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Precision cutting on heavy-wall vessels is not normally considered a robotic application, but the unique requirements of one project required some new thinking.

Enpro Systems, a Houston vessel manufacturer, recently fabricated a 49-foot-6-inch-diameter hemi-spherical vessel with 2 7/8-inch steel walls. The vessel had 28 connections of 16.5-inch pipe through the wall. Cutting round holes isn't usually a major problem, except these holes were parallel and located between the "pole and the equator." So to cut a round hole required a complex torch path to follow the angle of the wall and the curvature.

Standard circle-burning machines could not handle the 20 inches of rise and fall of the torch path cutting on the vessel's curvature. The hole angle meant that the cut was actually more than 5 inches thick. The thickness and accuracy requirements ruled out manual cutting, and the customer's tight deadline precluded fabrication of a custom cutting system.

Enpro ultimately chose a different approach--robotics--partnering with Arc Specialties, a Houston-based company specializing in automated manufacturing systems. Arc Specialties proposed to use an ABB IR1400 welding robot as a programmable burning system.

The Project
From proposal to completion, engineers only had 10 days to complete the job. With only 28 holes to cut, Enpro elected to rent the robot. While Enpro fabricated a movable robot platform, Arc Specialties converted the arc welding robot into an oxyfuel cutting machine.

Several test cuts confirmed dimensional accuracy and cut quality (Figure 1). To simulate the inclined surface of the vessel, a 2 7/8-inch plate was angled to match the cut angle on the hemisphere. Any irregularity in torch motion would have created unacceptable gouges in the surface.

Ultimately, a low-pressure, high-capacity cutting torch running on oxygen and natural gas was able to make cuts at 10 inches per minute. Cut quality was very good (Figure 2), proving that robotic motion control is more than satisfactory for oxyfuel cutting.

With robotic burning, the cut could be started perpendicular to the plate, at the thinnest point. Once the cut is established, the torch moved to the desired bevel angle before the circle is cut. Preprogrammed paths with reference points allowed the robot operator to quickly reprogram the start point to coincide with the starting hole. The start hole did not need to be accurate relative to the cut. The robot could also retrace the cut path in the event that a cut was lost. This feature allowed the operator to retrace the path, and then pause the motion while the cut was reestablished.

To reduce the setup time for each cut, programmers created a macro to allow the operator to enter the hole diameter, the position relative to the top of the vessel and the diameter of the sphere. Using Robot Studio simulation software, engineers determined the robot would be able to reach two holes before needing to reposition for the next two holes. Once repositioned, the robot's program-shift feature allowed the operator to reorient the robot relative to the desired hole centerline without reprogramming.

On the project's fifth day, the cutting procedure was accepted and the robot platform was completed. That meant the robot could be moved to the vessel-fabrication site. At this stage, the vessel was only partially assembled, with half the sphere sitting on the shop floor.