Flap Discs For Metal, for Welding Prep and Smooth That Weld

By Bill Byers / April 19, 2019
Flap Discs For Metal Title Image

Flap Discs for Metal are;

1. Used for Preparation - by shaping and cleaning your metal surfaces before welding.

2. Used between Welding passes and after Welding - by taking off welding slag between welds. Perfect for after weld clean up and smoothing down of your metal by blending in your welding bead ahead of painting and galvanizing.

3. Selected for the type of metal you plan to use them on. Be it mild steel, stainless steel or Aluminum.

4. Chosen by shape, grit size and by surface type. Depending on the project and level of grinding you need to do.

​You have your metal - mild steel, stainless steel or Aluminum and you need to get it cleaned up ready to weld.

You've heard that flap discs for metal are the thing to buy but you're not sure why. Let me lead you through why, getting some flap discs for your metal is going to make your welding preparation and finishing off so much easier.

Four Areas You Can use Flap Discs on Your Metal

1. In Preparation.

Whether you're flux core welding, stick welding, MIG or TIG welding, before you start to weld, good clean metal can be crucial.

I've listed the welding processes in specific order as with flux core welding you can get away with more contamination of your metal.  The same when stick welding, but with MIG and especially when TIG welding clean metal is a must.

And unfortunately even metal that you have bought new, that you think must be clean, can have an oxide layer on the top that needs to be cleaned off before you weld.

2. Fix those Metal Weld Joints With a Flap Disc

Should the joints on your metal be a little jagged or burred. Deburr with your flap disc and prepare those metal joints so you have nice, even joints to weld together,

3. Flap Disc Use Between Welding Passes

Welding on thick gauges of metal will likely involve you welding multiples passes to build up the weld in the joint. Between each weld pass any welding slag or mill scale needs to be removed before you lay down the next weld bead on that metal.

A flap disc run over the joint before the next pass cleaning out the welding slag, mill scale or metal contaminants bought to the surface by your filler metal, sets you up nicely for the next pass.

4. Once You Have Done Welding

Clean up your metal with a flap disc. Unless you know that you're particularly skilled and can lay down some smooth dimes straight off the bat. For the rest, you know once you have welded you're going to want to smooth down those weld beads.

Particularly if the joint is going to be seen and you want it to look good or because you're wanting to paint it up or spray galvanize to finish off your project.

Flex Steel Deburring Mild Steel

​Points to Think About Before you Choose The Flap Disc for Your Metal

What is your aim? And by that I mean.

1. Is this a project that needs to look great? For example you're welding on some light gauge steel to fill that hole rust has caused. When you're done you want it look shiny and new like you never touched it.

2. Or that doesn't that matter because, for example, it's a lawn mower part that's broken off. You just need to smooth up the edges, clean down to the metal so that you can weld it up good as new. It's a lawnmower so you don't really care how it looks. You just need it functional.

3. Do you have a restricted budget and a small project where you need to clean up the metal before you weld and do some post weld tidy up before you paint or spray galvanize.

4. Or is time more important to you, you want what ever flap disc you choose to just work and quickly get the job done.

5. Does the project have lots of tight angles and corners you need to get into?

6. Or is the metal curved and you want to follow the curves and not dig into them with something that is going to mark up the metal?

7. Is there a wide expanse of mill scale and dirt removal to grind off? So you want something that is going to cover a lot of distance and fast.

Tiger Paw Flap Disc

​Best Practice When Using a Flap Disc On Different Metals

Mild Steel or Low Carbon steel

Select a flap disc with a surface abrasive designed for mild steel. One that suited to the surface you need to grind. Lots of rust and mill scale? Look for a flap disc aimed at removing heavy mill scale and rust, if that is what you are dealing with. Flat metal, pick a flap disc with a flat profile. Curved metal, choose the conical type of flap disc.

I have researched and put together a list of the flaps discs I consider the best for Mild Steel check it out if you're curious.

Stainless Steel

In addition to the points I mention above on mild steel, it is best not to swap your flap discs between grinding on a mild steel or low carbon steel surface and your stainless steel.

Because minute particles, or even not so minute bits of mild steel will be on your flap disc's surface. And as you grind on your stainless steel preparing the surface or doing your after weld clean up you'll be embedding those bits of low carbon steel into your stainless steel.

The whole idea of using stainless steel is for its stainless and rust resistant properties.

Mild steel and low carbon steel do not have those features.

Do you really want small particles that are susceptible to rust embedded in your stainless steel?

Or indeed have those particles to weld through after you have so carefully chosen your stainless steel welding wire for its stainless steel features. I suggest it is best to dedicate a flap disc to your stainless steel. ​

Aluminum

Is well known as a soft metal, much softer than mild, low carbon or stainless steel. Aluminum also has a different melting point to those metals. When preparing and cleaning off surface contaminants from your Aluminum you will want a flap disc that can keep cool while working.

In addition Aluminum can easily coat the surface of a normal flap disc clogging up the flap's abrasive surface. You'll be scraping your precious Aluminum up onto the flap disc instead of where you want it in your project.

It is best to use a flap disc that is rated for working on Aluminum. It'll keep your metal where you want it, yet remove the dirt on the surface.  Just like stainless steel it is best to dedicate the flap disc you use on Aluminum to Aluminum. The last thing you want is mild steel or stainless steel which are much harder metals embedded into your Aluminum surface. And not only that causing your arc to stutter as you MIG or TIG weld risking porosity from contaminants in your weld.

Enduro-Flex Grinding Mild Steel

​Why Use a Flap Disc On Your Metal?

If you are used to using grinding wheels and their discs you may wonder why use a flap disc at all.

It is worth trying out using a flap disc for preparing and finishing your steel, stainless steel or Aluminum as;

Flap discs are lighter weight and are so easier to use in your grinder as they move lightly over your metal's surface.

They operate with lower vibration and noise. A real advantage when you have a lot of metal to prepare, all that noise can get pretty wearing after a while, not to mention potential damage to your ears.

Flap discs are popular because they tend to last longer than grinder wheel discs. This is because as the surface area of the flaps wear down new abrasive surfaces are revealed.

Flap discs are perfect to use on thinner metal gauges as they work gently on the metal. The last thing you want on thin dirty metal is to remove too much metal, making the metal weaker. You want to remove the surface contamination down to the good clean metal below. No more, no less.

Flap Disc Types For Metal

There are two main types of flap discs for use on mild steel, low carbon steel, stainless steel and Aluminum - type 27 and type 29.

Type 27

Known as flat flap discs have in fact a 5 to 15 degree angle on the face. They are ideal for preparing your metal and or cleaning off welding slag.

And after MIG welding removing mill scale and accessing tight angled spaces. For example filet welds.

Got large flat metal surfaces? Use a type 27 flap disc for blending and finishing off after you have welded.

Type 29

​Are known as conical flap discs and have a depressed center. The surfaces of these flap discs are angled at 15-35 degrees. Use on metal where there is heavy surface contamination and you need a more aggressive brushing action. Type 29 flap discs are known for removing more metal than flat flap discs.

Use on metals that have a curved or contoured profile, as the more angled shape of the flap disc will glide over those surfaces fitting the contours well. Type 29's are also selected when you have a large surface area to cover and need speed or where you have edges to deburr or bevel before welding.

Flap Disc Grit Sizes for Use on Metal

Flap Discs are available in multiple grit sizes. The grit is the abrasive surface on your flap disc. The more common sizes are 24, 40, 60, 80 and 120 grit. There are higher grit sizes available for very fine work.

The lower number grit sizes are the rougher and more aggressive on metal. 

24 Grit is very rough and will remove a lot of metal quickly.

40 Grit is used for removing old welds, grinding the edge of metals, chamfering and beveling.

60 grit is selected for rust and paint removal. It'll take off dirt on your metal, deburr and round the edges of your metal and is good for finishing off.

80 Grit is used for light grinding. Use this grit surface to blend your weld into the surrounding metal

120 grit is used for very fine surface finishing when you are looking to put a nice shine on your weld and metal.

Density Differences in Flap Discs And the Affect on Metal

In addition to types and grit surfaces, flap discs come in different densities. Density in the flap disc world signifies the number of flaps on the disc and the spacing of those flaps across the surface.

High density jumbo flap discs for example, have more cloth on the flap discs and are thicker and softer. Perfect for finer blending of your weld with the metal.

Standard density flaps are more aggressive in their action and are better to use when you need to work quickly across your metal.

Tiger Paw Grinding Mild Steel

​Flap Disc Surface Types for Metal

Aluminum Oxide

Is the original flap disc surface and has been around for years. A cheap surface and you'll find this surface on the lowest cost flap discs. It doesn't last very long. Select an Aluminum Oxide flap disc when your metal preparation task is small, you're working to a tight budget and working on low carbon and mild steel. ​

Zirconia and or Zirconia-Alumina

Is a more premium flap disc surface and that is reflected in the cost. It is longer lasting than Aluminum Oxide. Pure Zirconia is the better surface as it lasts well. Zirconia-Alumina combines Zirconia and Aluminum Oxide. A great surface choice for mild and low carbon steel when you need great performance. This surface has better heat resistance and so does well on stainless steel.

Generally Zirconia and Zirconia-Alumina surfaces last well and keep their cutting power for longer. Look for the surface designs rated for stainless steels or Aluminum when grinding on those metals.

Last Words

And there you have it, flap discs for metal, for welding preparation and smoothing your welds after you have welded. I've covered the different ways you can use flap discs. Not only for preparation, between welds and finishing up your steel, stainless steel and Aluminum. I have discussed flap disc on metal best practices as well as the types of flap discs suited to metal. I hope you enjoyed my article and that I have helped answer your questions.


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