More Efficient Turbine Blade Milling

It’s tempting to think of a multitasking machine tool as being primarily a lathe. Some of these machines can also serve as full-function milling machines. In fact, according to Mazak development engineer Mike Finn, a machine that is capable of both turning and milling is actually better than a traditional machining center for certain workpieces that require milling alone.

Turbine blades are perhaps the prime example of this, he says. The video here illustrates a Mazak Integrex 300-IV multitasking machine milling a turbine blade in a cycle that few machining centers could replicate. Typically, the part would be machined on a VMC using a rotary table. But on the Integrex—as the video shows—the turning spindles provide C-axis motion, while the B-axis sweep keeps the tool continually normal to the surface, even as the surface contours rapidly flow by.

Until very recently, data processing limitations would have ruled out this kind of fast, complex, coordinated motion, Mr. Finn says. But today, “Anytime we see a turbine blade, we think Integrex.”

The opposing second spindle is part of the reason why the multitasking machine is effective for this part. With opposing rotary spindles, the Integrex can clamp and apply torque at both ends of the workpiece. The resulting setup is more rigid than what a rotary table would permit. In fact, the machining center’s rotary-table setup often requires blades to be machined to final dimensions with just a single heavy milling pass—because a nearly finished part would be too flimsy for further machining. By contrast, the Integrex’s setup easily allows separate roughing and finishing operations for a faster and more accurate process.

Mr. Finn says the multitasking machine also permits the use of bullnose end mills in place of ballnose tools. Many turbine blades are milled on three-axis machines with ballnose mills. The cutting diameter of this tool type continually changes because the complex, contoured surface of the turbine blade meets the tool at varying points along the ball. The resulting change in contact diameter changes the cutting speed (in sfm), and the effect of this change can be seen in the final part’s surface finish. However, by keeping a bullnose tool normal to the surface during simultaneous five-axis machining, the Integrex can maintain constant cutting speed. What’s more, the bullnose can take wider passes than the ballnose without increasing cusp height. The result is faster metal removal.

One further advantage of the multitasking machine potentially brings turning into play, Mr. Finn says. The blade could be machined out of barstock, with rough turning used to quickly remove some of the initial material. Through this approach, a barfeeder could enable the Integrex to run several consecutive blade workpieces overnight within a single, unattended cycle.