Robots make grinding and finishing look easy. But it takes a lot of work and careful consideration to get a robotic cell operating at peak performance. It’s challenging to design a perfect automated process from the start, but if you and your system integrator keep the below factors at the center of the design you will have a greater chance at success. 3M robotics experts have developed this list of technical factors that should be considered during the design phase.
Along with the robot, a robotic cell contains a lot of ancillary equipment that makes the process work. This includes, but is not limited to, belt backstands, pedestal grinders, gripping equipment, measurement and inspection equipment, and part racks. These will all contribute to the footprint and upfront cost of your robotic cell.
The payload of a robot is the maximum of the amount of weight it will carry plus the amount of force it will apply in the operation. The carried weight includes everything that attaches to the robot arm – force control and vision systems, the abrasive or gripping element, the part being manipulated, etc. It’s important to oversize the robot against the expected payload, since running a robot at its maximum payload can limit acceleration and agility. Understand that the footprint of the robotic cell will grow along with the payload and size of the robot, so be aware of your floorspace constraints when designing your system.
Much like the robot payload, it’s essential that the motors on your ancillary equipment have enough power to perform the required application. For example, you’ll need a motor with at least 40 HP for gate grinding. Trying to grind with only 10 HP will result in a significant reduction in efficiency, resulting in higher cost and reduced throughput. It’s also important to keep the duty cycle of your motors in mind – the amount of time a motor is designed to run uninterrupted.
Ignoring these important factors can result in frequent and costly motor replacement.
Every abrasive is designed to run optimally at specific speeds, dependent on application. Running abrasives at their optimal speed for the application is very important to achieving the best results in your abrasive process. For example, you wouldn’t want to run a fiber disc too slow under load, so make sure your equipment is able to run your abrasives at the optimum speed. If you run at too low of a speed, the abrasive performance will suffer.
This is an area where you will need to account for changes in abrasive performance as the abrasive is used. For example, abrasive belt performance will typically change over time as the abrasive grain dulls, and the surface speed of abrasive wheels will decrease as they wear due to the decrease in diameter. This surface speed decrease will affect abrasive performance, and a motor with variable speed settings will be able to compensate for these changes.
As noted above, as abrasives wear over time your robot will need to account for that change in cutting efficiency or wheel diameter. As the cut-rate drops, a robot can be programmed to increase force or RPM to compensate.
Furthermore, you must account for the process of changing from the worn-out abrasive to a fresh product. In many cases this process can be fully automated, or semi-automated with some operator engagement. If automating the abrasive changeout is not possible, the cell can be shut down and a human operator can manually change the abrasive.
Unless you are designing a one-step robotic abrasive process, you and your system integrator will need to determine the best way to handle the sequencing in your robotic cell. Will the robot manipulate the part or the abrasive?
If the robot is manipulating the part (known as “part in hand"), applying a sequence of abrasive steps may simply entail the robot bringing the part to multiple abrasive machines, each with the appropriate abrasive mounted. If the robot is manipulating the abrasive (known as "abrasive in hand"), you may choose to use tool changers to enable the robot to grab the appropriate abrasive for each particular step.
You can also choose to use a separate robot for each step, but you will need to consider part handoff (if part in hand) as well as robot cell footprint, and the upfront cost associated with purchasing multiple robots.
Just like in a manual abrasive operation, you will need to consider how to handle dust collection in your robotic cell. Whether employing a wet or dry collection method, it’s essential that dust is mitigated in the cell to ensure peak performance of motors and cleanliness of finished parts. If you don’t account for dust collection, it will build up more quickly and increase downtime in the robotic cell as you clean the cell or perform maintenance on components.
Unlike a human operator, a robot cannot sense its environment and use its judgment to adjust as needed. It must be programmed to travel a specific path and carry out repetitive motion. That’s why sensing technologies like force control and vision systems are crucial in many operations.
Without force control, achieving consistent results in an abrasive process may be difficult. Most abrasives are designed to work optimally within a specific pressure range. Force control allows the robot to apply a more controlled amount of force than is capable using only robot arm position control. There are several force control technologies available to account for a range of part and abrasive variations. Passive force control is the simplest and lowest-cost option, but it cannot as easily account for changes in part geometry and the impact of gravity as the robot moves around a complex part. Active force control leverages feedback to control force as critical variables such as gravity impact the actual applied force. With the right system, it’s also possible to program changes in applied force based on the location of the abrasive/part interface.
Vision systems similarly allow a robot to make process adjustments to account for external factors. These systems detect the orientation of incoming parts and adjust the end-of-arm gripper to correctly pick up the part. They can also measure the size of a finished part or the size of a gate after grinding to ensure the proper contact is generated.
A less-common sensing technology that plays an important role in certain situations is temperature measurement equipment. This is typically used on heat-sensitive substrates and is used to measure the temperature of the part to help avoid overheating of the substrate.
Setting down and picking up a part multiple times (re-gripping) may be required when multiple surfaces of a part must be oriented to the abrasive. For processes where a part is being brought to an abrasive – rather than the robot bringing the abrasive to the part – re-gripping can have a substantial impact on cycle time. The more times your robot needs to re-grip each part, the more your cycle time will suffer.
You will also need to account for grip force. The gripper at the end of the robot arm must be strong enough to handle the force of the application. For example, a light-duty gripper will fail to properly secure a part in high-pressure robotic applications like gate grinding.
Your operators will occasionally need to interface with the robot – changing abrasives, removing swarf, or other occasional tasks – and their safety must be accounted for. “Lock out, tag out” procedures, robot guarding equipment, as well as other methods to ensure safety such as interlocks and proximity switches should be considered in the robotic cell design process.
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