Blade machining has its fair share of idiosyncrasies. While problems usually stem from the software or the machinery used during the milling of the required blade curvature, the main challenge these days is the blade material. A wide range of materials may be required to equip a turbine with blades, opening up the different possibilities in terms of tools and strategies.

High temperatures are present in the high-pressure area, meaning that high-temperature materials are in particular demand. However, cast irons or titanium alloys are also used in the low pressure area.

In this area, it is not the temperatures but the centrifugal force that determines the material; other differences occur because of the type of blades. For example, rotating rotor blades are subjected to different types of stress than those of stationary guide blades, which is the reason why the materials vary.

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.

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 the person responsible for turbine blade machining technology says: “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 five-axis machining. “These tools,” he continues, “take care of 70 to 80 percent of the entire machining process, resulting in the majority of metal removal occuring 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 roughing stage.

When the blade has been completely roughed out, semi-finishing of the blade is usually the next step. A typical tool for semi-finishing is a shoulder mill with large corner radius or a tangential shoulder mill, both cutter types having the ability to leave a surface for fine finishing. Ball nose cutters such as the exchangeable insert type or solid carbide ball nose mills and conical ball nose copy mills then take care of the machining of the transitional radii.

Blade root finishing is usually a task for milling cutters with inserts with a large number of cutting edges. Since good surfaces are required at this stage of machining, the machine operators replace the cutting edges even if the slightest amount of wear is present. Ideal tools for this are special octagon cutters, for example.

Finishing the turbine blade is an extremely important process. The type of surface and the curvature of the blade ultimately determine the flow characteristics and therefore its efficiency. “Two concepts for this are available from Walter,” explains Mr Schaarschmidt. “(They are) milling with conventional solid carbide end mills and milling with our modular ConeFit system with solid carbide changeable heads. We use standard or special cutters depending on the application.” The special versions have a greater number of teeth, eg: 10 teeth on a 16 mm diameter cutter. This means high feed rates and therefore good cost efficiency are possible.

Ceramics Have The Edge

Walter collaborates closely with the Institute for Production Engineering (IfP) at the University of Applied Sciences in Zwickau, Germany, in developing tools and suitable machining strategies for the different materials.

Turbine blade machining is one of the institute’s core competencies.

The blade specialist reveals that only ceramic cutting material has achieved maximum performance in the machining of the more frequently occurring high temperature alloys.

Ceramics are a component in an overall concept. Cutters with ceramic indexable inserts take care of the roughing operations, ie: the lion’s share of the machining, and cutters utilising carbide carry out the semi-finishing and finishing work operations. With the current state of technology, ceramics are only suitable for roughing.

Tools for other processes are also under development. These also include all-ceramic end mills. “It will be possible to make additional significant machining time reductions when ceramic tools for prefinishing and finishing are available,” predicts Lucas Günther, research assistant at the IfP.

However, this development is totally dependent upon the turbine manufacturers. The extent to which ceramics can be introduced as a finishing material depends on the amount of heat that is introduced into the material. If excessive temperatures occur, they may change the surface structure in a non-permissible way. The manufacturer’s requirements and the possibilities from the point of view of machining are initially two undefined variables.

Back to what is currently feasible, the high cutting performance of ceramics is based on special cutting material characteristics. This includes extremely high hardness of up to 3,000 HV10 (carbide: up to maximum 2,500 HV10), high temperature resistance of approximately 1,100 deg C (carbide: maximum 1,000 deg C), a low friction coefficient and low adhesion wear.

However, these beneficial properties can only be utilised if several conditions are met. It all depends on stable clamping of the workpiece and low tool projection length. The machine must be designed for spindle speeds of about 15,000 to 30,000 rpm in order to be able to use the relevant strategies with high cutting speeds. It should also be equipped with an extraction system and encapsulated guideways because of the ceramic particles that are produced.

In the meantime, the IfP has tested both grades in optimised cutter bodies and developed appropriate machining strategies. According to Walter, the optimum cutting data for the WIS10 cutting material is a cutting speed of vc = 1,000 m/min with feed per tooth fz = 0.1 to 0.4 mm and infeed depths of ap = 1 to 2 mm.

“It is extremely important to determine which cutting data and machining strategies are suitable for the type of cutting material,” explains Mr Schaarschmidt, “our customers therefore always receive a complete package consisting of tool and know-how.”

As well as the strategies, the design of the tool body is another way of fine-tuning performance, continues the expert. For example, an important detail is the clamping of the indexable inserts. He estimates that about 30 percent of the performance of a ceramic mill is based on having an optimised tool body.