Finishing processes: honing, lapping, superfinishing

Why it matters

Engine cylinder bores, hydraulic cylinders, bearing races, gauge blocks and seal faces need finishes and form accuracy that grinding alone cannot give reliably. Honing, lapping and superfinishing remove only a few micrometres, but they set the final size, roundness, flatness and surface texture — and with it friction, leakage, wear and fatigue life.

Key ideas

All three are low-stress abrasive finishing processes: low speed and low pressure compared with grinding, very small stock removal, and little heat. They follow a previous operation (boring, turning, grinding) and correct its small errors and surface damage. They cannot correct large errors of position.

Honing

• A honing head carries several bonded abrasive sticks (stones) that are expanded radially against a bore, usually hydraulically or mechanically, under controlled pressure.
• The head rotates and reciprocates at the same time, so each grain traces a helix. Paths from up and down strokes cross, giving the characteristic cross-hatch pattern, which holds oil films in engine cylinders.
• The tool usually floats (is not rigidly located), so honing corrects roundness, taper, bell-mouth and waviness of the bore and its size, but not the position or straightness of the bore axis set by boring.
• Stock allowance is typically a few hundredths of a millimetre on diameter; finishes of roughly Ra 0.1–0.8 µm are common. Abrasives: aluminium oxide or silicon carbide sticks, and diamond or CBN for hard materials. Ample fluid (kerosene-based oils) flushes the swarf.
• The stroke should overrun each end of the bore by about a quarter to a third of the stone length; too little overrun leaves a barrel-shaped bore, too much gives bell-mouth.

Lapping

• Loose abrasive grains in a carrier (oil, paste or water) are trapped between the work and a softer lap (cast iron, copper, brass). The grains roll and are partly embedded in the lap, so they abrade the harder work in a random multi-directional pattern.
• Produces the best flatness and finish of the three (finishes down to a few hundredths of a micrometre; flatness to a fraction of a micrometre) — used for gauge blocks, valve seats, seal faces, optical flats and fuel-injector parts.
• The lap must be softer than the work so grains embed in the lap, not in the workpiece. Soft, ductile work is hard to lap because grains embed in it.
• Material removal follows Preston's law: removal rate is proportional to pressure and relative sliding velocity.

Superfinishing

• A fine-grit bonded stone is pressed lightly against a rotating (usually cylindrical) workpiece and oscillated axially with a short stroke at high frequency, with a viscous lubricant.
• As the surface becomes smoother, an oil film builds up between the stone and the peaks and the cutting action stops by itself; the process removes only the amorphous, damaged surface layer and the peaks left by grinding.
• Used on bearing races, rolls, crankshaft journals and shafts running in seals; it improves bearing ratio, wear and fatigue life. It does not correct size much.

Comparison — honing: bonded stones, internal bores, size plus form correction, cross-hatch. Lapping: loose abrasive, flat or cylindrical, finest finish and flatness. Superfinishing: bonded stone, short-stroke oscillation, self-limiting, mainly external cylinders.

Formulas

• vr = π·D·N / 1000 — rotary (peripheral) speed of the honing head, m/min; D bore diameter in mm, N rev/min.
• v = √(vr² + vo²) — resultant honing speed, m/min; vo = axial reciprocating speed, m/min.
• θ = 2·tan⁻¹(vo / vr) — included cross-hatch angle, degrees (each helix makes tan⁻¹(vo/vr) with the circumferential direction).
• V = (π/4)·(D2² − D1²)·L — volume removed when a bore of length L is honed from D1 to D2 (mm³).
• t = V / MRR — honing time, s, when the volumetric removal rate MRR (mm³/s) is known from trials.
• dh/dt = Kp·p·v — Preston's law for lapping and polishing: dh/dt removal rate (m/s), p contact pressure (Pa), v relative sliding velocity (m/s), Kp Preston coefficient (1/Pa), found experimentally for a given abrasive, lap and work.

These are kinematic and empirical relations; removal rates and Kp must come from trials or a data book.

Worked examples

Example 1 (standard) — honing kinematics and time. A cylinder bore of 80 mm diameter and 120 mm length is honed. Head speed N = 150 rev/min, reciprocating speed vo = 15 m/min. The bore is enlarged from 79.95 mm to 80.00 mm, and trials give MRR = 10 mm³/s. Find the resultant speed, the cross-hatch angle and the honing time.

1. vr = π·D·N/1000 = π × 80 × 150/1000 = 37.70 m/min.
2. v = √(vr² + vo²) = √(37.70² + 15²) = 40.57 m/min.
3. θ = 2·tan⁻¹(vo/vr) = 2 × tan⁻¹(15/37.70) = 2 × 21.70° = 43.4°.
4. V = (π/4)·(80.00² − 79.95²)·120 = 753.7 mm³.
5. t = V/MRR = 753.7/10 = 75.4 s.

Example 2 (GATE level) — lapping by Preston's law. A hardened steel seal face is lapped at contact pressure p = 20 kPa and mean sliding velocity v = 0.5 m/s. The Preston coefficient for this lap and abrasive is Kp = 1.0 × 10⁻¹² Pa⁻¹ (given from trials). (a) How long does it take to remove 5 µm? (b) If the pressure is raised to 30 kPa and the speed lowered to 0.4 m/s, how long does it take to remove 3 µm with Kp = 1.5 × 10⁻¹² Pa⁻¹?

1. (a) dh/dt = Kp·p·v = 1.0 × 10⁻¹² × 20 000 × 0.5 = 1.0 × 10⁻⁸ m/s = 0.010 µm/s.
2. t = h/(dh/dt) = 5 × 10⁻⁶/1.0 × 10⁻⁸ = 500 s (8.3 min).
3. (b) dh/dt = 1.5 × 10⁻¹² × 30 000 × 0.4 = 1.8 × 10⁻⁸ m/s.
4. t = 3 × 10⁻⁶/1.8 × 10⁻⁸ = 166.7 s = 2.78 min. Note the units: Kp (1/Pa) × p (Pa) × v (m/s) gives m/s.

Common mistakes

• Expecting honing to correct the location or straightness of a bore axis — a floating hone follows the existing axis; only boring fixes position.
• Quoting the cross-hatch angle as tan⁻¹(vo/vr): that is the helix angle of one path; the included angle between crossing paths is twice that.
• Mixing m/min and mm/min, or diameter and radius, when computing vr and stock volume.
• Using a lap harder than the work: grains then embed in the workpiece and scratch it.
• Thinking superfinishing removes large stock — it is self-limiting and removes only a few micrometres.
• Treating lapping and honing as interchangeable: honing uses bonded sticks; lapping uses loose abrasive.

For GATE PI

• MCQs matching process to application (cross-hatch in bores, gauge blocks, bearing races) and to abrasive form (bonded stick vs loose grain).
• Statements on what each process can and cannot correct (size, form, position).
• Short numericals on honing speed, cross-hatch angle and stock volume, and on Preston's law for lapping time.
• Practise comparing achievable surface finish and stock removal across grinding, honing, lapping and superfinishing.

Quick check

1. Which process gives the cross-hatch pattern in an engine bore?
2. Can a floating hone correct a bore whose axis is out of position?
3. Bore 100 mm, head speed 120 rev/min, vo = 20 m/min: find the included cross-hatch angle.
4. In lapping, should the lap be harder or softer than the work?
5. Why does superfinishing stop removing material by itself?

Answers: 1. Honing. 2. No — it follows the existing axis. 3. vr = 37.70 m/min; θ = 2 × tan⁻¹(20/37.70) ≈ 55.9°. 4. Softer, so grains embed in the lap. 5. As the surface smooths, an oil film forms between stone and work and lifts the stone off the peaks.