Sky high demand

NCMT and ATI Stellram recently toured the country, taking their titanium machining message to several interested groups. Machinery sat in on one of the events
The two big UK drivers for titanium machining are its use on the F32 Lightning II programme (previously called JSF – Joint Strike Fighter) and Airbus' A350. And, in April 2008, one of the world's largest titanium producers is quoted as saying that demand for titanium in airframes is expected to approximately double by 2015.

Not surprising, then, that Makino, claiming to be the world's largest produce of horizontal machining centres, has developed two new machines specifically for machining large titanium components, it was revealed – the T4 (4 m in X; 4,000 by 1,500 mm pallet), launched in March, and T2 (2 m in X; 1,250 by 1,250 mm pallet), not yet 'on the shelves'. But the pedigree behind the machines was emphasised. Makino first introduced a titanium-focused machine in 1985, and it set up a global aerospace committee in 1992 to feed into the design of machines for that sector. Latterly, NCMT has been performing combined titanium machining trials with ATI Stellram at the former's Thames Ditton site, using a Makino a81M machine – more of that later.

PRECISELY TUNED MACHINES
The two new machines, emphasised Adrian Maughan, NCMT engineering director, have resulted from conversations with end users about not just aerospace, but a sector within that, namely airframes. The machines are, therefore, precisely tuned to the machining needs of titanium airframe parts, he advised.

To set the scene, Mr Maughan explained the challenges that titanium presents. It has a 30 per cent higher specific cutting energy than does stainless steel; the thermal conductivity is 65 per cent lower than stainless steel, so the tool is exposed to more heat; titanium is reactive, oxidising and adhering to the cutting tool; it work hardens; and it has a lower modulus of elasticity, so it is springy – twice as springy as stainless steel, in fact, which means that thin webs at tool break out, for example, may spring into the tool and cause tool damage and, because of that, component damage. (Particular cutting strategies must be employed to avoid such 'bad' situations.)

So, the engineering director says: "We started with a blank sheet of paper and asked what machine features are required to prolong tool life and reduce consumable costs. Vibration is the enemy of tool life, so rigidity is required, as is high damping. And although we wanted a 5-axis machine, it needed to have the higher capability of a 4-axis machine, as regards metal removal. We would need through-spindle coolant to remove heat from the cutting zone, and compact A and C axes [T4 only; T2 is A at the spindle with B on the table], plus high dynamic stiffness at the spindle."
The company started with the spindle, Mr Maughan explains, and built the machine around that. First off, roller bearings, rather than ball bearings, were used, as they have 2.7 times the capability of ball designs. The 150 kW, wide torque range motor is able to deliver 1,500 Nm (1,000 Nm continuous) up to 1,000 rpm via a twin, integral motor design. The stressed benefit is that, typically, maximum torque would fall off after 400 rpm. And the integral motor design, with the elimination of gears, contributes both to a reduction in drive-chain-induced vibration, and delivers increased moment of inertia – double that of a geared spindle, it was claimed. All this taken together provides the required 'grunt' and, equally as important, rpm stability as cutters move in and out of cut.

Through-spindle coolant, pumped at 200 litres/min, resulting in 175 litres/min on the machine side, features. But, in addition, there's another 200 litres/min pumped through nozzles surrounding the spindle, and a further 200 litres pumped for shower and Z-axis flushing.

A compact, high torque A-axis design was next. Makino drew on its backlash-free, trunnion-style design first introduced in 1982 for this, providing drive at either side of the spindle, and delivering 10,000 Nm versus a traditional worm and wheel value of 4,000 Nm.

A vibration monitoring system during cutting is also used. This function enables vibration thresholds to be set for warning level and machine stop. The objective of this technology is to stop the machine before cutter breakage, thereby avoiding/minimising damage to the cutter body and machine tool. This technology is very useful during prove- out, as warnings are recorded for interrogation where cutting data is found to be too aggressive.

The avoidance of these situations is achieved by changing the cutting technology – cutting depths, speeds, feed rates – to an area of lower harmonics. But while this is easier in softer materials, where there is a wide band of cutting parameters, in the case of titanium the parameters are narrower. For example, in aluminium this could mean changing surface speed by up to 300 m/min, but for titanium, surface speeds for roughing are between 35-90 m/min, while for finishing they are 100 to 150 m/min, so there is "no big window to play with," Mr Maughan offers, adding that you therefore require a highly rigid and damped machine tool which offers a wide stable application window.

A particular machine feature highlighted in this regard is box guide ways, four of them supporting the pallet (Z-axis), each approx 300 mm wide and approx 2.5 m from one side to the other – lots of surface, so lots of carefully controlled friction and, therefore, damping. But friction isn't so good when you want a slide to move. To counter this, Makino's air pocket system is employed to provide some lift between the surfaces, with this effect controlled via an 'air micrometer'. Additionally, the Y-axis (vertical movement) features a counterbalance to ensure equal dynamics, up and down.

STABILITY IS COOL
Thermal stability, achieved via the pumping of liquid through the column and table, is yet another technology employed, while volumetric error compensation can also feature. Of course, the machine could further be housed in a temperature-controlled environment to provide greater stability.