accuracy, and that means more manual intervention, higher costs and squeezed profit margins.”

Advanced functions

One of the key features of Okuma machine tools that swayed Mr Fox towards buying the Multus U3000W is the manufacturer’s collision avoidance software, which runs in the proprietary OSP control. The entire machining area — including the spindle head, slideways and even the guarding — is monitored in real time; and if interference is predicted, the machine stops instantly to prevent damage.

Another Okuma software function that supports Norjon’s ability to produce high-precision parts is Machining Navi, which detects cutting conditions caused by vibration. Either a spindle speed change is recommended, so that it can be set manually to suppress chatter, or speed adjustments are applied automatically.

The machine can produce components up to 1,500mm long and is fitted with an 8in chuck. Its specification includes a 22kW main spindle and opposed spindle (both with a C axis), and a -30 to +210deg swivelling B-axis tool spindle rated at 22kW 12,000rev/min and featuring a Capto C6 tool interface. Feed rates are up to 50m/min for the X and Z axes, and 40m/min for the 250mm Y axis. Linear scales can be specified.

In order to meet urgent deliveries, the Multus was programmed on the shopfloor using the OSP control for the first couple of months of its operation at Gosport — a process that Mr Fox describes as straightforward for his highly skilled operators, one of whom has many years’ experience operating Okuma machine tools. However, he says that the company will move to the off-line preparation of programs in the near future, using its FeatureCAM programming software.