Tower Automotive of Elkton, MI, is a Tier One automotive supplier and one of the largest contract stamping companies in the United States. The company also produces the final assembly from the stampings.
One of Tower''s latest jobs is to take an automotive lift-gate that was originally produced in plastic and make it in a more rugged steel part. To accomplish this task, Tower turned to Bay City, MI-based Newcor to design, engineer, and produce an integrated part assembly system that could yield 112 parts per hr. Galvaneal Type 1010/1008 steels are used for the parts.
To develop the needed structural integrity, two stampings are used: the outer lift-gate and an inner panel. Before they are joined together, several reinforcement brackets are welded to the inner panel. At the same time, a robot picks up the outer panel and maneuvers it while a second robot applies a bead of two-part epoxy adhesive in various areas around the panel. The part manipulation robot twists the panel in four axes, keeping it level to the floor to reduce adhesive sag. The robot then presents the panel for the application of antiflutter adhesive dollops.
Mounted on the adhesive dispensing robot is a Perceptron laser system, which monitors the thickness and quality of the bead. Then an operator manually marries the inner panel. The part is transferred with an overhead pick-and-place unit through the system where the inner and outer panels are hemmed together and welded using a conventional spot welding system in areas that don''t require a critical surface finish. For critical surface finish areas, Newcor''s patented Hy-Pak welding system is used to achieve a class-A, nonblemish surface.
The Hy-Pak incorporates a resistance welding process that uses a combination of high-energy, unidirectional pulse with rapid electrode follow-up. The duration of the weld current pulse is so short that the high current is concentrated at the weld point and minimal heating is transmitted to the adjacent component and electrode material.
To use this welding process on the panels, a sharp linear projection is produced on one of the panels where the weld takes place. Newcor designed the fifteen-station line to put the projection in the parts for the Hy-Pak process instead of adding them in the stamping process. By doing this, Newcor found that the projection quality is much better, because the projection location can be adjusted and the projection dimensions can be easily controlled. Also, if the punch and die set needs changing, it is easier to do it on the assembly line. If one of the punches breaks, it can be detected quickly because the change in weld quality is immediately noticeable.
"Without the Hy-Pak weld, the inner and outer panels would shift no matter how tightly they squeeze the hems together in the hemming process," says Merlin Farver, chief engineer at Tower. Part repeatability is important. Panel tolerance is 0.5-0.75 mm. "We collect dimensional data and provide it to the customer. We track the dimensional tolerances of the part so we know if there is a problem with consistency. We can identify the problem during manufacturing and make the appropriate adjustments to maintain proper specifications," adds Farver.