manufacturing technology A New Approach In its quest for a better way to machine today’s turbine
blades Walter looked at traditional tooling issues such as cut- ting tool materials and geometries, but also went beyond that
the greatest potential for cooling. The savings anticipated in the production of stainless steel turbine blades were as high as 40%. Building on this pioneering basic research, and in close cooperation with Starrag AG (Rorschacher- berg, Switzerland), Walter became the fi rst tooling specialist worldwide to implement the principle of cryogenic cooling in an application-based, ready-for-industry solution. This was successfully presented for the fi rst time in March 2013 under the name of “Cryo·tec.” At EMO 2013, Walter and Starrag received the MM Award for the most innovative exhibit in the milling category for their solution.
A Special Milling Cutter For the tool, the two partners chose a version of the Walter
F2334R round insert copy mill that was optimized specifi cally for blade machining. 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 fi ve-axis machining. It covers 70–80% of the entire machining process, covering the majority of metal removal in the turbine blade area. The use of fi ve-axis machin- ing allows optimum geometric and cutting force ratios to be set in all positions and permits a close approximation of the fi nal contour to be achieved even during roughing. The F2334R is equipped with two channels, which can be used to supply two different coolant types at the same time: one channel for CO2
and a second channel for MQL, compressed air or emulsion. The CO2 is fed through the tool
in liquid form at room temperature under 870 psi (6 MPa) pressure and is released in the direct vicinity of the cutting edge. The CO2
the desired cooling effect is achieved.
Chart indicates potential savings with Cryo•tec for roughing of turbine blades made of austenitic stainless steel over dry machining.
to study the cooling process as well. The high temperatures that occur during turbine machining stress the carbide sub- strate and limit performance. Unlike steel, the heat produced when machining diffi cult-to-cut materials is not primarily dis- sipated via swarf. Instead, it penetrates into the cutting edge, thereby softening the cobalt and tungsten carbide alloy. At the start of 2013, Walter got together with the Institute
for Production Technology (IfP) at Germany’s West Saxon University of Applied Sciences Zwickau to perform basic research on cryogenic machining with liquid CO2
. During
these experiments the cutting edges were cooled to –78.5°C. The experiments in Zwickau were performed with an external coolant supply. Various cooling strategies, MQL (minimum quantity lubricant), emulsion, CO2
were compared with one another. The results showed a clear winner: the CO2
approach has
“An important developmental step with Cryo·tec is the use of a two-channel system for feeding coolant through the tool instead of an external supply,” said Thomas Schaarschmidt, who is responsible for turbine blade machining technology at Walter. “With fi ve-axis machining, externally fed CO2
then expands at this point to form dry ice, and
never re-
ally gets to where it is needed, that is, as close as possible to the cutting zone, where the high temperatures actually occur. When working with titanium, lubrication is also required in ad- dition to pure cooling. The approach here is to separate MQL and CO2
using a two-channel tool. Starrag AG developed the , and combinations of these
spindle for this, while Walter provided the optimum tools.” Schaarschmidt said the advantages of the cryogenic cooling concept can be summarized as follows: Longer tool life thanks to lower tool wear rate, greater productivity thanks to improved cutting parameters, higher component qual- ity thanks to better surface fi nish and reduced border zone infl uence. Costs associated with maintenance and disposal
76 — Energy Manufacturing 2015
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