§1Rows of tiny teeth
A file is a hardened steel blade covered in many small teeth, each a miniature cutting edge. Drawn across the work, the teeth shave off fine chips — the same wedge-and-chip action as any cutting tool, multiplied over hundreds of edges.
What makes the file special is control, not power: hundreds of tiny edges each take so little that the operator can remove metal a whisker at a time, feeling and steering the cut by hand. That suits the jobs machines struggle with — breaking a sharp edge, fitting one part to another, dressing a weld, finishing a curve, deburring. The same principle, spun rather than stroked, gives the rotary bur (§6) for powered hand work in a die grinder. Both are governed by the same three things this page covers: the tooth pattern (§2), the coarseness (§3), and the hardness that lets the teeth cut at all (§4).
Contents§2Single-cut and double-cut
Files carry their teeth in one of two patterns, and the choice sets whether the file cuts fast or finishes fine (the hero).
A single-cut file has one set of parallel rows of teeth running diagonally across its face; it removes metal more slowly but leaves a smoother surface, and is used for finishing, for sharpening edges, and on harder materials. A double-cut file adds a second set of rows crossing the first, breaking the teeth into a field of individual sharp points; it cuts much faster because each point acts alone, but leaves a rougher surface, so it is the choice for rapid stock removal on softer metals. The rule is simple: double-cut to take metal off quickly, single-cut to finish. Curved-tooth and rasp patterns extend the family for soft materials and bodywork, but the single/double distinction covers the common engineering files.
Contents§3Coarseness grades
Independently of the cut pattern, files are graded by how closely the teeth are spaced — their coarseness — from coarse to fine.
| Grade | Teeth spacing | Use |
|---|---|---|
| Rough / coarse | widely spaced | heavy, fast removal on soft metal |
| Bastard | medium-coarse | general rough work — the usual first file |
| Second-cut | medium | intermediate shaping and truing |
| Smooth | fine | finishing to size and surface |
| Dead-smooth | very fine | final finish |
| The teeth also grow finer, at a given grade, as the file gets shorter, so grade is relative to length. The working sequence mirrors machining — start with a bastard to bring the work close, move to second-cut to true it, and finish with a smooth file to size — the coarse file for speed, the fine file for finish, exactly as roughing precedes finishing on a machine. | ||
§4Harder than the work
A file cuts steel because it is harder than the steel it cuts — hardened to around 64 on the Rockwell C scale, near the top of what steel reaches.
The materials section set out the rule that a cutting tool must be markedly harder than its work, and the file is a pure case: file teeth are hardened to roughly HRC 64, well above the HRC 20–30 of the mild steel and the softer metals a file is used on, so the teeth bite in rather than skate off. The corollary is a real limitation — a file cannot cut anything approaching its own hardness, so hardened steel simply glazes or ruins a file, and such work must go to a grinding wheel (an abrasive harder still) instead. It also means files are brittle: that same high hardness makes them snap if bent or dropped, so a file is never used as a lever or pry bar. Hard enough to cut soft steel, too hard and brittle to survive misuse or to touch hardened work.
Contents§5Filing technique
A file cuts on the forward stroke only, and using it well is a matter of stroke, pressure and keeping the teeth clear.
The teeth point forward, so the file cuts as it is pushed and should be lifted, or at least unloaded, on the return — dragging it back under pressure only dulls the teeth. Cross-filing, pushing across the work for stock removal, is followed by draw-filing, drawing a smooth file sideways along the work, for a fine, straight finish. Pressure is applied on the push and eased on the return. And the teeth must be kept clean: soft metals, especially aluminium and lead, clog the teeth (“pinning”), and the trapped chips then scratch the work, so the file is cleared with a file card (a wire brush) and, for sticky metals, rubbed with chalk to stop the pins forming. Stroke forward, finish by draw-filing, and keep the teeth clear — the whole craft of the file in three habits.
Contents§6Rotary burs
A rotary bur is a small toothed cutter spun at high speed in a die grinder — the powered equivalent of a file for shaping, deburring and blending in awkward places.
Burs run fast — commonly 15 000 to 35 000 rev/min — and because they are small, even those high speeds give a modest rim speed. A 6 mm carbide bur at 25 000 rev/min has a rim speed of π × 6 × 25 000 ÷ 60 000 = 7.85 m/s; a smaller 3 mm bur must spin faster to cut well, reaching about 4.71 m/s even at 30 000 rev/min. The lesson from the same N = 1000 V/(π D) relationship used throughout machining is that small tools need very high spindle speeds to reach a useful cutting speed — which is why die grinders spin so fast. Carbide burs cut steel and cast iron; high-speed-steel burs suit softer metals; and the right speed keeps the bur cutting cleanly rather than rubbing and glazing.
§7Quick reference
The working core of the page on one card rack.
Cut pattern
single-cut → finish
double-cut → fast removal
Coarseness
bastard → second-cut → smooth
Hardness
teeth ~HRC 64
can't cut hardened steel
Technique
cuts on push · draw-file to finish
clear pinning with a card
Burs
15 000–35 000 rev/min
6 mm @ 25 000 → 7.85 m/s