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New world record!

At EMO 2023, Starrag presented the prototype of the hydrostatically guided Starrag S1250 HD 5-axis machining center. With its exceptional rigidity and damping, the machine already achieved cutting parameter records in titanium roughing at that time. The current standard version increases these records to an unbelievable level, as proven in initial tests. An even more rigid structure, a new milling head with 15,000 Nm, and a gear rotary table with 25,000 Nm torque, are responsible for the enhancements.

For many years now the Starrag STC series has been setting standards in the efficient machining of structural components made of titanium. This extremely tough material with its poor thermal conduction coefficient which is difficult to cut subjects tool cutting edges to high stress and prevents high speeds. Successful machining requires low speeds and high torques, which in turn demand high rigidity and damping of the machine. This is why Starrag decided to offer its horizontal STC 1250 machining center in a variant in which the horizontal linear axes are guided hydrostatically. Friction-free, wear-free, extremely rigid, and highly damped – with these properties the new machining center under the new name Starrag S1250 HD is appropriate for the five-axis heavy-duty cutting of titanium structural components. In roughing tests the prototype already achieved up to three times the material removal rate of the version with roller bearings. The improved damping and dynamic performance also ensured excellent precision and surface quality during finishing. The fact that this new concept is a hit is confirmed by the order from Airbus for the Varel site. After some endurance tests with the prototype machine, the company placed an order for five Starrag S1250 HD machines – with a few special requests.

High-torque rotary table and improved output in the head

“Our customers were impressed by the tests conducted two years ago”, reports Alexander Fitz, Sales Director for Aerospace & Turbine at Starrag AG. “Because the Starrag S1250 HD promised considerably more efficient cutting of titanium structural components than ever before. In order to optimally fulfill the special requirements of titanium cutting specialists, we decided to make some modifications to the series machine.” The modifications mainly involved the rotary table and the milling head, both of which were adopted from the standard STC 1250 for the prototype of the HD variant. Alexander Fitz explains: “We do not need a fast-moving, direct-driven table for the high-performance milling of titanium structural components. This is why we changed it for a gearbox variant, which can better meet the requirements with its enormous torque of 25,000 Nm and 32,000 Nm clamping torque.” Other than a torque increase in the table, the customer also wanted changes to be made to the milling head. “Our swivel milling head used in the standard STC machines is very compact and allows optimal accessibility to the workpiece and thus the use of short tools”, says Alexander Fitz. “We modified this slightly for the Starrag S1250 HD. By enlarging the swivel milling head on one side we created space for a 50% stronger A-axis with 12,000 Nm torque and 20,000 Nm clamping torque – while retaining the previous good accessibility.”

For many years now the Starrag STC series has been setting standards in the efficient machining of structural components made of titanium.

Starrag’s own robust 53 kW gear spindle is used as a work spindle, which provides 1,600 Nm torque and a speed of 4,500 rpm. A new feature here is the HSK-B160 interface. “The large contact surface with 160 mm diameter ensures high tool rigidity”, argues Alexander Fitz, “which is important for large cutting depths – especially for tools with a wide overhang.”

The previous tests also underlined that at the moment it is not the machine but the tool that is the limiting factor.

The true values are reflected in practice

The initial tests of the series produced machine showed very promising results. Dr. Markus Ess, Director of Technology of the Starrag Business Unit HPS, highlights one test that was conducted in Rorschacherberg in the presence of the titanium specialists from Airbus, Varel: “We made a so-called performance cut, which is performed with ideal clamping and tool ratios. Roughing with short 80 mm Igel milling cutter, 68 mm depth in the full cut; with 80 mm/min feed and 160 turns. Then the tool capacity was exhausted. The machine ran very smoothly – the power consumption at the spindle was only at roughly 60 to 70%.” Further performance cuts in titanium (Ti6Al4V) on the new Starrag S1250 HD yielded results, which, according to the Starrag milling specialists, indicated a world record in titanium machining: With the carbide cutter with Ø40 and 18 teeth, developed by Starrag and designed for heavy-duty machining, a titanium block was roughed with the following cutting parameters: cutting speed vc = 120 m/min, tooth feed fz = 0.45 mm, cutting depth axial ap = 98 mm, and cutting depth radial ae = 2 mm. The speed was 955 rpm, the feed speed vf was 7,735 mm/min, and the average undeformed chip thickness hm was 0.101 mm. The result: An unbelievable metal removal rate Q of 1,516 cm3/min, which promises enormous productivity and demonstrates the advantages coordinated systems with machining centers, tools, and technology bring to the customer. The previous tests also underlined that at the moment it is not the machine but the tool that is the limiting factor. Therefore, further joint tests are planned – with our own tools, but also with tools from leading tool manufacturers – in order to achieve more world records for the Starrag S1250 HD.

Long tools for deep cavities

The developers of the Starrag S1250 HD are already keeping an eye on future demands. That’s because modern processes for producing near-net shaped blanks are likely to reduce the amount of roughing that is performed in coming years. The focus then shifts to the challenge of entering the complicated and deep cavities with tools up to 500 mm long and producing a high level of performance there. But it also essentially comes down to the tool, whereby the large planar support of the HSK-B160 retainer offers the best support.

Initial tests show that vibration-damped versions are far superior to monolithic tools. “The structure of our HD machine is also ideally suited for this type of machining”, says Markus Ess confidently. “We have built an extremely rigid, good titanium machine, with which we can perform excellent roughing tasks and also work in complicated cavities with very long tools – a future-proof mix.”

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