Grinding

Grinding removes material with abrasive grains for high accuracy and fine finish. Learn wheel specification, surface/cylindrical/centreless grinding, wheel speed, specific energy, G-ratio, dressing and grinding burn.

When a part must be accurate to a few micrometres or hardened beyond what cutting tools can handle — bearing races, gear teeth, shafts, cutting tools — it is ground.

A grinding wheel has millions of tiny, randomly oriented cutting edges (abrasive grains) held in a bond.

An analogy

Sandpaper on a spinning wheel: each grain takes a microscopic scratch, and millions of scratches produce a smooth, precise surface.

Wheel specification

A wheel is specified by abrasive (A = aluminium oxide for steel, C = silicon carbide for cast iron/non-ferrous, CBN for hardened steel, diamond for carbide/ceramics), grit size (coarse 24 … fine 600), grade (hardness of bond: soft E … hard Z), structure (porosity) and bond (vitrified, resin, metal).

Processes

Surface grinding (flat faces), cylindrical grinding (between centres), internal grinding (bores), centreless grinding (high-volume pins and shafts without centres), creep-feed grinding (deep cuts, slow feed — turbine blade roots).

Dressing and truing

Grains dull and pores clog (loading). Dressing with a diamond tool re-sharpens the wheel; truing restores its geometry. Correct dressing is essential for finish and to avoid burn.

Energy and thermal damage

Grinding needs 10–100× more specific energy than turning because of small chips, negative rake grains, rubbing and ploughing. Most energy becomes heat in the workpiece surface — risk of grinding burn, softening, rehardening and tensile residual stresses (cracks). Coolant, sharp wheels, CBN and controlled feeds prevent it; Barkhausen noise testing detects burn.

Formulas

• Wheel speed: v_s = π D N / 60 000 [m/s, D in mm]
• Surface-grinding MRR: MRR = a_e · b · v_w
• Grinding power: P = u · MRR
• G-ratio: G = V_work removed / V_wheel worn

Worked example: Wheel speed

Given: Wheel Ø200 mm; Target v_s = 30 m/s

Find: Spindle speed.

• N = 30 × 60/(π × 0.2) = 2865 rpm

Answer: ≈ 2865 rpm (never exceed the wheel's max rated rpm)

Worked example: Surface grinding power

Given: a_e = 0.02 mm, b = 20 mm, v_w = 10 m/min; u = 40 J/mm³

Find: MRR and power.

• MRR = 0.02 × 20 × 10 000 = 4000 mm³/min = 66.7 mm³/s
• P = 40 × 66.7 = 2.67 kW

Answer: 4 cm³/min, ≈ 2.7 kW

Common mistakes

• Wrong: Taking deep cuts with a dull, loaded wheel. — Right: Dress regularly and keep depth small in finishing to avoid burn.
• Wrong: Exceeding the wheel's maximum rated speed. — Right: Always check the wheel label; bursting wheels are lethal.

Applications

• Bearings: Races and rollers ground and superfinished to sub-micron roundness.
• Automotive: Crankshaft and camshaft journal grinding, gear-tooth grinding for quiet EV gearboxes.
• Tooling: Carbide drills and end mills ground with diamond wheels.

Key takeaways

• Grinding = precision + finish on hard materials.
• Specify abrasive, grit, grade, structure, bond.
• P = u × MRR — watch for burn.
• Dress wheels; respect maximum speed.

Going further

• Superfinishing & honing: Stone oscillation produces plateau-honed cylinder bores and mirror bearing finishes.
• ELID grinding: Electrolytic in-process dressing of metal-bonded wheels for nano-finishes.
• Grinding temperature models: Moving heat source theory predicts surface temperature from energy partition to the workpiece.