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Can You Use Flap Discs to Grind Aluminum?

Jul 27th 2020

Aluminum is a beautiful metal that has a lot of great uses. Though, that beauty and utility can possibly create some major headaches, especially if you aren’t prepared with the right tools. When it comes to surface grinding aluminum, we highly recommend using a flap disc for aluminum grinding to get the job right the first time while avoiding damaging the aluminum.

Issues with Grinding Aluminum

Aluminum is one of the softest and lightest metals, which are two major reasons this is such a commonly used metal. It is an incredibly flexible material that can more easily be fabricated and shaped than most other metals. Due to its lightweight properties, it can also be shipped at a fraction of the price of other metals, like steel that is about three times the weight.

With all of those benefits of aluminum comes some drawbacks when it comes to working with this soft metal.   

Heat Tolerance/Low Melting Point

The melting point of aluminum alloys ranges from approximately 865-1240°F. Compare that to the 2750°F melting point of stainless steel or 2800°F for iron and you can see how sensitive aluminum is to heat. Using the wrong sanding disc or trying to grind the metal too aggressively can raise the temperatures quickly to those temperatures which can destroy the aluminum.

Loading/Chip Adhesion

That low melting point also makes is all too easy to clog up the porous surface of any abrasives. Grinding aluminum at high temperatures causes bits of aluminum to get stuck between the abrasive grains and embed themselves into your grinding disc. As more and more aluminum gets embedded, you’ll eventually be grinding the aluminum stuck on the wheel against the aluminum surface you are working on. This can cause the temperatures to rise much quicker which will damage the aluminum and risk an exploding grinding disc if the temperature gets too high.

Oxidation/Aluminum Oxide

Aluminum does not rust like iron or steel, but it does corrode in its own way. Bare aluminum quickly oxidizes when exposed to oxygen. Instead of getting a rust color that flakes off, aluminum oxide forms a white-colored surface skin. Not only is this a bit of an eyesore, but it can also be a nightmare for welders since it causes the welding arc to stutter and increases porosity in the weld. This is why you should always grind the surface to prep for welding aluminum.

Contamination

Even though aluminum doesn’t rust, it doesn’t mean it’s impossible to get a rusty aluminum surface. Using the wrong abrasives can easily ruin the surface of the aluminum. Because of its softness and low melting point, bits of abrasive grains from sandpaper, sanding discs, or even wire wheels can contaminate aluminum. You risk having rust spots (after-rust) eventually show up on the surface if you embed iron or carbon-based materials into it. The contamination can also cause another issue for grinding aluminum welds. 

Can You Use Flap Discs for Aluminum?

It turns out, flap discs are possibly the best grinding discs for aluminum.

By design, flap disks grind metal surfaces at much lower temperatures than other abrasive discs. The abrasive cloths don’t absorb the heat like a solid grinding disc is prone to, and the spacing provides additional airflow that keeps the surface cooler than the alternatives.

What Flap Discs are Best for Aluminum Grinding?

Despite running much cooler than other abrasives, flap discs are prone to loading and still generate heat that can damage aluminum. However, there are specialized flap discs for aluminum that are made to avoid these challenges.

The first thing you should be looking for is an abrasive that is contaminant free and won’t risk after-rust caused by metals like iron, sulfur, chlorine, or carbon steel.

Secondly, you’re going to want a flap disc that has a no-load coating, like calcium stearate. A calcium stearate coating will liquify as the disc is being used, forming a protective coating for your aluminum surface and cooling the disc to prevent loading.

Secondly, you’re going to want a flap disc that has a no-load coating, like calcium stearate. A calcium stearate coating will liquify as the disc is being used, forming a protective coating for your aluminum surface and cooling the disc to prevent loading.

Want to know which flap discs checks both of those boxes? Check out this T29 flap disc for aluminum or this T27 aluminum flap disc. Both discs are made of high-quality aluminum oxide abrasive grains with the protective load-resistant calcium stearate coating to effectively grind aluminum at lower temperatures. They are also contaminant-free, so you don’t have to worry about after-rust or interference with your welds.

What are Flap Discs?

Flap discs are abrasive wheels made up of overlapping “flaps” of cloth sanding strips. Each strip is coated in abrasive grains that wear away with use, exposing new grains on each flap to expand the life of the tool. They are made to work with angle grinders and die grinders.

Common flap disc uses include grinding, deburring, stock removal, blending, and finishing. In regards to aluminum, they are a great tool to prep for polishing aluminum.

Flap Disc Benefits

  • Economical/low-cost
  • Long-lasting
  • Multi-purpose
  • Gentler on soft metals, like aluminum
  • Less vibration than grinding wheels
  • A consistent rate of removal
  • Lightweight 

Alternative Abrasives for Grinding Aluminum

While we recommend flap discs for aluminum grinding, you do have other options if you prefer a different tool. Just like flap discs, you need to choose abrasives that grind at cooler temperatures, are load-resistant, and contaminant free.

Grinding Wheels

If you decide to use these grinding discs, it is incredibly important to get one made for grinding aluminum specifically. A grinding disc that isn’t specifically for aluminum will not only likely damage your material, but also puts you at risk of a disc breaking from overheating. A grinding wheel for aluminum will have a carefully blended mix of aluminum oxide and silicon carbide grains that will effectively grind soft metals while running at a lower temperature.

Sanding Drums

Abrasive sanding drums come in a variety of options that make them a great choice for sanding large areas of aluminum. They attach to surface contouring machines or burnishing tools that allow the user to quickly grind large surfaces rather quickly. An interleaf flap wheel drum naturally runs cooler and also simultaneously conditions the surface of metals while it grinds.

Wire Wheels and Brushes

Wire abrasives can be used on aluminum if you need aggressive rust, paint, or weld spatter removal but aren’t recommended when you are looking to get a smooth surface. Wire brushes and wire wheels can leave marks on the surface that would likely still need to be sanded out by another tool. In addition, if you are using a carbon fiber wire wheel, you are likely going to deal with after-rust. Stainless steel or brass brushes will avoid after-rust.

Tips for Grinding Aluminum with Flap Discs

Apply the Proper Pressure

Even though flapper discs for aluminum are manufactured to create less heat and avoid loading, improper use can still somewhat nullify these benefits. You can reduce loading and heat buildup by applying light and even pressure while grinding aluminum.

Choosing Flap Disc Types

Flap discs come in two different types, T27 and T29.

T27 flap discs are flat and meant for grinding at a 0-15° angle. They are more suited for flat sanding and for smoothing and blending surfaces.

T29 flap discs are conically shaped with a 15° angle from the center of the disc towards the edge. They are meant to be used as a 15-25° angle to the surface. They are better for aggressive grinding and for contouring and shaping surfaces.

Avoid Contamination

In order to avoid cross-contaminating your aluminum surfaces, it is recommended that you only use your aluminum flap disks for aluminum projects. If you use these discs on another project, like grinding cast iron for example, you'll risk contaminating the aluminum with pieces of iron your abrasives have picked up.