April 1, 2010
When it comes to grinding, a more productive worker is often a safer worker, if given the right tools and training.
Even people who sell hand power tools for a living aren't shy about an obvious truth: No child dreams of one day working in a fab shop's grinding department. The job can be dirty. It can be loud. And without the proper tools, protection, and training, it can be dangerous.
On the other hand, the job of finishing metal has never been more important. "Good enough" just doesn't cut it anymore. Shops may spend millions on new and complex machine tools, such as tube cutting lasers and automated punching and bending cells. Automation can do wonders for efficiency and quality, but it's all for naught if a subpar grinding operation forces excessive rework and late deliveries.
Some grinding is expensive and often not practical to automate. A blank can be finished in a flat-metal graining and deburring machine. A mechanized stroke sander can be used for some applications. And a torch-cutting bug system or even a stationary router or specialty mill can automate weld prep in some cases. But if a job shop has welded assemblies of various shapes being sent to the finishing department, cost-effective automation options are limited.
For this reason, manual grinding pervades the industry. It's a basic shop tool. According to sources, creating a safe grinding operation should be the first step in making life easier for the grinder and the entire operation more efficient. Operators should be trained and given the right tools for the job (see Figure 1). Once this happens, more can be accomplished in one shift with less operator effort. When they work safer, their jobs become easier and more productive.
Quality tools and abrasives may have a greater upfront cost, but they can help support a safer, more productive, and less costly work environment. Although this goes against some conventional thinking, numbers often support the story.
Scott Saunders, a salesperson for CS Unitec, explained it this way: Consider an operator who makes $50 an hour, when considering his hourly wage, overtime, insurance, and benefit costs. He uses a 6-amp grinder with a low-end abrasive wheel that costs $5. This disk can remove 5 linear feet of material in one hour. So the total hourly cost is $55, or $11 per foot of weld metal removal. Now, replace that abrasive wheel with one that costs twice as much, but can remove 6 linear feet in one hour using a 12-amp grinder. This makes the total cost $60 ($50 an hour, plus $10 abrasive), but the operator just removed 6 linear feet. That makes it $10 per foot of weld.
"Upfront, an abrasive might cost 50 percent less, but the total cost for the job is more," Saunders said.
An added benefit: A more powerful tool, matched to the specific job, requires less operator effort, meaning less fatigue, which reduces the chance of injury.
Powertool-makers have gone to great lengths to ensure operator safety. Jens Gschloessl, product manager for FEIN Power Tools Inc., explained that operators will duct tape the pedal switch on the side, and even glue the side safety switch into a permanent "on" position, all in a quest to reduce fatigue and improve efficiency. Gschloessl described some models with four touch pads, all of which need to be activated to start the tool. "They're embedded into the motor housing, so it's difficult to duct tape and lock the tool [in the on position]."
Other tool features include restart protection, in which the tool won't start immediately when plugged in if it's switched on. Some safety guards also are designed so operators can't remove them without a special tool.
Gschloessl added that some grinders have safety clutches that engage to shut off the tool under extreme load conditions, such as when a cutoff disk becomes stuck in a material (see Figure 2).
Some tools use specialized fixtures on flat plate, tubes, or other surfaces to provide support (see Figure 3). "These fixtures always keep the tool at the same setting," said Iwan Antonow, executive vice president of Saar-Hartmetall USA LLC, "so you cannot overgrind or undergrind it. They also reduce the fatigue factor and block all access to the grinding wheel during operation."
Shops have established safety rules, but that doesn't mean they're always followed, even when operators are using tools with advanced safety features. Workers aren't trying to hurt themselves; they're just trying to meet efficiency goals and make their job easier, sources said. As with any safety issue, the best approach is to design out the hazard from the process.
Why are operators taping buttons? Their hands hurt. Why? They may have the wrong tool for the job. Why are they removing the guard? They may be using a standard angle grinder and contorting themselves to reach into a tight space. Here, a belt file or another tool designed for tight spaces might help.
Abrasive wheels of specific grits operate best at specified RPMs, offering efficient metal removal and long abrasive life. An operator need only apply a firm, consistent force during operation, Gschloessl said. Excessive force reduces the abrasives' effectiveness, shortens consumable life, and adds operator fatigue. If the operator needs to use excessive force, he's probably using the wrong abrasive or tool for the job. To ensure the most efficient abrasive use, some grinders have electronics that helps the tool maintain RPMs even under increased pressure from the operator, Gschloessl said.
As Saunders explained, an efficient grinding operation is one where managers have chosen the right abrasive of sufficient quality and matched it with a portable grinder of sufficient power. If a part requires a No. 8 finish, and a grinder uses a 40-grit abrasive, "you're putting 40-grit scratches that now have to be taken out to get that No. 8 finish," he said, adding that in this instance, getting the right finish may require a finer grain disk, such as a 300-grit disk.
If a grinding tool doesn't have sufficient power for the application, an abrasive easily can become loaded or glazed, because the power tool is not producing enough heat or pressure to break the grain. Especially common with aluminum and other nonferrous metals, loading happens when the base material becomes attached to the wheel. Glazing happens when abrasive grains don't fracture, but instead roll over to produce almost a burnished coating over the abrasive disk.
"So what does the operator do? He pushes harder," Saunders said. The operator becomes tired, which in turn can lead to unsafe practices as he struggles to get more finished by the end of his shift.
It's good to take a holistic approach. For instance, not only should finishers be given the right tools for the job, tools should be organized and within easy reach. Say a part has one feature that's difficult to reach. An operator may just remove the guard on his angle grinder to reach in to finish the job. In this case, the operator should have a safer, more productive tool close at hand, so he can switch quickly to the right tool.
Here's another question: Why do operators spend so much time grinding each piece in the first place? As Saunders explained, "Sometimes we see some battleship welds on parts that may not need them."
In fact, excessive weld metal can be detrimental, contributing to weld distortion and shrinkage. Welds should be of a certain height from the base metal surface. Any more could affect weld quality and make for a very laborious grinding operation, no matter how good the power tool is. The more laborious the grinding operation, the greater the operator fatigue, which decreases safety.
Another tactic, Saunders said, is to give operators ownership over their grinding tools, literally. If an operator is told he can own a grinding tool after a year of service, he may take better care of the power tool and be more likely to abide by the safety rules.
"Ultimately, a safer working environment will increase productivity," Saunders said, "if a company has a culture that embraces safety and training. Without formal training, workers learn grinding through trial and error, with limited success. So if we teach them how to grind, and give them the right tools for the job, will have a much safer and productive grinding environment."
Portable grinding tools are ubiquitous in fab shops for a reason. They can accomplish various applications. Certain grinders can reach those hard-to-reach weld seams effectively and safely. They get the job done. But for some beveling and weld seam removal applications, there's more than one way to get the job done with a portable hand tool.
Portable milling tools can be a viable option. Instead of abrasive grit, these tools use a carbide cutter to do the grunt work. "Basically, they're like a wood router, only for metal," said Iwan Antonow, executive vice president of Saar-Hartmetall USA.
Most models work as beveling and weld prep tools for plate edges. They can bevel relatively quickly, can be set to cut specific angles, and the consumables can last a shift or more, depending on the application.
Their biggest limitation is plate thickness. If you have 1-in. plate to bevel, a portable milling tool generally won't work. But they have several advantages from a safety perspective. First, they eliminate metal dust, because the cutter produces chips that are heavier than air. Second, "the tool rotates so it always moves away from you," Antonow said.
Other milling tools are available to finish weld seams on flat plate and outside edges, such as an outside weld on a box. Such portable tools operate on the same principle as the beveling mill tools, except the carbide cutting tool is positioned to cut flat. The end mill in these tools uses eight square inserts, positioned horizontally so their edges (not the corner) cut away at the weld seam.
The tools' shoe fixture, with a horizontal and vertical guide plate, allows them to be positioned on a box edge. "The tools have two shoe plates on each side of the cutter," explained Phil Heck, president of Heck Industries. "This bridges the weld bead. The tool has half-inch carbide inserts. So if you have a 0.75-in.-wide weld bead, it would be able to take it off in two passes."
For beveling applications, nibbling hand tools are another option. Certain nibblers adjust for bevels of various degrees, and the cutting tool moves up and down, nibbling away at the metal hundreds of times a minute.
Operators can start their cut in the middle of a workpiece. "This will allow the tool and the cutter to be used forward and backward, utilizing two sides of the rectangular cutter," said Tony Mirisola, product group manager for TRUMPF Inc., Power Tool Div.
Each edge of the rectangular tool can cut about 1,000 ft., depending on the material and application requirements. This means that an operator can cut up to 2,000 ft. using one side (that is, two cutting edges) of the tool. Once those two edges become dull, the operator flips the tool 180 degrees, reinserts it, and he's good to go for another 2,000 ft.
Also, the tool can be ground up to a depth of 0.375 in., "so you can get many feet of operation out of one cutter," Mirisola said.
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