Emerald Group Publishing Limited
Copyright © 2008, Emerald Group Publishing Limited
Subcontractor moves into five-axis machining
Article Type: Aerospace world From: Aircraft Engineering and Aerospace Technology: An International Journal, Volume 80, Issue 4
Next time you see a bomb-disposal robot in action, hopefully on a television screen, it will probably be carrying a stainless steel component machined by Turntech Precision Engineering on its new OKK five-axis vertical machining centre (VMC).
Supplied in September 2007 by Whitehouse Machine Tools, UK agent for the Japanese machine builder, the VP400 machining centre is the first five-axis prismatic metalcutting machine at the subcontractor’s Wimborne facility. A workpiece setting probe and tool breakage detection from Renishaw have been fitted to speed set-up and minimise scrap.
According to Managing Director, Tony Pigott, the OKK was bought primarily to help Turntech meet inexorable “cost-down” demands imposed by the aerospace industry. About a quarter of the firm’s business is in this sector, so serious attention has to be paid to maintaining profitability when machining the diverse range of components, many of them complex.
Commented Mr Pigott, “We produce a typical aerospace part in two hits on the five-axis machine whereas it previously required three, four or even five operations, often on two different machine tools.”
“Actual cycle times are not much different, but savings in handling and reclamping components make an enormous difference to the cost of production.”
“I estimate that if we cut the numbers of ops down from three to two, there is a 25 per cent cost saving, and if the part previously needed five ops, the saving is more like 50 per cent.”
In one exceptional case, an aluminium component had to be machined on six faces in six separate clampings on a three-axis VMC. The same component is now produced in two set-ups on the OKK. Economies on this scale are allowing the subcontractor to win more business, pay for the five-axis machine and maintain a reasonable margin on aerospace contracts.
When interviewed in February 2008, Mr Pigott said that the machine had been used only in three-axis simultaneous mode, using the rotary table and its supporting swivelling trunnion in fixed positions to orientate components for more efficient access. However, he is actively looking for fully interpolative five-axis work to maximise the machine’s potential.
Having won the bomb disposal component contract, Turntech realised that the required cutting cycle was deceptively complex and ideal for producing in two operations on the OKK five-axis machine. The part is made from high-tensile strength 420 S 37 stainless steel that is difficult to machine. Particularly, testing was the stipulation that two closely spaced holes had to be drilled into the curved side of the cylindrically shaped component to a pitch limit of +50 μm, −0.
As to his choice of the OKK machine rather than one of the other, similarly configured five-axis machining centres on the market, Mr Pigott cited the machine’s quality and rigidity as well as its small footprint, which is important with land prices high in the south of England.
He also pointed to the good relationship formed with the supplier since Whitehouse sold the subcontractor an Italian-built Biglia Y-axis, mill-turn centre in 2004 and subsequently two Brother turret-type machining centres.
The first of these, a TC-S2C, was installed in February 2007 to improve the efficiency of small prismatic parts manufacture. One aluminium medical component in particular was causing a problem, as the three VMCs at Wimborne were too large to machine them efficiently. What was needed was a nimble machine with fast rapid traverses and quick tool changes to reduce cycle times.
A 30 per cent like-for-like saving in machining time was achieved and in addition, by loading the components 16 instead of eight at a time, an overall increase in productivity of 40 per cent resulted. Batches of 1,000 per month of each of two mating parts are produced to 50 μm total tolerance on critical dimensions.
Turret-type machining centres are often considered suitable only for light duty machining applications, but the next job put on Turntech’s TC-S2C disproved that view. It was a 316 stainless steel valve block machined on three faces, one of which required two 1/4 in. NPT ports to be drilled from solid and thread milled. The machine regularly produces batches of 500-off without any problems. It even machined 1/2 in. NPT ports on one occasion.
As the TC-S2C became busy with the valve work, another turret-type machining centre was needed to take over production of the aluminium medical components and to increase overall capacity for this type of work. A Brother TC-S2A was duly installed in August 2007, which Mr Pigott regards as very good value.
It is an entry-level model reintroduced by the Japanese manufacturer mainly for the Chinese market and costs nearly 40 per cent less than the TC-S2C. Admittedly it has a more basic control, lower spindle power and no through-tool coolant, but it is as fast as the more expensive machine and produces the aluminium parts just as quickly to the same accuracy. It even drills the side mounting holes in the stainless steel valve blocks.
Speaking generally of Brother turret-type machining centres, Mr Pigott says that their lower mass compared with traditional VMCs means that they are more quickly up to operating temperature and stay there reliably.
Speeds and feeds are faster and so also is tool change time. These factors lead to higher productivity across all of Turntech’s smaller components. The footprint of the machines is compact, saving space on the shop floor.
The subcontractor’s first dealings with Whitehouse was in 2004, when Mr Pigott was focusing on reducing the number of operations needed to produce stainless steel valve blocks. The production method at that time involved four operations on two different machines, a lathe and a machining centre, and Mr Pigott was aiming at single-hit production. A twin-opposed spindle turning centre with Y-axis and live tooling was identified as the way to achieve it, but none of the five CNC lathes on site had that capability.
Concluded Mr Pigott, “We looked at a few suppliers and compared cost and specifications, but it was the approachability of the Whitehouse staff that decided us to select them as the supplier and opt for the Biglia B 301 YS mill-turn centre.”
“The decision has paid off, as not only did we halve floor-to-floor time producing certain valve bodies in one hit on the Biglia lathe, but it was also the start of a successful relationship that has seen our subsequent installation of highly productive prismatic machining equipment from the same source.”
Details available from: Whitehouse Machine Tools Ltd, Tel: +44 (0)1926 852725, Fax: +44 (0)1926 850620.