Nexus Precision Engineering is focused on producing highly complex components in often very-difficult-to-machine materials for customers in the oil and gas sector.

Indeed, many of the parts it produces are used in sub-sea applications, which means that the level of quality achieved by the Scottish company is monitored by independent assessors.

The ability to consistently meet demanded high levels of quality puts machine selection high on the company’s agenda; it has also resulted in a close working relationship with TW Ward CNC Machinery Ltd (www.wardcnc.com).

Nexus’s need for highly rigid machines capable of consistently maintaining high levels of precision has resulted in the installation of several Hartford machines from the Sheffield-based machine tool supplier. These include an HCMC-15AG open-fronted vertical machining centre, a PBM-115AG column-type horizontal boring and milling centre, and a Sumo VMC-3100 open-fronted vertical machining centre.

Following its acquisition in 2009 by the Sheffield-based Gabbro Precision group of companies, Nexus has been going from strength to strength. Over the last 20 years, Gabbro has established three operating divisions: Engineered, Down-hole and Sub-sea Products, with each providing a first-class service to the oil and gas trade throughout Europe and Asia.

Nexus is one of the four companies in the Engineered Products Division; at its four 20,000ft2 units in Broxburn (where it employs 36 people on the shopfloor, 15 in support and two apprentices), production is focused on components such as valve bodies, isolation sleeves, casing hangers and conductor housings. It also produces assemblies and kits of parts for ROVs (remotely operated vehicles).

Major investment

Nexus invested over £1.7 million in facilities and machine tools in 2012-13, bringing its installed base to 14 machines. The company now has a turning capacity for parts up to 1,500mm in diameter x 3,200mm long, as well as four- and five–axis CNC milling and boring capabilities with work envelopes up to 1,500 x 1,600 x 3,100mm.

The recently installed Hartford HCMC-15AG open-fronted vertical machining centre has a work envelope of 1,500 x 820 x 700mm, a 26kW 6,000rev/min spindle and a 1,650 x 820mm table that accommodates loads up to 2,200kg. Control is via a Fanuc 0iMD CNC featuring the Hartrol Al-100 Advanced Programming System. Nexus has also invested in Ward CNC-supplied machines for its manufacturing site in Malaysia; these include a Hartford Sumo VMC-3100 and a Hyundai-Kia KH63 NCRT horizontal machining centre.

Operations manager Steven McGeachie says Nexus installed its first Hartford Sumo six years ago. With its 3m bed capacity, this machine allowed Nexus — for the first time — to cost-effectively machine large workpieces in a single set-up, including down-hole gauge carriers.

Made from 4140 or 420 stainless steel, the carriers are received by Nexus as offset-bored and honed ‘blanks’ in a variety of sizes, from 100 to 150mm in diameter and from 1.8 to 2.5m long. Once the journals are turned, each carrier is loaded onto the Sumo for a series of milling and drilling routines, as well as threading — after which the parts are pressure-tested.

As Nexus’s business grew, the type of com-ponents produced became progressively riskier to produce. They were more complex, required tighter tolerances and finishes — and were being made from increasingly difficult-to-machine materials. “When we sought to compare other suppliers for future machine purchases, there was no advantage in having a different brand of machine to perform the type of work that the Hartford had already set the benchmark for,” said Mr McGeachie.

Difficult materials

A wide range of materials is machined by Nexus’s Hartfords, including Inconel, Duplex, 4130, 4140, 8630 and various stainless steels such as 316, 410, 420 and 17/4. In addition, carbon steels, aluminium, brass and bronzes are machined; these can be supplied as forgings, as well as rolled/forged bar.

Batch sizes range from two to 50, and around half of the batches produced are repeat orders; up to 70% of the work is exported (destinations include Houston in Texas, Singapore, Malaysia, Brazil, Norway and South Africa).

Typical of the parts machined on the Hartford PBM-115AG is a highly complex valve body produced from a block of 4130 steel measuring 525 x 320 x 285mm. The part is initially rough- and semi-finish-machined, which takes some 112hr.

The operations include spade drilling a hole that is 85mm in diameter x 418mm deep, interpolating seal features with form cutters, interpolating a large internal cavity and a host of milling cycles — plus drilling, tapping and thread-milling holes.

The valve body then has Inconel inserts welded in at five locations, to create high-tolerance seating and sealing features for valves and oil seals. These features pose a production engineering and programming nightmare — the finish-machining of two very dissimilar materials.

This takes a further 160hr and includes the production of a series of line-bored internal reverse counter-bores in the Inconel on each side of an internal machined void (diameters of 76, 74 and 70mm) to a surface finish of Ra 0.2.

This void has an 88±0.13mm dimension between its internal faces, with a squareness tolerance to the centre line of the bores of 0.08mm.

The internal faces of the counter-bores are also subjected to parallel geometric tolerances of just 0.05mm. Here, the combination of the Hartford machine’s rigidity and precision allows the tooling to contend with machining the Inconel inserts and the 4130 steel of the main body.

To achieve the tolerances required, a special D’Andrea twin-line boring head with a 15deg approach angle is run at 150m/min on the steel but 25-30m/min when engaged with the Inconel. The final pass removes just 0.3mm of stock at
a feed rate of 0.07mm/rev.

The Hartford PBM-115AG, which is also controlled by a Fanuc 0iMD CNC with the Hartrol AI100 Advanced Programming System, has a 26kW 3,000rev/min four-speed geared headstock with automatic gear changing. The 110mm-diameter spindle has a BT-50 taper and 20-bar through-spindle coolant supply. The work envelope is 2,000 x 1,600 x 1,500mm, the table measures 1,400 x 1,600mm, and there is a 40-tool magazine.