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Innovative turn-cutting simplifies complex well component production


07-12-2012

Author: Admin

£2.2 million FMS installed in Scotland by NCMT
 
Controlling the flow of oil from a well is the function of a piece of equipment known as a Christmas tree. In the case of a deepwater well, it sits on the sea floor where external pressures can be up to 15,000 psi. Manufacture of the hydraulic nerve centre of the apparatus, the subsea control module or SCM, to withstand the extreme environment is crucial to safe and successful operation at the wellhead.
 
FMC Technologies, a world leading manufacturer serving the oil and gas industry, has just invested £2.2 million in a flexible manufacturing system (FMS) at its Dunfermline factory to produce SCM components to accuracies measured in tens of microns. The turnkey production cell was supplied by NCMT and comprises elements from four supplier companies for which it acts as sole UK agent.

One is Japanese machine builder, Okuma, two of whose MA-600HB horizontal machining centres form the centrepiece of the system. 
 
Supplied fitted with high pressure coolant systems, their phased delivery was in August and December 2011. Within their 285-tool magazines are toolholders from BIG Daishowa as well as alumina deburring tools from Xebec, both of which are Japanese manufacturers. Tool presetting is carried out using an STP46 from Speroni, Italy, the fourth NCMT principal involved.
 
Linking the two Okumas is an automated storage and retrieval system from Finnish company, Fastems, which incorporates two workpiece set-up positions and an input/output station for raw material. Of high capacity compared with its footprint, the computer controlled store can hold 24 machine pallets and 32 pallets of near-net-shape billets on two levels.
 
Defined by the production schedule downloaded from the host computer, a new billet is delivered automatically by a 3-axis CNC, rail-guided stacker crane fitted with telescopic forks for pallet handling to whichever machining centre becomes available first. The correct NC program is sent to the relevant Okuma control and instigated when the pallet reaches the machine. Finished components are delivered back into the racking or directly to one of the set-up stations for removal and dispatch to Norway for assembly. Order information is transmitted back upstream after completion of each machining cycle.
 
The main window in the Fastems control displays a mimic of the entire system and uses colours and icons to advise the real-time status of all system elements. Operators effectively work in a paperless environment, as set-up sheets, tooling data and setting details are stored electronically.
 
As all SCM parts must be particle-free prior to assembly, a Vixen washing system has been purchased to stand alongside the FMS, in addition to which a manual finishing station is positioned next to the second Okuma.
 
Novel profile cutting is fundamental to short cycle times
 
An unusual, patented process called Turn-cutting is employed to turn bores and seal grooves during demanding machining cycles on two SCM components, the 316 stainless steel hydraulic manifold block and a mild steel base plate. The other two elements of the module are stainless steel high-pressure and low-pressure adapter plates. Around a dozen variants of the set of four components are required, according to size of SCM.

Thirteen holes on the underside of the square-section manifold block are machined to within 50 microns tolerance on diameter using Turn-cut software running in the proprietary OSP-P200M control on the Okuma machining centres. Turn-cut features are a 160 mm central bore, M58 holes around it that require thread relieving at the back, and six smaller ports along one edge of the square.
 
The Turn-cut process, which is carried out in the same clamping as prismatic machining operations, involves circular-interpolation of the machine's X and Y axes using up to 60 m/min linear feed rates as the spindle moves back and forth in Z. While the spindle head is describing a circle, a turning tool clamped in the spindle is made to rotate at precisely the same speed and in synchrony with the interpolated motion. The tool cuts at the correct rake angle at every point throughout the 360 degrees to ensure efficient and precise metal removal.
 
The technique is faster as well as more cost effective and accurate than if a facing head had been used, or if transfer to a turning machine had been necessary. Moreover, tapers and profiles can be generated simply by varying the amplitude of circular interpolation.
 
The SCM component in production on the day that FMC Technologies was visited was the steel base plate. This also requires Turn-cutting of three small bores and a 635 mm diameter, profiled groove around the periphery to within 50 microns. The X/Y feeds are programmed to produce circular movement at 38 revs/min while a 4 mm wide grooving tool, turning at the same speed in the spindle, cuts a 4.5 mm deep groove in one roughing and one finishing cycle. Renishaw probing between the operations provides feedback data to adjust the offset for the final pass. The same probing system checks that the billet is datumed correctly at the outset, after automatic pallet transfer, before machining commences.
 
Although the above Turn-cut feature is circular, the flexibility of synchronising X/Y interpolation with tool orientation in the spindle and combining these motions with Z-axis infeed allows different shapes to be machined. Indeed, a nominally square groove with rounded corners, positioned inside the circular groove, also needs to be machined into the base plate. It is produced by another technique known as Tool Groove Function and its implementation at Dunfermline is believed to be a first in the UK.
 
In this case, an axial face-grooving tool is held static in the spindle while machining the first straight. At the first corner, the tool enters Turn-cut mode to machine a 90-degree arc before the start of the next straight. The process continues in a similar manner three times more until the closed path is complete. During the 24-minute operation, the groove is first rough milled, finish machined using the Tool Groove Function at 35 m/min, taking a 50 micron depth of cut per revolution, then deburred with a 100 mm diameter Xebec brush.
 
The latter has proved to be an economical way of removing the slight burrs produced during previous operations, as the cost of manual finishing has been removed. The brush comprises hundreds of white alumina fibre rods, each consisting of hundreds of 15-micron diameter filaments. The rods are clamped in a holder and prevented from splaying by a metal sleeve that reaches almost to the tip, where the cutting action is. The sleeve is retracted manually from time to time to compensate for rod wear, requiring periodic offsets to be entered into the Okuma control.
 
Applications engineering
 
A considerable amount of applications engineering input from NCMT was required to make the FMS work to the required levels of consistency and accuracy and to achieve economic cycle times. To perfect the above special boring and grooving techniques, engineers from the supplier undertook early trials on one of the Okumas in its own technical centre in Thames Ditton before the second machine was delivered to FMC Technologies.
 
NCMT engineers subsequently worked in Dunfermline for several weeks during the first quarter of 2012 to optimise the FMS for lights-out production, consistent with achieving a process capability standard of Cpk 1.67.
 
Innovative ideas included upgrading the Okuma control to augment spindle control performance for Turn-cutting and Tool Groove Function; and going against the recommendations of a front-end tooling supplier by developing a pecking cycle for solid carbide drills to maximise productivity during the many occasions when interrupted cutting occurs across hole intersections.
 
Another innovation, designed to simplify FMS communication, involves the use of a chip attached to each BIG Daishowa toolholder containing 10 lines of information on the cutter, including its length and radius offsets and remaining life. When a tool is exchanged into an Okuma spindle, the data is corroborated by a Renishaw NC4 laser probe supplied with the machines.
 
The Fastems control system has an integral tool management module to track the movement of tooling throughout the system via its connection with the Okuma CNCs. It is not necessary to link the Speroni toolsetter with either the machining centres or the Fastems control to manage the tooling within the FMS, as comprehensive information is resident within each tool's chip.
 
                       
 









Conclusion
 
When a management decision was taken stainless steel manifold in-house, as well as that of its three sister components, early attempts to machine the part in multiple operations on existing plant in the factory resulted in a lot of manual intervention and cycle times of over 100 hours. Today, that has been reduced to 35 hours, with a further reduction to 30 hours firmly in sight, while the process is far more reliable and repeatable.
 
When laying the foundations for the FMS, provision was made to add a third Okuma MA-600HB horizontal machining centre, ensuring sufficient capacity for all SCM modules well into the future. Additionally, the manufacture of other high-added-value parts may be transferred to the FMS. Production of new components is already being considered, one of which has a drawing tolerance of 17 microns on diameter, ideal for Turn-cutting.
 
The new technology will further improve the Dunfermline manufacturing facility's reputation and performance. Production Manager, Adam O'Sullivan, concluded, "This project marks a step-change in our production capability. The investment is recognition of the world class performance of the Dunfermline factory and keeps us at the forefront of manufacturing technology within the FMC group."
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