February 19, 2001
For a fabricator to enjoy the benefits afforded by a robotic welding system, the parts to be welded and the system itself must be designed properly.
|This dual robot system uses a large "Ferris Wheel" positioner to give the robots access to the underside of the parts to be welded.|
Automated robotic arc welding systems are used in all types of manufacturing. They can provide several benefits in certain welding applications.
The consistency and repeatability of robotic welding systems can reduce production costs in various ways. For example, the precise movement of the robot arm and torch may result in fewer scrapped parts or parts that need rework.
Also, consistent welding parameters, such as wire feed speed and travel speed, reduce consumables usage by eliminating the speed variation inherent in manual welding. This consistency reduces the amount of filler wire, electricity, and shielding gas used for each welded part.
The robotic welding system's consistency and repeatability can also improve product quality. Repeatable travel speeds and torch angles generate a more consistent weld penetration and weld strength. The perfect movements of the robot and torch also generally produce a superior cosmetic appearance of the welded part.
Robots can also help increase productivity, because an automated system achieves more throughput than a manual system, and robotically made welds are more consistent than those produced manually.
For a fabricator to achieve these benefits, however, the parts to be welded and the system itself must be designed properly. Adapting an existing part or designing a new part to be welded on a robotic system can present some complex challenges. However, these challenges can typically be overcome by following the guidelines discussed in this article.
This design process can be a time-consuming and frustrating endeavor for someone without the proper experience, so some fabricators might find that consulting with a welding automation and/or tooling expert will be cost-effective.
The design issues discussed in this article are critical to the success of any automated robotic welding system. Many of these concepts can also be applied to the design of parts for manual welding.
The tolerance for each component in a robotic arc welding system affects the overall tolerance of the system.
For instance, the welding robot may have a position tolerance of ±0.004 inch, while the positioner, which is the component that holds and rotates the part, may have a tolerance of ±0.010 inch. The fixture, which is the component used to clamp and hold parts, may have a tolerance of ±0.012 inch, while the parts to be welded may have a tolerance of ±0.015 inch.
When all of these tolerances are combined in one system, the location of the weld could end up being out of place by ±0.041 inch.
Positioning equipment manufacturers state their products' repeatability tolerances in various ways, based on the location of the part to be welded in reference to the center of the positioner axis. For this reason, the size and shape of the part should be evaluated in relation to the type of positioner to be used.
As the size and capacity of the robot, positioner, fixtures, and parts increase, the accuracy of the system decreases, and the tolerance for the weld location increases. Incorporating precise components with a high degree of accuracy and maintaining low tolerances are critical to ensuring the success of a robotic welding system.
The weld joint (fillet, lap, butt, "T," etc.) is an important consideration and should be chosen based on the intended load requirements. Once the weld joint is selected, several methods can be used to reduce welding costs and increase finished part quality.
Welding close to the neutral axis (centerline) of the part reduces the potential for distortion. This can eliminate secondary processes such as bending or straightening, which could be required to correct the welded parts.
With prepared weld joints, (single or double grooves, beveled edges, etc.), using the minimum root opening and the smallest included angle needed reduces the amount of filler material required and increases the travel speed of the welding robot. On thicker plates, using a double bevel instead of a large single bevel can reduce the amount of filler material needed by as much as 40 percent (see Figure 1).
With prepared weld joints, using the minimum root opening and smallest included angle reduces the amount of filler material required and increases the robot's travel speed. On thicker plates, using a double bevel instead of a large single bevel can reduce the amount of filler material by as much as 40 percent.
Long, small welds may be stronger than large, intermittent welds and put less heat into the part. This can reduce equipment costs by requiring smaller torches, power sources, and other components. In addition, keeping the amount of welding to a minimum reduces the use of consumables such as filler material, shielding gas, and electricity. Reducing the weld size also minimizes the heat put into the part, potential distortion, and internal stresses that may occur.
Designing the weld joint for torch access is another important consideration. Maintaining the proper torch angle throughout the weld improves weld quality and reduces heat input to the part. Ample space must be allowed for the robot arm and torch. This will make programming the robot weld path much easier and will reduce the chance that the robot, the torch, or even the welded part could be damaged.
Tooling devices, including jigs and fixtures, are also critical design considerations. Welding fixtures and tooling can be used to decrease fabrication time and improve part location for the welding process.
It is very important to determine if the tooling and fixturing equipment will be used simply to hold a pretacked subassembly or to hold the individual pieces in their exact locations while they are being welded together.
If the fixture holds a subassembly, the part must be located accurately and consistently for the robot to perform the necessary welds. Holding individual pieces in place during a robotic welding sequence is much more complex. The fixture must hold each piece in the same location as the others during loading. The individual pieces must be held securely as the positioner moves and rotates at high speeds. Finally, the tooling must be rigid enough to withstand heavy clamping forces.
Designers must make sure that the parts can be loaded and unloaded quickly and easily for increased production output. Automatic clamping is advantageous for higher throughput and can be sequenced to reduce interference while the part is being welded. Clamping devices can be situated to increase the reach and accessibility of the robot arm and torch.
Robotic arc welding automation is not suitable for every manual welding operation. The manual operation being considered for automation must be a repeatable process with precise tolerances built into the parts to be welded.
A good return on investment (ROI) must be demonstrated to offset the initial cost of a robotic system, and other considerations, including floor space and production volume requirements, must also be addressed.
The design concepts listed in this article are just some of the typical issues to consider when designing for a robotic welding system. Understanding and implementing these concepts during the design phase will help improve a fabricator's chance for producing a successful, productive robotic workcell.
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