An abrasive works by cutting with hard grains, so its grain must be harder than the work: quartz sand and sandstone cut wood, bronze and soft iron, corundum and emery cut hardened steel, and silicon carbide cuts glass and the hardest tool materials. A 300 mm wheel turning at 1,500 RPM has a rim speed of about 23.6 m/s, fast enough that a burst wheel is a lethal projectile. Ring-test every bonded wheel before mounting and guard all of it except the working face.
Part of Advanced Ceramics
Abrasives are hard mineral grains, loose or bonded into wheels, stones and papers, used to cut, shape and polish harder or softer materials by wearing them away.
Hardness Decides What An Abrasive Can Cut
Cutting works only if the grain is harder than the surface. On the Mohs scale, quartz is 7 and corundum (aluminium oxide) is 9, with silicon carbide above corundum and diamond at 10. Hardened tool steel sits in the lower range of that scale, so quartz sand is marginal on it and corundum cuts it easily.
| Abrasive | Source | Good for | Note |
|---|---|---|---|
| Quartz sand, sandstone | Quarried | Wood, copper, soft iron, stone | Fine, even sandstone grinds best |
| Emery | Natural corundum and magnetite | Steel | Metamorphic rock deposits |
| Corundum / alumina | Mineral or made from bauxite | Hardened steel | Tough |
| Silicon carbide | Sand and carbon fused in an arc furnace | Glass, carbide, hard tools | Needs electric power |
Grinding Wheels gives the practical wheel types: the traditional sandstone wheel, run wet and slowly at 100-200 RPM for a 300 mm wheel, and bonded-abrasive wheels using shellac, sodium silicate or clay-feldspar vitrified bonds. Refractory Ceramics lists silicon carbide, rated to 2,200 °C.
A Bond Holds The Grain And Wears Away On Purpose
In a bonded wheel, worn grains must break out so fresh sharp ones are exposed. A bond that is too strong holds dull grains and the wheel glazes and burns the work. A bond that is too weak sheds grains and wears fast. Choose the bond for the work: hard bond for soft materials, soft bond for hard materials.
Surface Speed Sets The Hazard
Rim speed = π × diameter × rotations per second.
Worked example, using the grinding-wheels article’s figures: at 1,500 RPM a 300 mm (0.3 m) wheel makes 1,500 / 60 = 25 revolutions per second. Rim speed = 3.1416 × 0.3 × 25 = 23.6 m/s. A sandstone wheel at 100-200 RPM runs at 3.1416 × 0.3 × (100 / 60) = 1.6 m/s to 3.1416 × 0.3 × (200 / 60) = 3.1 m/s. Burst energy rises with the square of speed, so the faster wheel is far more dangerous, and a rated speed marked by the maker must never be exceeded. A homemade bonded wheel has no rating until it is proof-tested well below its intended speed behind a barrier.
Choose Grit Coarse To Fine
Cut quickly with coarse grit, then step to finer grit, removing the previous scratches at each stage. Lenses applies this from coarse sand through 400-plus grit to a polish with rouge. The same sequence is used to prepare a specimen in Metallography.
Bursting wheels, dust and sparks
A cracked or overspeed wheel bursts. Ring-test it by tapping with a wooden handle: a clear tone is sound, a dull thud means a crack, as described for bricks in Ring Test. Guard the wheel except at the working face, stand to the side at start-up, and wear eye protection. Grinding stone and silica-bearing grit creates dust that scars the lungs, so grind wet where possible. Grinding steel throws sparks, so keep it away from flammables.
Common Mistakes
- Pressing too hard — it burns the work and glazes the wheel.
- Skipping the ring test — a hidden crack is the usual start of a burst.
- Mixing grits on one surface — a single coarse grain scratches a polish.
What’s Next
- Learn next: Cermet — tools tipped with grains too hard for ordinary wheels
- Combine with: Grinding Wheels — building and using a wheel