Auto supplier expands robotic cells

Manufacturing Engineering, Jan 1999

When PK U.S.A. Inc. needed to boost its production by 25% to meet an Asian customer's request, the Shelbyville, IN-based automotive parts maker turned to Motoman Inc. to expand and redesign its existing robotic systems.

PK U.S.A., a supplier of metal body parts, chassis, and plastic injection parts for U.S. and foreign automotive companies, has a large volume of equipment from Motoman Inc., (West Carrollton, OH), in its 318,000 ft2 plant. The inventory includes more than 75 arc-welding robots, 12 materialhandling robots and 24 spot-welding robots scattered throughout the plant's automated assembly and production lines.

The company's latest robotic expansion came after the Asian automobile customer's request for higher volumes of the PK U.S.A.'s upper link subassemblies used in front suspensions. Michael Hancock, assembly process coordinator, said expanding and redesigning one of PK U.S.A.'s many Motoman robotic systems provided the solution.

"Originally, this Motoman robotic workcell was designed for one level of production capacity that allowed for some fluctuations in scheduling," notes Hancock. "But to handle the 25% volume increase that these new product variations generate, we had to make modifications or bring in an additional robot."

PK U.S.A. began the upper link subassembly project with one Motoman SK120 robot. Besides expanding the original workcell with installation of a Motoman SK6 robot to handle the increased production, PK U.S.A. faced the additional challenge of making the changes while using just-in-time manufacturing procedures that maintain its overall production schedule.

"The task of the SKI 20 robot was to run a three-step operation encompassing material handling, arc welding, and spot welding," notes Hancock. "We accomplished some of the upgrade on weekends."

Designed to produce sets of the upper link subassembly (including right and left parts of a front suspension) at the rate of 620 sets per day, the workcell increased production to 830 sets per day with the additional SK6 robot, says Hancock.

Precision welding is crucial for the upper link subassembly, which is part of the suspension that serves as a hinge point allowing the front suspension to move and "step" over bumps in the road. The welding procedure, done in sets on the right and left side of the upper link assembly, must mirror each other exactly in order to perform properly.

In the current robotic process, an employee loads a small clip and two reinforcement parts for each left and right upper link subassembly onto a part positioner. An employee initiates the work cycle of the SK120 and SK6 robots, which are both integrated with welding packages to perform arc welding. The SK120 robot is also equipped with a three-prong gripper to perform material handling duties.

Robots perform a plug weld to attach a clip to one of the reinforced parts, and two linear welds to join the three pieces of each part firmly together. After the positioner rotates 90, a circular weld pattern applied by the robots reinforces each part. Filler metal of 0.045' (11.43 mm) copper-coated steel wire is used and the shielding gas is a mixture of 80% argon and 20% CO2.

At this point, the SK6 machine withdraws from the arc-welding workspace to allow the SKI 20 unit to transport both subassemblies to the spot-welding station. Completion of the operation is accomplished by the robot rotating its S axis 90 and using the material handling end-effector to grip each part individually and move it to the spot-welding station.

A peripheral piece of equipment in the workcell, the spot welder receives the signal to begin spot welding from a PLC. A sixaxis SK120 robot transfers each subassembly to the spot welder to receive five reinforcement welds, and the robot then transfers the two pieces, one at a time, to the hydraulic-powered date-stamp embosser. Upon completion, the SK120 transfers the parts to an ejection chute. Operators place the sets onto racks and ship them to final assembly.

Hancock says adding the SK6 robot to the workcell increased the speed of the operation by executing half of the arc welding formerly done by the SK120. While the SK120 still performs arc welding on the left side of the part, the SK6 simultaneously welds the right side, so that the SK120 can move the completed parts to the spot-welding area more quickly. Circle 224.