Manipulators – Part 3 of 8 Articles
The Job of the Manipulator
Before choosing a manipulator, we need to fully understand what is expected of it. How will it add to productivity? What is its purpose?
Will the manipulator control the gun, the workpiece or both? In the majority of thermal spray applications, the manipulator controls the gun. The piece being sprayed is fixed, mounted to a turntable or mounted to a lathe. The primary question is where will the system fit within the production operation?
The Job of the Manipulator by Plasma Powder and Systems, Inc. “The first rule of any technology used in a business is that automation applied to an efficient operation will magnify the efficiency. The second is that automation applied to an inefficient operation will magnify the inefficiency”. Bill Gates Before choosing a manipulator, we need to fully understand what is expected of it. How will it add to productivity? What is its purpose? Will the manipulator control the gun, the workpiece or both? In the majority of thermal spray applications, the manipulator controls the gun. The piece being sprayed is fixed, mounted to a turntable or mounted to a lathe. The primary question is where will the system fit within the production operation? In one application, where the welded end-pipe on truck mufflers needed a wire-arc coating, it initially appeared that the manipulator should control the gun. However, when the production operations were considered, it was best to use a robot to pick the fabricated mufflers off the production line and poke the muffler end through an opening where a fixed wire-arc gun coated the weld area. The robot rotated the muffler during the coating operation. In another application, hydroxylapatite was applied to small medical implants. The application used a robot to pick an implant from a tray and position it in front of a fixed plasma gun. The arrangement and tooling insured that the robot did not become coated during the operations. Another unique application was two robots being used for coating aerospace parts: one controlled the plasma gun while the second controlled the part used during the coating operation. Again, the robot programming had to take into consideration the direction of the spray to avoid coating the part-handling robot. In all of these cases, the application was very specific. These systems would generally not be suitable for a thermal spray job shop. What is the weight to be manipulated? The weight to be manipulated needs to include not only the gun but a portion of the weight of the connecting hoses, mounting fixture, cables and sensors. One robotic system for coating a weld zone in large hydro turbines required an HVOF gun mounted on a three-foot extension to allow coating between the blades. The robot had to be sized to handle the weight of the cantilevered gun, gun extension, hoses and cables. Some production shops use one robot for a variety of guns. Each gun is attached to a keyed quick-disconnect mounting half with the mating half on the robot. This not only allows for rapid changeover of the gun but also precise positioning of the gun on the robot. The weight of the quick-disconnect needs to be included in the manipulator payload requirements. What motion is required? Roll covering shops often use only a single-axis gun controller that traverses along with a lathe. Many job-shops require only a two-axis manipulator that provides an X-Y motion in front of a turntable with a fixed standoff. For the coating of complex shapes, especially where the part to be coated is fixed, a five- or six-axis robot is usually required. What range of motion is required? Not only is it necessary to consider the size of the largest area to be coated, but it is also necessary to consider what range of motion will be required for thermal spray operations. For example, a robot for coating the face and cheeks of steel rolls will need to extend well past the face of the roll. In this case, a drawing of the robot in the extreme coating positions is needed to verify suitability. What speeds will be required? Manipulator speed is generally not of major concern when coating parts mounted on a turntable or lathe. The surface speed needed for coating is achieved by the speed of the turntable. The speed capability of the manipulator becomes a significant factor when the part to be coated is fixed, especially when HVOF coating is applied. In such cases, robot speeds up to 275 feet per minute may be required. Such speeds are possible with present-day robots but safety becomes a major issue to ensure that no one is around the robot during high-speed motions. The next and fourth article in the series explores “The Qualifications of a Manipulator”. |