Consistent fixturing ensures consistent robotic welding
August 1, 2009
Don't forget a component of the system sometimes can dictate whether a robotic welding integration succeeds or fails: the fixturing.
Specifying and purchasing a robotic welding system can be a confusing process requiring predictions about future volumes and types of jobs. Along the way, the robotic system manufacturer and the integrator, if you are working with one, will help specify the best system for your application. However, an often-forgotten component of the system sometimes can dictate whether a robotic welding integration succeeds or fails: the fixturing.
Some may think that all of the loose pieces of an assembly can be fixtured entirely in the robot cell, without pretacking parts together before they arrive at the robot cell. Although this can be the case for simple parts, more complex assemblies may need to be tacked before the robot operation. Without pretacking a complex assembly, the fixture sometimes can become so complex that it impedes part loading and even blocks access to weld joints. Pretacking speeds the loading time for the robot and makes the fixture simpler, thus ensuring the robot can access all of the weld joints. So, be sure to take a detailed look at your parts—ideally, before purchasing the robotic welding system—to determine whether pretacking would be beneficial.
Many also think robots are most applicable to high-volume jobs, but not low-volume work. However, given the proper process and flexible fixturing, many companies have been very successful using robots for low-volume jobs. The robot does not know or care that a job is low-volume. It's the process and fixturing that will enable your success.
One of the key components is the positioner. The type you choose, be it a straightforward stationary table or a complex multiaxis rotary system, helps determine the flexibility and adaptability of the entire robotic system.
Headstock and tailstock positioners have a spine that spans the distance between the powered end (headstock) and the nonpowered (or sometimes powered) end (tailstock). The fixture and parts mount on the spine. Ferris wheel-style positioners have a similar configuration and share many of the same requirements.
The distance between the headstock and tailstock, the weight capacity, the center of gravity, and the swing diameter are particularly critical (see Figure 1 and Figure 2). The distance between the headstock and tailstock will affect the size and number of parts that can be fixtured. If you plan on welding multiple assemblies simultaneously, then you must allow sufficient distance between the headstock and tailstock for this setup.
The positioner's swing diameter must allow for 360-degree rotation of the parts without crashing into obstacles. Even if you do not plan on rotating the workpiece 360 degrees, you still should allow for full positioner rotation to prevent accidental crashes. Also, the swing diameter must account for the dimensions of not only the parts but the fixturing as well, including clamps and other components used to locate the parts (see Figure 3).
Also consider the positioner's weight capacity. The positioner must support the weight of both the part(s) and the fixture. This often-overlooked aspect can become a severe limitation of a robotic system.
Another overlooked characteristic is the center of gravity, or CG. The farther the CG is away from the centerline of the headstock/tailstock positioner, the more torque (or horsepower) will be required to roll the fixture and the part. This usually means the fixture and part need to shift downward to maintain the CG near the positioner centerline. Looking at Figure 3, you can see that although there is no problem with the swing diameter, the CG is too high. Figure 4 shows how the unit can be shifted downward to improve the CG, but now the fixture is outside the swing diameter. In this case, you must adjust the fixtures to maintain the proper CG while still working within the envelope of the swing diameter, as shown in Figure 5.
Turntable positioners—also called indexing or lazy Susan positioners—generally have two or more stations (see Figure 6). One is for part loading/unloading, while the other station is for robotic welding. When welding is complete, the table indexes, and the process repeats. The diameter of the table will affect the size and number of parts that can be fixtured simultaneously. And like the headstock/tailstock positioner, turntable systems must also handle the weight of both the part and the fixturing.
Stationary positioners do not move the parts being held, but rather allow the robot to move (sometimes down a track) and perform welding while the parts are held in place (see Figure 7). For the most efficient setup, ensure that the robot can reach all of the weld joints without excessive handling and part repositioning.
Modular fixturing allows you to fixture parts in various configurations consistently—important for robotic welding systems, which expect parts to be in the same position every time. Using a modular system allows for flexibility: welding one assembly, changing to another, changing back to the original, and so on. Such fixtures have repeatability within ±0.005 inch.
The goal should be to ensure that you end up with a system optimized for your environment and process. By taking an objective look at your assemblies, you should be able to define or refine your process to maximize efficiency, and answer critical questions about your fixturing before you buy your system:
Although critical to the success of the entire system, fixturing is sometimes overlooked. Ideally, it should be considered during the first stages of specifying the system requirements. To ensure a successful robotic welding process integration, it's essential to consider how fixturing will affect the specifications of the overall system. Simply put, the more consistently fixtured the part, the better a robot can weld it.