Choosing coated-abrasive flap discs. Flap disk grinder

How to Select the Right Flap Disc for Your Project

Are you exploring flap discs to use for your next project? Maybe you’ve been using grinding wheels all this time and have overlooked the benefits of a flap disc. Flap discs are highly versatile tools and can meet your needs in a variety of applications — from stock removal to grinding and finishing. Many operators actually prefer flap discs over grinding wheels during operation, and when it comes to your next project, flap discs may be just the solution you need.

Keep reading our flap disc buying guide to learn more about these tools and how to select the right flap disc for your project.

What Are Flap Discs?

Flap discs are made from multiple overlapping pieces of abrasives or ‘flaps,’ which is how they got their name. Flap discs are designed for right angle grinder applications ranging from heavy stock removal to surface blending and smooth finishing.

While the first abrasive flap discs for high-speed angle grinders were developed towards the end of the 1970s and were somewhat basic, today’s versions offer an array of diversity. You should strongly consider a flap disc over a grinding wheel if you need a superior finish and greater ease of use during operation.

Advantages of Flap Discs

Flap discs are versatile: they can grind, blend and finish. They are also lightweight, easy to maneuver, and require less change over time. When performing a job, many operators prefer flap discs over grinding wheels due to lower vibration and noise levels. Plus, they offer cooler cutting with minimal scratching.

For instance, grinding wheels have to be discarded even after a small area of abrasive is worn out. Comparatively, flap discs remain useful even after the flaps erode, which results in a longer operating life. Compared to a flap disc, grinding wheels are cheaper on a unit cost basis. However, if you consider the versatility, durability and ease of use provided by flap discs, they are more cost-effective in the long run. Because of these advantages, the popularity of flap discs has soared in the last several years.

How to Select the Right Flap Disc

When it comes to flap discs, there’s a variety of discs available in today’s marketplace. Let’s start by understanding the various components of a flap disc, so you know how to select the right disc for the right task:

Flap Disc Shape

Is stock removal your primary objective, or do you want a smooth finish? Choosing the right disc shape is the single-most important variable, and the shape will help you effectively achieve your results. Flap discs are almost always used on right angle grinders, and they are applied to your work at an angle or parallel to it. Flap discs are available in two shapes: conical or flat-shape.

  • Conical flap disc: Conical flap discs are your best friends when you need to remove a lot of material in a small amount of time. They can be used for edge-working as well as on contoured applications. The flaps in a conical flap disc are angled. As a result, these discs offer a greater surface area for stock removal on horizontal surfaces.
  • Flat flap disc: While conical discs are great for Rapid stock removal, flat flap discs are best for blending and superior finishing. They are mostly used on flat surfaces. The flaps of a flap disc are adhered to a backing plate which provides stability during operation.

Flap Disc Material

The backing plate material is also an important variable to consider when choosing flap discs for your application. Fiberglass, plastic and metal are the most popular backing plate materials:

  • Fiberglass: Being strong, durable, lightweight and safe, fiberglass is the most popular material choice. Fiberglass creates a strong bond with adhesives and does not contaminate the working surface. This type of plate also gets consumed during use, and it absorbs vibration very well. Remember, fiberglass backing is made out of layers of fiberglass that are mesh-bonded and pressed together. With more layers and a higher mesh density, the backing will be stronger and more durable — this is an important consideration when you’re examining the detailed specs of a flap disc.
  • Plastic: Another popular backing material is plastic, and nylon is the most commonly used plastic. These backings can be trimmed which allow longer usage of flaps, specifically during blending and finishing. These days, plastic is becoming an increasingly attractive choice due to its conformability and costs.
  • Metal: Metal backing plates are the safest choice, and they’re great when you need extra strength and firm support. Aluminum is a commonly used metal. Since metal plates are expensive, they should also be used where they make the most sense. For example, when working with flap discs on concrete or stone applications, use flap discs with metals for strong support and better performance. Metals don’t get consumed during use, but metal plates can be easily recycled when a flap disc is past its useful life.

Abrasive Flap Densities

What does abrasive flap density mean? Think of the density as the total amount of abrasive area provided by the flaps on a flap disc. This area depends on the quantity of flaps on a disc, their angle relative to the center of the disc and how far they are spaced. Remember, each variable can impact the amount of disc area available to work on your job.

  • Standard density: Standard density flaps are optimal for fast stock removal and heavy-duty applications.
  • High density: High-density flaps are best-suited when working on curved or uneven jobs, as well as during finishing. Don’t take the description of flap disc density at face value. Consider the number of flaps, angle and spacing to differentiate between two discs, both of which may specify “standard density flaps” or “high-density flaps.”

Abrasive Grit Material

Flap discs can be used for a variety of applications, whether it’s metal or woodworking, concrete grinding or finishing, stone smoothing or finishing, paint or rust removal, and more. In order to get the maximum benefits out of flap discs, it’s important to choose the right abrasive grit material for your specific job needs. Let’s look at the most commonly used abrasive grit types:

  • Ceramic Alumina: This material is great for stainless steel or alloy metals application. With Ceramic Alumina, the grit material ruptures at a micro level during operation. This produces a constant supply of sharp cutting surfaces. As a result, it enables faster cutting while allowing the entire grain to be used. As the entire grain is getting used up in the cutting process, these discs offer higher durability.
  • Zirconia Alumina: This is a blend of Zirconia and Aluminum Oxide grain, and it’s great for carbon and mild steel application. Zirconia Alumina costs less than Ceramic Alumina, and it provides a great cut rate for the cost.
  • Aluminum Oxide: This is the original grit material used in the 1970s when flap discs were first introduced. It’s also the lowest-cost option. Today, it is recommended for smaller jobs where the product being produced is low-value.

Grit Size

You’re probably familiar with grit size if you’ve been using grinding wheels. Grit size is the final variable you have to choose based on your ultimate goal and what you’re trying to achieve. For stock removal or common grinding, use abrasives with lower grit numbers. On the other end of the spectrum, use higher grit sizes if you’re trying to achieve smooth finishing.

Flap Disc Uses

Flap discs initially became popular for use on metals, especially in welding applications. Today, various flap discs are available to use for different surfaces:

  • Flap discs for aluminum: Compared to other metals, aluminum has a lower melting temperature and melts easily. This causes the aluminum material to coat the flap disc during grinding, covering the grit and exposing only bits of aluminum. For stock removal, use a T29 conical disc at a 15-degree angle to provide maximum surface contact. If you need to surface-clean or provide a smooth finish, use a T27 flat disc that’s parallel to your work area. For best results, use light and even pressure to optimize the grinding process and reduce loading.
  • Flap discs for wood: Flap discs are great tools for working on wood. The flap discs designed for use on wood are similar in nature to the ones designed for use on metal. You can use aluminum oxide grits for wood applications. For wood, you should also use the flap discs on your angle grinder just like you would use a grinding wheel. In order to avoid deep scratches, start with a heavy grit and work your way up to the lighter grits (100) for a final finish. For wood sanding, work your way through 120, 150, 180 and 220-grit sizes to achieve a furniture-grade surface.
  • Flap discs for paint removal: Do you have a metal object that’s rusted but you know you can still extend its life? Do you have old, chipping paint on your car that you need to remove to give it a like-new appearance? Flap discs, especially non-woven discs, are ideal tools for removing paint and rust. Non-woven flap discs, or the ones with aluminum oxide, can be used for paint or rust removal applications. As with flap discs in general, these discs can grind and finish in a single operation while offering a smooth and controlled grind.
  • Flap discs for concrete: You’ll need silicon carbide or diamond flap discs for aggressive stock removal on concrete. Silicon carbide and diamond are some of the hardest materials in the world. Using these discs will allow you to work on concrete surfaces without the need for high pressure. These flap discs have rigid backings, and they can also be used on other surfaces such as engineered stone, granite, marble and ceramics.

Getting Started With Your Flap Discs

Before you start using any shop tool, you should always make sure you:

  • Understand your tool
  • Read the user’s manual
  • Wear the proper safety equipment
  • Ensure a safe working area
  • Know what you’re using your tool for

With flap discs, you need to consider the size and scope of your project. Do you have to remove stock aggressively, or is smooth finishing your goal? Or do you want a grind that’s somewhere in between? Whatever your answer, there’s a flap disc that’s right for your situation.

Don’t limit flap disc to the common metals. Flap discs can also be used across various surfaces, including aluminum, wood, concrete, engineered stone, granite and more. For each of these applications, whether you’re grinding or finishing, make sure you choose the right flap disc. Remember, conical shaped flap discs are great for stock removal and flat flap discs are best for finishing.

Flap disc backing material is important as it provides support during operation. Use metal backings for concrete or engineered stones, and use fiberglass or plastic backings for most metal or wood jobs. Also consider your abrasive grit material, and choose your grit size to achieve the desired results. For common grinding, use abrasives with lower grit numbers. For smooth finishing, use higher grit sizes.

Using a flap disc instead of a traditional tool can greatly enhance the quality of your job. You can also benefit from lower noise and vibration. Flap discs can lead you to a world of new applications, while helping you achieve new levels of efficiency and effectiveness.

Purchasing Your Flap Discs

When it comes to purchasing flap discs, you have plenty of options. However, a reputable company that cares about you and your project will help you experience a better result. You should also feel comfortable asking for samples when you’re trying new tools like a flap disc.

National Abrasives, Inc. has a large variety of tools, accessories and supplies to meet your immediate needs and your needs in the future. We’re a family owned company that offers same-day shipping and discounted pricing for bulk orders. Browse our large selection of brand-name flap discs, including Walter Abrasives flap discs, along with angle and bench grinder tools and accessories. If you have any questions, our team can guide you to the right tool for your specific job. Contact us today to get started!

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.

choosing, coated-abrasive, flap, discs, disk

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

SMT vs. CMT Quick Change Flap Disc | KLINGSPOR Abrasives USA

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.

choosing, coated-abrasive, flap, discs, disk

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.”

choosing, coated-abrasive, flap, discs, disk

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 “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.

choosing, coated-abrasive, flap, discs, disk

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

Norton Abrasives North America (508) 795-2183

Pacific Abrasive Supply Co. (800) 755-2042

Superior Abrasives Inc. (937) 278-9123

3M, Abrasive Systems Div. (866) 279-1288, ext. 1258


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.

How to Mount a Flap Disc on an Angle Grinder | Norton Abrasives


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.


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.


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.


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.


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.


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.


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.


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.

Flap disk grinder

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.

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