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Nexus Precision Engineering’s dedication to producing highly complex components in often very difficult-to-machine materials for customers in the oil and gas sector sees many of the parts produced being installed in sub-sea applications, which indeed means that the level of quality being attained by the Scotland-based company is also monitored by independent assessors for the industry.

The ability to consistently meet such demands therefore makes machine selection very high on the company’s agenda - and the in-built rigidity and ability to consistently maintain precision has led to multiple Hartford machine installations by T W Ward CNC Machinery (Ward CNC). These include a HCMC-15AG open-fronted vertical machining centre, a PBM-115AG cross bed infeed, column-type horizontal boring and milling centre, and a Sumo VMC-3100 open-fronted vertical machining centre.

Nexus has invested over £1.7 million in facilities and machine tools in 2012-13 to bring its installed base to 14 machines with a turning capacity up to 1,500 mm diameter by 3,200 mm long as well as four- and five– axis CNC milling and boring up to 1,500 mm by 1,600 mm by 3,100 mm. Due to the critical nature of the components produced, Ward CNC was selected to be the prime source for its large-capacity milling and boring equipment.

Most recently, a Hartford HCMC-15AG open-fronted vertical machining centre has been installed. Offering X, Y and Z axes travels of 1,500 mm by 820 mm by 700 mm, the 26 kW/6,000 revs/min machine offers a table capacity of 1,650 mm by 820 mm and able to accommodate loads of 2,200 kgs. It is controlled by a Fanuc 0iMD CNC featuring the Hartrol Al-100 Advanced Programming System.

This machine complements a Hartford PBM-115AG cross bed infeed, column-type horizontal boring and milling centre, and a Hartford Sumo VMC-3100 open-fronted, vertical machining centre.

In addition, Nexus has also invested in Ward CNC-supplied machines for its manufacturing site in Malaysia – a Hartford Sumo VMC-3100 and a Hyundai-Kia KH63 NCRT horizontal machining centre.

According to Operations Manager Steven McGeachie, Nexus installed its first Hartford Sumo six years ago, justified against a price for specification advantage to meet a growing order book. With a three metre bed capacity, the machine enabled Nexus for the first time to cost-effectively machine large workpieces in a single set-up, especially downhole gauge carriers. After turning, the 4140 or 420 stainless steel carriers enter the Sumo for a series of milling and drilling routines, including component-length slot production, to tolerances of +/- 0.05 mm using ball nose cutters.

“With solid hardened and ground ways and ample power to the 50 taper spindle, the Hartford machines boast high-level rigidity that ensures we can maintain tolerances of a few microns with good repeatability.”

A wide range of materials pass through the Nexus 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 ally-bronze are machined as forgings, rolled/forged bar and heavy-duty blocks.

Typical gains from the capability of the Hartford PBM-115AG, for example, is the machining of a highly complex valve body from 525 mm by 320 mm by 285 mm 4130 steel blocks. The body is initially rough and semi-finish machined which takes some 112 hours. The operation involves spade drilling an 85 mm hole by 418 mm deep, the interpolation of seal features with form cutters, interpolation of a large internal cavity, a host of milling cycles plus drilling, tapping and thread milling pitch circle holes.

The valve body then has Inconel inserts welded in five critical areas, to create high-tolerance seating and sealing features for valves and oil seals. These featured areas effectively create the scenario for a potential production engineering and programming nightmare in that the requirement is to finish machine two very dissimilar materials.

This finish machining takes a further 160 hours which includes, for example, features such as the production of a series of line bored internal reverse counterbores in the Inconel of 76 mm, 74 mm and 70 mm with Ra 0.2 surface finishes (the larger diameters are internal) on each side of an internal machined void.

This void has an 88 mm +/- 0.13 dimension between its internal faces, with a squareness tolerance to the centre line of the bores of 0.08 mm. The internal faces of the counterbores are also subjected to parallel geometric tolerances of just 0.05 mm.

Here, the combination of the Hartford machine’s rigidity and precision is well-proven to Nexus engineers, enabling the tooling to contend with machining the Inconel inserts and the 4130 steel of the main body, which also involves the difficult area around the skin of the weld and the varying material thicknesses.

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