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Fixturing for robotic welding

Specifying and purchasing a robotic welding system can be challenging and requires predictions about future part volumes and types of jobs. A robotic system manufacturer and integrator, if you are working with one, will help you specify the best system for your application. However, how parts will be fixtured often is forgotten in this process. Yet these fixtures sometimes can dictate the success or failure of an installation.

It is critical that fixturing be considered starting at the very first stage of specifying the system requirements. Characteristics such as the weight capacity and swing diameter of the positioner must account not only for your parts, but also for the fixturing.

Fixture Misconceptions

In the excitement of purchasing a robotic welding system, fabricators might have misconceptions that can prove costly during the implementation and usage of it.

People often think that all the loose pieces of an assembly can be fixtured entirely on the robot, without the need to tack them together first. Although this can be the case for simple parts, complex assemblies may need to be tacked together before being placed on the robot. If a complex assembly is not tacked, the fixture often becomes so complex that it impedes the loading process and blocks access to the weld joints.

Tacking a complex assembly can speed the loading time for the robot and significantly reduce the complexity of the fixture on the robot, thus increasing the robot’s access to all of the weld joints. Take a detailed look at your parts before purchasing your robotic welding system to ensure you streamline your process by determining tacking needs ahead of time.

Robots often are thought of as being most applicable to high-volume jobs. But using the proper process and flexible fixturing, many companies have been very successful using robots for low-volume jobs as well. It’s the process and fixturing that will determine the profitability of such an approach.

The perceived requirement for quick changeovers suggests to most people the need for dedicated fixtures. However, modular fixturing is available that can provide you with accurate, repeatable, and flexible options for your system. Understanding the flexibility of the fixturing you are investing in can help you plan to introduce more parts to your robotic welding system in short order.

Positioner Options

One of the key components of your robotic welding system is the positioner, whether it is a straightforward stationary table or a more complex multiaxis rotary positioner. The characteristics of the positioner will be a decisive factor in the flexibility and adaptability of your robotic welding system.

There are many different types of positioner, each with its own set of characteristics. Here we discuss the types you are most likely to consider when preparing your welding system, although there are other options.

Headstock/Tailstock Positioners

These positioners have a spine that runs the span between the powered end (headstock) and the non-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.)

Critical characteristics of this type of positioner include the distance between the headstock and tailstock, the weight capacity, the centre of gravity (CG), and the swing diameter.

The distance between the headstock and tailstock affects 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 swing diameter of the positioner must be sufficient to allow 360-degree rotation of the parts without crashing into obstacles. Even if you do not plan to rotate 360 degrees, it is highly recommended that you specify a system to allow for full rotation as this will prevent crashes.

The swing diameter must not only account for the dimensions of your parts, but must also allow room for the fixturing, including clamps and other components used to locate your parts. In Figure 2 we see a diagram representing an end view of a headstock/tailstock positioner. The dashed circle is the swing diameter of the positioner. As you can see, both the parts and the fixture fit inside the swing diameter.

The weight capacity of the positioner is vital. The positioner must not only be able to support the weight of your parts, but it also must bear the weight of the fixturing. This is an often overlooked aspect that can become a severe limitation of the robotic system.

CG is frequently overlooked too. The farther the CG is away from the centreline of the positioner, the more torque (or horsepower) will be required to roll the fixture and part. Most of the time this means the fixture and part need to move down to maintain the CG near the centreline of the positioner. Looking again at Figure 2 you can see that although there is no issue with the swing diameter, the CG is too high. Figure 3 shows how the fixture could be shifted downward to improve the CG. However, now the fixture is outside the swing diameter.

Adjustments to the fixtures are required to have a proper CG while still working within the envelope of the swing diameter. Figure 4 shows a corrected setup that satisfies the requirements for both CG and swing diameter.

Turntable or “Indexing” Positioners

Turntable positioners generally have two or more stations. One station is for part loading/unloading, while welding occurs in the other station. When the process is complete, the table indexes and the process repeats. Another common term for this style of fixture is a lazy susan.

Critical characteristics of the turntable positioner include the diameter of the table and the weight capacity.

The diameter of the table affects the size and number of parts that can be fixtured simultaneously.

The weight capacity of the positioner also is critical. The positioner must be able to support the weight of your parts as well as the weight of the fixturing.

Stationary Positioners

Stationary positioners do not move the parts being held, but rather allow for the robot to move (sometimes down a track) and perform welds while the parts are held in place.

The main concern when considering this type of positioner is to ensure that the robot can reach all of the weld joints without excessive handling and repositioning of the parts.

Fixturing Options

Many companies are finding that modular fixturing is a perfect match for the flexibility, repeatability, and accuracy of a robotic welding system. It allows your robotic welding system to fabricate an ever-broader selection of parts. These fixturing systems are designed such that you can get the same repeatability and accuracy as you do with your robotic welding system even as you change parts in and out. Repeatability within 0.005 in. is readily achievable.

Ultimately, you have many variables to consider when specifying a robotic welding system. To ensure the successful integration of the robot into your processes, it’s critical to answer key questions about your fixturing ahead of design and implementation.