CitationDownload as .RIS
Emerald Group Publishing Limited
Copyright © 2009, Emerald Group Publishing Limited
New machining centres for cutting titanium
Article Type: Aerospace technology From: Aircraft Engineering and Aerospace Technology: An International Journal, Volume 21, Issue 6
As new aircraft have to fulfill ever-higher standards of efficiency to lower emission levels, increasing use will be made of titanium due to its lightness and strength. Of the extra-wide-body version of the Airbus A350 by weight, 15 per cent will be titanium, compared with 5 per cent of the A300 and 9 per cent of the A380. Titanium is used widely in all three variants of the F-35 Lightening II (Joint Strike Fighter) and Lockheed Martin has identified its machining as a significant cost driver.
With this growth potential in mind, as well as increasing applications in other sectors such as marine and petrochemical, Japanese machine tool builder, Makino, has introduced two new five-axis, horizontal machining centres specifically for manufacturing components efficiently from titanium and its alloys. Designated Makino T4 and T2, the machines are available in Britain and Ireland through sole agent, NCMT.
Titanium has high strength-to-weight ratio, even at high temperatures, and has high-corrosion resistance compared with other alloys. However, it is notoriously difficult to machine, not least because it has a low elastic modulus and so is easily deformed during metalcutting. Since titanium is much springier than steel, it causes chattering when milling and problems when drilling and tapping.
Low thermal conductivity, one-third that of stainless steel, makes it difficult to remove heat from the point of cutting, increasing 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 – exactly what aircraft manufacturers do not want. Excessive vibration and temperature are the main enemies.
There are numerous requirements when designing a machine capable of overcoming these problems and processing titanium faster and hence more cost effectively, while still maintaining good surface quality and tool life. The principal requirements are: high-feed forces and a rigid structure to counter those forces; good damping to resist vibration; a dynamically stiff, high-torque spindle; high-volume coolant delivery and efficient evacuation of cutting chips.
Makino was quick to take up the challenge of delivering a commercially viable solution. The manufacturer claims that its T-Series machines combine the roughing capacity of a four-axis machine with five-axis finishing capability, so only one machining centre is needed rather than two, reducing capital cost.
Working envelope of the T4 is 4,200×2,000×1,000 mm and axis feed rates are up to 16 m/min, allowing the efficient production of sizeable airframe parts such as bulkheads, edge frames and pylons.
The machine's HSK-A125, integral-drive spindle is Makino's most powerful to date, providing 1,000 Nm of continuous torque (1,500 Nm peak), 150 kW of power and a tool clamping force of 10 tonnes, all values significantly higher than for previous integral spindle motors.
The two-axis spindle is of compact design, combining the latest spindle motor technology to deliver high torque over a wide speed range from 20 to 1,000 rpm. The spindle is supported by a generously sized (170 mm diameter) roller bearing at the front to ensure high rigidity.
Absence of gears helps to minimise vibration as well as reducing energy loss, making the machine less expensive to run. A gearless drive is also more compact and offers twice the moment of inertia of a spindle driven through a gear train, smoothing entry of the cutter into the material and its subsequent exit.
Combined A-axis movement of ±110° and 360°, continuous C-axis rotation allow good accessibility to the component being machined. Drive to the A-axis is from both sides rather than just one, enabling a torque of 10,000 Nm. Preloaded, tapered gears eliminate backlash. Vibration monitoring in three planes and the setting of maximum thresholds allows automatic cessation of machining if parameters are exceeded.
The T4 has a very rigid structure to minimise cutting vibration, under even the toughest cutting conditions. Workpieces up to 5 tonnes are supported by the 4×1.5 m pallet, which travels 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 damp vibrations. To counter the increased friction, Makino has developed an air pocket system to provide some lift between the surfaces, controlled by an air micrometer.
A high-pressure, high-flow coolant system delivers 200 l/min of water-soluble coolant at high pressure (70 bar) through the spindle to the cutting zone. Porcupine cutters commonly used to rough titanium have up to 35 coolant holes, so a large coolant volume is needed to ensure a reasonable flow through each.
An additional 200 l/min of coolant is delivered via an overhead shower plus a further 200 l/min through nozzles around the spindle. The high-coolant volumes and pressures ensure the all-important temperature reduction at the point of cutting, as well as efficient chip evacuation, underpinning accurate machining of large titanium parts over long periods.
The T4 can be equipped with an automatic pallet changer for continuous operations, eliminating costly downtime for part changeovers. The machine can also be integrated with an automatic pallet transfer and storage system in a flexible Makino machining complex for extended periods of unattended operation. This automation system assigns work and initiates operations automatically, maximising spindle uptime for increased productivity.
Tests in Japan have demonstrated the T4's impressive metal removal rate. Using an 80 mm diameter porcupine mill at 245 rpm with 600 Nm of torque, 500 cm3 of Ti-6Al-4V was removed in 1 min, five times faster than the industry average of 100 cm3/min.
Most recently introduced for machining titanium is the more compact (7×9.8 m footprint) Makino T2, which has half the travel in X of 2,000 mm, the same in Y (2,000 mm) but a larger Z-axis travel of 1,600 mm.
The machine is available in two versions. One is similar to the T4 with an identical A/C-axis spindle but with additional indexing capability on the table. The other version has a spindle with A-axis movement only, the table and 1.25×1.25 m pallet performing the B-axis movement in 0.0001° increments (Figure 1).
Details are available from: NCMT Limited, Tel.: +44 (0) 20 8398 4277, Fax: +44 (0) 20 8398 3631.