Flap discs

Type 27 is nearly flat and is used at about 0 to 15 degrees, suiting finishing and blending welds flat. Type 29 is conical and used at about 15 to 25 degrees, engaging more grain for faster stock removal on edges and large surfaces. Choose based on whether flatness or removal rate matters more.

Aluminium oxide is low cost and suits mild steel and light work. Zirconia is durable under pressure and widely used on steel and stainless. Ceramic cuts coolest and fastest when pushed with enough pressure, suiting stainless and difficult alloys, but too little pressure stops it self-sharpening.

Grit 40 is for aggressive removal such as weld levelling, 60 balances removal and finish, and 80 to 120 is for blending and surface preparation before painting. Work from coarse to fine rather than one grit for every step. A flap disc can replace a grinding wheel for many weld-levelling jobs and leaves a smoother finish in one step.

Fibreglass backings are rigid, widespread and wear away with the flaps, so they mark the surface less as the disc wears. Plastic backings can be trimmed to use flaps closer to the root and reduce vibration. With either type, stop when the flaps are nearly down to the backing to avoid it rubbing the workpiece.

With a flat Type 27 disc, about 0 to 15 degrees suits finishing broad surfaces; a conical Type 29 disc works well at around 15 to 25 degrees for heavier stock removal. Too steep an angle wears the flap edges quickly and gouges the workpiece. A steady angle helps the flaps wear evenly and keeps exposing fresh grain.

Replace it when the flaps have worn close to the backing plate, when flaps come loose, or when removal rate falls noticeably despite adjusting pressure. Continuing with worn flaps lets the backing rub the workpiece, causing scratches and heat. Discs that have been dropped, have a cracked backing or are oil-soaked should also be discarded.

It can be used on the same angle grinder if the disc's maximum speed is not below the tool speed and the arbor hole fits. However, guards for Type 1 cutting and for Type 27 grinding can differ, so the correct guard must be fitted for the task. Always check the flange and nut when switching.

Moderate pressure that keeps the grinder close to its rated speed works best. Pushing too hard slows the tool, builds heat, glazes the flaps and reduces cutting. Ceramic grain needs enough pressure to fracture and form fresh cutting edges, so follow the manufacturer's recommendation.