Choosing coated-abrasive flap discs: General Industry Coverage

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.