Grinding and finishing processes

In short

Grinding is abrasive machining: thousands of hard, randomly shaped abrasive grains held in a bonded wheel each take a very small chip, so the process gives close tolerance (IT5-IT6, commonly ±0.005 mm) and fine surface finish (Ra 0.1-1.6 µm) on materials too hard to turn or mill. Finishing processes - honing, lapping, superfinishing, polishing, burnishing and mass finishing - take over below that, correcting geometry or roughness with little or no stock removal. Together they are the last and costliest operations on a part, so they decide whether it is accepted or scrapped. Examined heavily in GATE (ME) under Manufacturing, in university Manufacturing Technology / Machine Tools papers, and asked in interviews at bearing, auto-component and tooling firms (SKF, Schaeffler, Timken, Bosch, Rane, Sundram Fasteners, Bharat Forge, Grindwell Norton, Carborundum Universal, HMT, TAL).

Explained

Abrasive machining and the grinding wheel

• A grinding wheel is a multi-point cutting tool of random geometry: grains have highly negative effective rake, large negative clearance in places, and only a fraction actually cut - the rest plough or rub.
• Three actions occur together: cutting (chip formed), ploughing (material pushed sideways into ridges) and rubbing (pure friction, all energy to heat).
• Bonded abrasive = grains fixed in a bond (grinding, honing); loose abrasive = grains free to roll in a carrier (lapping, polishing, buffing).
• A wheel has three phases: abrasive grains, bond and pores; the pores carry coolant and chips. Volume fractions are set by the "structure" number.
• Conventional abrasives: aluminium oxide Al₂O₃ (Knoop ~2,100) - brown/regular (tough, general steel), white (friable, pure, hardened and heat-sensitive steel), pink/ruby, and seeded-gel / microcrystalline SG (self-sharpening sub-micron crystals, high removal rate); silicon carbide SiC (Knoop ~2,500) - black (cast iron, brass, aluminium) and green (carbides, ceramics).
• Superabrasives: CBN (Knoop ~4,700) for hardened steel, HSS, tool steel and superalloys; diamond (Knoop ~8,000) for tungsten carbide, ceramics, glass, stone, concrete, CFRP. Diamond is not used on ferrous material - carbon diffuses into iron at the grinding temperature and the grain wears chemically.
• Friability (ease of fracture) matters more than hardness: a friable grain self-sharpens by micro-fracture; a tough grain holds its edge but dulls.
• Grit (grain) size is a mesh number - larger number means finer grain: coarse 8-24, medium 30-60, fine 70-180, very fine 220-600 and finer (micron grades for superfinishing).

Wheel specification, selection and the IS marking system

• The standard marking sequence (Bureau of Indian Standards, IS 551 for bonded abrasive products; essentially the same as the ISO/ANSI system - always check the current edition, as BIS numbers and revisions change):

• So A 60 K 5 V 17 = aluminium oxide, medium grain, soft-medium grade, fairly dense structure, vitrified bond. Superabrasive wheels add concentration and abrasive depth of the rim.
• Grade is not grain hardness - it is the strength with which the bond holds the grain, i.e. how readily a dull grain is released.
• Bonds: *vitrified* (clay fired to glass, ~75% of wheels, rigid, porous, coolant-resistant, speed limit typically ~33 m/s, up to 45-60 m/s for high-speed grades); *resinoid* (tough, used to 45-80 m/s, cut-off wheels, roll grinding); *reinforced resinoid* (fibreglass mesh, portable cut-off and snagging); *rubber* (thin, flexible, centreless regulating wheels, polishing); *shellac* (cool, fine finish, roll and cam grinding); *silicate* (mild action, large wheels); *metal and electroplated* (single-layer diamond/CBN, high speed); *oxychloride* (disc grinding).
• Selection rules (examinable):
  • Al₂O₃ for high tensile strength materials (steels, wrought iron); SiC for low tensile strength / brittle materials (grey cast iron, non-ferrous, ceramics, carbide).
  • Hard work → soft grade wheel (dull grains must break free); soft work → hard grade wheel.
  • Hard, brittle work → fine grain; soft, ductile work → coarse grain (chip clearance).
  • Large area of contact → coarser grain and softer grade; small contact area → finer grain, harder grade.
  • Higher wheel speed lets a softer grade be used; higher work speed makes the wheel act softer, so pick a harder grade.
  • Wet grinding permits one grade harder than dry grinding.
  • Open structure for soft/gummy metals, deep forms and creep feed; dense structure for brittle material and fine finish or form holding.

Mechanics, forces, power and the G-ratio

• Chip formation: only grains projecting far enough engage; the chip starts as rubbing, then ploughing, then cutting once the grain depth exceeds a threshold. Chips are tiny and often leave as sparks (burnt iron).
• Size effect: because undeformed chip thickness is ~1-10 µm, the specific energy u of grinding is 20-60 J/mm³ against 2-5 J/mm³ for turning the same steel - grinding is energy-intensive per unit volume and u falls as chip thickness rises.
• Force ratio: normal force is 1.5-3 times tangential force (typically Fn ≈ 2 Ft) because of the negative rake and rubbing; this normal force causes deflection of slender work and of the wheel spindle.
• Nearly all the energy becomes heat concentrated in a tiny contact arc, and most of it goes into the workpiece (opposite to turning, where the chip carries it away) - hence thermal damage risk.
• Grinding ratio G = volume of work removed / volume of wheel worn. Typical G: 10-60 for vitrified Al₂O₃ on steel; several hundred to several thousand for CBN or diamond. High G means low wheel cost but often higher force, power and burn risk.
• Three wheel-wear mechanisms: *attritious (grain dulling)* by rubbing and chemical attack; *grain fracture* (good - self-sharpening); *bond fracture* (grain pull-out, keeps wheel cutting but loses form).
• Creep feed grinding: very low table speed (10-100 mm/min) with full depth (1-10 mm) in one pass; long contact arc, soft open wheel, flood coolant at high pressure; used for fir-tree slots in turbine blades, form tools. HEDG / peel grinding uses high wheel speeds (>100 m/s) with CBN for very high removal rates.

Grinding operations and machines

• Surface grinding: (a) horizontal spindle + reciprocating table (periphery cuts, line contact, best accuracy and finish); (b) horizontal spindle + rotary table; (c) vertical spindle + reciprocating table; (d) vertical spindle + rotary table (wheel face cuts, large area contact, highest removal, poorer finish). Work held on a magnetic chuck, vice or fixture.
• Cylindrical grinding (external): work between centres or in a chuck, rotating against the wheel. Traverse grinding (work traverses axially, small infeed per pass) for long parts; plunge grinding (wheel wider than the ground length, radial infeed only) for shoulders, grooves, crank pins. Dressing the wheel to a form gives form grinding.
• Internal grinding: chucking type (work rotates), planetary (work fixed, wheel orbits - large or odd-shaped work), or internal centreless; wheel is small, so very high rpm is needed and wheel wear is fast.
• Centreless grinding: work rests on a work-rest blade between a fast grinding wheel and a slow regulating (feed) wheel, with the work centre above the line of wheel centres (typically 7-15°) so it is not round-ground into a triangle. Variants: through-feed (regulating wheel inclined 0.5-8° to pull the bar through - plain cylindrical parts, pins, rollers, bars), in-feed (headed or shouldered parts), end-feed (tapers). Very high output, no centre holes needed, but no concentricity control relative to a bore or existing feature.
• Tool and cutter grinder: universal machine for sharpening milling cutters, reamers, drills; cup and dish wheels with tooth rests.
• Others: thread grinding (single-rib or multi-rib wheel, hardened lead screws, taps, gauges), gear grinding (form wheel or generating - Reishauer-type threaded worm wheel for hardened gears), double-disc grinding (both faces at once - washers, vanes), abrasive belt grinding (coated abrasive, high removal, cool cutting), snagging (rough weld/casting dressing with portable or swing-frame grinders), cut-off / abrasive sawing, bench and pedestal grinders (off-hand tool sharpening).

Wheel wear, dressing, coolant, thermal damage and safety

• Loading: work material clogs the pores (soft, gummy metals - aluminium, copper, mild steel); glazing: grains dull and the surface becomes shiny and smooth (wheel too hard or speed too high). Both raise force, power and heat, and cause burn and chatter.
• Truing restores concentricity and the required form/profile; dressing sharpens and opens the surface by fracturing and removing dull grains and clogged bond. In practice the same tool often does both.
• Dressing tools: single-point diamond, cluster/blade diamond, diamond form roll (rotary, for CNC production and crush-form profiles), star-wheel (Huntington) dresser for off-hand wheels, abrasive dressing sticks for superabrasives. ELID (electrolytic in-process dressing) keeps metal-bonded fine diamond wheels sharp for mirror finishes on ceramics.
• Dressing parameters control finish: fine dress lead (small axial feed per wheel revolution) and shallow depth → smooth wheel → fine finish, lower removal rate; coarse dress → free cutting, rough finish.
• Balance and mounting: blotters (paper washers) under flanges, flange diameter at least one-third of wheel diameter, correct torque, and dynamic balancing on precision machines - unbalance causes chatter marks and wheel burst risk.
• Grinding fluids: soluble-oil emulsion or synthetic for general grinding; neat oil for thread, gear and form grinding; functions are cooling, lubrication, chip flushing and corrosion protection. Delivery matters more than quantity - a nozzle must break the air barrier carried by the wheel, so high-pressure coherent jets, scrubber nozzles and through-the-wheel delivery are used. Filtration (magnetic separator, paper band, centrifuge) and fluid-management hygiene (microbial control, mist extraction) are part of the job.
• Thermal damage (surface integrity):
  • Oxidation / temper burn - visible temper colours, loss of hardness, softened layer.
  • Rehardening burn - a hard brittle untempered-martensite white layer with soft tempered zone beneath.
  • Residual tensile stress and grinding cracks (often perpendicular to the grinding direction) - these cut fatigue life badly in bearings, gears and crankshafts.
  • Detection: nital etch, Barkhausen noise, microhardness traverse, X-ray diffraction residual stress.
  • Remedies: lower depth of cut, lower wheel speed or softer/open wheel, frequent dressing, more and better directed coolant, CBN (higher conductivity, sharper for longer).
• Safety and statutory practice: never exceed the maximum operating speed marked on the wheel (and check it after the wheel wears, since rpm may have been raised); perform a ring test on vitrified wheels before mounting (a tap with a non-metallic tool should give a clear ring - a dull thud means cracks); run a new wheel at speed for about a minute behind the guard before grinding; keep wheel guards in place; on off-hand grinders keep the work-rest gap ~1-2 mm and the tongue guard ~6 mm from the wheel and limit the exposed arc. In India this is enforced under the Factories Act 1948 (fencing and safe use of machinery) and the state Factories Rules, which carry a schedule on abrasive wheels; the Act is being subsumed into the OSH and Working Conditions Code, 2020, whose provisions have been notified in stages - check the current position. Personal protective equipment (eye/face protection), dust extraction for dry grinding (silica and metal dust) and noise control are mandatory. Wheels are also certified against IS/ISO safety requirements for bonded abrasive products.

Finishing processes: honing, lapping, superfinishing, polishing, burnishing, mass finishing

• Honing: bonded abrasive stones in an expanding mandrel (hone head) rotate and reciprocate inside a bore, giving a cross-hatch pattern (typical included angle 20-60°, often ~45° for engine bores) that holds oil. Stock removed 0.01-0.2 mm; corrects taper, waviness and small roundness error but not bore location; Ra 0.1-0.4 µm (plateau honing finer). Used on IC engine cylinder liners, hydraulic cylinders, gear bores, gun barrels.
• Lapping: loose abrasive (Al₂O₃, SiC, B₄C or diamond paste) in oil or grease between the work and a cast-iron lap charged with grains; low pressure (10-200 kPa), low speed; three-body abrasion with rolling grains. Gives flatness and finish rather than dimensional change: Ra 0.012-0.1 µm, flatness to a fraction of a light band. Used for gauge blocks, valve seats, seal faces, optical flats, silicon wafers (CMP is its chemical-mechanical relative).
• Superfinishing (microhoning): fine bonded stone pressed at low pressure with a short, rapid oscillation (1-5 mm, 200-1,500 cycles/min) against rotating work; removes the amorphous/ damaged surface layer, Ra 0.01-0.1 µm, high bearing area. Standard on bearing races, pins, crankshaft journals, camshafts.
• Polishing uses abrasive bonded to a flexible wheel or belt to remove scratches; buffing uses loose fine abrasive in a wax compound on a soft cloth wheel - polishing levels, buffing brightens (no appreciable stock removal).
• Burnishing (roller/ball burnishing, ballizing): no chips - a hard roller or ball plastically smooths the peaks, giving Ra below 0.1 µm plus work hardening and compressive residual stress that improves fatigue life. Related surface treatments: shot peening, laser peening.
• Mass (bulk) finishing: barrel tumbling, vibratory finishing, centrifugal disc, drag finishing with ceramic/plastic media and compounds - deburring, edge radiusing and brightening of many small parts at once; cheap per part, limited control of geometry.
• Advanced / nano-finishing (syllabus-level awareness): abrasive flow machining (AFM), magnetic abrasive finishing (MAF), magnetorheological finishing (MRF), elastic emission machining, electrochemical honing - used for internal passages, dies and optics where a tool cannot reach.

Key points

• Abrasive machining - cutting with many hard grains of random geometry and highly negative rake; the only practical way to machine hardened steel, ceramics and carbide to close tolerance.
• Wheel marking - abrasive, grain size, grade, structure, bond (IS 551 order), e.g. A 60 K 5 V.
• Grade - strength of the bond's grip on the grain (soft A-H, medium I-P, hard Q-Z), not grain hardness.
• Structure - grain spacing/porosity: 1-8 dense, 12+ open; open structures clear chips and cut cool.
• Al₂O₃ for high tensile strength materials, SiC for brittle/low tensile strength ones; CBN for hardened ferrous, diamond for carbide, ceramics and non-ferrous - never diamond on steel.
• Hard work → soft wheel; soft work → hard wheel - the single most-asked selection rule.
• Specific energy 20-60 J/mm³ - far higher than turning because of the size effect, so grinding is heat-dominated.
• Most grinding heat enters the workpiece, unlike turning where the chip carries it off - hence burn, cracks and tensile residual stress.
• G-ratio = work volume removed / wheel volume lost; high for superabrasives, low for conventional wheels.
• Loading = pores clogged by work material; glazing = grains dulled and smooth - both cured by dressing and correct wheel choice.
• Truing restores shape and runout; dressing restores sharpness; dressing lead and depth control the finish achieved.
• Centreless grinding - work above the centre line, inclined regulating wheel feeds it through; highest output for plain cylindrical parts, but no concentricity datum.
• Creep feed grinding - full depth at very low table speed with an open soft wheel and heavy coolant; used for turbine blade forms.
• Honing gives a cross-hatched bore and corrects form; lapping gives flatness and the finest finish with loose abrasive; superfinishing removes the damaged layer with a short-stroke stone.
• Burnishing improves finish by plastic flow and adds compressive residual stress - no chips.
• Ring test, speed marking, flanges and blotters, guards - the four non-negotiable wheel safety checks under the Factories Act / state Factories Rules.
• Typical capability - rough grinding Ra ~0.8-1.6 µm, fine grinding 0.2-0.4 µm, honing 0.1-0.4 µm, superfinishing/lapping 0.01-0.1 µm (indicative, machine- and material-dependent).

Formulas and examples

1. Wheel speed and worn-wheel safety. A 300 mm vitrified wheel runs at 1,800 rpm.

• V_s = π × 0.300 × 1,800/60 = π × 0.300 × 30 = 28.3 m/s - safe for a standard vitrified wheel (~33 m/s limit).
• Worn to 250 mm at the same rpm: V_s = π × 0.250 × 30 = 23.6 m/s - the wheel now acts softer (each grain takes a deeper chip and is torn out sooner), so finish worsens and the wheel wears and loses form faster, though burn risk falls.
• To restore 28.3 m/s: N = 60 × 28.274/(π × 0.250) = 1,696.5/0.7854 = 2,160 rpm - permissible only if the wheel's marked maximum speed in m/s is not exceeded.

2. Surface grinding: MRR, power, force, chip thickness. Horizontal-spindle surface grinding of hardened steel: wheel D_s = 300 mm at V_s = 30 m/s, width of cut b = 12 mm, depth d = 0.025 mm, table speed v_w = 15 m/min.

• v_w = 15,000/60 = 250 mm/s → MRR = 12 × 0.025 × 250 = 75 mm³/s.
• With specific energy u = 35 J/mm³: P = 35 × 75 = 2,625 W ≈ 2.6 kW (motor rating must allow for spindle and coolant losses, so ~4 kW).
• F_t = P/V_s = 2,625/30 = 87.5 N; F_n ≈ 2 F_t = 175 N - this is what deflects thin work and causes taper.
• Grain depth of cut with C = 2 grains/mm², r = 15, D_e = D_s = 300 mm:

(4 × 0.25/30)/(2 × 15) = 0.033333/30 = 1.1111 × 10⁻³ mm²; √(0.025/300) = 9.129 × 10⁻³; t_max = √(1.1111 × 10⁻³ × 9.129 × 10⁻³) = √(1.014 × 10⁻⁵) = 3.2 µm.

• Same settings in external cylindrical grinding of a 50 mm shaft: D_e = (300 × 50)/(50 + 300) = 42.9 mm, √(0.025/42.9) = 0.02414, so t_max = √(1.1111 × 10⁻³ × 0.02414) = 5.2 µm - thicker chips, rougher surface, lower specific energy than surface grinding.

3. G-ratio and wheel cost per part. A 250 mm × 25 mm wide vitrified wheel removes 20,000 mm³ of hardened steel while its diameter falls from 250.00 mm to 249.90 mm.

• Wheel volume lost = (π/4)(250² − 249.9²) × 25 = 0.7854 × (62,500 − 62,450.01) × 25 = 0.7854 × 49.99 × 25 = 981.7 mm³.
• G = 20,000/981.7 = 20.4 - typical for Al₂O₃ on hardened steel.
• Usable wheel volume from 250 mm down to a 200 mm discard diameter = 0.7854 × (62,500 − 40,000) × 25 = 441,787 mm³. At ₹2,500 per wheel that is ₹0.00566 per mm³, so wheel cost for this part = 981.7 × 0.00566 ≈ ₹5.6 (indicative prices only). A CBN wheel with G ≈ 1,000 would consume 20 mm³ per part - the reason its high first cost is often repaid in volume production.

4. Centreless through-feed output. Regulating wheel D_r = 300 mm, N_r = 30 rpm, inclination α = 4°, bars 100 mm long fed end to end.

• Work peripheral (rotation) speed = π × 0.300 × 30 × cos 4° = 28.274 × 0.99756 = 28.2 m/min.
• Axial feed = π × 0.300 × 30 × sin 4° = 28.274 × 0.06976 = 1.97 m/min = 32.9 mm/s.
• Output = 1,972/100 = 19.7 pieces/min ≈ 1,183 pieces/hour (before allowances) - shows why centreless grinding is used for pins, rollers and shafts in bearing and fastener plants.

5. Honing cross-hatch angle. An 80 mm bore is honed at 300 rpm with a reciprocating speed of 20 m/min.

• Peripheral speed = π × 0.080 × 300 = 75.4 m/min.
• tan(θ/2) = 20/75.4 = 0.2653 → θ/2 = 14.9° → θ ≈ 30°.
• For the 45° cross-hatch often specified for engine liners: tan 22.5° = 0.4142 → required reciprocating speed = 0.4142 × 75.4 = 31.2 m/min (raise stroke speed or reduce rpm).

Where it is used

• Bearing manufacture (SKF, Schaeffler, Timken, NRB, NBC Jaipur): race grinding, centreless roller grinding and superfinishing to sub-micron roundness and Ra - the classic high-volume grinding industry in India.
• Auto components and powertrain (Bosch, Rane, Sundram Fasteners, Bharat Forge, Mahindra, TVS group): crankshaft and camshaft grinding, valve-seat and injector lapping, cylinder-liner honing, gear grinding after case hardening.
• Tool rooms, die and mould shops, and HMT/TAL-type machine builders: surface and cylindrical grinding of jigs, dies, punches and slideways; tool and cutter grinding; thread and form grinding of gauges and lead screws.
• Abrasives and machine-tool industry (Grindwell Norton, Carborundum Universal, Wendt India, Micromatic/ Praga, Bhagwansons): wheel manufacture and selection support, dressing systems, grinder design and retrofits.
• Aerospace and power (HAL, BHEL, GTRE suppliers): creep-feed grinding of turbine blade root forms, CBN grinding of superalloys, lapping of valve and seal faces.
• Job roles: production/manufacturing engineer owning grinding cycle time and burn-free process windows; process engineer selecting wheel specification and dressing parameters; quality engineer judging surface integrity and roughness; maintenance/safety officer for abrasive-wheel compliance; applications engineer with an abrasive supplier.

Interview questions

• Q: Why is the specific energy in grinding so much higher than in turning? A: The undeformed chip thickness is only a few microns, so a large share of energy goes into rubbing, ploughing and the size effect in material strength rather than into chip formation.
• Q: Which wheel for grinding hardened tool steel - soft or hard grade, and why? A: A soft grade, so dull grains break out and expose fresh cutting edges; a hard wheel would glaze, heat the work and burn it.
• Q: Why is a diamond wheel never used on steel? A: At grinding temperatures carbon from the diamond diffuses into the iron, so the grain wears chemically and the wheel fails quickly; CBN is used instead.
• Q: What does A 46 L 6 V mean? A: Aluminium oxide abrasive, 46 mesh (medium) grain, L grade (medium-soft), structure 6 (fairly dense), vitrified bond.
• Q: Difference between truing and dressing? A: Truing restores the wheel's concentricity and profile; dressing removes dull and loaded grains to make it cut freely - the same diamond tool often does both.
• Q: What are loading and glazing, and how do you cure them? A: Loading is clogging of the pores with work material (soft, gummy metals) and glazing is dulling of the grains to a shiny surface; cure by dressing, an open-structure or softer wheel, more or better-directed coolant, and correcting speeds.
• Q: Why is the work held above the line of the wheel centres in centreless grinding? A: Offsetting the work above the centre line prevents lobing - at the centre line the high spots are reproduced and the bar ends up out of round.
• Q: What is the advantage of creep feed grinding? A: Full-depth, very slow feed with a soft open wheel gives high removal rate on deep forms with lower peak temperature and long wheel form life - used for turbine blade slots.
• Q: Why does grinding most of the heat go into the workpiece, and what damage follows? A: Chips are minute and the contact arc small, so the chips carry little heat; the result can be temper or rehardening burn, tensile residual stress and grinding cracks that cut fatigue life.
• Q: When would you hone, and when would you lap? A: Hone to finish and correct the form of a bore (bonded stones, cross-hatch, measurable stock removal); lap to achieve flatness and the finest finish on flat or mating surfaces with loose abrasive and almost no size change.
• Q: What is the G-ratio, and does a very high G-ratio always mean a better process? A: It is work volume removed per unit of wheel volume lost; a very high value can also mean the wheel is too hard and not self-sharpening, which raises force, power and burn risk.
• Q: How do you check a grinding wheel before mounting it? A: Inspect for cracks and chips, perform the ring test on vitrified wheels, confirm the marked maximum speed against the spindle speed, use blotters and flanges of at least one-third the wheel diameter, then run it up behind the guard for about a minute.

Common mistakes

• Saying grade means the hardness of the abrasive grain - it means the strength of the bond's hold on the grain.
• Inverting the selection rule: students regularly answer "hard material, hard wheel". It is hard material, soft grade wheel.
• Using the wheel diameter as D_e in cylindrical grinding chip-thickness problems instead of the equivalent diameter D_e = D_s D_w/(D_w ± D_s).
• Unit slips: table speed given in m/min used directly with mm depth and width, or wheel speed in m/min used where m/s is needed - MRR and power then come out 60 times wrong.
• Treating grinding as a cold process and ignoring thermal damage, then explaining a fatigue failure or a cracked race as a "material problem".
• Grinding on after the wheel glazes or loads instead of dressing, and dressing too coarsely when a fine finish is required (dressing lead and depth set the achievable Ra).
• Confusing honing, lapping and superfinishing, and claiming any of them can correct bore position or large geometric error - they refine the existing geometry.
• In the lab or shop: exceeding the marked wheel speed, grinding on the side of a straight wheel, removing the guard, or leaving a wide work-rest gap - each is both a safety violation and a statutory one.

Related topics

Theory of metal cutting · Machining and machine tools · Non-traditional machining · Metrology and quality control · Jigs and fixtures · Engineering materials and heat treatment