How to choose an abrasive disc

Get the disc right and it cuts clean, lasts and stays safe. Get it wrong and it wears out in minutes, tea-stains your stainless, or shatters. The job, the material and the rated RPM are what separate the two.

We carry the Rocket range - cutting, grinding and flap discs in the 115mm and 125mm sizes most angle grinders run, plus 355mm for the drop saw, in both steel and stainless (INOX) grades. Three things get you to the right one and keep you safe: what you're doing to the metal, what the metal is, and the numbers stamped on the disc.

Match the disc to the job (cut, grind, or blend and finish), match the grade to the metal (mild steel or stainless/INOX), then check the disc's max RPM meets or beats your grinder before you fit it.

1.Which disc for the job

Rocket 115mm INOX Cutstainless, iron-freeShop these discs

Three discs cover the work. Pick by what you're doing to the metal.

2.Match the metal and the size

Rocket 125mm Flap 40Gcoarse, fast blendShop flap discs

Two things beyond the job decide the disc: the metal you're on, and the fit to your grinder.

Metal. Mild steel takes a standard aluminium-oxide disc. Stainless is the one to get right - use an INOX disc, made iron-, sulphur- and chlorine-free, or the contamination will make the cut edge rust and tea-stain later. (Masonry and concrete are a different job again - they need a silicon-carbide disc or a diamond blade, not a metal disc.)

An INOX disc is defined by what it leaves out - kept under 0.1% iron, sulphur and chlorine so it won't seed rust in the stainless. The grain matters too on harder metals: standard aluminium oxide is the everyday choice and what the Rocket discs use, zirconia is a tougher self-sharpening grain that runs cooler on stainless and hard steel, and ceramic is the premium micro-fracturing grain for the longest life on high-tensile alloys.

Size and fit. Rocket discs come in 115mm (4.5") and 125mm (5") for angle grinders, plus 355mm (14") for the drop saw. The 115 and 125mm run a 22.23mm centre bore (the metric 7/8" arbor), so match the diameter to your grinder and the bore drops straight on. On flap discs the grit does the finishing: EHI stocks 40, 60, 80 and 120 grit - low numbers (40) are coarse and fast for stock removal, high numbers (120) are fine for blending out. Grit carries a "P" prefix on the label (the FEPA grading standard).

3.The safety numbers

Abrasive discs spin fast and fail hard, so these checks are non-negotiable before every job.

• RPM. The max RPM printed on the disc must equal or beat your grinder's speed - a disc spun past its rating can burst. As a guide, the 80 m/s peripheral-speed limit puts a 115mm disc at about 13,300 rpm, a 125mm at about 12,250 rpm, and a 355mm drop-saw disc at about 4,400 rpm.
• Certification. The current Australian standard for bonded discs is AS EN 12413:2022 - it replaced the old AS 1788.1 back in 2022, so don't reject a disc just because it isn't stamped "AS 1788". Look for the oSa mark as well: it means the disc has been burst-tested, usually to around 1.5 times its rated speed.
• Guard on. Never pull the guard to fit a bigger disc - fit the right size instead.
• Right disc, right way. Never grind on the flat face of a thin cutting disc; the side load shatters it. And check the expiry - resin-bonded discs have a shelf life of about three years, stamped on the centre ring as a "V" date (V04/2027 means expire April 2027); past it, the bond can't be trusted.
• Dust and face. Cutting or grinding masonry, concrete or coated steel throws respirable dust - wear a P2 respirator (AS/NZS 1716), not a paper mask. And wear a face shield to AS/NZS 1337.1 over your safety glasses; a fragment off a failing disc is faster than glasses alone will stop.