Five-axis gantry does it all

Dayton, OH --By combining compact, rigid construction with an innovative drive, a new five-axis gantry offers fast, accurate digitizing and machining. The huge system plows new ground for automakers, shipbuilders, and airframe manufacturers with its unprecedented combination of size and speed.

The gantry enables manufacturers to digitize large parts at speeds as high as 400 inches per minute, feed the design information to CAD/CAM systems, and then mill the parts in the same location. Combining speed, size, and rigidity, it eliminates the need for painstaking hand polishing and grinding of parts.

Prior to the development of the new system, manufacturers hand-polished large molds and dies. The reason: Their machining centers typically lacked the necessary size, speed, or flexibility to do the job.

Reportedly, the DMT1200 Series Gantry Center changes all that. Engineers at Dayton Machine Tool Co. managed to combined the seemingly incompatible qualities of size and speed. The gantry moves at unusually high speeds, despite a work envelope that measures an impressive 20- by 11- by 5-feet.

To achieve their objectives, they incorporated innovative designs in the gantry's drive system and spindles. One of the keys is a dual-pinion gearbox that uses timing-belt technology. Working in conjunction with the rack end of a rack-and-pinion system, the dual pinions provide linear movement of the gantry in both directions along the x-axis. Adjustable preloads on the gearbox's pinions take the "spring," or non-compliance, out of the system, enabling it to accelerate rapidly without losing its position.

Most critical to the drive's design, however, is the use of the timing belt. By employing a timing belt, rather than special gearing, Dayton Machine Tool engineer G. Dan Siler dramatically reduced the inertia of the system.

Dual-pinion gearboxes, available for half a century, typically use special gearing. But the special gearing adds to the inertia around the motor area, which is normally the greatest contributor of inertia to a gantry-type system. Siler estimates that the timing belt system reduces that inertia by as much as 60%. Timing-belt technology also cut the cost of the gearbox by close to $40,000, he says.

As a result, the DMT1200 gantry can accelerate far more rapidly than other large systems. The dual-pinion gearbox drives the gantry at feed rates of 1,200 inches per minute, and acceleration rates of 50 inches/second/second using ac servomotors.

The dual-pinion gearbox also provides a viable alternative to ball screw technology. Because of the long x-axis travel, Siler feared that ball screws would suffer from "stretch," and therefore compromise precision. Using the dual-pinion gearbox, rather than a ball screw, solved that problem. "We need to accurately accelerate this large machine over a broad envelope," Siler says. "The dual-pinion gearbox lends itself well to that application." A new patented feature of the gearbox also balances its output in both directions, thus making it possible to run at high speeds.

To carry out the machine's all-encompassing theme, Siler also designed a special spindle. The spindle is critical to the machine's performance for two reasons. First, its compact size adds rigidity, which, in turn, enables high-speed gantry movement and

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