Chesterfield-based BG Engineering operates three Japanese-built Okuma horizontal machining centres; all are equipped for carrying out Okuma’s novel turn-cutting process, which is used extensively at the sub-contractor’s facility to machine gas and steam regulator valves, as well as offshore seals and components for gas storage equipment.

Turn-cutting is substantially different from conventional turning operations on a machining centre, which normally involve a static turning tool in the spindle and a component rotating on a ‘torque table’.

Instead, the component remains stationary on the table of an HMC, while a lathe boring bar mounted in the machining centre spindle describes a 360deg motion to effect the turning action.

This is achieved using circular interpolation of the X and Y axes to generate almost any size of feature — up to 0.5m in diameter on BG Engineering’s Okuma — while feeding the tool forward in the Z axis. At the same time, the tool tip is continuously orientated by the spindle, which turns at the same rotational speed as the interpolated circular motion to maintain the correct rake angle.

The main advantage of turn-cutting is that a standard boring bar for a lathe can be programmed to turn outside diameters and bores, which can be straight, tapered, stepped or profiled; it can also machine radii, chamfers, grooves and faces, eliminating the need for special tooling or attachments. The process can be seen in action on YouTube (www.youtube.com/watch?v=6HDMoafaRWc).

Minimised tooling costs

BG Engineering managing director Antony Holmes says: “Before we had turn-cutting, we used expensive boring heads of specific sizes on our HMCs to machine each of the different-diameter ports in a valve body, plus deep internal features.

"Furthermore, separate tools were needed for corner radiusing, chamfering, grooving and face milling. All of these features can now be produced by turn-cutting, using a single boring bar with standard ceramic indexable inserts. Even the seal face can be ‘gramophone-turned’ to produce a single flat spiral, avoiding the chatter associated with plunge milling.

“The cost benefits of not having to buy 10 or 15 different tools when we start a new job are considerable; and when you bear in mind that we typically machine fewer than 10 of any part — exceptionally up to 100-off — high expenditure on tooling can make a contract uneconomic.

"Additionally, cycles can be faster and more efficient if turn-cutting is used, as the number of tool changes is drastically reduced.”

Immediately after Thames Ditton-based NCMT Ltd (www.ncmt.co.uk) installed the first Okuma MA600 HMC at BG Engineering in 2008, the sub-contractor was able to quote a customer zero tooling cost for a new project, as just one lathe boring bar — already on the shopfloor — was sufficient to complete the job.

The business was won as a result. Otherwise, the initial tooling would have cost £9,000 and made the first-off components prohibitively costly. Moreover, a design alteration may necessitate buying further conventional tools, whereas a simple change to the turn-cutting program allows the same boring bar to produce the new features.

Machine replacement

The initial BT50 Okuma MA600, which has a twin 630mm pallet changer and a nominal working volume of 1m cube, was purchased to replace an ageing HMC of a different make. It was acquired when BG Engineering bought another sub-contractor’s machining facility in 2006 and moved into its Chesterfield factory.

At the time, the MA600 was the second machine in the UK to have turn-cutting software in the Okuma OSP control.

Integral cooling of the ballscrews and nuts is included in the machine specification, as the long periods of circular interpolation at up to 300rev/min would otherwise result in thermal growth and inaccuracy.

Various metals are machined at Chesterfield, ranging from aluminium to steel, stainless steels and Duplex, but most components start out as ductile-iron castings.

Around half of BG Engineering’s throughput is high- and low-pressure gas regulator valves (mostly for Honeywell, which is located in an adjacent unit). Steam regulators, petrochemical seals, cryogenic gas movement equipment and 70-bar gas storage system components for offshore applications account for the company’s remaining business.

In 2012, BG Engineering purchased a second Okuma MA600 2APC machine with turn-cutting capability, but this one had a higher spindle speed — 12,000rev/min — to meet increasing demand from customers and to reduce the amount of overtime being worked.

In 2013, a 400mm-pallet 0.5m-cube machine with an HSK-A63 15,000rev/min spindle and a 10-pallet pool was installed.

Designated MB4000H, this machine is also capable of turn-cutting and has taken over multiple set-up machining of regular low-volume components that are really too small to put on the larger machines.

Most components produced on the three Okumas include turn-cutting in the program, which accounts for an average of 20% of total cycle time over the course of a year.

BG Engineering also operates two Okuma CNC turning centres — namely, a Multus B400-W multi-tasking machine, with B-axis milling head and 710mm-diameter turning capacity, and an LB4000-EX MY bar-fed lathe. These machines are positioned opposite each other on the shopfloor and are run by one setter/operator.

NCMT has also supplied a Speroni tool pre-setter, which reduces non-cutting times by eliminating on-machine tool setting. It currently serves the two BT50 Okuma MA600 HMCs and the Multus lathe, which has Capto 6 tooling.