§1The abrasive family
Grinding is the best-known abrasive process, but it heads a family that runs from heavy stock grinding down to the finest lapping — all cutting with hard, small, many-edged abrasive rather than a defined tool.
What unites them is the abrasive grain: a hard particle that scratches away a tiny chip, used in vast numbers. What separates them is fineness and purpose — grinding removes stock and holds tolerance; honing corrects and finishes bores; lapping and superfinishing chase the last micron of size and the smoothest surface. This page covers the tool of grinding first — the wheel, and the five-symbol code that describes it (§2–4) — then the finer processes that carry on where grinding leaves off (§5–6). Together they take a surface from as-machined to mirror-finished, each process picking up at the finish the one before it can reach.
Contents§2The wheel marking system
A grinding wheel’s character is captured in a standard five-part code — abrasive, grit size, grade, structure, bond — so that a wheel can be specified and reordered exactly, like the four-digit code of a steel.
Take the marking A60K5V (the hero). The letter A is the abrasive — aluminium oxide. The number 60 is the grit size — a medium grain. The letter K is the grade, or hardness of the bond — here medium-soft. The number 5 is the structure — how open the spacing of the grains. And the final letter V is the bond — vitrified. Read in order, the five symbols tell you everything about how the wheel will behave: a medium-grit, medium-soft, vitrified aluminium-oxide wheel — a general-purpose steel-grinding wheel. Just as an engineer reads 1045 as a medium-carbon steel, a grinder reads A60K5V as a wheel’s full specification, and orders another identical one by that code.
§3Abrasive types
The first symbol — the abrasive — is chosen to suit the material being ground, and four abrasives cover almost everything.
| Abrasive | Symbol | Grinds |
|---|---|---|
| Aluminium oxide | A | steels and steel alloys — the general abrasive |
| Silicon carbide | C | cast iron, non-ferrous metals, carbide, ceramics |
| Cubic boron nitride (CBN) | B | hardened steels and superalloys |
| Diamond | D | cemented carbide, ceramics, glass |
| The rule of thumb is that the abrasive must be harder than the work and chemically suited to it: aluminium oxide for steels, silicon carbide for the harder-but-more-brittle job of cast iron and carbide, and the superabrasives — CBN for hardened ferrous metals, diamond for carbide and ceramics — where nothing softer will cut. Diamond is not used on plain steel, since at grinding temperatures carbon reacts with iron; CBN takes that role. Matching abrasive to material is the first grinding decision, and the wheel code’s first symbol records it. | ||
§4Grade, structure and bond
The remaining symbols — grit size, grade, structure, bond — tune how the wheel cuts, wears and finishes.
Grit size is the grain fineness, a mesh number: coarse grits (say 24) remove stock fast and leave a rough finish, fine grits (120 and up) remove little but finish smoothly — the direct trade of removal against finish, chosen like feed in turning. Grade is how firmly the bond grips the grains — a “hard” wheel holds grains longer (good for soft work, but glazes if too hard), a “soft” wheel releases dulled grains readily to expose fresh ones (good for hard work, but wears fast); it is the lever over the grinding ratio of the previous page. Structure is the spacing of the grains — open structures give chip room for soft, gummy materials and heavy cuts, dense structures suit fine finishing. Bond is what cements the grains — vitrified (glass-like, most common, rigid and porous), resinoid (tougher, for cut-off and rough work), and others. Together these four dial the wheel from a coarse, free-cutting, stock-removal wheel to a fine, hard, finishing one.
Contents§5Honing and lapping
Where grinding leaves off, honing and lapping take a surface finer still — two low-speed abrasive processes for the closest sizes and smoothest surfaces.
Honing uses bonded abrasive stones, carried in an expanding tool rotated and stroked in a bore, to correct and finish it: it removes the slight inaccuracy and roughness a grind or ream leaves, producing a precise, round, straight bore with the characteristic cross-hatch pattern of an engine cylinder. It is a low-speed, large-contact-area process — the opposite of grinding’s fast, small contact — which is why it corrects form so well. Lapping uses loose abrasive, in a paste or slurry, rubbed between the work and a soft lap (often cast iron) to remove the very last microns and produce an extremely flat, smooth surface — the finish of gauge blocks, sealing faces and optical flats. Both are slow, gentle, tiny-removal processes for accuracy and finish beyond grinding, honing chiefly for bores and form, lapping for flatness and the ultimate smoothness.
Contents§6Superfinishing and the finish ladder
The abrasive processes form a ladder of ever-finer surface finish, each picking up where the last leaves off — from grinding down to a mirror.
Roughly, turning and milling leave a surface around a micron or two of roughness; grinding takes it to about 0.4 µm Ra; honing to about 0.2 µm; and lapping and superfinishing to 0.05 µm and below — a mirror. Superfinishing, like honing, uses a fine bonded stone but with a light, oscillating action that removes the last peaks of a ground surface, leaving the smooth, load-bearing finish wanted on bearing journals and races. Each step down the ladder removes less metal, takes longer and costs more, so a surface is taken only as far as it needs to go — most parts stop at turning or grinding, and only those that must seal, slide or roll with minimal friction earn the honing, lapping or superfinishing above. The ladder is the map: choose the process by the finish the part actually requires, no finer.
Contents§7Quick reference
The working core of the page on one card rack.
Wheel code
abrasive·grit·grade·structure·bond
A60K5V
Abrasive
A steel · C iron/carbide
B hardened · D carbide/ceramic
Grade
hard holds grains · soft releases
sets the G-ratio
Finer processes
honing (bores) · lapping (flats)
Finish ladder
grind 0.4 → hone 0.2 → lap 0.05 µm