Flap Discs: The Right Selection and Technique Can Save Time and Money

By Rick Hopkins, senior product manager – Welding & Fabrication, Weiler Abrasives Group and Ron McCarthy, Regional Sales Manager, Director of Marketing Oliver H. Van Horn Co


In any metal grinding or finishing application, it is important to have the correct product for the job — it minimizes the risk of removing too much material and reduces the time and cost for the process. When working with thinner materials, flap discs are often a good choice. Flap discs offer reliable grinding action, while also allowing for more delicate blending and finishing work.

Flap discs are constructed with the same grain types as found in bonded abrasives, but their layered construction gives them a much softer, more forgiving feel. As a result, they are ideal for grinding and finishing a product — at the same time — which helps save time and money. They do not, however, work as well for finishing uneven surfaces. A grinding disc is better suited here, as the flap disc’s cloth can catch and/or tear too easily. 

These discs feature coated abrasives that adhere grains to a backing cloth on the flap disc, typically a cotton, polyester, or blended material. This abrasive cloth is then cut into smaller flaps and layered radially to form the flap disc. How a flap disc performs is largely related to the abrasive material used and the grit size. There are three primary types: aluminum oxide, zirconia alumina and ceramic alumina.

The flaps wear away as the grains deplete, which exposes fresh and sharp grains below. When used properly, a flap disc can essentially be used until the cloth is almost completely worn down to the resin.

In general, flap discs are designed for use on right-angle grinders at angles from 5 to 35 degrees. A Type 27 flap disc is best for finishing and applications that require lower grinding angles ranging from 5 to 15 degrees. A Type 29 flap disc is best for more aggressive, higher-angle grinding, between 15 and 35 degrees.

Choosing the best flap disc for the job and maximizing its effectiveness and longevity depends on interweaving elements. The three main factors are: 1) the surface finish requirements and desired aesthetics, 2) the time, productivity and efficiency requirements and 3) the labor and consumable costs involved.

Selecting the best grain and disc size

It is important to choose a grain that is appropriate for the metal being cut and a grit appropriate for the application. In addition to single-density flap discs, there are also jumbo or high-density flap discs. These flap discs have more cloth on them, which makes them thicker, softer and longer lasting. Flap discs are made in different sizes, too, ranging from two to seven inches. A two-inch disc essentially replaces a blending disc, and gives an operator up to 15 times the use.


Choosing the abrasive material

Aluminum oxide is the traditional abrasive used when a consistent, low-cost product is needed, but the most common grain used today is zirconia alumina. A blend of zirconia and aluminum oxide grain, this type of flap disc provides the most versatility for the lowest cost benefit. Although it is slightly more expensive upfront than an alumina oxide flap disc, it is more heat resistant, tough and durable, and provides more self-sharpening grain capabilities. The third choice of abrasive material is ceramic alumina. The grains on these flap discs micro-fracture to enable a continuous supply of the sharpest edges, and are more resistant to heat. Ceramic alumina flap discs are also the most expensive, but they last the longest out of the three types.

When deciding whether to use a ceramic flap disc, an operator needs to decide whether the increased longevity is worth the extra money, factoring in details such as the reduction in downtime for changeover and the reduction in inventory for extra products.In the end, the goal is to find the flap disc that accomplishes the application’s requirement within a desired timeframe and budget.


Using the correct technique

To maximize the effectiveness and longevity a flap disc, it is important to use the proper pressure at the proper angle. Putting too much pressure on a flap disc when it does not work fast enough can cause premature wear or lead to gouging, undercutting or burning — when the real issue is the need for a coarser grain. Using too much pressure can also undercut or reduce the grain cut rate, inadvertently glazing the disc and making it too smooth. Once this happens, the disc must be discarded, even if it is only partially used.

Similarly, grinding at too steep of an angle causes the operator to work lot harder because they’re not engaging the full width of the cloth. It can also use up the edge on the flap disc and ultimately end its life prematurely. It is equally important to use the correct pressure for the grain selected. For example, a 120-grit disc may provide a delicate finish, but it would take the operator a very long time to complete the job. On the other hand, using a more aggressive grain with lighter pressure could accomplish the same task in a shorter amount of time.

When finishing thin weld material, in particular, it is important not to remove too much of the metal and thereby create weaknesses. This is another reason why it’s so important to use a disc that is not too aggressive for the job, especially when tighter tolerances are required. Undercutting or gouging the thin material could ultimately ruin the workpiece. In applications where these issues are real concerns, it is usually best to use a 60 or 80-grit disc rather than a 36- or 40-grit disc.

When using flap discs, don’t try to judge visually whether the product is used up to the point of ineffectiveness. Given that a flap disc can often be used down to the resin, the only real way to judge whether it needs to be replaced is when it is no longer effective. This single insight, when shared with all operators, can save significant cost for product every year.

Remember, even though flap discs are disposable, there are steps that can be taken to lengthen their usability. The result is less downtime, more work and greater savings.

How to trim a flap disc
Some flap discs feature a trimmable backing plate that can be removed progressively to establish the desired flap overhang. This feature provides the operator with a more versatile product for multiple jobs and it extends the life of the product. Every flap disc manufacturer has its own recommendations for trimming. A common one is a maximum trim of 3/8 inch from the edge of the original backing plate.


To achieve the best results, follow these steps:

  1. Select a sharp piece of scrap metal.
  2. Ramp the flap disc up to speed.
  3. Slowly lift the flap disc until the flap’s top (or overhang) contacts the scrap metal.
  4. Push directly into the corner of the metal with consistent, controlled pressure.


The process may take some practice to perfect, but in the long run it can save time and money.