Machining turbine blade

The majority of the production costs for steam and gas turbines are apportioned to the blades. The geometrically complex structures are the only parts of a turbine with a certain large-scale production character. The increased use of high temperature alloys makes machining even more complicated and therefore more expensive. Innovative tool solutions are therefore required. The energy production specialists at Walter AG in Tübingen have met this challenge with new cutters, carbide grades and ceramic cutting tool materials.

Blade machining has always been a science of its own. The machine operators used to have problems with the software or the machinery when they were milling the required blade curvature, but the main headache nowadays is the blade material. A wide range of materials may be required to equip a turbine with blades, and therefore different tools and strategies are needed. High temperatures are present in the high-pressure area, meaning that high-temperature materials are in particular demand. Cast irons or titanium alloys are also used in the low pressure area. Here, it is not the temperatures but the centrifugal force that determines the material; other differences occur because of the type of blades: Rotating rotor blades are subject to different types of stress than stationary guide blades, which is the reason why the materials vary in this case.

As stated above, the greatest focus is on the high-pressure area of the blades. In order to increase the efficiency of their machines, the system designers are allowing for increasingly high input temperatures. For machine operators, this development means higher manufacturing costs for blades and related components. Because extreme temperatures require special materials, conventional ferritic, martensitic or austenitic turbine steels are often insufficient, making nickel-based high-temperature alloys necessary. The increased use of these difficult-to-cut materials leads to significantly longer machine operating times. The best way of keeping costs down is to use optimised tools with maximum performance.

“Energy production has been one of the main industries that we have been concentrating on for decades,” emphasises Andreas Elenz, who manages the Business Development area at Walter. “We are a full service provider for all key components, which therefore includes blades. This means that we supply all of the tools (and in the blade machining area that means quite a few), from indexable insert roughing cutters for maximum metal removal to solid carbide finishing end mills for the smallest transitional radii.”

Optimised tools
Whereas some low-pressure blades are forged or cast, the majority of other blades (particularly medium- and high-pressure blades) are created by milling from a solid block on special machines with highly dynamic axes. Between 60 to 85 percent of the block material is machined away.

Thomas Schaarschmidt, Business Development Energy team leader and person responsible for turbine blade machining technology: “The preferred cutters for roughing are initially copy mills with round inserts such as our type F2334R, which has been optimised for blade machining.” The R stands for reinforced design. A main feature of this type of cutter is its high level of stability and therefore process reliability, and the fact that it is capable of 5-axis machining. “These tools,” continues Schaarschmidt, “take care of 70 to 80 percent of the entire machining process, thus the majority of metal removal occurs in the turbine blade area.”

Simultaneous milling using five axes is now the standard, because it means that the best possible geometry and cutting force relationships can be established in any orientation; it is also possible to get close to the required finish contour during the roughiSOCIETY