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.

Stock Removal

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.

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.