It was the use of the recently introduced Aurora DLC 1500 (diamond-like carbon) insert coating in combination with the SumiDrill WDX series of drills from Sumitomo Electric Hardmetal Ltd, Princes Risborough (
www.sumitomo-hardmetal.co.uk), that transformed troublesome drilling operations on a range of lithium aluminium aerostructure components at Magellan Aerospace UK — a £70 million-turnover company based in Wrexham.
Although Magellan had successfully standardised many of the processes involved with machining wing ribs, butt straps, thrust fittings, access panels and wing spars, there remained a recurring drilling problem on its rib production.
Despite significant process changes and trials using a variety of tools from different suppliers, no tool could better 10 holes before inserts needed changing or total tool failure occurred. Even the use of solid-carbide tools, which made heavy inroads to the tooling budget, did not come close to providing an acceptable solution.
With some parts machined on a recently installed Dorries Scharmann DST Ecospeed F2060 fixed-column horizontal machining centre having machining cycle times of more than 14hr, maximum tool utilisation was important. The Sumitomo WDX drill combined with its DLC-coated inserts immediately overcame the problems Magellan had been experiencing — namely, built-up edge, which in turn caused immediate overheating, welding of swarf in the drill flute, and seizure of the tool in the hole being processed.
Previous supplier trials involving different tool types and processes included solid-carbide drills, pilot drilling, slot drilling and revised speeds and feeds, but all had failed to overcome these problems. Even with the use of MQL (minimum-quantity lubrication), swarf control was proving very difficult, but Sumitomo engineers say that this system works very efficiently as part of the new drilling set-up.
Bolt-down holes
Magellan produces a variety of wing ribs; these are milled from 100mm-thick plate that can have as many as 150 bolt-down holes, some with counter-bores that are 14, 20 or 26mm in diameter and up to 70mm deep. In fact Magellan’s Wrexham site is currently having to produce up to 1,500 such holes a month.
The root cause of the problem was the change to lithium aluminium alloy on some of the aerostructure parts. This advanced aluminium alloy has a high strength-to-weight ratio, a lower density and an improved elastic modulus; it contains 2-3% lithium copper magnesium alloying elements. While this material gives the designer the benefits of greater resistance to factors such as fatigue cracking, it is very difficullt to drill — especially on deeper holes.
The Aurora DLC 1500 coated insert was originally developed for non-ferrous materials in order to present a high hardness with a lubricated cutting surface (it creates a co-efficient of friction of between 0.05 and 0.2). As Magellan engineers have found, this highly effective coating allows the inserts to precisely control chips and suppress the formation of built-up edge. As a result, a consistent penetration rate can be maintained — plus the in-cut life and reliability of the tool are greatly extended.
When used in the Sumi-Drill WDX high-feed-rate drill body, the adjacent inserts with inner and outer cutting edges provide a balanced cut and the ability to drill from solid on a plain surface. Moreover, the drill is so rigid and stable that it can even be used to mill an outside diameter, drill with half the wall of a hole exposed and be started on an inclined surface or in a pre-drilled hole.
Diameter-to-depth ratios can be up to 5¥D, and the design of the tool is such that it creates an almost flat-bottomed blind hole, which is ideal for counter-boring applications.
Chip formation John McGrail, Magellan’s project engineering manager, said: “The initial trial using a 26mm-diameter WDX drill produced 200 holes with no apparent wear on the insert.
"Chip formation was excellent without having to resort to a pecking action; we were subsequently able to increase speeds and feeds and run the tools 25% faster. On certain holes that have deep 20mm-diameter counter-bores, we are now running 60% faster than before.”
Magellan is currently achieving insert cutting life of 50min per edge on the Ecospeed machine; and because the inserts have four cutting edges, they are now programmed to be changed after 200min.
As part of the company’s drive towards standardisation of processes and improved tool inventory control, the use of Sumitomo’s SumiDrill WDX and insert solution will be progressively introduced for drilling ‘normal’ aluminium aerostructure components.
On the lithium alloy components, the WDX drill body with DLC 1500 coated inserts is being run at 13,500rev/min with a feed of 0.075mm/rev for 14mm-diameter drills producing holes 50mm deep. When drilling 20mm-diameter holes that are 70mm deep, the tool is run at 10,000rev/min and 0.15mm/rev feed.
In addition to its site at Wrexham, Magellan has others in Blackpool (where hard-metal components up to 1m cube are produced), Bournemouth (for machining aluminium parts from 500 to 2,000mm cube), Chalfont (for aluminium components up to 500mm cube), and Greyabbey (for wing ribs up to 4m long).
Adrian Young — the operations manager at Wrexham — says: “The ability to produce large, complex aerospace components for Airbus has allowed our operation to grow from £20 million in 2004 to £70 million this year.” The site employs 400 people and is investing some £4 million a year in new equipment; it is expected that the business will achieve a turnover of some £85 million within the next five years.
This means that machine utilisation is critical in helping to control capital expenditure, and solutions such as the Sumitomo drilling application are important in achieving our objectives,” concludes Mr Young.