Cutting up on the bus
Laser cutting of bus components pays off at I.C. Corp.
Laser cutting of bus components pays off at I.C. Corp.
You might not expect to find a high-tech, state-of-the-art robotic laser cutting system in Conway, Arkansas, a town about 20 miles from Little Rock. But that is just what I.C. Corp., a wholly owned subsidiary of International Truck and Engine Corporation, now uses to perform laser trimming and cutting of 16- and 20-gauge galvanized sheet metal stampings and complex deep-drawn parts used in the manufacture of school buses. Dual six-axis Motoman (West Carrollton, OH) UP20 robots provide accurate laser cutting of more than 40 shapes, trimming the perimeters of both flat and contoured parts which range in size from 20.9 to 47.2 inches wide by 93.7 inches long by 6 to 9 inches deep.
“The state-of-the-art robotic production cell is all new technology for the plant. The people at the plant have embraced the robots,” says Jim Krieger, project manager for the Center for Manufacturing Excellence that supports all the divisions. “The sheet metal parts fabrication facility runs two shifts, producing up to 82 sets of parts per shift, and the robots have proven to be extremely reliable.
“The flexible production cell was designed to laser-cut four types of school bus parts-outer end caps, inner end caps, bulkheads, and wheel pockets-that are used by both the Conway bus assembly plant and a sister bus assembly facility in Tulsa, Oklahoma,” Krieger says.
Installed in late 2003, the Motoman robots are equipped with TRUMPF (Plymouth, MI) HL703D Nd:YAG laser packages, and are mounted on steel pedestal support structures. Mounting the robots from these support structures saves floor space and improves access to the large, complex parts, which are fixtured on a Motoman MSR-1000 AC two-station 180-degree indexing turntable positioner. This heavy-duty positioner features an H-frame tabletop design, 2,200 lb. capacity per side, and a 7-second index time when fully loaded. A light-tight front wall in the center of the positioner and unique automatic seal safeguard operators from laser radiation. The robots are completely enclosed in a CDRH Class 1 laser workcell enclosure with a safety-interlocked access door. The process is monitored using a CCTV camera and monitor package. The laser-related equipment is housed in a separate room.
Dual robots use lasers to trim part perimeter and then accurately laser cut various shapes into wheel pockets for school buses.
Motoman also provided two sets of flexible fixtures, one per side of the indexing positioner. Parts are cradled by locators on the outside form and are held in place by vacuum cups. Although the same type of part is generally run on both sides of the positioner, the system is capable of running different parts on the two sides. Cycle time varies by part, with the outer end caps requiring approximately 300 seconds and wheel pockets, which require fewer cuts, taking around 110 seconds.
Changeover between the four part styles-outer end caps, inner end caps, bulkheads, and wheel pockets-is manual and requires less than 15 minutes to switch the self-locating fixtures. Parts are batch-processed. An operator changes part numbers by selecting the appropriate robot program for the part to be run using a PC-based color touch screen located on a standalone pedestal. An Allen-Bradley SLC 5/05 Programmable Logic Controller (PLC) provides overall cell control and I/O monitoring. The Motoman XRC 2001 robot controllers control the robots and positioner.
“Initially we envisioned that the outer end caps, which take the longest amount of time to process and are the primary parts run on the system, would be produced on the first shift. And the other three parts-inner end caps, bulkheads, and wheel pockets-would be produced on the second shift. However, the system is often used to produce outer end caps for a couple of days a week, then other parts,” says Krieger.
With the two robots working simultaneously, laser cutting the bathtub-shaped outer end caps that require the most cuts takes about 300 seconds, while wheel pockets that require fewer cuts take just 110 seconds.
Two operators per shift are required to load and unload the larger parts produced in the robot cell. Operators load a part, clear the loading area that is monitored by the dual laser scanners, and activate the cycle start buttons on the operator station. The cell turntable automatically indexes, presenting the next part to the robots when the previous cycle is complete. Robots perform perimeter trimming and between 6 and 110 laser cut features per part. The operator unloads the cut part and loads a new uncut part into the fixtures, and the cycle repeats. With the laser system, form tolerance is ± 0.030 inch and location tolerance is ± 0.060 inch.
The cell also includes a 12-inch-wide by 16-foot-long magnetic belt conveyor with scrap chutes to collect and discharge laser cut slugs. An exhaust system with 9,000 CFM blower, plenum, and ductwork removes fumes from the cell enclosure.
“The Motoman system achieved cycle times that are 10% faster than our planned requirement,” says Krieger. “We’ve been really impressed by the smoothness of the installation, the reliability of the robots, the quality of the laser cuts, and Motoman’s support,” he adds.
“Before the robots, production of these parts was extremely manual-labor-intensive. Every part feature was cut by hand using a plasma torch, manually loaded into a punch press, and trimmed with pneumatic shears. Trim line patterns had to be traced on the parts prior to cutting. The process required multiple stages, multiple operators, and a lot of material handling in between operations. With the robot cells, much of the in-process material handling has been eliminated,” Krieger explains.
“Robotic automation increased the quality of the final product. The precise control reduced the amount of waste material around each part, thus the process allowed us to significantly reduce the cost of each part, while maintaining production flexibility,” he continues.
“One of the major reasons we decided to automate when we started the new school bus project was that the two pieces of the outer end cap would be very difficult to separate manually. The old manual method, a very loud and ergonomically difficult process, used a band saw to cut the deep-draw, bathtub-shaped outer end cap into two pieces-one for the front of the bus, and one for the rear. With the new model, less than 1 inch would be available for trimming-not 6 inches between the halves, like it was before,” Krieger explains.
“The laser systems we chose require very little maintenance, and consumables costs are much lower than the plasma-based cutting methods we investigated,” he concludes.
Payback for the robotic system is approximately 2 1/2 years.
Mary Kay Morel is a staff writer with Motoman, Inc., West Carrollton, Ohio (www.motoman.com).