Choosing coated-abrasive flap discs. Die grinder flap disc

06.05.2023 Beanrad choosing, coated-abrasive, discs, flap

Choosing coated-abrasive flap discs

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.

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.”

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.

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

abrasive

Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.

alloys

Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

aluminum oxide

Aluminum oxide, also known as corundum, is used in grinding wheels. The chemical formula is Al2O3. Aluminum oxide is the base for ceramics, which are used in cutting tools for high-speed machining with light chip removal. Aluminum oxide is widely used as coating material applied to carbide substrates by chemical vapor deposition. Coated carbide inserts with Al2O3 layers withstand high cutting speeds, as well as abrasive and crater wear.

backing

Flexible portion of a bandsaw blade. 2. Support material behind the cutting edge of a tool. 3. Base material for coated abrasives.

carbon steels

Known as unalloyed steels and plain carbon steels. Contains, in addition to iron and carbon, manganese, phosphorus and sulfur. Characterized as low carbon, medium carbon, high carbon and free machining.

coated abrasive

Flexible-backed abrasive. Grit is attached to paper, fiber, cloth or film. Types include sheets, belts, flap wheels and discs.

fatigue

Phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

flash

Thin web or film of metal on a casting that occurs at die partings and around air vents and movable cores. This excess metal is due to necessary working and operating clearances in a die. Flash also is the excess material squeezed out of the cavity as a compression mold closes or as pressure is applied to the cavity.

grinding

Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

grinding wheel

Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.

grit size

Specified size of the abrasive particles in grinding wheels and other abrasive tools. Determines metal-removal capability and quality of finish.

lubricity

Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.

metal-removal rate

Rate at which metal is removed from an unfinished part, measured in cubic inches or cubic centimeters per minute.

metalworking

Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.

Author

Alan holds a bachelor’s degree in journalism from Southern Illinois University Carbondale. Including his 20 years at CTE, Alan has more than 30 years of trade journalism experience.

Die grinder flap disc

In this second part of our “Training the Apprentice” series on grinders and their various accessories, we’ll be looking at the physical, as well as applicational differences between grinding wheels and flap discs. In the first part, we looked at grinders and their various accessories and applications. This article will take a more detailed approach to two of the more common grinder options to see which one will work best for you.

At some point, you might find yourself in the hardware store, knowing that there’s some grinding work ahead of you. Maybe you’ve got some welding beads to knock down and smooth out. Maybe the job ahead is sharpening mower blades. Or, you might be doing some woodworking and need to bevel some edges. Maybe the paint job on your car has seen more glorious days, and you’ve got some rust and paint removal ahead of you. So, you find yourself looking down the aisle at your available grinding attachment options. Where do you start? Considering the materials you’re working on, let’s say you narrow your options down to two: hard grinding wheels and flap discs.

Grinding Wheels And Flap Discs

Both options have similar functions; they both are abrasive cutting tools that consist of thousands of sharp grains and a bonding agent to hold all the grains together. Both spin at a high RPM, moving against the workpiece to cut away little chips of material. They both work, to some degree or another, for stock removal, grinding, sanding, and finishing applications. Both can be used on a variety of materials, though typically they find the most use with wood and metal.

But which works best? Is there a clearly preferable option for your particular application? The simplest answer is yes, there is. The answer might even be more definitive than you’d think. So, let’s look at some of the options, so you can inform yourself without having to spend a bunch of time tracking down the sales associate.

Grinding Wheels

Aluminum oxide is the most common abrasive for the grinding wheel. Zirconia alumina is another common and durable abrasive. Both generally work well for steel and steel alloy applications. Silicon carbide wheels cut non-ferrous materials, like stone, brass, soft bronze, and rubber. There are other abrasive options as well, but these represent the most typical.

Grinding Wheel Bonds

The bond is the material that holds the grains together and offers support so that the grains can cut effectively. Like the grains, the bonding agent wears away over time to expose new sharp grains.

Conventional grinding wheels typically use one of three types of bonding agents. Most employ vitrified bonds, which consist of a mixture of clays that have been fired to form a type of glass that holds the grains together. Vitrified bonds tend to be hard, but brittle, and wear down through grinding pressure. They can work through water, acids, and oils, and they don’t lose efficiency because of temperature changes.

Resinoid bonds, made from synthetic resin, soften under friction heat, and they can operate at higher speeds. They typically get used for applications that call for a better, finer finish.

Rubber bonds provide even better finishing, but may only be ideal for smaller areas of stock removal or pieces that require a high-quality finish.

A general rule of thumb for selecting a grinding wheel is that hard, brittle materials require a fine grit and softer bonds. Sure, using a finer grit seems counter-intuitive, but it actually cuts faster, as there become sharper grains more readily available as the bonding material wears away more quickly. Coarser grit and a harder grade bond will penetrate better with softer material, providing heavier cuts. Of course, the amount of stock material to be removed should also be considered.

Flap Discs

Flap discs are made from overlapping pieces of sandpaper which attach to the central hub. They work well for a variety of applications because they excel at grinding, blending and finishing. They are also lightweight and maneuverable. Many users prefer the flap discs over grinding wheels because of the lower vibration and noise levels. They also offer cooler cutting temperatures and less scratching than grinding wheels.

They come in a couple different shapes, conical and flat. The angled flaps in a conical flap disc offer a bigger surface area for stock removal on horizontal surfaces. They tend to work better for contoured surfaces and edge working as well. Flat flap discs work best for blending and finishing applications. They generally offer more stability, as they are mounted to a backing plate for support.

Like grinding wheels, flap discs offer a variety of different coarseness ratings and densities, as well as different grit materials. As a matter of fact, the grain materials listed in the section on grinding wheels could just as well be copy and pasted here; the information is basically the same. And, like the grinding wheels, use lower grit numbers for the coarser grinding and higher grit counts for the finishing work.

However, unlike the grinding wheels, flap discs will need to be mounted to a backing plate. Where the bonding agent offered cutting support for the grinding wheels, backing plates offer support for flap discs. You have a couple of options here. Use metal backing plates for stone and concrete applications. Fiberglass and plastic backing plates work well with most metal and wood applications.

Safety Concerns

As always, be mindful not to go over the recommended wheel speed suggested by the wheel manufacturer. Breakage can happen if you’re not careful, and the last thing you probably need is half of a 30-grit, vitrified wheel thrown through your eyeball. Hopefully, you have the good sense to wear eye and face protection as well, but why not take every precaution anyway, right?

Final Thoughts

So, back to the original question: between hard grinding wheels and flap discs, which grinding option is right for you? While researching this topic, both online and through talking to professional welders, shop owners, and woodworkers, there seems to be an almost overwhelming consensus. Of the two options, flap discs are generally better at everything.

There are a few advantages to the grinding wheel, but only a few, and in very specific instances. Say, for instance, you’ve got a large piece of sheet metal that needs cleaning up. Maybe it doesn’t need to look great, just free of crud. A grinding wheel might be your best bet, as they tend to run cheaper than a flap disc, and they tend to be more durable. There also might be some merit to using the grinding wheel in tight corners. Grinding down some welding in a corner might be easier accomplished with the firmer edge of a grinding wheel, but it will likely gouge at the metal, and it won’t look as finished as using a flap disc would.

If you care at all about aesthetics, and if spending a couple dollars more doesn’t bother you that much, flap discs do seem like the go-to option for many professionals. It almost seems as if the flap disc has, by and large, replaced the standard grinding wheel in almost every application. They tend to leave a better finish, and they accomplish the task more quickly than the grinding wheel will. They do tend to be more expensive, and they generally wear out more quickly than hard grinding wheels. But, if you buy in bulk from the right places, some of those costs can be mitigated and you’ll wind up with a cleaner looking end product.

Chris Boll

You’ll find Chris behind the scenes at Shop Tool Reviews. When he doesn’t have his hands on tools himself, he’s often the man behind the camera lens making the rest of the team look good. In his free time, you might find Chris with his nose jammed in a book, or tearing out his remaining hair while watching Liverpool FC. He enjoys his faith, family, friends, and the Oxford comma.

Flap Discs for Angle Grinder

It always pays to use the right tools for the job. When working with metal, it becomes very important to use tools that are safe and effective. If you are engaging in metal grinding or finishing, then you will want to consider using a flap disc for your angle grinder.

What is a Flap Disc for Angle Grinder?

A flap disc is an abrasive disc made of small sheets of overlapping sandpapers arranged in a circular shape. The layered overlapping segments or “flaps” have small grains of abrasives such as aluminum oxide. These abrasive grains are attached to a backing cloth which is normally made from polyester, cotton, or blended material.

The cloth is cut into smaller sections or flaps and layered across the disc. This is where the flap disc gets its name. These flaps are glued onto a backing plate made of plastic or fiberglass.

How the disc itself performs when being used will depend on the type of abrasive material used and its grit size. This is an important step because the selection of abrasive material and grit size will determine the effectiveness of the finishing or grinding you intend to do on the metal.

You can select between single and high-density flap discs. The high-density versions have more cloth which makes them thicker, and they will last longer compared to the single versions. You can also select the size of the flap discs which range from two to seven inches in diameter. Many people use the two-inch disc to replace blending discs as they tend to be far more durable.

Advantages of Flap Discs

The main benefit of using a flap disc is its low wear and ability to apply varying degrees of finish with the same disc. Each flap on the disc touches the workpiece surface at a different angle. This not only reduces swirl marks but also prevents repetitive damage to the surface, which is common with flat sanders such as palm sander.

In the case of a flat sanding sheet, you may have to replace it when a portion of the sanding surface is damaged. With the flap disc, the wear of abrasive particles is spread across several flaps; and damage to one flap doesn’t affect the performance of the disc.

During grinding the highest point of the flaps touch the workpiece. As you continue to do the machining, the abrasive grains here will get depleted and the fresh abrasives of the flap behind them will be exposed. This process continuous and enables you to grind without losing the quality of cutting and also extends the life of the disc.

The grinding wheel on an angle grinder is aggressive and can cause gouging and digging. While the flap discs can remove a lot of material quickly, it is much more forgiving than grinding.

Cons

The downside is that a flap disc is not well suited to work on uneven surfaces. This is because the flap disc uses cloth that can catch on the uneven surface and tear, resulting in having to change out the disc which only costs you more money. In that case, you will need a polishing disc or a die grinder to do the job.

Flap Disc Uses

What are flap discs used for?

Flap discs are primarily used for metalworking for grinding, blending, clean up welds, chamfering, etc. They can be used for finishing metals by quickly removing burrs and producing a smooth finish on the edges. Flap discs are also used to remove paint, rust, and corrosion from metal surfaces.

Although not as aggressive as a grinding wheel, the flap disc can remove a lot of material than one would imagine. The first time I used a 40grit flap disc on an angle grinder, I was surprised how quickly it removed the excess stock.

Contouring

High-density flap discs are excellent for contouring, blending, matching, and finishing. When you apply pressure, the overlapping flaps compress to form a stronger disc and removing a higher amount of material. With the same disc, if you apply light pressure, it can be used for finishing.

Cleaning up Welds

This is perhaps one of the most common uses of these discs. Since you can grind and finish the weld with the sample flap disc, it saves you a lot of time. This also means that you spend less money to get the same result if you were to purchase a grinding and finishing disc.

Paint and Scale Removal

It is also an excellent tool to remove rust and corrosion scales, heat treatment scales, etc. You can use a flap disc with an angle grinder to clean old paint, rust, and other materials without damaging the surface underneath. It can also be utilized to remove stickers or other adhesive materials from surfaces like concrete or metal.

Sanding Wood

Although it is mainly a metalworking tool, you can use a flap disc for sanding wood and preparation for refinishing. Here is a detailed guide where I discussed the methods of using an angle grinder as a sander.

Automobile

This type of abrasive disc also works well for auto body repair and restoration work. Since these discs are much more forgiving you can use them for grinding and finishing the car’s metal body and parts.

Types of Flap Discs

Before you buy the discs, you should know about the different types of flap discs. The two main types of flap discs are Type 27 and Type 29.

Type 27 vs. Type 29 Flap Discs

The differences between the Type 27 and Type 29 flap discs are in the angle. The type 27 disc has a flat face whereas the type 29 flap disc has a conical shape with flaps angled towards the edges. The difference is subtle, but important since they are designed for different uses. That is why you should carefully consider the work being performed before choosing which one is right for the job.

The Type 27 is a flat disc and is best used for finishing applications where the angle of use will be from 0 to 15 degrees. You can also do some mild grinding with the Type 27 disc at this low angle, but this is more of a finishing disc when most of the grinding has been completed.

The Type 29 has a beveled edge and is better for high angle grinding, which is normally between 15 and 25 degrees. This type of disc is best used for grinding purposes, especially on tough metal or when a considerable amount of grinding needs to be done.

Abrasive Types

There are different abrasive types that you will need to consider before making a purchase.

Aluminum Oxide: This is the traditional abrasive that was first widely used on flap discs. Today, aluminum oxide is most common on low-priced flap discs.

Zirconia Alumina: Another very popular type of grain used on flap discs, zirconia alumina is a blend of both zirconia and aluminum oxide grains. The result is a disc that is more versatile compared to the standard aluminum oxide, but it is more heat-resistant, durable, and has self-sharpening grains so that it does not wear out nearly as quickly.

Ceramic Alumina: These are the most expensive of the three types of grains, but they are also the most durable. Because the grains on the disc break apart into micro-fractures, they provide a continually sharp edge for grinding and finishing. This means that the ceramic alumina flap disc has longer tool life and needs to be replaced less often. This improves productivity.

However, the price of ceramic alumina discs can be steep. So it may be more economical to purchase a cheaper type depending on the requirements of your job. If you are only working for a short time or intermittently on different types of grinding or finishing work, then the zirconia alumina or aluminum oxide may be a better choice. But if you are a professional or are doing long-term work, then investing in a ceramic alumina flap disc is probably the best choice.

Ceramic and Zirconia Alumina Blend: This as the name suggests is a blend of both ceramic and zirconia. It provides the best of both substances for all-purpose use. The main advantage is that it grinds with less effort compared to all other types. However, it is also expensive, so you will need to look at your budget before you decide.

Grit Size

The size of the abrasive grain determines the quality of finish. A higher the grit number means finer abrasive grains and hence smoother finish.

Grit Size	Common Uses
24 – 40 grit	Heavy Stock Removal
40 – 60 grit	Grinding, Weld clean-up
80 grit discs	Cleaning, Sanding
120 git and above	Finishing

Flap Disc for Metal

You can use all four types of flap discs for metal. This is because the flap disc is primarily designed to both grind and finish metal. You will need to decide what type and grit to use in getting the results that you desire. Be sure that you are placing the flap disc at the proper angle when grinding or finishing.

Flap Disc for Wood

Although mostly associated with metal, the flap disc is perfect for grinding or sanding wood. It will not provide a finish like it does for metal, but it can sand down wood with great efficiency. You should choose a coarser grit version that can really grind into the wood to remove imperfections. However, if you are prepping wood for painting, then you can use a finer grit to lightly take off the surface.

Keep in mind that it will be very difficult to get a perfectly flat face on the lumber with an angle grinder. For flattening wood, I strongly recommend you use a belt sander or a drum sander.

Flap Disc for Paint Removal

The best type of flap disc is arguably the Type 27 that can be used at a nearly flat angle with fine grit. This will allow the flap disc to take off the paint while not digging into the surface too much.

As mentioned before, there are some grit components that work better than others. In particular, the ceramic and zirconia alumina are quite good and will last a long time with proper use. But they are also expensive, particularly the ceramic and the blend. If you are a professional who will use the flap discs regularly, then you probably want to invest in the more expensive versions.

If you are a woodworker or hobbyist who occasionally delves into grinding or finishing materials, then you might consider a cheaper version such as aluminum oxide. In addition, cheap flap discs may also be used by professionals if they believe that the additional price may be worth the purchase.

How to Use a Flap Disc?

Select the Right Gran and Grit Size

As I explained earlier you can use a single flap disc to remove material (grinding) and finish it (sanding) by varying the pressure applied on the workpiece. A flap disc is perhaps the only sanding device with which you can achieve higher grades of finish with lower grit wheels. That means you can use a 40grit disc and with delicate touches, you can achieve the finish of 60-grade sandpaper.

However, if you need to remove a lot of stock it is better to start with a coarse grain disc. When you are finishing the material, you can switch to a finer grit to complete the job.

Clamp the Disc and Check the Rotation.

This step is self-explanatory. Before you switch on the grinder, rotate the disc with your hand to ensure that it spins smoothly.

Safety

Adjust the safety guard on the grinder such that the direction of the sparks will be away from your body. Make sure that the guard will not cause hindrance as you keep moving the grinder with the disc spinning at high speed.

Grinding

Start the angle grinder and slowly bring the flap disc onto the workpiece to be ground or polished. To ensure that you use a flap disc properly, it must be held at the proper angle with the right amount of pressure applied to achieve the results that you desire. If you grind at an angle too steep, it may use up the edge of the disc rather quickly and cause it to wear out at an accelerated rate. Once you do this, the flap disc will have to be discarded.

The wrong angle will not remove the excess material in the proper manner. And if you are trying to grind away unwanted material, the wrong angle may only ruin the project. If you want to complete the work faster, it is better to stick to a proper angle and use a coarser grit flap disc which will take away more material.

Thin Sheets

You must be careful when using the flap disc on thin sheet metals for finishing purposes. The material removal rate could be more than what you expect and could create holes in thin sheets.

Too little pressure is placed on the flap disc and it will take longer for it to achieve the desired results. While applying too much pressure may cause the disc to slip or that you take away more material than desired. Too much pressure can also wear away the grains on the disc faster, causing it to create gouges or burn marks in the material.

Overall, flap discs are quite handy and useful in finishing metal and grinding metal, wood, and other hard materials. While angle grinders are arguably not the best devices for grinding and finishing, they are quite handy, portable, and can fill in adequately when needed.

What is a Flap Disc for Angle Grinder?

Advantages of Flap Discs
Cons

Stock Removal
Contouring
Cleaning up Welds
Paint and Scale Removal
Sanding Wood
Automobile

Type 27 vs. Type 29 Flap Discs
Abrasive Types
Grit Size
Flap Disc for Metal
Flap Disc for Wood
Flap Disc for Paint Removal

Select the Right Gran and Grit Size
Clamp the Disc and Check the Rotation.
Safety
Grinding
Thin Sheets

Flap Discs for Angle Grinder

What is a Flap Disc for Angle Grinder?

The cloth is cut into smaller sections or flaps and layered across the disc. This is where the flap disc gets its name. These flaps are glued onto a backing plate made of plastic or fiberglass.

Advantages of Flap Discs

The grinding wheel on an angle grinder is aggressive and can cause gouging and digging. While the flap discs can remove a lot of material quickly, it is much more forgiving than grinding.

Cons

Flap Disc Uses

What are flap discs used for?

Contouring

Cleaning up Welds

Paint and Scale Removal

Sanding Wood

Automobile

Types of Flap Discs

Before you buy the discs, you should know about the different types of flap discs. The two main types of flap discs are Type 27 and Type 29.

Type 27 vs. Type 29 Flap Discs

Abrasive Types

There are different abrasive types that you will need to consider before making a purchase.

Aluminum Oxide: This is the traditional abrasive that was first widely used on flap discs. Today, aluminum oxide is most common on low-priced flap discs.

Grit Size

The size of the abrasive grain determines the quality of finish. A higher the grit number means finer abrasive grains and hence smoother finish.

Grit Size	Common Uses
24 – 40 grit	Heavy Stock Removal
40 – 60 grit	Grinding, Weld clean-up
80 grit discs	Cleaning, Sanding
120 git and above	Finishing

Flap Disc for Metal

Flap Disc for Wood

Keep in mind that it will be very difficult to get a perfectly flat face on the lumber with an angle grinder. For flattening wood, I strongly recommend you use a belt sander or a drum sander.

Flap Disc for Paint Removal

The best type of flap disc is arguably the Type 27 that can be used at a nearly flat angle with fine grit. This will allow the flap disc to take off the paint while not digging into the surface too much.

How to Use a Flap Disc?

Select the Right Gran and Grit Size

As I explained earlier you can use a single flap disc to remove material (grinding) and finish it (sanding) by varying the pressure applied on the workpiece. A flap disc is perhaps the only sanding device with which you can achieve higher grades of finish with lower grit wheels. That means you can use a 40grit disc and with delicate touches, you can achieve the finish of 60-grade sandpaper.

However, if you need to remove a lot of stock it is better to start with a coarse grain disc. When you are finishing the material, you can switch to a finer grit to complete the job.

Clamp the Disc and Check the Rotation.

This step is self-explanatory. Before you switch on the grinder, rotate the disc with your hand to ensure that it spins smoothly.

Safety

Grinding

Thin Sheets

You must be careful when using the flap disc on thin sheet metals for finishing purposes. The material removal rate could be more than what you expect and could create holes in thin sheets.

What is a Flap Disc for Angle Grinder?

Advantages of Flap Discs
Cons

Stock Removal
Contouring
Cleaning up Welds
Paint and Scale Removal
Sanding Wood
Automobile

Type 27 vs. Type 29 Flap Discs
Abrasive Types
Grit Size
Flap Disc for Metal
Flap Disc for Wood
Flap Disc for Paint Removal

Select the Right Gran and Grit Size
Clamp the Disc and Check the Rotation.
Safety
Grinding
Thin Sheets

Flap Discs Vs. Grinding Wheels: When To Use A Flap Disc

Flap discs offer benefits such as fast stock removal and the ability to grind, blend, and finish with a single product, which can improve project timeline without compromising on results. In general, you’re better off using a flap disc over a grinding wheel when abrading metal and applying a smooth finish.

When using an angle grinder, choosing the right attachment for the job is critical to a successful outcome. The most common options are flap discs and grinding wheels. While they are often confused due to some overlapping characteristics, the two different abrasive products are not the same. In this blog, the team at Red Label Abrasives explains what they are, the recommended applications for each one, and when you should opt for a flap disc over a grinding wheel for your project.

What is a Flap Disc?

A flap disc is an abrasive product used to contour and shape metal. It consists of overlapping abrasive flaps glued to a backing plate and is regularly used for welding, machining, heavy-duty equipment work, and industrial maintenance. Common applications include:

Cleaning flash from molds and castings
Removing rust
Edge grinding
Deburring
Blending weld seams

Flap discs offer benefits such as fast stock removal and the ability to grind, blend, and finish with a single product, which can improve project timeline without compromising on results.

When Should You Use a Flap Disc?

Flap discs are the recommended choice when you’re working with metal, especially when you intend to make right angle cuts. Being flexible, these discs make it easier to achieve contours in the metal.

For grinding, apply heavy pressure and for finishing, apply light pressure. You thereby avoid the downtime caused by switching discs between tasks. Other benefits include:

Cooler operation, minimizing the risk of scorch or heat marks
Reduced vibration and fatigue for a more comfortable experience
Safety is improved because there are no pieces that break or fly off
With less gouging, the finish is better

What Are Grinding Wheels?

Grinding wheels are one of the most commonly used abrasive products. Made from thousands of tiny abrasive grains, they remove material to both shape and refine a workpiece.

Different types of grinding wheels are available, and each type serves a different purpose. Some are sharpeners and cutters, while others are polishers and smoothers.

When Should You Use a Grinding Wheel?

Grinding wheels are great for general sharpening tasks, such as restoring edges on worn-down shovels and garden tools or performing an initial grinding on lawn mower blades, shears, hatches, and axes. They can also be used for material removal, but aren’t as great for finishing work due to their tendency to gouge surfaces.

When To Use a Flap Disc Over A Grinding Wheel

In general, you’re better off using a flap disc when abrading metal and applying a smoother finish. Although they’ve long been used with high-speed angle grinders, advances in both design and materials have brought flap discs to the point where they can carry out grinding, blending, and finishing jobs much more quickly and with less noise than grinding wheels- layered flaps constantly expose new grain and act as a cushion, resulting in less noise and vibration.

Users have better control over flap discs, making damage (and rework to repair it) much less likely.
Operators tend to find flap discs more comfortable to use, so they’re a recommended option for longer grinding jobs.
Since flap discs don’t gouge the workpiece as fast as grinding wheels do, a less-skilled operator can use them more efficiently without damaging the work piece

Grinding wheels can play an important role in your project, particularly during material removal, but when you’re working with metal and need a tool that can achieve results during each stage of the project, flap discs may be your best and most efficient option.

Questions? Speak With An Abrasive Specialist

At Red Label Abrasives. we are a leading and trusted provider of specialty abrasive products, including sanding belts. discs. rolls. and flap discs. Whether you need the right abrasive for your application or advice on how to get the most out of your product, our technicians are here to help. For more information or help in placing an order, please call 844-824-1956 or fill out our contact form.

Sources:

https://www.ctemag.com/news/articles/choosing-coated-abrasive-flap-discs
https://shoptoolreviews.com/metalworking/grinders/grinding-wheels-flap-discs/30371/
https://www.finepowertools.com/grinders/flap-discs/
https://www.finepowertools.com/grinders/flap-discs/
https://www.redlabelabrasives.com/blogs/news/flap-discs-vs-grinding-wheels-when-to-use-a-flap-disc