As the aerospace industry’s demand for titanium continues to climb – according to some sector observers, global usage could grow by 40% this year – the Starrag Group (0121 359 3637) continues to reinforce its world technology lead in the machining of titanium, a material that offers high tensile strength-to-density ratios and is highly resistant to corrosion and fatigue.
During the benchmarking event, Starrag gave three demonstrations on:
- BTP 5000/2 twin-spindle Profiler
- Five- and 6-axis horizontal machining centres STC 800–1800
- Five-axis titanium profilers EcoForce Ti 9, 13, 24 and 30
Each machine showed impressive chip removal capability, as well as cost-effective production routines – with the claim that each was unrivalled in its class.
Taking centre stage, the BTP 5000/2 5-axis twin-spindle Profiler was shown simultaneously machining two airframe spar segments of Ti6Al-4V. Measuring about 3,300 by 700 by 80 mm, the parts start life as 317 kg billets and are reduced to 15 kg finished parts in 28.3 hours.
The 37 kW (52 kW optional), 8,000 rpm Profiler, which boasts spindle torque of 940 Nm, machines at depths of cut of up to 100 mm, achieving a material removal rate of 750 cm3/min per spindle/min, compared to the usual 200-300 cm3 for hard metal machines.
The twin-spindle BTP 5000/2 Profiler was a highlight of the demonstrations
A number of innovative machine features contribute to this performance, with these supported by Starrag applications engineers’ balancing of cycle times with tool life/cost and workpiece quality.
Such machine features include a geared spindle and steel-on-steel worm gears, as well as the clever use of a damping brake within the highly rigid A-axis swivel head, which operates in much the same way as a car clutch system. With this system, heavy roughing becomes a reality, even in 5-axis simultaneous operation.
In addition to that, 100-bar, temperature-controlled through-tool coolant allows for higher cutting speeds and tool life. It is complemented by the use of flood coolant to minimise the effects of the inherent excessive build-up of heat at the cutting face, which could lead to distortion of the workpiece and reduced tool life.
In support of its machine technology and expertise, Starrag engineers also adopt best-in-class tooling, taking in Ceratizit, Kennametal, Mitsubishi and Walter brands, together with the company’s in-house tool production expertise, to achieve the most cost-efficient production solution to each unique scenario.
The BTP Profiler demonstration involved:
- Initial roughing, using a 160 mm diameter face-mill at a cutting speed of 70 m/min and a depth of cut of 8 mm
- Contour roughing, using a 63 mm diameter porcupine cutter at speeds of 40 m/min and taking 65 mm depths of cut
- Pocket roughing, using a 25 mm end-mill at 70 m/min and a 50 mm depth of cut
- Finishing, using a 25 mm end-mill at 40 m/min and a 30 mm depth of cut.
BTP machines are available with pallet lengths from 4,500 to 8,000 mm, and X-, Y- and Z-axis travels of 5,000 to 8,000 mm, 1,300 to 1,850 mm and 1,200 mm, respectively. The A-axis is ± 100º and C-axis is 360º continuous. Feed rate for all linear axes is 25 m/min, with 14 rpm possible in both rotary axes.
Starrag also offers a range of STC machines, extending from the 800/130 model with axis strokes of 1,450by 1,200 mm, up to the STC 1800/130 for components of 3,300 by 2,200 mm.
The STC’s articulated head is available with the same geared spindles as the BTP, ranging in speed from maximum 5,600 to 12,000 rpm, offering 37 to 52 kW spindle power and continuous torque of from 620 to 1,300 Nm.
Crucially, the use of the optional 2-axis head (as used on the BTP machines) effectively transforms STC models into 6-axis machines, when a B-axis table is fitted.
The machine line-up is completed by a series of single-spindle titanium profilers, the Ecoforce Ti machines. The Ecoforce Ti 9 offers the same 2-axis head as the BTP, while the Ecoforce T1 24 offers a 75 kW/5,500 Nm horizontal spindle or fork-type head of 50 kW/2,400 Nm. X, Y and Z travels are up to 6,300, 3,000 and 3,000 mm.
Starrag also used the Ti Benchmark Days to highlight how its Uptime Package, which can be embedded into the machine’s CNC system, helps avoid unpredicted downtime.
Innovative software routines continually condition monitor machine characteristics such as temperature levels, power consumption and frequencies across all main axes, as well as overall process conditions that includes monitoring of potential collisions. Where necessary, automatic alerts highlight any areas of concern.
PROPERTIES OF Ti6Al-4V
Described as the workhorse alloy of the industry, Ti6Al-4V is the most common grade of titanium alloy, with over 65% of all titanium produced being this grade. And since it is the most common, over 70% of all alloy grades are a sub-grade of Ti6Al-4V, according to www.azom.com. More generally, titanium makes up close to 10% of the empty weight of aircraft such as the Boeing 777, according to the Titanium Information Group.
According to Seco Tools, Ti6Al-4V has a uniquely attractive combination of strength to weight (the best of any material, in fact), corrosion resistance and formability. It is readily hot worked, but cold working is a challenge because, like most titanium alloys, it is very elastic, which mean spring-back is a characteristic.
The uses of Ti6Al-4V itself span many aerospace frame and engine components. Seco Tools says that about 17% of the weight of an engine is Ti6Al-4V, for example. It also has uses outside the sector, within the medical, marine, offshore and power generation industries.
In terms of machining characteristics, related to its elasticity, the machining of parts having thin walls is difficult, as deformation followed by spring-back occurs in thin wall areas. Metal build-up on the cutting edge is common, resulting in tool damage and poor surface finish. And Ti6Al-4V chips are flammable, too.
Positive rake chip grooves in combination with light insert edge hones are said to be advantageous, with PVD-coated carbide inserts having been the choice for the last 20 years. But, adds Seco Tools, today there is a trend back to uncoated grades in combination with high pressure coolant.
This article was originally published in the August 2015 Challenging Materials supplement of Machinery magazine.
