Choosing coated-abrasive flap discs

When choosing among coated-abrasive flap discs to remove metal and finish parts, grit size is just the beginning.

When it comes to abrasive tools, one is causing a bit of a flap. The flap disc category has been the fastest growing one in metal fabrication over the last decade, according to David Long, director, marketing and strategy for Norton Abrasives North America, Worcester, Mass. “From a product performance standpoint, flap discs offer fast stock removal and provide grinding, blending and finishing with one product,” he said.

For example, a right-angle grinder (a hand-held machine with handles 90° apart) might use a type-27, depressed-center grinding wheel to clean a weld and then change to a bonded-abrasive fiber disc for finishing. A flap disc, however, can be employed on a right-angle grinder for both operations. 

Courtesy of Superior Abrasives

Superior Abrasives offer a full line of flap discs, such as these ceramic alumina ones.

Courtesy of Coated Abrasive Fabricators Association

A typical coated abrasive has a backing (cotton, polyester, poly-cotton or paper); base coat (hide glue, urea resin or phenolic resin); size coat, which is the top layer of a coated abrasive (also made from hide glue, urea resin or phenolic resin); and top size with a grinding aid, such as a stearate, to reduce friction and heat.

Flap discs are also used for a variety of other applications, such as cleaning flash from molds and castings, removing rust, edge grinding, deburring and weld-seam blending. Flap discs are constructed with cloth coated-abrasive material, cut into squares and glued onto a backing plate.

Coated abrasives provide better grain protrusion than bonded abrasives. That’s because electrostatic coating is the most common method of applying abrasive onto the backing, according to a white paper from the Unified Abrasive Manufacturers’ Association. With that method, the abrasive is placed on a grain conveyor belt, which brings the grits into an electrostatic field at the same time the backing passes through the field, above the abrasive. As the grains pick up an electrostatic charge, they are propelled away from the belt and onto the adhesive-coated backing. The process leaves the grains standing upright, perpendicular to the backing, with the sharper ends of the grains pointing up and away from the backing (see illustration below).

Step It Down

Even when a user only applies fiber discs, the application might require several grit sizes employed in a “step-down” method. That involves, for example, starting with a 24-grit fiber disc, switching to a 36 grit and then a 50 grit and so on until the desired finish is imparted. “What you’re doing is removing the scratch patterns left by the previous disc with a finer and finer disc,” said Shane Miller, executive vice president of Superior Abrasives Inc., Dayton, Ohio. “When it comes to a flap disc, you can skip a couple grains sizes because you can take a 60 grit and get it to finesse like it’s an 80 grit or a 120 grit by adjusting the pressure and allowing the grains and the backing to break down.”

Although a flap disc can reduce the number of products needed for an operation, James Norman, marketing manager for metal fabrication at 3M, Abrasive Systems Div., St. Paul, Minn., emphasized that flap discs, fiber discs and grinding wheels are three distinct product categories. Users primarily buy a flap disc when imparting a fine surface finish is the main goal, whereas a fiber disc provides a fast cut to quickly produce parts, and long life is the main attraction for a wheel, he added.

Lyle Rawlins, general manager of Pacific Abrasive Supply Co. (PASCO), Buena Park, Calif., concurred that flap discs are the product of choice when a fine finish is required. “I was just amazed the first time I ran a flap disc to see the finish,” he said, attributing the result to the cushioning effect provided by the layers of individual abrasive segments. “Even a coarse grit will give you a finish you won’t believe.”

In addition to imparting a fine finish and reducing the number of operations, a flap disc aids the operator of the right-angle grinder. “Ergonomically, a flap disc is a much more comfortable tool to use because you don’t get a lot of the bouncing that you get with a depressed-center wheel,” Long said.

Courtesy of Norton Abrasives

A flat, or type 27, flap disc (top) is for blending and smoothing flat surfaces while a conical, or type 29, shape is best when speed and heavy stock removal are the primary considerations and when working on contours and edges.

He added that, compared to depressed-center wheels, flap discs weigh less, are easier to control and require less downtime for tool changes, especially when equipped with a ⅝-11 threaded, quick-change attachment instead of the standard ⅞" center hole. Less vibration and resulting fatigue, as well as reduced noise, also benefit operators, according to Long.

With the benefits flap discs provide, it’s important to know how to select the right one by understanding the variables and how they apply to a metal fabrication application. Those variables include disc size, shape, backing plate material, abrasive flap densities, type of abrasive grit and grit size.

Size and Shape

Flap discs are typically available in 4 ", 4½ ", 5 " and 7 " sizes. To reduce weight of the right-angle grinder and therefore operator fatigue, Rawlins recommends a 4½ " disc even though it has a slower metal-removal rate than larger ones. Smaller discs also generally cost less, he added.

End users apparently agree, as 4½ " discs are the most popular, representing about half of all sales, Long noted, followed by 7 " discs. In Canada, however, 5 " right-angle grinding machines are popular.

Courtesy of Coated Abrasive Fabricators Association

Electrostatic coating is the most widely used process for applying abrasive onto the backing of coated-abrasive products. The process leaves the abrasive grains standing upright, perpendicular to the backing, with the sharper ends of the grains pointing up and away from the backing.

Standard flap discs are available in two shapes: conical (type 29) and flat (type 27). Long recommends the flat shape for blending and finishing flat surfaces with a 60-grit abrasive or finer. The conical shape features 15° to 25° angled flaps and is for aggressively grinding contours and edges because the angle increases surface contact. Conical is the best choice when speed and stock removal are the primary considerations, he noted.

Superior Abrasives’ Miller added that the conical shape is also more effective for beveling and weld grinding. “If you need to bear down and get rid of material, a type-29 disc will work the best,” he said, noting that the two types cover about 90 percent of the market.

As previously noted, operator comfort plays a role, and conical flap discs reduce stress on the back and arms, according to Rawlins.

Back It Up

Another consideration when selecting a flap disc is the type of backing plate, which the abrasive flaps adhere to. The plate provides stability during use. 

There are three basic backing material types: fiberglass, plastic and metal—typically aluminum. “Fiberglass tends to be the most common,” said Erik Vanstrum, technical service manager for 3M. “Metal is for extreme applications.”

Courtesy of 3M

A giant flap disc (top) is for working on uneven or curved surfaces whereas a standard-density flap disc is for rapid stock removal.

Fiberglass is the most popular backing plate material because of its high strength and light weight, according to Long. Fiberglass backing plates help absorb vibration and can be consumed during operation without smearing the workpiece, he added. “The fiberglass will erode with the flaps as you grind at a 15° to 20° angle,” he said, “but if you flatten the disc and take it to a 5° angle or less, then the fiberglass won’t necessarily erode as easily.”

According to Long, plastic backing plates are becoming more popular because they provide some conformability and, unlike other available material types, are “trimmable.” Being trimmable allows more use of the flaps, especially when blending and finishing. (To view a Norton video about how to trim plastic backing plates, visit hpmvideo.saint-gobain.com/wsi-ab-sga-na/video/trimmable_backing.MPG.) “In many cases, you trim the backing plate of a flap disc so you can expose the leading edge of the disc,” Long said.

Metal backing plates are the third choice and the least popular, according to Long. They are for applications that demand extra-high strength and rigid support. Although not consumed during use, an aluminum plate can be recycled, he added.

Although a metal plate can scratch the workpiece, this product line has its following. “Some people are traditionalist and like that metal hub because they feel it adds rigidity,” Miller said. “We like the fiberglass backing. It gives great support and strength and is cost-effective while not scratching the work surface.” Plastic also doesn’t scratch during use.

Regardless of the backing plate material, Norman noted that the flaps are designed to wear down to the plate.

Density and Grit

The quantity, angle and spacing of the flaps on the backing plate can vary substantially, which is referred to as flap density. The basic densities are standard and high. The standard density is for heavy-duty applications and rapid stock removal, and the high density is for abrading uneven and curved surfaces, as well as for blending with finer grits, Long explained. “A high-density disc gives you a bit more flexibility for doing slight contours and radius work,” he said. “The higher the disc is stacked, the more cushioning or compression you have.”

Norman noted that high-density, or giant, discs also last longer because 3M’s giant flap discs, for example, contain up to 25 percent more abrasive material than standard discs.

Manufacturers offer flap discs with a host of abrasive grits, including silicon carbide, diamond, garnet, emery, chrome oxide and crocus. For metalworking, three grits are effective: aluminum oxide, zirconia alumina, or alumina zirconia, and ceramic alumina.

Courtesy of Norton Abrasives

Three types of abrasive grains are suitable for metalworking: aluminum oxide (left), zirconia alumina (middle) and ceramic alumina.

Al₂O₃ is generally the low-price alternative, Vanstrum said. However, he added, zirconia is the most common mineral, so it seems to be the lowest cost option, and alumina-zirconia grits self-sharpen to provide longer life than Al₂O₃ grits, which “dull out.”

Metalworking abrasives microfracture to self-sharpen during operation, but Al₂O₃ grains expose rounder edges as they wear compared to the other two types, according to Vanstrum.

Al₂O₃ is suitable for low-alloy steels, carbon steels and general-purpose grinding.

Zirconia alumina provides a good cut-rate-to-cost ratio and is effective on high-alloy steels and high-tensile materials. Zirconia alumina is also effective for stainless steel, Miller noted. “It helps to decrease discoloration of the workpiece and provides an excellent cut rate.”

Rawlins pointed out that PASCO sells more zirconia-alumina discs than it does Al₂O₃ discs. “Our cost is the same, so you might as well get the zirconia because you’re going to get a little longer life,” he said.

The best of the three metalworking abrasives is ceramic alumina because it cuts twice as fast and lasts up to 10 times as long, according to Long. That’s because the grain microfractures to allow a continuous supply of the sharpest cutting edges, providing the best utilization of the entire grain for longest life, especially on stainless and high-alloy steels.

The abrasive grains for metalworking often have a grinding aid applied. This enables them to run cooler and add lubricity to help minimize disc loading. 

A typical grinding aid is cryolite, according to Miller. “Just like any cutting tool, heat is the enemy and dulls a cutting edge,” he said. “The longer you can keep that edge cooler, the more aggressive and sharper it will be. In abrasives, you want the edge to eventually break or fracture once it begins to dull. This creates a new, sharp contact point.”

Vanstrum noted that grinding aids are particularly effective when finishing 300 series stainless steel and nickel-base alloys. “Grinding aids are lubricants and they react with the metal, so they help make better metal chips for more efficient grinding,” he said.

Although grinding aids help minimize disc loading, Miller explained that aluminum has a high tendency to load and there isn’t a lot a user can do other than knock the loading off. “It just loads by the nature of the beast,” he said, adding that Al₂O₃ is a good abrasive for aluminum because it’s cost-effective.

When loading is an issue, Vanstrum recommends applying a lubricant, such as Accu-Lube from ITW Rocol. Lubrication options include a wax applied to a disc while it is moving or stationary and a liquid in which a small quantity is sprayed at the disc/workpiece interface.

The Nitty Gritty

Grit size is the final component an end user must select to match a flap disc to an application. The standard range is from 24 to 120 grit. “Eighty grit is the most popular size we carry,” said PASCO’s Rawlins.

When performing heavy metal removal, selecting too coarse a grit can cause shelling, cautioned Norman of 3M, which offers abrasive as coarse as 36 grit. “Shelling means the grit is just falling off the edge of the flap before you’ve actually broken it down and got full usage of it,” he said. “Sometimes customers find longer life with a 40 grit than a 36 grit because it has better mineral retention.”

Courtesy of Norton Abrasives

Operators can select how coarse or fine the grit is based on the flap disc application.

At the other end of the spectrum, users seeking an abrasive finer than 120 grit to remove, for instance, a coating without damaging the underlying metal surface, probably require a duplex flap disc. That type of disc contains nonwoven, surface-conditioning material blended with traditional coated material. Miller noted that Superior Abrasives recently introduced duplex flap discs that are as fine as 180 grit. “In one fell swoop, you can remove a weld on stainless and produce a No. 4 finish very easily,” he said.

Long pointed out that a flap disc with nonwoven material can impart a finish finer than 10 rms, but hand-held operations using right-angle grinders generally don’t require that fine a finish. “That’s why, at this point in time, we think nonwoven hasn’t increased in popularity,” he said.

Once users understand the variables for selecting the correct flap disc for an application, they will be able to realize the tool’s effectiveness. “If you have not tried flap discs yet, I recommend you do so as they are a versatile solution to most metal fabrication applications,” Long said. “Be sure to consult your sales representative to help understand their total line to provide you with the best grinding, blending and finishing solution.” CTE

About the Author: Alan Richter is editor of CTE. He joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.

Contributors

Norton Abrasives North America 
(508) 795-2183
www.nortonindustrial.com/metalfab/muscle.aspx

Pacific Abrasive Supply Co.
(800) 755-2042
www.pacificabrasive.com

Superior Abrasives Inc. 
(937) 278-9123
www.superiorabrasives.com

3M, Abrasive Systems Div.
(866) 279-1288, ext. 1258
www.3m.com/abrasives