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Machining of titanium means milling at the limit

Difficult-to-machine metals such as titanium alloys, nickel-base materials or stainless steels have become standard in today's aerospace industry. To enable economic-efficient machining of these materials, manufacturers have come to use high torque values at moderate speeds – one of the many requirements that make HELLER solutions practically tailor-made for these operations. In-house trials have shown that many components involved in the machining process can come up against their limits during such operations. That is why comprehensive process control is essential for achieving high quality and economic efficiency in these types of high-performance machining operations.

As a typical system developer, HELLER does not only supply machine tools and manufacturing systems but also comprehensive solutions to cater to the needs of many different user industries. Apart from machine competence this requires profound process competence in terms of the machining task at hand. The goal is to offer customers the best cost-efficient overall solution for their specific workpiece. To achieve this, extensive basic research can be necessary. A typical example are difficult-to-machine materials such as titanium alloys used in the aerospace industry. Due to the comparatively high machining content of typical aerospace components, the industry demands high chip removal rates and thus machines providing an equally high performance. At the same time, manufacturers are looking for high machining flexibility and a high degree of automation. Additionally, 5-axis machining will also be playing an increasingly important role in this field of application in the future.

Despite their excellent suitability for the given tasks, the machines and systems made by HELLER have not been considered first choice by companies from the aerospace industry for some time. "The market mostly perceives us as a premium supplier and solution partner for the automotive industry. However, we are now working on raising the market’s awareness of the fact that for more than 75 years we have also been a qualified supplier to aerospace manufacturers offering commercially successful machining solutions for this sector," explained HELLER Technology Expert Werner Kirsten at Farnborough Air Show in June 2010. On the occasion, HELLER impressed aerospace manufacturers and suppliers with high performance and precision in titanium machining. Although a small number of other machine manufacturers were present at the fair, the Nürtingen-based machine-tool specialist was the only one to exhibit a fully functional machining centre operating under 'real-life' machining conditions. For the demonstration, a 5-axis machining centre type FT 2000 PCU equipped with HSK63 spindle taper providing 242Nm torque from the standard machine range was used. The swivel-head unit PCU 63 provides 44kW power and is currently the most powerful 5-axis spindle equipped with HSK63 on the market. The Ti-6Al-4V workpiece that was machined was characterised by narrow honeycombs and grooves, lean concave webs and slightly concave inclines. 

In order to establish the limits of feasibility, HELLER conducted comprehensive trials in the company's own laboratory and production workshops in the forefront of the fair using HSK-100 and HSK-63 systems, motor-driven and gear-driven spindles, 4-axis and 5-axis setups. For these trials, the machining specialists from Nürtingen mainly used standard components that were tested to the limit again and again. Process parameters, for instance, were increased in increments, until one of the components failed. Subsequently optimisations were performed in order to push the limits even further. The trials clearly showed that, apart from the tool clamping system and coolant supply, NC-path generation in corners and grooves needed special attention. As a particularly tough and long-chipping material, titanium requires specially-adapted cycles in order to prevent vibration or even tool breakage.

However, throughout all trials, the above described configuration of the HELLER F series was able to demonstrate its exceptional abilities. The excellent damping of the machine provides very smooth running qualities and enables outstanding surface qualities, high tool life and maximum process stability, even in demanding operations such as the machining of titanium. The powerful spindle provides the same cutting power in horizontal and vertical position and at any solid angle. The FT 2000 incorporates a coded direct measuring system, high resolution rotary encoders and YRT bearings with integrated measuring system. The proven 5-axis kinematics provided by the tool in X, Y, C and/or A and two axes provided by the workpiece in the Z and B axes offer combined precision and dynamics. As a founding member of the Machining Innovations Network, HELLER presented the machine to trade visitors at the MIN booth at EMO 2011. 

In order to optimally cater to the specific requirements of heavy-duty cutting in the future, HELLER teamed up with a renowned expert in this field in the past year: the US-based ATI Group provides comprehensive know-how in the area of special materials for the aerospace industry and is considered one of the pioneers in the use of titanium alloys. A first result of this international best-in-class partnership was exhibited at last year's EMO in Hanover at the booth of tool manufacturer ATI Stellram - by their own account premier supplier in the US when it comes to difficult-to-machine materials. The machine was a HELLER 4-axis machining centre model H 6000 equipped with HighPowerCutting (HPC) gear-spindle providing 6,000RPM, 60kW and 2,292Nm. The machining operations demonstrated at the booth included machining of a 50mm groove in a TiAl6V4 titanium block using a porcupine milling cutter size 80 at a cutting speed of 50m/min. What made this demonstration particularly impressive is the fact that the machine only has to utilise 30 percent of its full capacity for the operation. 

For many decades HELLER has demonstrated that roughing and finishing operations can be performed on a single machine. With the new H series, HELLER is also redefining the roughing process with a machine concept that has been specifically designed for robust processes. At last year's EMO, HELLER set new standards in the machining of titanium with this series, especially 4-axis machining centre model H 6000. At the same time, the company was able to illustrate the significance of optimising the interaction between all components involved in the machining process. For the finishing operation, a 25mm diameter tool with 10 cutting edges and 5xD protruding length was used. Tools of this length would usually start to vibrate during machining. Therefore the protruding length is usually reduced in practical application. However, due to the often adverse machining conditions this is usually not feasible in aerospace applications. However, HELLER was able to prove that using a Heavy-Duty Chuck made by Haimer this problem can be eliminated by redistribution of mass.

The performance of this standard milling centre is boosted by means of a controlled U axis enabling classic turning operations. It can also be used for other machining tasks without any problem. In power engineering, for instance, it is now often necessary to machine flanges with grooves for seals in a single setup. Until now, these grooves were machined in a second setup on a turning machine. However the controlled U axis and integrated out-facing slide also enable classic turning operations. HELLER's unique out-facing slide comprises a drive, a facing head and the tool including all the required interfaces. The drive for the U axis required by the control operates in parallel to the Z axis through the centre of the spindle and is coupled to the facing head and the tool. Compared to mechatronic solutions this system has a significantly shallower depth and thus provides a reduced tilting moment. This has a very positive effect on tool change and cutting capacity. Another advantage of this configuration is the fact that heat-inducing drive motors are located far from the point of machining.

Experience gained in heavy-duty cutting so far provides impressive evidence of the high structural dynamic stiffness of the H series. Due to their modularity, the machining centres from the H series provide great flexibility allowing to optimally configure them for the application at hand. The same applies to HELLER's range of 5-axis machining centres from the MCH-C series with 800Nm torque making them an excellent choice for the machining of titanium. Our investigations have also shown that given the appropriate process competence, a machine equipped with HSK63 spindle taper often presents a more cost-effective solution than an HSK100 machine. HELLER's universal concept provides the right solution for any for requirement - from high speeds, high dynamics and high stiffness through to high torque combined with adequate damping characteristics. However, the investigations have also shown that machining of titanium means milling at the limit. That is why every link in the process chain is of crucial importance. Apart from using optimal technology, quality and costs are determined by the level of process control.



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