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Can Fabricators Really Automate Grinding?

A fabrication machineWalk into a typical fabricator and you might see various forms of automation, from material handling towers to bending and welding robots. You’ll likely see a fair amount of flat-part deburring and graining automation too. But enter the weld grinding and surface conditioning department and you’ll probably witness a very manual operation.

Increasingly, though, some are exploring automated options. Many grinding applications still aren’t practical or cost-effective for automation; the product mix or part complexity is just too great. But as technological advances proliferate, so too does the potential for automated grinding.

The Business Case for gAutomated Grinding

Often a fabrication operation has a team of highly skilled welders who send parts to the finishing department, a perennial bottleneck. A fabricator might consider automation in the finishing department, but the variability that comes with a manual welding operation adds to the cost of automated grinding.

Moreover, the business can hire welders. It might not be easy to find them, but it’s a craft that people learn in school and perfect throughout their careers. Conversely, fewer people want to make a career out of grinding. If welders grind their own work, it’s usually their least favorite part of the job. If a fabricator has a dedicated grinding and surface conditioning department, it often has a hard time retaining employees.

All this is what makes automated grinding so attractive and yet so elusive at the same time. Yes, in most cases automating a welding process makes automating a grinding process a lot easier, but automated welding is no longer an absolute prerequisite for automated grinding.

In fact, the latest abrasive technology has opened up the possibility of more automation. Generally speaking, new abrasive media can reduce grinding time significantly, which in turn reduces the number of passes and the heat generated. Automation also opens up novel paths for the grind tool to take—ones that would be uncomfortable or impossible for a human operator to perform.

General Abrasive Processing Factors

If you’re thinking of automating a grinding process, you still need to consider the basic factors of any abrasive application, including abrasive speed, application force, torque requirements, horsepower, contact wheel construction (for belts), backup plates (for discs), expected life of the abrasive, part temperature, and, of course, the part finish that’s required.

The optimal feed rate breaks down the minerals in the abrasive media so they self-sharpen. If your feed rate is too slow, the abrasive mineral will dull and so will your part. Similar thinking goes for pressure. Insufficient pressure—applied by a person or a machine—can lead to the minerals dulling and part burning, simply because the media isn’t breaking down and self-sharpening as designed. A telltale sign is when you see parts with “burned” finishes, yet when you pull off the belt or other abrasive media, it looks like it hasn’t been used.

Conversely, if you apply too much force, you’ll break the mineral down too rapidly. And in belt grinding, you also run the risk of breaking the belt and potentially damaging your contact wheel.

Media and Positioning Factors

Automated grinding can save a fabricator money on grinding media, but the savings don’t come through purchasing lower-cost discs, wheels, and belts. In fact, premium products often make more sense. Their performance is predictable, their life is long.

If you choose media with less predictable performance and shorter life, frequent media changeover might be the least of your problems. The pressure a robot applies to a workpiece needs to change as the abrasive wears, and subpar media might lead to frequent parameter adjustments.

If the wear rate isn’t predictable, the robot cell won’t grind pieces properly, which could increase your scrap and manual rework rates. And even if you do successfully adjust the robot parameters frequently enough, in doing so you’re hindering the cell’s throughput. In other words, you’re defeating one of the key reasons for moving to automation.

Automation can use most of the grinding media a manual operator can, including depressed-center grinding wheels (Type 27), nonwoven media, fiber discs, and flap discs. But they also can use large abrasive belts, and this presents some key advantages, longer life being the most significant. There’s simply more abrasive media on a 132-inch belt than there is on a 7-in. fiber disc. You also can use more horsepower. In a robotic application, the arm itself can be designed to handle the payload of a high-horsepower belt grinder. For even more horsepower, you can integrate an abrasive belt on a stationary machine, then secure the work in a fixture and bring it to the grinding machine.

Contact wheels behind the abrasive belts come in a variety of sizes, from 0.25 in. diameter all the way to 30 in. diameter or even more. The contact wheel shapes also can be adjusted to handle various workpiece geometries.

Like in welding automation, grinding automation can approach a fixtured workpiece, but it also can be brought to a workpiece, especially when the workpiece is too large or unwieldy to maneuver easily. For instance, certain tank grinding applications today use an overhead gantry-mounted belt-grinding system that traverses across a workpiece more than 12 feet long.

At the same time, traditional articulating-arm robots can be integrated to use not only disc media, but also change over media when required, as the operation progresses from weld removal to surface conditioning. This approach can also mitigate the effects of short tool life, especially if the grinding application is aggressive.

A cell can have a queue of discs at the ready. Automated tool changing is obviously faster than relying on an operator to manually change out the media, especially if tool changes are frequent. That said, the abrasives still need to wear predictably, which means the abrasive media needs to be of a certain quality. The part geometry and fixturing need to be consistent as well.

Opportunities for Grinding Efficiency

When the robot arm approaches the workpiece using a fiber disc, you might notice a grinding operation that at first glance looks a bit odd, especially if you regularly observe or perform manual grinding.

Consider a grinding application with three large stitch welds on the corner of a box. To perform this manually, an operator would probably approach each weld individually and approach each at a consistent angle, going back and forth many times to remove weld metal and feather the edges.

A robot might approach this completely differently. It could start at a low angle of attack and, in a single sweeping motion, grind the weld metal and at least roughly feather the edges—all in one pass. It’s a feat that would be uncomfortable or outright impossible to perform manually.

When manual grinders use a depressed-center wheel, they approach the workpiece at a 20- to 30-degree angle. They do this not only for ergonomics but also to get the most out of the abrasive wheel. The wheels are designed for the impact wear that occurs with that angle of attack. Lower the angle, and the operator reduces processing speed and wheel life significantly. This applies whether the grinding is manual or automated.

Not so with a fiber disc. Operators attack the workpiece at about a 5- to 10-degree angle not for the abrasive’s sake, but for reasons related to control, safety, and ergonomics. Depending on the application, lowering the angle of attack could make for more efficient grinding, especially if applied force can be increased. In fact, with the latest abrasive media, the increased contact area reduces process time and, with the correct applied force, could wear the abrasive so that it fractures the abrasive grain, revealing more sharp edges, speeding material removal even more, shortening the cycle time, and reducing overall heat input.

 

But manual operators cannot and should not do this. A low or almost flat angle of attack with a fiber disc would create a high amount of operator stress trying to control the disc, resulting in discomfort and vastly increasing the chance of serious injury. Ergonomics and safety considerations are critical in any manual grinding operation. For example, operators often choose softer backing material that absorbs the vibration.

With automated grinding, you have no manual operator, so these ergonomic considerations go away. Application designers can choose abrasives, backing material, and toolpaths based solely on what makes the automation most efficient and repeatable. A robot might approach the workpiece at a very low, almost flat angle of attack. And the abrasive backing material might be harder, perhaps with ridges that help concentrate the force that the abrasive undergoes. This helps to wear and expose sharp edges quickly and boost material removal rates. (Again, as always, specific parameters and abrasive choice depend on the application.)

Many grinding operators stay away from ridged backing material for comfort reasons, and they should certainly be free to do so for optimal safety and ergonomics. But when you robotize the operation, ergonomics isn’t a factor.

Automation Factors for Grinding

More often than not, automation will give you better abrasive product utilization, simply because you can control the processing conditions so tightly. The process will likely induce less heat into your part, not necessarily because automation has a “lighter touch,” but because the machine can apply a pressure far more consistently than a manual operator could—especially when you employ a number of different operators, each with their own method of grinding.

Of course, all this is true only if the automation is integrated and programmed properly. For instance, the robot or machine program needs to take into account abrasive wear. You have to consider that from the time you start using your abrasive product, whatever it is, it’s changing. This applies from the beginning all the way to the end of the abrasive life. You might need to adjust application force, toolpath, and speeds to maintain a consistent process. This dovetails back to an earlier point: A high-quality abrasive is designed to have a predictable performance, so you’ll have less unexpected downtime adjusting the force and toolpath during the abrasive life.

If your automated grinding system uses a robot, integration isn’t always straightforward. Some have attempted to take a robot that was designed for welding and insert a force control device and a servomotor to handle the abrasive. This is usually not the best way to go. The servomotor and force control device alone can add significantly to the robot’s payload. Then you need to consider the grinding forces. Sometimes they can be as low as 2 or 4 foot-pounds, but they could also be as high as 50 or 75 ft.-lbs., especially if you want to grind quickly. You then need to deal with the vibration that abrasive grinding produces.

Overall, abrasive grinding is a much more aggressive process than welding, and the robot will need to be designed to suit. Depending on a job’s weld removal and surface conditioning requirements, if you use a robot to weld a certain part, chances are that the robot grinding the part will be larger.

Automate or Not to Automate?

Today’s grinding automation can handle more variation than it used to, but variation still adds cost and complexity. If a grinding operation can be made simpler in a cost-effective way, it should be.

When deciding whether or not to automate a grinding application, consider the usual suspects first. Are the part geometries consistent? If part geometries vary greatly, or if those geometries are very complex, there’s less of a chance that automation will make sense. Also consider the part family. Automating grinding in a high-volume environment with only a few part numbers will likely pay off very quickly.

Can the part be fixtured and presented to the automation in a consistent manner? In a manual operation, the part basically can be anywhere, as long as it’s secured and the operator can access the areas he needs to grind and finish. This isn’t true with robots. The good thing about robots is the same as the bad thing about robots: They do exactly what you tell them to do. If you don’t have good fixturing consistency, an operator moving parts in and out of the cell could crash the robot.

The same is true for the toolpath. If the part isn’t fixtured well or the part geometry isn’t consistent, the toolpath will need to change more frequently, which makes automation more difficult.

If a weld is clean and consistent, perhaps produced by a robot upstream, automated weld grinding has serious potential. If a weld has spatter, pits, and valleys, automated grinding probably isn’t an option. A weld that’s slightly inconsistent might be a good candidate for automation, though it might affect your approach to the project.

Also consider the finishing tolerances. It’s very easy for an experienced operator to blend and feather surrounding material perfectly. It’s not quite so easy for a robot—but, as we’ll soon see, it’s becoming possible, even if weld sizes vary somewhat.

Got Variation? You Still Can Automate

Grinding automation can occur using specific control techniques. With position control, you program the path to account for expected workpiece geometry variations. Know, however, that if you’re automating with a robot, its positioning accuracy isn’t as accurate as a CNC. Of course, problems arise if you have unexpected workpiece geometry variations.

With force control, the automation will apply a specified force within a certain range of foot-pounds or newtons, and the robot or end-of-arm tool adjusts its position ever so slightly to maintain it. As recently as two years ago, force control wasn’t as useful if you had highly variable welds or other workpiece geometries. This even applied to the variability seen in many manual welding operations. If one welder produced a weld or tack one way while another welder produced a weld or tack another way, that variability could be enough to cause problems for an automated grinding system.

This still is sometimes the case, but advances in processing knowledge, combined with new abrasive media, are starting to change the game. Under the right conditions and with the right products, automated systems can manage the variation. New abrasive media has increased material removal rates significantly. This in turn has decreased the heat input, which minimizes one significant hurdle to grinding automation, and has shortened dwell times. Put all this together and you build a processing-parameter recipe that can handle some weld size variation.

It also has allowed engineers to develop processing parameters for effective feathering, even if weld sizes vary somewhat. That said, the best feathering and surface conditioning still require another step, like with a disc of nonwoven media. But the look of ground welds, even after the robot has made its first passes with abrasive fiber discs, has come a long way in a few short years.

Just the Beginning

A quick disclaimer: Compared to welding automation, grinding automation is a new field, as is the science behind how exactly new abrasives make grinding automation more effective. Engineers are learning more about how advances in abrasive media open up new possibilities for automated grinding.

Software is playing a big role too. New software increases the potential for automation in applications with varying weld sizes by telling the robot to dwell for a certain period over a weld’s high spots and then dwell for less time in the lower spots. Other software also simplifies work entry—that is, the moment at which the media touches metal. In some systems today, the programming automatically instructs the robot to initiate grinding at a low force level, then quickly ramp up to a specified force level.

Despite all these advances, the old rule still applies for most applications: Minimize your variability upstream, and you increase your chance of grinding automation success. But with technological advances in software, vision, and abrasives—combined with new processing knowledge on how to make best use of them all—exceptions to the rules are starting to proliferate in a big way.

Source: https://www.thefabricator.com/thefabricator/article/finishing/can-fabricators-really-automate-grinding

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