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ROBOTS/ AUTOMATED HANDLING |
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Factors
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Better lead times, more efficient and manpower usage, greater volume and product mix, greater tolerance requirements, and safety improvements are all critical in specifying a robotic work cell. By Nicholas Trick, Motoman, Inc. Standard, modified-standard and custom-designed robotic welding cells can each be suitable for present needs with the capacity to handle anticipated growth. A cell’s suitability for particular types of projects is determined during the concept stage. Often, robotic manufacturers will ask to see sample parts or drawings to determine what type of robotic welding cell is needed. For fabricators working with high volumes and short delivery times, and a lower cost per unit, a standard system is usually the low-cost answer if it can handle the parts. Some standard robotic welding cells can be installed within a few hours. These systems can easily fit into an operation to replace a no-longer-available or no-longer-adequate manual operation. Along with the six-axis arm equipped with the appropriate end-effector tool, many standard systems and cells consist of:
Each standard robotic welding solution, or "World" as Motoman, Inc. (West Carrollton, OH) calls them, lends itself to a particular type of welding project. (e.g., simple linear welds; welding parts on two sides, front and back; long, tube-like parts with saddle welds; etc.). End users have the option of selecting from different Motoman robots with variations on welding processes, work envelopes, power sources (CV, pulse, multipurpose, aluminum-specific) and controllers. Motoman systems include a total safety environment that meets and exceeds safety standards. If some of the standard system components aren’t adequate for a particular operation, a modified-standard system should be considered. Special handling equipment to position parts might be required; or three weld stations rather than two; or enclosures that allow for cell installation where building accommodations are needed. The operation might require specific positioning equipment to join a small part to a large, heavy part. Switching between two or three types of welding may be required. The increase in productivity, quality control and volume, from adding distinct features to meet particular needs, can more than justify the additional investment. At the top of the robotic complexity scale is the custom-designed system. This is the most sophisticated robotic system and will require the longest lead time from ordering to operation. It requires concentrated engineering effort to develop the system, from concept, design and build to run-off. When required, the increase in versatility and usability is well worth the effort, since such a system also offers the greatest potential for improving operating cost per unit and quality control, increasing volume, saving material, and using people more efficiently. Training and Off-Line Programming People with manual welding experience often make the best robot programmers. Their manual welding experience provides good background for troubleshooting. Operator training should be conducted by instructors certified by the International Association for Continuing Education and Training (IACET), and should provide continuing education credits for those who pass the course. Ideally, it is best to buy your system from a vendor that offers a complete solution, including plenty of hands-on training. One of the useful features now available in robotic welding cell packages is off-line programming (OLP). In the past, manufacturing operations might have to be interrupted to program the robot for each new part using the programming pendant. OLP enables the operator to program the robot off-line while the system continues producing parts on-line. Although OLP has been available for some time, cost may have been a deterrent. Recently, software package prices have dropped, making increases in robotic welding productivity practical for many more users. Using OLP also results in a sizable reduction in downtime for shops that take their robots off-line for testing purposes. Motoman’s ROTSY, version 4.3, (Robot Off-line Teaching System of Yaskawa) can model a proposed robot cell and perform test cycles even before the cell is built. ROTSY features several functions that confirm the most efficient robot system configuration and enhance programming efficiency. These functions include advanced layout design, cycle time calculations, job analysis and off-line teaching technology. Operators can calculate cycle times on the PC instead of actually using the robot. The interference function verifies the workpiece, robot arm and peripheral equipment locations, and indicates any problem areas. The data transfer functions eliminate the need for job switching and enables the operator to create job data easily, then verify those jobs on the robot simulator. Motoman, Inc. About the Author: Nicholas Trick is a marketing communications specialist for Motoman, Inc., manufacturer of robotic solutions for arc welding, assembly, coating, dispensing, material cutting, material handling, material removal and spot welding applications. He can be reached for comment at 805 Liberty Lane, West Carrollton, OH 45449-2158; telephone 937/847-3300, or fax 937/847-3288. Robotic Welding Cell Tooling Shouldn’t Be An Afterthought The tooling of a robotic welding cell is a critical aspect of the cell design. Fixtures (tooling) consist of locators, clamps, sensors, turntables and other systems needed to hold parts and ensure precise welding by the robot. If the tooling is inadequate, inaccurate, worn out or malfunctioning, parts will not be positioned accurately; the robot will then apply an inaccurate weld; and the resulting piece will be scrap. Tooling costs can range from a minimal investment to a significant fraction of the cost of the entire welding cell, sometimes equaling the cost of the robot. Attempts to reduce cost by doing the tooling in-house can be successful, but this may prolong the installation by limiting use of the cell or causing problems with other parts and assemblies being produced by the cell. "Some companies can produce their own tooling cheaper than they can buy it, but to gain the full use of the robot the tooling needs to be very, very accurate," says Chris Anderson, product manager for Motoman, Inc. (West Carrollton, OH). "For many of our customers, Motoman provides the robotic cell in a turnkey fashion to the end user. We design the tooling, debug the fixtures and take responsibility for making accurate parts. I recommend this method for first-time investors in robots to get the robot up-and-running and producing parts in the shortest amount of time." Designing the tooling requires a thorough knowledge of the part manufacturing sequence before it reaches the welding cell. The engineering drawing of the completed assembly shows components in a finished condition. There may be holes, ridges, edges, bends, etc. in the finished piece. Knowing when these features are added to the part makes tooling design easier. The welding process also can affect the design of the tooling. Spot welding two pieces together has a much wider tolerance range than arc welding, since spot welding is applied in a general area, while arc welding must join two edges. Another consideration is that the pieces being welded may change dimensions or locations during the welding process. Welding applies heat, causing the pieces to expand, while cooling causes them to contract. These changes in dimensions must be taken into account when designing the tooling. One advantage of robotic welding is that the weld is consistent, making part distortion consistent as well. In some instances, Motoman has built fixtures that hold parts outside of tolerance, allowing weld distortion to bring the parts to nominal dimensions. Motoman has routinely provided robot systems capable of meeting critical dimensions within three-sigma or a Cpk of 1.3. The robotic vendor knows the requirements for robot movement and positioning during the welding operation. Vendors can inform users about the space needed for the welding torch to reach the joint. If the assembly is rotated during the welding operation, experienced robotic welding tool designers have the knowledge to maintain the correct welding torch clearance during rotation. The tooling vendor also has the experience to determine if the tooling should be manual, with the operator controlling the clamping; or automatic, with a controller checking for proper loading and positioning, and performing the pneumatic clamping sequence. More comprehensive tooling packages often involve pneumatic clamping; some include electronic sensing to verify part presence and clamp open/closed conditions. Automatic clamping makes loading easier, results in faster load/unload times and improves ergonomics for the operator. Part sensors help prevent the operator from making mistakes in part loading and can detect parts that are not clamped properly. Part sensors also can indicate if a left-hand or right-hand part is loaded so that the robot executes the proper program. |