Nathan Jackson, Application Engineer II, Norton | Saint-Gobain
Originally published in Welding Journal, an American Welding Society (AWS) publication. Reproduced with permission.
Learn tips for automating weld prep and finishing
As more companies are faced with the need for labor, it’s no secret that automation provides a potential solution for mitigating workforce challenges. More than 517,000 industrial robots were installed worldwide in 2021, a 31% increase from the previous year (Ref. 1). The first three quarters of 2022 saw a 24% increase over the same time period in 2021, setting up yet another record-breaking year for industrial robotics (Ref. 2). In most applications, automation serves as a viable solution for handling the labor-intensive jobs that are suffering from both a need for operators and a significant increase in labor costs.
Weld grinding is an example of one of these applications because many companies are struggling to hire and retain operators due to skill levels and the highly physical demands of this process. Weld grinding automation systems can not only help take the burden off of labor challenges, but they can also lead to improvements in safety, output, cycle time, and part consistency. The key to fully optimizing and benefitting from an automation system is ensuring that the robotic cell is built around the most effective process for the application.
Deconstructing Automated Weld Grinding
When it comes to welding processes, it is helpful to review the two phases of automated grinding: weld preparation and weld finishing.
Weld preparation typically involves removing paint or rust from the surface and adding a bevel to the workpiece to provide an opening for the weld filler. Since paint and rust are usually only found in job shop situations, the cleaning phase is not a common application for automation. However, using automation to grind a bevel ensures there is a clean, straight opening that will allow for a more constant, reliable weld. Flap discs or depressed center wheels are the best-suited abrasives for weld preparation.
Weld finishing is the process of removing excess stock from the weld and polishing the welded area to match the surface finish of the surrounding area of the part. This is generally performed as a two-step process consisting of grinding and blending. For the grinding step, coarse-grit abrasives are chosen for high-stock-removal rates. Abrasive belts, flap discs, and depressed center wheels are the optimal tools for weld grinding. Since the purpose of the blending step is to provide the best possible finish, less-aggressive abrasive products in a fine grit are ideal. Flap wheels, nonwoven discs, wheels or belts, and paper discs on an orbital sander are the best products for weld blending.
Approaching Automation
As discussed previously, automating weld grinding processes have many benefits. However, before diving into automation, it is critical to define the objectives. First, consider which parts you will be automating, how much you are willing to spend on an automation system, what qualifies as an adequate cycle time, and the measurements used to determine if a finished part is acceptable. These considerations exemplify a concept in automated systems that is often referred to as the 80/20 rule, meaning, in many cases, that automating the last 20% of your process accounts for 80% of the cost. The higher cost components can be for a variety of reasons, such as the following:
- Requiring additional fixtures for hard-to-reach welds
- Unique tooling for odd-shaped parts
- Additional vision and sensors needed to finish grinding inconsistent areas of the parts
It is important to consider how much of the process you are willing to pay to automate. In many cases, the best solution is to automate the bulk of the grinding operation while leaving the final steps to be performed by operators, making the best use of the operators’ time and skills.
One of the easiest ways to simplify the weld grinding process is to improve the consistency of welds upstream. If welds are consistent, the same cycle of grinding will be able to produce the same finished product time after time. Large variations in the size or shape of the weld may require identifying the grind zone with a machine vision system, which will increase the price and complexity of the automation system.
Determining Grinding Cell Composition
A grinding cell has options for one of two types of systems: abrasive to part or part to abrasive.
In the abrasive-to-part system, the part is mounted within the cell and the robot uses end of arm tooling (EOAT) to bring the abrasive to the part. For a weld grinding cell, the most common forms of EOAT are spindle motors, pneumatic tools, and custom-built belt grinding heads. This type of system is generally used when the part is too large for a robot to pick up safely or when the part has tight areas to be ground that a large abrasive belt or wheel cannot reach. Alternatively, in a part-to-abrasive system, the robot picks up the part using a gripper or a custom fixture and brings it to a grinding head. This type of system is generally used for smaller, high-volume parts.
Compliance
Compliance is a key feature to have in any automated grinding cell, and it is especially critical when it comes to weld grinding. Compliance is defined as the adjustment of pressing force via motion modification. Essentially, a compliance device uses pneumatic pressure to allow an abrasive to maintain a constant force at any point within a specific mechanical stroke. Compliance makes programming a robot simpler, as it reduces the accuracy requirement for points in the toolpath. It also helps accommodate for variations in the part, allowing the robot to still move its intended point rather than crashing in the event that there is extra material on the weld. Using compliance to ensure the grinding force is constant will give a more consistent finish while reducing cycle time and increasing the abrasive life. This is because maintaining ideal force provides an optimal material-removal-to-abrasive-wear ratio. As such, it is recommended to use premium abrasives, which will provide the longest life in an automated cell, reducing abrasive changeover time and maximizing the uptime.
The compliance in the grinding cell can either be on the grinding head or the end of the robot arm. In an abrasive-to-part system, the force control will come from a compliance device placed between the spindle or abrasive tool and the robot arm. In a part-to-abrasive system, the compliance generally comes from a pneumatic cylinder on the grinding head, allowing the grinding force to remain constant regardless of the contact point between the part and the abrasive.
Automation Expertise
Although automating your weld grinding process may seem daunting, a proof-of-concept lab from an automation partner can greatly reduce your risk. By testing different parameters and being able to see a completed part firsthand, you can be sure that your automation system will be able to deliver the results you need. With proper planning and process development, automation can provide the perfect solution your company needs for improving your weld grinding processes.
For more welding resources, visit the Norton metal fabrication page.
References
- International Federation of Robotics. 2022. ifr.org/ifr-press-releases/news/wr-report-all-time-high-with-half-a-millionrobots-installed
- Wessling, B. 2022. Robot sales on track to hit a new high in 2022. WTWH Media. therobotreport.com/robot-sales-on-trackto-hit-a-new-high-in-2022