Arbor

Flap discs come either with a plain circular arbor which require a locking nut or a 5/8-11 threaded hub which allows for a quick change over.

Backing Plate

Flap discs come with aluminum, fiberglass or plastic backing plates. Fiberglass is the most common. Aluminum is sometimes chosen due to no potential fiberglass dust hazards. Plastic backing is becoming more popular with some manufacturers suggesting that the plastic backing can be trimmed to increase working life.

Shape

Flap discs are broken down into flat and angle face. Following grinding wheel terminology flat discs are type 27 while angle discs are type 29. Typical angle faced flap wheels are about 10°. Angle faced discs are more aggressive and are more suited to stock removal while flat faced discs are better on flat surfaces and will produce a more consistent finish.

Density

Flap discs are available in regular and high density. High density discs, often called jumbo discs, have more flaps and are thicker. Jumbo discs tend to be a little softer and are therefore not as aggressive. Jumbo discs will last longer due to the extra abrasive.

Flap discs are a 3 dimension abrasive product. They consist of abrasive flaps glued radialy to a backing plate. As the product is used the abrasive wears down and new abrasive is exposed underneath. Zirconia is the most common mineral for flap discs due to its cool cutting action. Ceramic is sometimes used but the price usually cannot be justified for the small performance increase.

Flap discs are used in heavy stock removal, usually weld removal. They outperform Resin Fiber Discs due to their increased lifetime. Resin Fiber discs are only two dimensional, once the abrasive surface wears down the product is finished.  To achieve a finish equivalent to a resin fiber disc always use a flap disc one grit coarser. E.g. if the operator is running a 60 grit resin fiber disc try a 40 or 50 grit flap disc.