Abrasives and tools
An abrasive is a very hard material in the form of small grains. Each grain scratches the surface and removes a tiny chip. A tool must be harder than the material it cuts.
- Alumina (Al2O3): hardness about 9. General grinding.
- Silicon carbide (SiC): about 9.5. Sharp and hard, used for ceramics, stone and glass.
- Cubic boron nitride (CBN): very hard, used for hard steels.
- Diamond: hardness 10, the best for ceramics. Used in saw blades, drills and wheels.
Grit size: the grit number tells the grain size. A bigger number means smaller grains. Coarse grit (about 60) cuts fast and leaves a rough surface. Fine grit (1000 or more) cuts slowly and leaves a smooth surface.
Tool forms: bonded wheels (grains held in resin or glass-like bond), coated papers and belts, loose grit in a slurry for lapping, and diamond saws and drills. Water or coolant carries away heat and chips.
Ceramic processing: cutting, grinding, lapping, polishing
Fired ceramic is hard and brittle: it does not bend, it cracks. So shape it as much as you can before firing, when it is soft. After firing, only abrasive or special methods are used.
- Cutting: diamond saw to size.
- Grinding: a wheel removes material fast to reach the size. Coarse grit first, then finer.
- Lapping: a flat plate with loose fine grit makes the surface very flat.
- Polishing: very fine grit gives a mirror-like surface.
Other methods for hard shapes: ultrasonic machining, laser cutting and waterjet cutting.
Care with brittleness: use light pressure, keep it cool with coolant, and avoid knocks. Small scratches can start big cracks, so finishing also makes the part stronger.
Key formulas and definitions
- Time = thickness to remove ÷ removal rate
- Wheel surface speed v = π × D × N ÷ 60 (D in m, N in rpm, v in m/s)
- Number of passes = total removal ÷ removal per pass
- Roughness reduction % = (Ra before − Ra after) ÷ Ra before × 100
- Tool must have hardness greater than the ceramic
Worked examples
1. A wheel removes 0.5 mm per minute. How long to remove 2 mm?
Time = 2 ÷ 0.5 = 4 minutes.
2. A wheel of diameter 0.2 m turns at 3000 rpm. Find its surface speed. (π = 3.14)
v = 3.14 × 0.2 × 3000 ÷ 60 = 31.4 m/s.
3. A block is 10 mm thick and must be ground to 9.2 mm. Each pass removes 0.05 mm. How many passes?
Removal = 10 − 9.2 = 0.8 mm. Passes = 0.8 ÷ 0.05 = 16.
4. Lapping reduces roughness Ra from 2.0 µm to 0.2 µm. Find the percent reduction.
(2.0 − 0.2) ÷ 2.0 × 100 = 90%.
5. A grinding wheel can finish 400 parts. A plant needs 12,000 parts. How many wheels?
12,000 ÷ 400 = 30 wheels.
6. Can a steel tool of hardness about 5 cut alumina ceramic of hardness about 9? What should be used?
No. The tool must be harder than the ceramic. Use silicon carbide (9.5) or diamond (10).
Common mistakes
- Thinking a bigger grit number means bigger grains. It is the opposite: bigger number, finer grains.
- Pushing very hard to cut faster. Brittle ceramic cracks under heavy pressure or shock.
- Using steel tools on fired ceramic. Steel is softer and wears away.
- Machining after firing when the shape could have been made before firing. Shaping the soft piece is cheaper.