19. February 2014
If you showed me this part and asked me how I might support and machine it on a mill/turn center, my instinct would tell me to chuck the hub and clamp or support the other end with an actuated driver in a tailstock or secondary spindle, then rough and finish mill the complex surface. One thing I’ve learned about processing parts—there’s more than one way to make a Margarita. And experimentation sometimes leads to perfection!
Mazak Corporation coined the term “multi-tasking” to describe machine models that perform more than one type of operation. Presumably, “operations” meant turning, drilling, milling, boring, and other traditional machining tasks. I’m pretty sure they hadn’t envisioned stretching a raw forging as one of the tasks that the next generation of “Done-in-One” machines might tackle. But if you’ve got a feel for making parts and understand how new technology changes the way machines work, you might use that technology in unintended ways and end up with an all-new cocktail.
I learned about a propeller machining process on an Integrex E-420 H-S II, 7-axis multi-tasking machine that helped me understand how new technology can mean more than just faster cycle times, closer tolerances, and reduced labor cost. The second spindle on the right side of this horizontal turning/milling center is programmable in the plus and minus direction in line with the first spindle. The W-axis is the key to creating an ideal workholding condition that allows a parabolic-shaped propeller blade to be ideally supported during the rough and finish machining operations.
The raw aluminum propeller forging is about 40” long with parabolic curvature along the length of the blade, whose width varies from about four to seven inches. Operation 1 on another machine simply turns the 4” diameter hub, providing an accurate diameter and face on which to locate. Op 2 is performed on the Integrex, chucking on the turned hub on one end. The next couple of steps in the process are intriguing and in my experience, unique. The programmable W-axis second spindle approaches the unmachined end of the raw propeller forging in feed mode. Hardened needle-like inserts in a finger chuck fixture mounted to the second spindle nose are actuated closed. They bite into the raw forging end of the propeller with enough force to insure penetration of the forging skin and deep enough to secure the part during the rough milling operation AND this next step in the process before milling. After clamping on the raw forging, the W-axis spindle moves in the plus direction, away from the left spindle. That’s right—this is where we discover another way to make a Margarita. By moving the second spindle along the W-axis, away from the first spindle, the part is now being elongated or stretched slightly. “Slightly” is a subjective term that means different dimensions depending on the rigidity of the material, its elasticity and tendency to distort, the load during rough milling, and a few other variables. “Just the right amount” of stretch is found somewhere between the science and art of processing. Granted, actual test cutting to eliminate harmonics and chatter at the highest feed rate possible and establishing that the finished blade will spring back to an undistorted condition is a little more complicated than finding the perfect ratio of Agave nectar to Tequila and lime juice, but it’s the same idea. Torque load on the W-axis drive motor is monitored in real time as the second spindle gripping the forging stretches the part as it moves away from the first spindle. A torque limiter triggers the next command to stop and maintain the W-axis position.
The next step in the process begins optimal rough milling followed by finish milling, all completed without chatter and in 30% less time. A new process is born, and it’s perfect—with or without salt.