The grinding disc is probably the most commonly used abrasive discs on an angle grinder as they allow for quick material removal and is fairly cost-effective. Unfortunately, due to the rigid nature of a grinding disc, a grinding disc can sometimes take off too much materials (stock) when all you need is only to finish off a work surface or welding joints for work surfaces that must be cosmetically pleasing.

That's where the flap disc come into the picture. Flap disc as its name would suggest, is made up of multiple 'flaps' of abrasive cloth pieces bonded to a disc. The flexible nature of the flap disc tends to be more forgiving when the operator accidentally applies too much pressure on the work piece (middle) which would otherwise results in a grinding disc gouging (on the left) into the work surface.

Depending on the application, flap discs are available in different disc shapes - flat or angled, different backing materials - fibreglass, nylon or aluminium, different type of abrasive grits, grit sizes - from 36 to 120, as well as the number of flaps on a disc - more flaps are ideal for uneven surfaces. A cousin of the flap disc, the flap wheel, is essentially the same thing but is made for a bench grinder.

Compared to a grinding disc, flap disc also tends to vibrate less which means less operator fatigue. What flap disc gives the best results? That's all down to trial and error and lots of practice!