Titanium machining - tough challenge met

Makino has developed three horizontal machining centres machines, T2, T2.5 and T4, for machining titanium, and it was the T2 that saw its European launch at a July event held at NCMT's Coventry showroom. The machine is a 5/6-axis horizontal machining centre and there's, perhaps, only need of one particular statistic to underline its power – it is able to hog up to 500 cm3 of Ti-6Al-4V per minute and will be capable of much higher metal removal rates when cutting tool technology catches up. But even so, this current figure is five times faster than the industry average, it is claimed. However, the machine also sets itself apart by being able to additionally undertake 5-axis simultaneous finishing to very high precision. The T2, which has a 2,000 by 2,000 by 1,800 mm working envelope and a compact, 7 m by 9.8 m footprint, incorporates a significant technological advance, in that it has three rotary CNC axes, hence the 6-axis moniker. The ±110° A-axis and 360°C-axis are on the spindle head, while there's a B-axis table that provides 360° of continuous movement. Image: Demonstrations on the day, showing of Makino's T2's capabilities A CHOICE OF AXES Only two of the three rotary axes may be interpolated simultaneously with the linear axes, but the option of two pairs of rotary axes offers the user the most appropriate choice for the part under consideration. For machining airframe parts, interpolation of A and C axes, with positioning in the B-axis, is best; for engine casings and other circular components, A/B interpolation and positioning in C-axis is preferred. In either case, the sixth CNC axis can be repeatedly repositioned during a machining cycle. Benefits of being able to choose between the 5-axis modes to suit the job include reduced cycle times and better surface finish. Titanium has high strength-to-weight ratio, even at high temperatures, and has high corrosion resistance, compared with other alloys. It is notoriously difficult to machine, not least because it has a low modulus of elasticity and is easily deformed during metalcutting. Since titanium is much springier than steel, it causes chattering when milling, as well as problems when drilling and tapping. Perhaps the biggest problem when machining titanium, however, arises from its low thermal conductivity, which makes it difficult to remove heat from the point of cutting, raising the temperature and contributing to a chemical interaction with the cutting tool. The high shearing angle formed ahead of the cutting edge generates a large bearing load on the tool face. In addition, titanium tends to work-harden during machining, making it even more difficult to cut. Overall, it means that machining titanium alloys takes longer than other, more common, aircraft metals, and tools are worn out faster, resulting in high machining costs. Excessive vibration and temperature are the main enemies. There are numerous requirements when designing a machine to meet these challenges, the principal ones being: high feed forces and a rigid structure to counter those forces; good damping to resist vibration; a dynamically stiff, high torque spindle; and high volume coolant delivery for efficient cooling at the point of cutting and rapid evacuation of swarf. Makino has delivered on all these, alongside automatic pallet change, and integrated pallet transfer and storage options. NINE-FOLD IMPROVEMENT Mr Inoue, general manager of R&D at Makino in Japan, claimed in a presentation during the European launch, that T-Series machines, which include the larger T2.5 and T4 models, deliver a nine-fold increase in tool life when cutting titanium, compared with typical gantry-type machining centres. In addition, the technology allows four times faster surface speeds to be used. For an example component, a reduction from 45 hours to 11.1 hours was achieved when a gantry machine process was replaced by T2. The technology behind this performance was highlighted. The T2's HSK-A125, integral-drive spindle is Makino's most powerful to date, providing 1,000 Nm of continuous torque (1,500 Nm peak) up to 1,000 rpm, 150 kW of power and a tool clamping force of 9.8 tonnes, all values significantly higher than for previous integral-spindle motors. The 4,000 rpm (optionally 8,000 rpm) spindle is of compact, 2-axis design and is supported by a generously sized (150 mm diameter) roller bearing at the front to ensure high rigidity. Absence of gears helps to minimise vibration, as well as reduce energy loss, making the machine less expensive to run, it is said. Direct drive from the induction motor is more compact than if a gear train were to be used and offers twice the moment of inertia, smoothing entry of the cutter into the material and its subsequent exit. The A-axis is driven through dual helical gears from both sides, rather than just one, allowing a continuous cutting force of up to 20 kN; no longer is this rotary axis the weak link in the 5-axis chain, stressed NCMT's technical director, Adrian Maughan. Workpieces up to five tonnes are supported by the T2's 1 by 1.25 m pallet, which travels at up to 16 m/min on four, integrally cast and quenched box slideways in the Z-axis. The four slideways present a large surface area that increases the ability of the structure to dampen vibrations. To further improve matters, active damping in the machine's box slideways is another feature that minimises cutting vibration. The pallet and component float on an air cushion of approximately three microns thick, such that friction remains constant, irrespective of workpiece weight or cutting forces. Having minimised vibrations and maintained a rigid structure, cooling is the next issue tackled, and the T2 offers plenty of coolant. Two hundred litres per minute of 70 bar coolant is supplied through the spindle, while an additional 200 l/min is distributed via an overhead shower, with a further 200 l/min through nozzles around the spindle. Accuracy is further promoted by pumping fluid around the column to stabilise heat distribution and avoid the common problem of temperature differential around the structure. The linear and rotary axes are compensation mapped, giving volumetric accuracy of 5 microns throughout the entire machining volume, under controlled machining conditions. Image: Titanium parts like these can be produced more efficiently on the T2 and its sister machines PROTECTING THE INVESTMENT Makino's process protection technology is in use on the T2 to safeguard the machining centre, component and tool, so avoiding damage, as well as unscheduled stoppages in production. This sees the spindle head monitored for vibration and force in three planes. Setting thresholds allows a warning to be given, if values reach, say, 6 G, and automatic machine stoppage, if 8 G is exceeded. Similarly, cutting force is plotted in X,Y and Z, and the feed rate automatically adjusted to keep it within predefined limits. To prevent collisions, Vericut simulation of the entire machining area, including spindle, tool, component, fixture and table, is used, but differently from the norm. Instead of being run off-line, the relevant data is downloaded with the NC program and the virtual cycle runs almost in real time, lagging the actual cutting paths by a few milliseconds – just enough time to stop the machine, should a collision situation be predicted. Analysis of stoppage data, giving machine status and utilisation throughout a shift and reasons for periods of downtime, also forms part of the Makino process protection package. In addition to the machine, the European launch highlighted that Makino was working with Idemitsu Lube in developing coolant specifically for titanium machining and provide lubricity at the elevated temperatures of 600° or more. Certification for use in the aerospace industry is expected next year. In addition, Makino is working on the development of a fundamental model for titanium machining that will be implemented within NC units, aiding high performance machining of titanium alloys, with maximised tool life. This development will also likely appear next year. First published in Machinery, September 2011