Adding Tilt to Trunnion-Table Turning

Multi-process equipment is not always based on a turning platform. By locking the spindle in place and spinning the trunnion table, a properly configured five-axis machining center can consolidate setups by turning symmetrically round parts, particularly large aerospace and energy industry components. Some machines can even leverage two rotary axes at once, adding tilt to spin in order to get even closer to the workpiece surface. As is the case with moving from three- to five-axis milling, adding rotary motion can enable turning with shorter, more rigid cutters at more aggressive parameters to improve cycle times, surface finishes and tool life.

That is according to Gunther Schnitzer, vice president of sales and engineering at German machine tool builder Hermle’s North American arm, which is located in Franklin, Wisconsin. He emphasizes that the key phrase here is “properly configured.” Not just any five-axis machine can turn, let alone tilt and swing what is most likely to be a large, heavy part spinning at speeds as fast as 800 rpm. For Hermle, the groundwork for achieving the latter capability was laid long ago, he says, back when the company first standardized on a modified-gantry-design scheme for all its machine tools. Although developed with milling in mind, this machine configuration is considered essential to ensuring the high levels of rigidity needed to turn the trunnion table into what is essentially an adjustable lathe spindle, one that is capable of presenting work to tool at a wide range of precisely programmable angles.

At first glance, the only differentiating feature from other gantry designs is that the gantry structure rides on top of the sidewalls of the machine. However, the two sidewall-mounted guideways are not the only support for the Y-axis gantry and the X-axis slide mounted on top of it. Locating the guide system above and outside the workzone enables adding another guideway (or two in the case of larger machines) underneath the center of the gantry structure without interfering with the machine’s operation.

Located farther toward the back of the machine than the sidewall-mounted guideways, this third point of support completes a rigid, triangular foundation for the gantry structure, one that remains intact regardless of X-Y location. This configuration helps combat what Mr. Schnitzer calls the “horsehead” effect: that is, the tendency for the front of the gantry structure to sag as it moves farther along the Y axis and out over the workzone. Rather, rigidity is maintained even at the fullest extent of Y-axis travel. Meanwhile, centrally locating the drive unit in essentially the same area—at the apex of the support triangle—ensures even application of drive force across all guideways. “No matter where you are within the work area, you are always dealing with the same, rigid support structure,” he says.

The machines’ double-sided trunnion tables also offer benefits to rigidity and flexibility that are particularly important for turning, Mr. Schnitzer says. Twin points of connection with the machine, one on each side, provide a stable platform for heavy workpieces. Similarly to the linear axes, the motors and drives are located outside the workzone. An open workzone translates to a wide range of motion for the table: in this case, +100 to -130 degrees. The wider the range of table motion, the easier it is to manipulate the work in a way that enables more efficient turning (or milling, for that matter) of complex, contoured geometries, he says. 

Although these elements are common to Hermle’s entire line, certain design features are reserved only for machines capable of turning, which feature the designation “MT” (for “mill-turn”) in the product name. For instance, rather than the standard worm gears, MT machine trunnion tables employ 4,000-Nm torque drives that can handle high turning forces without stalling. MT machines also ship with a system of weights for precisely balancing the trunnion table. During setup, the CNC (a choice of Heidenhain or Siemens models) instructs the user in precisely how to shift the positions of these weights based on the weight of the workpiece, the C-axis rotational speed, and other part attributes and parameter settings. These adjustments take only a few minutes at most, Mr. Schnitzer says, noting that the basic concept is similar to balancing tires on a car.

This routine is just one example of CNC functionality originating not with Siemens or Heidenhain, but with Hermle, Mr. Schnitzer says. He adds that whatever the make and model of the machine, and regardless of whether it is capable of leveraging both rotary axes, the right CNC configuration is essential for any trunnion-table turning. For its part, Hermle paid particular attention to functions that help track the locations of critical datums, he says, citing tool center-point control (TCP) as one example. CNCs also have particular, turning-oriented settings for acceleration, deceleration and other essential parameters. Users can fine-tune these settings to emphasize speed or metal removal. 

MT models also feature the HSK-T spindle interface, which is designed specifically for stationary cutting tools. Although fully compatible with the HSK-A63 and larger HSK-A100 designs that are standard with Hermle machines, HSK-T provides a tighter fit between spindle key and toolholder keyway. This ensures that insert approaches work at the proper angle. Locking tool assemblies securely in place for turning is accomplished via a Hirth coupling for larger machines or a hydraulic sleeve for smaller machines.

Tool measurement is also a concern because turning cutters cannot be checked for wear or breakage by being spun inside a laser beam. That is why the Blum laser tool probes that come standard with MT machines are fitted with a static touch-off probe, Mr. Schnitzer says. He adds that this unit can double as a presetter for tools with unknown lengths and radii.

As for the tools themselves, Mr. Schnitzer recommends cutters with multiple insert positions to consolidate inventory and maximize toolchanger capacity. Rather than using a new tool, he explains, the same tool can be adjusted to present a new insert to the work.

Although a five-axis machining center must be configured differently for turning, there’s a common thread to Hermle’s design adaptations. Most ensure rigidity and operator safety in some way, building on the advantages already provided by the company’s standard design platform. The fact that this platform was originally developed for milling demonstrates the importance of rigidity for all machining operations, not to mention the extent to which conventional thinking about milling versus turning has broken down.