January MT Magazine Bonus Content
Improving the breed at Electric Boat
By Andrew DiFusco
My company, Electric Boat Corporation, is the prime contractor to the United States Navy responsible for the design and construction of nuclear submarines. We work with the navy on the newly designed Columbia-class submarine, as well as additional Virginia-class submarines (VCS). We’re working to leverage new technology to enhance how we build submarines now and in the future.
The submarine shipbuilding industry is a high mix-low volume (HMLV) manufacturing process that routinely requires multi-layer, multi-pass welding. Until recently, this has not been cost-effective to perform with welding automation. In the past, we were dependent on mechanized welding in lieu of robotic welding due to difficulties in the welding processes, and the production time of programs for the robotic systems. Due to the development of new technologies, we’re now realizing the opportunity to incorporate welding automation.
With advances in technology, the processes involved in heavy steel fabrication are now being looked at differently. New initiatives are enabling engineers to perform research and development (R&D) on emerging technologies to increase the efficiency of current HMLV manufacturing processes. At Electric Boat, we’re working to capitalize on this and implement a paradigm shift in automation and robotics. Recent projects focusing on improvements in offline programming, development of multi-pass heavy welding capabilities, the integration of 3D metrology processes, and build authority enhanced work packages have enabled the implementation of new solutions and spawned additional opportunities where we could use robotics.
Offline robot programming is a great advancement over traditional online programming. Online programming would require production to stop the robot while an operator uses a teach mode to drive the robot around while recording the movements for replay. Offline programming allows this teaching to happen in a virtual environment on a remote computer, thus enabling the robot to continue production work without interruption. This way, programs can be simulated ahead of time and saved in a library for repeated use or copied to other robot systems to scale up production with the same amount of programming time.
We use Robot Studio to complete the offline programs for the implementation of robotic systems. Manufacturing information is added to the computer-aided design (CAD) geometry and then the model is converted to a STEP file and transferred into Robot Studio. Robot Studio has a dimensionally accurate CAD model of the interim products robot cells, which is used as the virtual environment in which the offline program is developed. This ensures that the placement of the part on the tables, the reach of the robot arm, and other issues can be identified and simulated on a remote computer before starting the job, with the real robot being able to continue working another task all the while. Figure 1 is an image of offline programming being done in Robot Studio
Figure 1: Robot Studio offline programming
Development of capabilities
The multi-pass heavy welding capability is an essential tool for multi-bead, multi-layer adaptive welding. This welding process, developed in conjunction with robotic integrators, consists of a set of welding instructions and data types that can be used to make single or multiple welds based on deposition or percent fill. The process also provides a set of tactile searching instructions and data types using the welding wire as a probe for measuring and calculating area, root width, and top width of a weld joint.
The aim of the multi-pass heavy welding process is to achieve the set fill percentage with each pass. Given a particular wire size and weave length, the program adjusts two parameters—weave width and travel speed. The software will show if a limit has been reached and, as a result, the percent fill may not be obtained. The welding parameter limits govern the percent fill attributes to ensure the robot will not go outside the allowable weld parameters to achieve a percent fill. This is commonly referred to as adaptive mode or adaptive fill.
3D metrology software has been used in production for scanning as-built geometries to compare to the technical models and check the accuracy of the manufacturing process. The use of these advance measurement techniques has been crucial in the implementation of robotics in heavy steel applications due to the difficulty in exactly matching dimensions called out in technical models. Robotic welding processes rely on repeatable and predictable conditions of weld joints, which traditional ship fitting practice alone cannot accomplish. To overcome this, in-process measurement data is reviewed and assists in achieving the best possible fit-up of parts to provide an in-tolerance configuration of the assembly. This same data also is used to reverse engineer the weld joints in the best fit condition, which is critical to the execution of the welding process.
The use of bead planning in multi-pass robotic heavy welding applications is advantageous for controlling weld distortion. Our current robotics systems consist of bead planning algorithms developed by vendors to establish bead plans that achieve minimal weld distortion. These bead plans have the capabilities of controlling the amount of weld passes with respect to a specific joint configuration and material thickness. The process typically begins with a search or scan of the desired joint using metrology software such as laser scanning to map out the joint. The bead planning algorithm can then use the information from the scan to produce data that will establish a bead plan accordingly. Figure 2 is an example of bead planning for T-joints.
Figure 2: Example of bead planning T-Joints.
Enhanced work packages
At Electric Boat, we also have realized the importance of conveying technical data to the trades during construction and that doing so can help reduce cost and schedule. To enable production, technical data is modified to an enhanced state referred to as “build authority,” which contains information such as bevel types, extra stock, labels, and markings. Due to these different conditions, we’re implementing new technologies to accurately portray the assemblies compared to the “technical authority models” and drawings. The purpose of providing this build authority information to the trades is to better explain how to build a specific assembly and to improve the ability to integrate automation into the manufacturing process. The result of this is reduced re-work and improved cost and schedule by improving the quality of work packages provided to the trades and improved ability to implement automation.
We currently use mechanized welding systems to weld straight joints to reduce the start and stop times and quality issues in the current process. Typically, the mechanized system is mounted on tractors in way of the joint to be welded, and the operator controls the system remotely, via controls and sometimes a camera. This has been beneficial to remove the operator from an environment where environmental air temperatures can be in excess of 140°F and also sometimes to remove them from tight spaces that make it difficult to maneuver. The use of mechanized welding systems, as described, is beneficial for welded joints that are long, straight, and uninterrupted by attaching members. A mechanized system is not ideal for assemblies with more complex features.
Approximately 20 years ago, we made the first attempt to implement a robotic welding system known as Programmable Automated Weld System (PAWS). Although it was an older system, PAWS was capable of welding assemblies with these complex features. However, at the time the complexity of programming and the lack of in-process compensation during welding proved too difficult to overcome. Recently, recognizing the benefits of newer systems, we began investigating the use of robotics for more complicated assemblies such as curved web to flange connections. Our pursuit intended to leverage the many advantages robotic integration has to offer; such as:
- adaptive fill
- bead planning
- tool changing
- interpass cleaning
- safety enhancements
- offline programming
- through-the-arc sensing.
Interim products robot (IPR). Our most recent large effort for implementing robotic welding technologies was through a navy manufacturing technology (ManTech) project, “Robotic Welding of Virginia Class Submarine Interim Products.” The goal of this project was to research and prototype a robotic welding solution to robotically weld parts identified as interim products. The interim products are fabricated structural assemblies that are broken down by their product structure. Examples of interim products are “tee” and “I” assemblies; foundations; tank internals; and inserts and penetrations.
This is the first system we had that required qualifications for robotic welding using an adaptive welding process to perform the welding on nuclear submarine structures. We believed the best method for reducing welding costs would be to increase the amount of joints that could be robotically welded. The final solution would be an integrated robotic system, capable of offline programming to robotically weld the complex joints in different positions.
The project was successfully completed in 2014. The IPR is fully implemented at our Quonset Point, Rhode Island location, and has been shown to increase the welding efficiency (greater than 30% improvement) along with close to 100% first-time quality rates up to and including X-ray quality welds. Figure 3 identifies the IPR in the working cell.
Figure 3: IPR working cell.
Missile tube automation. Subsequent to the IPR, we developed a robotic system to support the completion of more complex welds. The purpose of this robot is to revolutionize the installation of missile tubes (MT) by using two robotic systems, one for cutting and one for welding. Figure 4 identifies the cut/bevel system and welding system.
Figure 4: Cut/bevel equipment and weld equipment.
The legacy process for installing MTs in the prior classes of submarines was a manual and very labor-intensive process that involved a single MT, which was hand-welded and then outfitted within confined spaces. The new process combines automation, technology, and robotics to weld and outfit multiple tubes concurrently in a safer and more manageable atmosphere.
The entire process is facilitated with the use of 3D metrology tooling and software to establish the best fit of the as-built conditions of the raw material. Data obtained from the measurements and scanning is processed in the software to accurately define and create the robotic cut path. The surface is then prepared for welding using specific grind media to achieve a repeatable and accurate fit-up. Welding is then accomplished by using an adaptive path planning process to deposit multi-pass welds.
We have been awarded funding to perform the R&D of robotic and automated solutions for multiple projects. The goal of these projects is to develop a prototype to demonstrate that the technology is achievable in a scaled-down version.
Robotic process for installing hull inserts. This new initiative is aimed at developing a prototype robotic solution to cut, grind, bevel, and weld inserts into the pressure hull. Currently, we install inserts in VCS hulls with a painstaking process that consists of multiple manual operations consisting of:
- Trades locating and laying out the insert on the hull
- cutting the opening in the hull with oxy-fuel processes
- beveling the hull weld joint with oxy-fuel processes
- grinding to desired surface finish
- semi-automatically welding the insert into place.
Installing hull inserts for VCS currently requires extensive labor hours. This expands manufacturing span time for the initial outfitting phase, and because weld quality is dependent on “tribal knowledge” and individual skill level, this often requires to rework. The goal of this project is to investigate and develop a prototype robotic solution that increases weld quality and drastically reduces the amount of labor hours. The project team is leveraging the technology used in the MT2K system, and applying it to this project. These hull inserts vary in shape, thickness, length, orientation, and so forth, and are required to be installed at an elevated height. The current proposed solution is to have two robotic systems. One system will robotically cut, grind, and bevel the pressure hull and hull insert, and the other system will robotically weld the hull insert into the pressure hull.
Portable robotic welding cell. This is another project on which we currently are working. The purpose of this project is to be able to bring a portable robotic welding cell to the assembly/part, rather than bringing the part to the robotic cell. There are several heavy steel assemblies for which it is not advantageous or possible to be transported to the robot in the submarine manufacturing process. This project will investigate, develop, and prototype a portable robotic welding solution that reduces welding labor hours and increases weld quality. The project team is planning to leverage existing capabilities of commercial-off-the-shelf welding robot technology to the greatest extent practicable. They will also work with the end-users and robotic integrators, including but not limited to those integrators who developed components of the MT2K system that was implemented at Electric Boat.
CAD enhancements for robotic programming. Our robotic team has been monitoring a series of related projects involving software enhancements to automating the programming process for robotic welding. Enhancing the software is advantageous for the manufacturing environment as well as for comparing the as-built condition to the technical authority models. 3D metrology can be used to scan the as-built condition of the assembly or work part that consists of obstacles not identified in the technical models such as strong-backs, tie-downs, and so forth. The robot can now recognize these obstacles as well as the as-built configuration and seamlessly avoid the obstacles while producing multi-layer welds. The software will ultimately be capable of generating a robotic welding program automatically for a specific work part. This tool will eliminate the need for a trained or beginner robotic programmer to spend time programming for each new assembly with the potential of reducing the cost of the robotic welding process.
Taken as a whole, the overall effort of implementing these kinds of new technologies to support manufacturing is a continuous improvement process for us. Our intention is to work with industry leaders to develop new technologies for the manufacturing processes. In this way, we can improve safety, quality, cost, and schedules during the manufacturing of the nuclear submarines that defend our country.
Andrew DiFusco is a supervisor of engineering at Electric Boat-Quonset Point.