When I worked at Matsu, I was on the second shift. One day when I came in, the production manager had noticed the robotic spot welds were off position, but the taught weld positions were correct. This robot was a Motoman K100. I had already noticed the welds were wrong as I walked past the robots, and the production manager was surprised that I knew about it before he told me. And I also knew that when he checked the positions by stepping through the robot program they were correct.
Just then, one of the self-proclaimed robotic welding experts told us that he had cut several seconds from the robot cycle time. I asked him if the time savings would pay to replace the robot wrists -- axes 4, 5, and 6.
Robot positions are taught with several parameters, one of which is the termination type. When the termination type is set to Fine, the positioning move is completed -- all axes have stopped moving -- before the next command is executed: the weld command. When the termination type is set to Medium, the following command is executed before the positioning is finished. The weld gun then closes before actually reaching the programmed position, and the weld is done. When the gun opens, the gun jumps about two inches. This stress is placed onto the robot joints, the weld gun, and the tooling. Running a robot like this for any length of time will break something. The fix is to step through the robot program and edit each weld position so that the termination type is Fine.
This entry was submitted by Glenn Aitchison and edited by Rob Spiegel.
Glenn Aitchisonís first field service job was in 1987. Since then he has worked in robotics, automotive, as well as industrial automation and machinery. He received his Certificates of Qualification as an Industrial Electrician and as an Industrial Mechanic (Millwright).
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