As The Turn/Mill Concept Evolves, Is New Terminology Needed?

Machine tools that combine milling, turning and other processes on a single platform go by many names. “Turn/mill” and “multitasking” are two of the most common labels. “Integrated mill turn center” is the term that Mori Seiki of Nagoya, Japan (U.S. headquarters in Dallas, Texas) is using for its new NT series of machine tools. The name reflects the design philosophy behind these machines. They are intended to provide the full turning capability of a pure turning center along with the full milling capability of a pure machining center. The combined capabilities can be used for completing complex workpieces in one setup. This design is distinct from those machines that add milling capability to what is fundamentally a slant-bed turning center.

The strategy, then, was to meld the design advantages of the company’s NH series of horizontal machining centers and its NL series of turning centers. The NH series HMCs feature box-in-box construction whereby twin ballscrews are situated to move the X and Z axes in virtually the same vertical plane. This keeps the center of gravity of moving components aligned one above the other, minimizing the tendency of these components to rotate against each other and create a twisting motion. The company calls this principle “driven from the center of gravity,” or DCG. Because this twisting motion is a main source of vibration in machining, avoiding the twisting allows for high rates of acceleration and deceleration without affecting accuracy or degrading surface finish. That’s the milling side of integration.

On the turning side, the NL series contributes two key features. For one, the headstock and main tool turret are arranged symmetrically above the bed to spread machining forces evenly and straight downward. The company says this enhances rigidity and reduces vibration. For another, the tool turret incorporates a built-in motor to drive the live tooling. Delivering power to the tool stations in this way reduces heat and disperses it evenly.

The company also turned its attention to the designs of the rotating milling head (the motorized unit that rotates the head in the B axis) and the ram that provides Y-axis travel. Here, one of the main challenges was maintaining rigidity as the ram extends over the lathe bed portion of the machine. According to developers, the answer emerged in the octagonal shape of the ram body. Thus, the ram’s slideway surfaces form a symmetrical “V” shape for rigidity and linearity without compromising the optimal width and height of the ram body’s cross-section. This design allows the rotating milling head to be “full-sized” to incorporate a direct drive motor that rotates the milling head, also described as “full size” for high-torque machining operations.

The resulting design is best visualized in the “carcass” of a typical NT series machine as shown in the illustration.

Finally, a new MAPPS III control system was developed with software capable of simulating the simultaneous motion of tool spindles, turrets, subspindles and milling head. This simulation helps detect any interference that might result in a tool crash. Other software features are said to facilitate programming in multiple axes and generate efficient tool paths for high speed machining.

The company claims that the NT series doubles the productivity of its existing turn/mill offerings. The NT series will come in three general size ranges: the 3000 series for 250 mm (9.8 inches) of Y travel; the 4000 series for 420 mm (16.5 inches) of Y travel; and the 5000 series for 510 mm (20.1 inches) of Y travel. Optional tailstock and subspindle configurations yield 66 different variations of nine basic models.