KUKA Robot Well-Suited to Flexible Motion Sequence in Cutting and Deburring Operation

Problem:

The user, a medium-sized business, works in three shifts throughout the year, producing toboggans and child seats. The company manufactures 70,000 to 80,000 toboggans per season. After plastic parts finish going through the blow molding process, holes are cut in them while they are still hot; after that, the resulting sharp edges must be deburred. Until now, five employees were required continuously 24 hours a day just for the deburring. The extreme stress that this work placed on the workers’ wrists was constantly resulting in disabling work-related injuries. An automated solution was therefore extremely desirable. The only kind of robot that came into consideration was KUKA’s KR 15 with six axes, since other designs, with their limited kinematics, are not suitable for this type of flexible motion sequence.

Solution:

Roboter Technologie GmbH, a KUKA systems partner, found an answer – a float-mounted tool unit, which can accommodate various cutter blades, for example straight, concave and convex cutters for various radii. To remove the plastic burrs, the blade must always maintain the correct cutting angle and a constant cutting pressure. To avoid having to provide a large amount of floor space for intermediate storage, the toboggans had to have the holes cut in them and be deburred immediately after leaving the blow molding machine. It would not have been economically justifiable to use either quick cooling, which would have caused structural changes in the plastic, or a cooling zone, which would have required extensive systems engineering work and additional handling. Therefore the machining had to take place while the plastic was cooling, and simultaneously shrinking by about two to three centimeters.

A KUKA KR 15 robot cuts the holes in the parts and deburrs them in a single operation. The cutter adapts to the contours of the workpiece. The advantage of this procedure over manual handling lies in reduction in time, consistent and significantly higher quality, and the minimal number of rejects.  During machining, the toboggans are gripped in a support fixture. The user gains an additional quality improvement due to the fact that in the fixture the toboggans retain their shape despite cooling.

Apart from the tool, the greatest demand placed on the robot was the need for high flexibility, to be able to process all products with a single system. Furthermore, the operator placed a high value on precise repeatability and a controller that was easy to use. The open PC technology and Windows interface offered by KUKA Roboter GmbH, based in Augsburg, Germany in this regard met his expectations.

As soon as a blow-molded part leaves the plastic-molding machine, a worker takes it, removes the flash and places the part in one of two form locations mounted on a rotary fixture. Then he operates a switch to clamp the toboggan. As soon as the robot located on the other side has finished its task, the fixture rotates. The new part thus enters the KR 15’s working area, and the completed part can be removed from the fixture, which opens automatically. The worker then packs the toboggan directly into a box that is standing nearby for that purpose. Cutout pieces are removed from the robotic cell on a conveyor belt to be ground up and re-used.

After positioning, the robot begins immediately with simultaneous cutting and deburring. As soon as the side parts of the toboggan have been cut out, the fixture tilts it to a vertical position, so that the KR 15 can also easily reach the toboggan’s topside. The robot completes machining of the cooling toboggan within 40 to 50 seconds. Thus the cycle time of the robotic cell, including the manual handling for loading and unloading, is significantly shorter than that of the blow molding process, which takes 120 seconds.

During the processing of each toboggan, the robot exchanges tools once at a tool changer that is integrated into the system. In addition to a convex cutter for the cutouts, a straight cutter is also used, with which the KR 15 gives the toboggan a smooth exterior contour.

The robot always begins with the interior contour. Changing back to the curved cutter at the same time as the rotary fixture is turning saves time. Reversing the sequence is ruled out because of the cooling process.

The toboggans are always produced continuously over a period of two to three months. The finished products are put in storage for the next season. When making a production changeover, the operator disassembles the robotic cell and moves it with forklift trucks to the other plastic-molding machine. Because of these transfers, the size of the cell was an important factor; in its assembled form it measures 4.80 meters x 4.80 meters x 2 meters. No programming work is required after a changeover; the robot identifies each individual piece by means of the exchangeable coded form locations and the programs stored in the controller.

The robotic cell paid for itself within one season. Since it is practically impossible for the user to find new employees due to the health hazards posed by fumes emitted from the plastics. In addition, official regulations would require making additional investments. In any event, the user is already thinking about using an additional robot for automatically feeding materials to the cell and removing the finished products.

Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld