How knowledge eliminates the grinding constraint in the metal fab shop
Jul 15, 2023
An operator uses a lightweight fiber disc to grind excess weld metal. Images: Norton │Saint Gobain Abrasives
Grinding sparks have become metal fabrication’s signature image; they epitomize metalworking. Yet grinding can be a fab shop’s greatest constraint. Grinding operators might lack the appropriate technique, and quite often, they’re not given the right tools for the job. That includes the tool that contacts the metal—that is, the abrasive itself.
Eliminating the constraint starts with uncovering its root causes. To do this, put yourself in the grinding operators’ shoes. Where do they retrieve their grinding media? Which media do they choose for what job? What’s their technique? Most important, do they have the time, knowledge, and tools they need to grind safely?
Central to all this is knowledge—about what exactly creates a safe environment, like proper ventilation; when to use a flap disc versus a fiber disc or grinding wheel; how to properly store and care for them; even the effectiveness of certain grain types. The more everyone knows, the safer and more effective a fabricator’s grinding operation can be.
If you’re grinding on the fab shop floor today, you’re likely using one of several grain types. The oldest is aluminum oxide. Until the 1970s, it was the only choice for grinding all types of metal. Strong and blocky, the grain is sharp initially but dulls rapidly. It remains dull until you apply enough pressure to fracture the grain and create a new cutting edge. That fracturing tends to lose a large percentage of the grain at once. This results in relatively slow cutting.
By the late 1970s, the industry had developed zirconia alumina grains. A fusion of zirconia and aluminum oxide, these tough grains are engineered for controlled fracturing, which in turn makes better use of each grain. This results in better cut rates (more metal removal) and longer life.
In the 1990s came ceramic alumina grain, which has a unique combination of hardness and toughness that helps control microfracturing. This in effect provides a continuous supply of sharp cutting edges during grinding.
Today, some of the latest abrasive media developments incorporate advanced ceramic grains that break down in specific ways to improve cutting action, reduce operator fatigue, and improve the overall grinding experience.
Improving the overall grinding experience extends not just to the abrasive grains, but also the products operators use. Using the wrong product for the job only exacerbates the grinding constraint.
Imagine you arrive in the morning, prepare your workstation, then retrieve your grinding media. This could be a grinding wheel, otherwise known as a bonded-abrasive thin wheel. Ideal for when surface finish is not critical, these wheels give you aggressive grinding action, such as for interior or exterior corner welds. You can use one wheel to bevel, notch, and grind welds. Such rigid wheels give you greater control at the point of contact, so you can avoid mistakenly grinding into areas that don’t require grinding.
Although grinding wheels are tough, you can damage them if you drop them on a table or hard surface. If a wheel is damaged, don’t use it. Using a damaged wheel at 10,000 RPM makes everyone in the room susceptible to a potential accident.
Nonwoven discs are ideal when overgrinding is a concern.
Grinding wheels also require more training than other media types. They’re efficient, but they can be hard to handle as they remove large cutting fragments from the workpiece. An operator using a Type 27 wheel should approach the work at a 30-degree angle, while Type 28 and 29 wheels require an approach of 15 degrees or less relative to the workpiece surface. Apply too much of an angle and the wheel likely will start breaking down prematurely.
Alternatively, you could be reaching for a coated-abrasive fiber disc. Ideal for when cut rate is the primary concern, these offer quick changeover and require less pressure to grind. They’re lighter, too, so they can make overhead work easier. They’re good for confined spaces, and they can be ideal for blending welds to the parent material without overgrinding. The coating method on the paper allows for an aggressive cut, similar to grinding wheels.
Note that backup pads for fiber discs must be in good condition. Most important, they need to be actual backup pads. Never use older fiber discs or cutoff wheels as backup pads. Doing so can be dangerous, because they’re simply not designed to do the job backup pads do.
If you want forgiving cutting action (to, for instance, avoid large gouges), and if you need to achieve an even better finish than you could achieve with a grinding wheel or fiber disc, you’d likely reach for a coated abrasive flap disc. These work especially well in weld blending, which allows grinding on either side of the weld. They offer a good combination of long product life and efficient cut rate. Those with 50 grit or finer can help you remove weld spatter without overgrinding.
The flaps that make up a flap disc are held down by a strong adhesive, but they’re not indestructible. If you use them at excessive angles, jab them into tight places, or apply them to an extremely sharp or rough workpiece, the flaps can come apart during use. This can create a dangerous situation if you aren’t wearing the appropriate personal protective equipment (PPE).
To use them effectively, you need to apply enough pressure to wear away the cloth backing and expose more sharp grain. A lot of operators actually end up disposing flap discs too early. If they applied just slightly more pressure—more than required for typical fiber discs—they’d wear away the cloth backing, expose the sharp grain, and discover that the flap disc has plenty of life still in it.
Eventually, insufficient force can lead to what’s known as capping. Under insufficient pressure, the abrasive builds heat as the metal adheres to the grain, preventing the grain from fracturing. If the grain doesn’t fracture, it can’t resharpen itself.
You also could be reaching for a nonwoven abrasive disc, which is good for small welds (¼ in. wide by ¼ in. high) susceptible to overgrinding (think thin sheet metal applications).
You use these discs for cleaning up discoloration, weld spatter, and difficult-to-reach welds like fillet welds. You also can use them to achieve decorative surface-finish patterns.
When eliminating the grinding constraint, the abrasive media is just one factor to scrutinize. Another is how you store that media. Again, imagine you arrive in the morning and reach for a fiber disc, and you notice a sand-side-in curl. That’s a sign of excess humidity. Specifically, the fiber disc’s backing absorbed the excess moisture. If you see a sand-side-out curl, the storage area’s low humidity likely caused the backing to become dry and start to shrink.
An operator uses a flap disc for weld blending and finishing.
If you try to set up and grind with a disc having a sand-side-out curl, you’ll find it difficult to lock the fiber disc to the backup pad. The product could be too dry and could possibly break. If the disc has a sand-side-in curl, the disc edges might snag on the workpiece. This could lead to workpiece damage or, even worse, operator injury.
Imagine the same scenario, but this time you reach for a grinding wheel. You inspect the wheels and find they have cracks. Alas, the wheels were stored out in the open, not in their original container. They shouldn’t be exposed to high humidity or freezing temperatures or left vulnerable to water or other solvents; such extremes can affect the bond of the wheel.
If you’re using flap discs, store them in an area that’s between 60 and 80 degrees F and 40% to 50% humidity. It’s best to store them in their original container. Also, rotate stock (first in, first out) just as you would with grinding wheels and flap discs. And of course, before use, inspect the discs and discard any with missing flaps.
Imagine it’s your first day on the job. You’re given a right-angle grinder and a few basic pointers, but no one delves into certain details. For instance, what should your grinder speed be for certain jobs? Spinning a grinding wheel too fast can cause the disc or wheel to come apart, while spinning it too slowly will cause premature wear.
Imagine on that first day you’re given a right-angle grinder—and you notice it has no guard. Wheel guards save eyes, fingers, and lives. They should be properly located so that if any abrasive becomes damaged, the debris will hit the guard and not you.
On that first day, someone should show you the inspection procedure before you fire up the grinder. Is the backup pad or wheel damaged? Never just drop or even place a right-angle grinder on the grinding wheel itself; doing so can damage the wheel. If you drop the grinder with a wheel installed in it, replace the wheel even if you see no visible damage.
When grinding, use the appropriate approach angle for the product you’re using, grasp the grinder with both hands to avoid slippage, and assume the proper stance. Use your middle and lower body to move the upper body; this can help relieve stress on your elbows and shoulders.
Finally, don’t forget the right PPE. Use goggles, safety glasses, and face shields to prevent small metal and abrasive particles from hitting your face and eyes. Gloves reduce the chance for injury, a heavy cotton or leather apron and long sleeves prevent debris from hitting open skin, and hearing protection prevents long-term hearing damage.
Of course, all these factors are just part of the story; the other part involves using the right grinder that has sufficient amperage for the job (a subject that deserves an article of its own). Put all this together—the grain; the right product, be it a wheel, fiber disc, flap disc, or nonwoven disc; the right storage and care; a well-ventilated environment—and you give operators the knowledge they need to make life on the floor easier. This in turn makes each operator more productive, mitigating or even eliminating the often overlooked grinding constraint.
Aluminum oxide, zirconia alumina, and ceramic alumina have comprised some of the most effective grinding media introduced over the past 30 years.