Innovative solutions simplify airfoil and turbine blade machining

The Starrag Group recently hosted Airfoil Technology Days 2014 in Rorschacherberg, Switzerland, to showcase its latest technologies and innovations relating to the machining and processing of turbine blades. With more than 150 invited guests from around the world, Starrag demonstrated its breakthrough technologies over a two-day period of intense breakout sessions, which also involved live metal cutting demonstrations.

 

Showcasing the many aspects of blade machining, Starrag demonstrated a unique approach to blade fixturing making airfoil clamping smooth and easy; the BestFit Process which integrates the tightest tolerances of the workpiece with pre-produced features; and the Cryo-process which creates greater tool life and reduces manufacturing costs. However, the capabilities of the LX series of machines that make possible the reality of simplifying blade machining generated the greatest interest.

 

Based on the goal of continuous improvement, LX machines are designed and built taking into account technical parameters such as axis strokes, forces, torques and acceleration according to the application requirements. Furthermore, the LX series of machines incorporates specific design features such as backlash-free rotary and linear axes, counterbalances on all vertical axes to provide the highest levels of dynamic contour accuracy, and specifically designed motor spindles for turbine blade machining.

 

Significantly increasing the productivity of the LX machines is the installation of an intelligence system. By means of this system, process analysis and evaluation is simplified by integrated sensors. In addition, critical parts of the program are automatically identified enabling systematic process optimization. Damages caused by collisions are drastically reduced due to rapid detection and machine shutdown. Furthermore, the imbalance of all tools and the state of the spindle bearings are automatically monitored. This intelligence system also measures and compensates spindle displacements due to centrifugal forces and/or thermal drift.

 

To further increase productivity, the LX machines have the capability to completely finish-machine a blade without an operator. Using a robot for bar stock material and programming an automated workpiece – re-clamping on the machine for an all-side operation or by using an automatic bar loader unit to feed long bars directly into the rotary axis – makes it possible to machine all sides without the need to manually reposition the part. Starrag refers to this as “blade machining in 1 go.” Additionally, automatic bar feeding units with a pass-through of up to 65mm offers the possibility to machine small blades from bars.

 

With all the design and engineering advancements of the LX series of machines, consideration must also be given to the CAM software. The path to perfectly milled blades must begin with the right CAM system to efficiently transfer the blade design into a functional product. Developed and tested at Starrag with the specific intent of producing efficient programming and machining techniques of turbine blades, RCS V7.2 Cam Software has universal interfaces to many systems and data formats.

 

Included in V7.2 is the TDM tooling interface. The tool list contains information for the tool definition as well as technology values such as cutting speeds and feed rates. One or more tools can be selected directly from the list and is automatically created in RCS to further reduce the time to chip.

 

Also new is RCS BestFit, which locates the workpiece and aligns the machine to the co-ordinate system of the workpiece. This system handles location finding not only of prismatic shapes but also complex free form airfoil surfaces. Additionally, the spindle of the machine is now used as part of the built-in workpiece simulation. The spindle type is automatically selected based on the postprocessor file that the project uses.

 

Last, but certainly not least, to the LX series is the Starrag “Dengeling” process to further reduce overall cycle time of the complete finishing process. Now it is possible to forego traditional time-consuming operations such as shot peening, hand polishing, and machine grinding to maintain contour accuracy on turbine blades. After the milling process is complete, a tungsten carbide ball oscillating axially with high frequencies smoothes the surface of the blade. This tungsten carbide-ball tool is actually loaded into the automatic tool changer and is activated after milling. The path of the tool is controlled within the RCS programming suite with a specialized module.

 

The Dengeling process is based on the mechanism of moving a coil electromagnetically. Due to the current flow, the moving coil creates a force (Lorenz-force), which acts in the direction of the coil. This force is proportional to the induced current and its effective direction follows the current flow direction. The mechanical vibration is generated through electromagnetic oscillation by using an AC-power system. Every time the Dengeling tool strikes the workpiece, the tool acts as a tiny peening hammer imparting a small indentation or dimple on the surface. In order for the dimple to be created, the surface layer of the metal must yield to tension. In most cases, this process can replace the manual and cost intensive process of polishing. Additionally, the process will maintain surface roughness of less than 0.3µm, and ensures total reproducibility, dimensional accuracy, and reliability.

 

For more information on turbine blade machining or to discuss specific engineering application development needs, be sure to visit the Starrag AG booth (#S-8696) in the machining pavilion at IMTS 2014, at McCormick Place in Chicago, Ill., Sept. 8 to 13.