Selecting the Right Flap Disc for your Grinder – Beginners Guide

Selecting the right products for your metal grinding or finishing applications is very crucial as it will save time and cost for the process.  This is especially important when you will be working on different materials as it will minimize the risk of removing or leaving too much material.

Flap discs and grinding wheels are essential components of a grinder, but most people tend to use a grinding wheel more often, overlooking the benefits of a flap disc. While both have a lot in common, they yield very different results depending on the project in hand.

Both the grinding wheels and flap discs are used to grind metals effectively, and they have great cut rates. Also, they have high rates of metal removal. However, flap discs are often a reliable option for projects that involve thinner materials. They are also lightweight and require less changeover time.

Although they are more expensive when compared to grinding wheels, they are more cost-effective when you consider ease of use, versatility, and durability provided by flap discs.

With that in mind, let’s begin by understanding what flap discs are and how to choose the right one for your next project.

Exploring Flap Discs

Flap discs are highly versatile tools which are made of several overlapping pieces connected to a central point hence the name flap. They are made with the same grains found in abrasives, but the arrangement of the grains in the discs gives them a much softer feel.

The grains adhere to a backing cloth on the flap disc; usually a cotton or blended material. This makes them appropriate for delicate grinding application and finishing projects. They are however not suited for finishing uneven surfaces as the cloth can tear easily.

Flap discs are designed to be used with right angle grinder operations, and they can meet your needs in several applications. While it takes a considerable amount of experience to use grinding wheels, flap discs have a smooth learning curve with a small margin of error.

As the cloth wears, the grains depletes exposing sharp and fresh grains underneath. However, when well maintained, the flap can be used until the cloth is completely worn out to the level of the resins.

How to Select the Right Flap Disc

1. Choosing the abrasive material

The material used on the abrasives largely determines the effectiveness of a flap disc. The primary types are:

  • i) Zirconium Alumina– This is a blend of zirconia grain and aluminum oxide with self-sharpening capabilities. This type of flap provides a high cut rate for the lowest cost possible. It is tough, durable, and resistant to heat, making it a perfect choice for mild steel and carbon applications.
  • ii) Ceramic Alumina– This grain is the newest innovation in abrasives, and it is especially effective on stainless and high-alloy materials. It provides the fastest cuts as well as maximum utilization of the whole grain.
  • iii) Aluminum Oxide- This is the oldest grain in metalworking, and it is highly recommended for small, low-cost tasks.

When deciding on the three types, you need to factor in details such as increased longevity, reduction in downtime, and reduction in inventory for additional products. In the end, the most important thing is to find the flap disc that meets the requirements of the specific application within the desired time and budget.

2. Grit size

Choosing the right grit size is going to determine how fine or coarse you can go. The sizes range from 4 to 7 inches to fit various right angle grinders. There are also custom flap discs that are made to meet specialized applications. Understanding the grit sizes will ensure that you have the best grinding and finishing solution for your metalworking applications.

3. Flap Disc Shape

As mentioned, flap discs are used on right angle-angle grinders, and they are designed with a central point that allows work to be performed on the flaps. The flaps can be aligned at angles from 5 to 35 degrees. A type 27(flat) flap disc ranges from 5 to 15 degrees, and it is primarily used on flat working surfaces. It is perfect for finishing and less aggressive grinding applications.

Type 29(conical) has its flaps positioned between 15 and 35 degrees, and it is mainly used on contoured surfaces. It is best for suited more aggressive grinding applications where speed and stock removal are the primary considerations.

4. Backing Plate Material

When selecting a flap disc, you should also consider the material used on the backing plate where the abrasive flaps adheres. The backing plate provides stability during usage, and it is available in the following material types:

  • i) Plastic Backing Plates- These plates are more popular due to their ease of use, and also because you can trim them. This allows maximum utilization of the flaps, mainly for blending and finishing.
  • ii) Fiberglass Backing Plates- fiberglass is known for its strength and lightweight. It creates a strong bond with the adhesives, and it gets consumed during operation. When used as backing plates, they help to reduce vibrations without smearing.
  • iii) Aluminum Backing Plates- although they are not as popular, aluminum plates are used for applications that require more strength and support, and the good thing is that they can be recycled as they are not consumed during use.

5. The density of the flaps

Last but not least, you should consider the density of the flaps on the backing plate. By this we mean, the quantity, spacing, and angle of the flaps on the backing plate. The arrangement can vary greatly from one flap disc to the other, and it is important to match this with your application needs to achieve great results.

These are;

  • i) Standard density: a standard density is best suited for light-duty applications that require fast stock removal.
  • ii) High density: as the name denotes, this density is applicable when working on high-demanding applications and finishing as well as curved and uneven jobs.


With the recent innovations, flap discs are available to use on various surfaces unlike in the past where they were only used on metals. Today we have flap discs that can work on wood, flap disc for removing paint and those that can work on concrete and more. For each of these applications, the most important thing is to choose the right disc to enable you achieve the desired level of efficiency and effectiveness.

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