Grinding: wheel specification and grinding mechanics

Why it matters

Grinding is the standard way to finish hardened parts — bearing races, shafts, gauges, cutting tools — to micrometre tolerances and fine finishes. Picking the right wheel from its specification code, and understanding why grinding needs so much energy per unit volume, prevents burnt, cracked or out-of-size parts.

Key ideas

A grinding wheel is a bonded body of hard abrasive grains with pores between them. Each grain is a tiny cutting tool with a random, usually strongly negative rake. Pores carry coolant in and chips out.

The five wheel parameters

1. Abrasive: aluminium oxide (A) — tough, for steels and ferrous metals; silicon carbide (C) — harder but more friable, for cast iron, non-ferrous metals, ceramics and carbides; cubic boron nitride (CBN, B) — superabrasive for hardened steels and superalloys; diamond (D) — superabrasive for carbides, glass, ceramics and stone, but not for steel (carbon diffuses into iron at grinding temperatures).
2. Grain size: mesh number; coarse ≈ 8–24, medium ≈ 30–60, fine ≈ 70–180, very fine ≈ 220–600. Coarse for high stock removal and soft, ductile work; fine for finish and hard, brittle work.
3. Grade (hardness): A (soft) to Z (hard) — how strongly the bond holds the grains, not the hardness of the abrasive.
4. Structure: 1 (dense) to about 15 (open) — grain spacing. Open structures for soft, ductile materials and large contact areas (more chip space); dense for hard materials and fine finish.
5. Bond: vitrified (V, most common, rigid, porous), resinoid (B, high speed, cut-off wheels), rubber (R, thin wheels, centreless regulating wheels), shellac (E, fine finish), silicate (S), metal (M, for diamond/CBN).

Standard marking order: (prefix) abrasive – grain size – grade – structure – bond – (manufacturer's record), e.g. 51 A 46 K 5 V 23 = aluminium oxide, size 46, grade K (medium), structure 5, vitrified bond.

Selection rule: hard work → softer grade (blunt grains must break out to expose fresh ones); soft work → harder grade. Large contact area → softer, more open wheel.

Wheel conditions. Glazing — worn, flat grains not released (wheel too hard or too slow work speed); the wheel rubs and burns the work. Loading — pores clogged with metal (soft, ductile work, dense wheel). Dressing removes the dull layer and opens the pores; truing restores the wheel's geometric form and concentricity. Both are done with a diamond dresser. Wheels must be balanced and never run above the rated speed.

Grinding operations. Surface (reciprocating or rotary table), cylindrical (between centres: traverse and plunge), internal, centreless (through-feed and in-feed, with a regulating wheel), creep-feed (deep cut, very slow feed) and tool-and-cutter grinding.

Mechanics. Grit chips are tiny (micrometres thick), grains have large negative rakes and much energy goes into ploughing and rubbing. So specific energy is very high — often 10–60 J/mm³ for steels, against 2–5 J/mm³ for turning (take values from data). Most of the heat enters the work, risking burn, tempering of hardened layers, residual tensile stress and cracks; ample coolant and a free-cutting wheel are essential.

Formulas

• vs = π·D·N / 60 000 (wheel speed, m/s; D in mm, N rev/min). Conventional wheels run about 20–35 m/s.
• MRR = vw·b·d (surface grinding, mm³/s; vw work speed in mm/s; b width of cut, d depth (infeed), mm)
• lc = √(d·D) (length of wheel–work contact, mm)
• t_max = √[ (4·vw) / (vs·C·r) · √(d/D) ] (maximum grit chip thickness, mm; C = active grits per mm² of wheel surface; r = chip width/thickness ratio, typically 10–20)
• P = u·MRR, Ft = P / vs (power W with u in J/mm³ and MRR in mm³/s; tangential force N)
• G = volume of work removed / volume of wheel worn (grinding ratio)

Worked examples

Example 1 (standard). A steel plate is surface ground with a 250 mm wheel at 2400 rev/min. Work speed vw = 15 m/min (0.25 m/s), width of cut b = 20 mm, depth d = 0.02 mm. Specific energy u = 30 J/mm³ (given). Find the wheel speed, MRR, power and tangential force.

1. vs = π × 250 × 2400/60 000 = 31.4 m/s.
2. MRR = vw·b·d = 250 mm/s × 20 × 0.02 = 100 mm³/s.
3. P = u·MRR = 30 × 100 = 3000 W.
4. Ft = P/vs = 3000/31.42 = 95.5 N. Compare: turning steel at 3 J/mm³ would remove the same 100 mm³/s with only 300 W — grinding uses about ten times the energy per unit volume.

Example 2 (GATE level). For the same set-up, take C = 2 grits/mm² and r = 15. Find the contact length and the maximum grit chip thickness. What happens to t_max if the work speed is doubled?

1. lc = √(d·D) = √(0.02 × 250) = √5 = 2.24 mm.
2. vw/vs = 0.25/31.42 = 0.007958; √(d/D) = √(0.02/250) = 0.008944.
3. t_max = √[4 × 0.007958/(2 × 15) × 0.008944] = √(9.49×10⁻⁶) = 3.08×10⁻³ mm = 3.08 µm.
4. Doubling vw multiplies t_max by √2 → 4.36 µm. Thicker grit chips raise the force per grit, so the wheel "acts softer" (grains break out sooner) — a practical way to make a glazing wheel cut freely.

Common mistakes

• Thinking "grade" means abrasive hardness; it is the bond's grip on the grains.
• Choosing a hard wheel for hard work — it glazes; hard work needs a soft wheel.
• Choosing diamond for steel; use CBN or aluminium oxide.
• Mixing up dressing (sharpening) and truing (correcting form).
• Using mm/min for vw with m/s for vs in the chip-thickness formula — the ratio must be dimensionless.
• Ignoring burn: high specific energy plus heat into the work causes tempering and tensile residual stress.

For GATE PI

• Decoding a wheel specification and choosing a wheel for a given material.
• NAT on wheel speed, MRR, power, tangential force and maximum grit chip thickness; effect of vw, vs, d, D on t_max.
• MCQs on glazing, loading, dressing and truing, centreless grinding, and abrasive–material matching.

Quick check

1. In A 60 J 7 V, what do J and 7 mean?
2. Which abrasive suits grinding cemented carbide tools?
3. Wheel 300 mm at 1910 rev/min: surface speed?
4. If wheel speed vs is increased, does t_max rise or fall?
5. What causes glazing?

Answers: 1. J = grade (medium-soft), 7 = structure (medium). 2. Diamond (or green silicon carbide). 3. π × 300 × 1910/60 000 ≈ 30.0 m/s. 4. It falls (t_max ∝ 1/√vs), so the wheel acts harder. 5. A wheel too hard for the work (or too slow a work speed), so dull grains are not released.