A 6-axis showcase

In the past four years, Northern Ireland-based tier one airframe component supplier Moyola Precision Engineering has invested £6.5 million in its aerospace division. Over two thirds have been spent on three high-speed, 6-axis machining centres from Dörries Scharmann Technologie (DST), the latest of which was installed in November 2009.

Having an extra CNC axis in addition to the more usual five axes results in significant productivity and cost benefits when machining aerostructural parts. The advantage here is that deep recesses and other awkward areas can be accessed and machined at high speed by short cutters. If the same parts were machined on conventional 5-axis machines, significantly longer tools would be needed for clearance with a consequent risk of chatter adversely affecting surface finish. The likelihood being that cutting speeds and feeds would have to be reduced which could compromise productivity.

The kinematically driven Sprint Z3 head on the two DST Ecospeed F2035 machining centres tilts through ± 40º in the vertical A-axis and horizontal B-axis, making it suitable for many machining tasks. However, the ability to automatically replace the in-line spindle with a right angle milling head in under three minutes allows steep angles up to 130º to be achieved. By interpolating the 360º C-axis rotation of the head with the X, Y, Z, A and B axes, even difficult to reach areas on aircraft parts can be machined easily with short, rigid tooling.

And whether the in-line or right angle head is in use, advantage can be taken of the 120 kW/30,000 rpm spindle on the latest F2035 and of the 80kW/30,000rpm spindle on a machine installed in 2005. Unlike many high speed machining centres, it's unnecessary to be operating at maximum rpm to achieve full power, which is available at speeds as low as 13,800rpm. This added flexibility enables cutting speeds, feeds and depths of cut to be optimised to achieve maximum performance for each cutting tool assembly.

Fit, form and function

Aircraft manufacturers demand interchangeability of components so they assemble first time without the need for fitting on the factory floor. Features must be machined to tight positional tolerances to ensure accurate assembly across different temperatures in a range of environments.

The DST machines are said to deliver this accuracy without any problem, but speed and precision of machining are not the only factors that dictate profitability. As machining costs go down, the value added by metalcutting is often less than the cost of the material. It's important to minimise billet wastage and maximise material utilisation by producing as many parts as possible from each billet.

This is done by Moyola engineers programming in Catia and highlights another benefit of the Ecospeed 2035s: namely the 2.5m Y-axis - larger than on any other machining centre on the shopfloor. More components can be nested into each billet, which can be up to 2m x 3.5m. The additional width is useful when machining batches of curved fuselage ribs, with smaller parts placed in unused areas close to the billet edges. Reduced frequency of set-up is a further benefit of using larger billets as it maximises spindle uptime and productivity.

“We achieve large savings by adopting nesting methodology,” comments managing director, Mark Semple. “The technique adds extra margin to every part we manufacture and can even make the difference between a job being profitable or not. Alternatively, larger single parts may be accommodated on the table, which gives us extra flexibility as it extends the variety of work that we can quote for.”

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