Assembly in Action: Robots Weld Corvette Seat Frames
Aluminum presents welding challenges due to its high thermal conductivity and susceptibility to problems, such as warping, burn through, contact tip burn-back and wire tangling.
When Kadee Metalfab LLC (Solon, OH) received a contract from Lear Corp. (Warren, MI) to manufacture seat frames for the Chevrolet Corvette, it had to face the usual problems associated with aluminum welding.
Kadee had to look beyond its manual welding methods to find a way to keep up with the large volumes required for the Corvette. The company also needed to explore ways to weld a very difficult application requiring the fit-up of eight aluminum pieces-for a total of 22 welds per completed seat frame. To solve both of these problems, Kadee turned to automation.
"When we received the contract from Lear, we knew that the fast cycle times it dictated could not be achieved with anything less than six to eight manual welding systems," says Ken Proboski, engineering manager at Kadee. "We were already welding seat frames for the Plymouth Prowler, but the volumes were low enough to only require one gas tunsten arc welder to complete the work. With more than 1,500 seat frames required per week for the Corvette, we knew that we needed a faster and more cost-efficient option."
Kadee turned to The Lincoln Electric Co. (Cleveland) for automation assistance. "Having easy access to Lincoln's technical support and welding expertise was key," explains Proboski.
Lincoln recommended FANUC Robotic's (Rochester Hills, MI) Arc Mate 100i, a six-axis robot, combined with Lincoln's Power Wave 450 pulsing power source.
From tests, Lincoln determined that a robot could weld each Corvette seat frame in a cycle time that met the contract requirements.
Each frame requires 22 welds of differing varieties-corner, fillet and butt. Eight aluminum parts are welded together, including two side rails, two brackets, two plates, and front and back beams. These parts are of grade 6005 aluminum that ranges in thickness from 2 to 4 millimeters.
The robotic cell contains two custom fixturing plates mounted at 45-degree angles from the robot. While the robot is welding one seat frame on the right fixture, the operator is loading the left fixture. There is no downtime associated with the process. In addition to the robot and power source, the cell uses a Synergic 7F wire feeder from Lincoln and a push-pull gun with drive rolls that feed the soft aluminum wire without tangling and "bird-nesting." The wire used for this application is 3/64-inch-diameter, 5356 wire with an argon shielding gas.
Though cell setup was easy, setting the right welding parameters was a little rough at the onset. Because there are so many pieces and thin plate had to be welded to thick beams, the whole assembly heats up very quickly due to the thermal conductivity of aluminum. For this reason, the first weld of the seat frame had to be done at parameters dramatically different from the last weld. To set the right parameters, the team made many modifications to the program and positioning of the torches and travel angles until the right adjustments were made.
Fit-up was another obstacle to be overcome. To fill in the gaps, the robot uses a weaving technique rather than welding in a straight line. To further aid with fit-up, intricate fixturing is used. The fixturing includes pneumatic clamps to compress the pieces and minimize problems.
Burn-back was another concern. Kadee started by using 150-pound drums of wire, but were experiencing tremendous burn-back. Switching to 16-pound spools eliminated the problem and also made it easier for changeovers.
Since the normal start-up challenges have been overcome and the correct settings achieved, the operation of the robotic system has been good. The robot runs for two, 10-hour shifts each day, and the goal of each shift is to produce 200 seat frames. "Thanks to the robot, we are able to meet our contract requirements and we feel that it provides us with excellent productivity," says Elizabeth Kissner, welding engineer at Kadee.
Once the robot welds the parts, they are then wire-brushed. Any minor imperfections are sent to the manual gas tunsten arc welding station for touch-ups. All welds must pass inspection tests before being sent for e-coating and final preparation.
For more information on welding, call 216-481-8100 or visit www.lincolnelectric.com.
For more information on robotics, call 800-47-ROBOT or visit www.fanucrobotics.com.