Machining high precision turbine blades
- January 28, 2015
The complete machining of turbine blades requires striking a balance between powerful roughing and ultra-precise finishing, a task for which the modern five-axis turning-milling centers are ideal, when robust machine construction is combined with high-quality drive and control engineering.
Experience is the basis on which Hamuel Maschinenbau GmbH & Co. KG, Meeder, Germany, can offer a great deal: 85 years of machine tool manufacturing, 35 years of CNC machining, 25 years of five axis simultaneous machining and over 10 years in the construction of five axis turning-milling centres. CEO Dr. Markus Stanik knows that experience by itself cannot be any yardstick for the quality of machine tools, noting, "it certainly helps us, together with the right partner companies, to design and build the machines so that our customers can be successful in the marketplace." This machine builder and its many customers have developed correspondingly positive accomplishments together.
Today, Hamuel stands on three standards:
- ·The production of machine-tool components (such as axis units, machine bases and machine bodies)
- ·The development and construction of CNC-gantry milling machines
- The production of five-axis turning-milling centers
The latter, as evidenced by Hamuel's HSTM (high speed turning-milling) series, has been marketed since 1999 and makes a decisive contribution to the sales and success of the company. Leading turbine manufacturers from the aerospace and power-generating industries have been particularly impressed by the dynamic response, capability and precision of these machines.
Even the smallest faults are unacceptable on turbine blades
The HSTM that runs on the Sinumerik 840D sl CNC controllers is used mainly to manufacture turbine blades and blisks (turbine disks) deployed in stationary steam and gas turbines as well as in mobile turbines for aircraft jet engines and large turbochargers. These products consist of high strength materials, such as titanium or high alloyed steels. Form deviations are taboo in such applications, because even the smallest error would significantly reduce the efficiency of the turbines. Correspondingly high and comprehensive are the requirements placed on the machine tools. Hamuel sales manager, Dipl.-Ing. Jochen Schaede, explains: "our customers normally manufacture large turbine blades longer than approximately 1000 mm from forging blanks. In contrast, shorter workpieces are milled from a solid piece of material with a stock-removal ratio of approximately 80 percent. This makes it obvious that our machines must not only rough-cut very productively but also finish with high precision." Whereas productivity takes precedence for rough cutting, accuracy is more important for the subsequent finishing of the spatial free-form surfaces. "With regard to accuracy, our high performance machining centres achieve a precision in the micrometer range coupled with excellent surface quality," adds Schaede.
Key to this capability is the combination of compact machine construction together with perfectly matched drive and control engineering. The basis is the robust machine design with a one-piece machine base–either as welded construction cast with special concrete or latterly directly as polymer-concrete made from a single piece of material. The careful distribution of the masses ensures the best dynamic rigidity. The experience of the developers at Hamuel paired with the knowledge of the Siemens mechatronic experts make such results possible, as Jochen Schaede says: "thanks to various simulation tools, the Siemens engineers could provide us with very exact data to which we could add or remove masses. It was certainly advantageous that we could procure all components required for the dynamics from Siemens starting with the spindle, including the axis drives, and ending with the CNC. This means we needed only a few iteration loops in order to obtain a perfect dynamic rigidity in the machines."
It is also important that all components are arranged so that the horizontally clamped turbine blades and blisks can be machined optimally. This is also facilitated by the 45° inclined orientation of the axes that ensures a favorable swarf removal further augmented by the permanent rinsing of the machine internal area. The accessibility has also been enhanced by this inclined axis orientation.
Such designed and built HSTM machines equipped with precise direct drives from Siemens achieve a maximum positioning accuracy of 5 µm with traversal speeds exceeding 40 m/min in all linear axes. Indeed, the extremely fast rotary axes have a positioning accuracy of 3 µm. This allows the user to achieve maximum productivity, accuracy and surface finishes in the range of Ra = 0.8 µm, and so fully meet the industry requirements placed on the blade machining.
CNC ensures optimized surface transitions at the inlet and outlet edges
One of the biggest challenges for the turbine blade manufacturing industry concerns the critical surface transitions (leading and trailing edges) of the turbine blades. To achieve a high quality, the milling paths described with curve interpolation points must have a constant machining speed. As a consequence, the leading and trailing edges demand extreme angular accelerations. The number of curve interpolation points to be described is correspondingly quite high.
The controller now faces the challenge to reliably process the generated NC steps that are supported by the fast block cycle times and the look-ahead function of the CNC. "Hardly any other controller can supply the drive information for all five axes in the required short time as reliably as the Sinumerik 840D sl controller," says Jochen Schaede. "The new Advanced Surface motion control that includes among other things an optimized look-ahead function is also excellent. And I would also like to mention the five axis high speed turning-milling of free-form surfaces requires a continuous spatial reorientation of the geometry vectors." This leads to the use of the Traori (transformation orientation) principle in the Sinumerik 840D sl. Hamuel's customers can therefore achieve reproducible results with the best-possible surface finish, exact contour accuracy and with the maximum possible machining speed.
In addition to the primary properties of precision, productivity and reliability, Hamuel also appreciates other properties of the Siemens drive and control engineering–an example: the DRIVE-CLiQ interface. When this digital connection is used, all appropriately equipped drive components are detected automatically. Only a cable for the connection is required to achieve this result. Equivalent circuit diagram data for the motors and the characteristic values of the installed encoder systems are all stored. This avoids the need for manual data input during the commissioning and so makes this task more reliable and much faster, according to Hamuel engineers.
Sales manager Schaede is also impressed with the "know-how" protection onboard the control. With the help of innovative software, his customers can protect all their programs and their application know-how, using individual passwords and so largely prevent unauthorized access.
According to Hamuel, the Teleservice offered by Siemens has proven to be an important advantage for its machine users who often operate globally. It allows not only software problems to be rectified but also updates to be installed fast and reliably without needing to be present onsite. Manufacturing operations save time and money and they always remain up-to-date with the latest software releases, via this service.
Powerful motor spindle with Hirth gearing
The capability of the turning-milling centre also depends greatly on the 54-kW motor spindle made by Weiss Spindeltechnologie GmbH (a Siemens subsidiary). Equipped with a HSK A63 tool holder, it is able to provide the high torque required for roughing plus speeds as high as 16,000 rpm for finishing. This means an optimum cutting speed is always guaranteed. As a practical detail, the Hamuel engineers also emphasize the integrated Hirth gearing that makes it possible to clamp the spindle and relieve load on the bearings during the turning process.
The machine sizes in the HSTM Series are oriented to the length of the workpieces to be produced in standard ranges from ≤ 500 mm (HSTM 300) to 1750 mm (HSTM 1500). On special request, Hamuel also builds larger machines. A project for machining blades as large as 2500 mm in length is currently running in the field. CEO Markus Stanik explains, "thanks to our modular machine construction, we are able to satisfy special requests and build turning-milling centres in these dimensions."
New developments already in the introductory phase are also evolving at Hamuel. For example, in the near future, a special gas will be able to be used to cool the tool in the HSTM machines. A machine base made of mineral casting is also now offered. One of the first machines of this type has already been presented at EMO. "This material not only exhibits excellent properties with regard to vibration suppression but we have also been able to reduce somewhat our dependency on steel suppliers," notes Markus Stanik.
For the machine user, this makes itself apparent directly in improved surface quality and a significantly longer tool service life.