Which Flap Disc Should You Use? Grit and Backing Guide

Start with the job, not the disc

Flap discs do three main jobs on an angle grinder: removing stock, blending welds and edges, and finishing a surface. Each job points to a different grit, and sometimes a different shape. Deciding the job first narrows the choice quickly.

Ask yourself these questions before you shop:

• How much material do I need to remove?
• Is the surface flat, or are there edges, corners and fillet welds?
• What metal am I working on: mild steel, stainless or something else?
• What does my grinder accept in disc diameter, arbor size and speed?
• Do I need one grit, or several steps in a row?

Choosing a grit

Grit sets how fast the disc cuts and how rough the surface looks afterward. Lower numbers are coarser. Higher numbers cut slower and leave a smoother surface.

Many jobs need more than one step. You might hog off a weld with 40 or 60 grit, then blend with 80 grit and finish with 120 grit. That is why several products on our best flap discs page sell mixed packs of 40, 60, 80 and 120 grit. The Pukamam set, for example, lists 24 discs with six of each grit.

Zirconia, ceramic and aluminum oxide

The abrasive grain decides how long the disc lasts and how fast it cuts. You will see three common types.

Aluminum oxide is the basic, general-purpose grain. It works for light jobs on mild steel and wood, but it tends to wear sooner on harder metals. None of the discs in our top ten list it as the main abrasive.

Zirconia is the common middle choice. Most discs on our comparison page use it, including the EZARC, Benchmark Abrasives, NEIKO and Pukamam discs. It suits general metal work and holds up better than basic aluminum oxide on steel.

Ceramic is generally chosen for faster cutting. The PFERD and Panlen discs use it. Ceramic grains are designed to break down in small pieces as you work, which keeps fresh cutting edges exposed. That makes ceramic a good fit for heavy removal on steel and stainless, though you should check each listing for which metals the maker says it suits. Not every listing names stainless.

• Zirconia vs aluminum oxide: zirconia is the better pick for steel and longer life.
• Ceramic vs zirconia: ceramic for faster cutting and demanding jobs, zirconia for general work.
• Ceramic vs aluminum oxide: ceramic is the heavy-duty end, aluminum oxide the light-duty end.

Backing and shape: flat, angled and curved

The backing is the plate that holds the flaps. Fiberglass backing is common, and several listings, such as EZARC and NEIKO, say it absorbs vibration. Less vibration makes the disc easier to control and can be less tiring on long jobs.

Shape matters as much as grit. A Type 27 disc is flat and suits flat surfaces. A Type 29 disc is angled, so the flaps meet the work at a working angle. The EZARC listing gives 15 to 20° for weld work, and the Panlen listing gives 15° to 25°. That angle helps with edges, corners and weld blending, and it makes it easier to avoid gouging.

The PFERD takes a different approach. Its radial CURVE design is built for fillet welds, and the listing says it offers 280 degrees of flap surface for top, side and bottom grinding. It is also the lightest disc on our list at 0.16 oz.

In short, use an angled or curved disc for edges, welds and contours. Use a flat disc when the surface is flat and you want even contact.

Check size, arbor and speed

A great disc is useless if it does not fit your grinder. Check these three points before you buy:

• Diameter: most discs here are 4 1/2 inches. The Benchmark Abrasives listing says it also fits 5-inch grinders.
• Arbor size: most discs use a 7/8 inch arbor. The PFERD uses a 5/8-11 thread, so it only fits grinders with that spindle.
• Maximum speed: the listed max speed is 13300 RPM for most discs in our top ten. Your grinder's no-load speed must not be higher than the disc's rating.

Safety before you grind

Grinding throws sparks and fast-moving debris, and a flap disc can still shed pieces if it is damaged or overspeeded. Wear safety glasses or a face shield, hearing protection, a dust mask or respirator, and gloves. Keep sparks away from anything flammable.

Keep the guard on the grinder. Unplug it or remove the battery before changing a disc. Check the disc for cracks or loose flaps before each use, and never run it above its rated speed. Hold the grinder with both hands, and clamp the work so it cannot move. Do not use a disc on its edge unless the maker says it is designed for that.

Quick picks by situation

If you are unsure, use this rule of thumb. For one disc that does most things on steel, take 60 grit. For heavy removal, take 40 grit. For finishing, take 80 or 120 grit. Add an angled or curved shape if you work on welds and edges.

For a side-by-side look at specs such as grit, abrasive, backing, arbor and speed, see our best flap discs comparison.

Frequently asked questions

Can I use the same disc on stainless steel and mild steel?

Often yes, but check the listing. Some makers name stainless steel as a suitable metal and others do not. Avoid cross-contaminating stainless with particles from carbon steel by using a separate disc.

How do I know when a flap disc is worn out?

When the flaps have worn down close to the backing plate, or the disc cuts slowly and runs hot, it is time to replace it. Stop if you see loose flaps or cracks.

Is a mixed grit pack worth it?

It can be, if one project needs several steps from rough removal to a finish. If you mostly do one kind of job, a single-grit pack usually makes more sense.

Can I use a 4 1/2 inch disc on a 5-inch grinder?

Only if the disc's listing says it fits and the grinder's guard allows it. Always match the disc's arbor size and speed rating to the grinder.