§1A different regime
Micromachining is conventional cutting — turning, milling, drilling — but with tools and features measured in tens or hundreds of micrometres. At that scale several things that are negligible in ordinary machining come to dominate.
The reason is that some quantities do not scale down with the tool. A cutting edge can only be ground so sharp — its rounded edge stays a micron or two across whatever the tool’s size — so on a hair-thin cutter that edge radius is no longer negligible against the chip (§2). Cutting speed still needs the same metres per minute, but on a tiny diameter that demands enormous revolutions (§4). And the tool’s stiffness falls with the fourth power of its diameter, so a micro-tool is breathtakingly fragile (§5). Micromachining makes precision micro-parts — medical, optical, electronic — but only by respecting that these once-ignorable effects now rule.
Contents§2The size effect
The size effect is the observation that cutting takes disproportionately more energy per unit volume as the chip gets thinner — small cuts are inefficient, because the blunt edge does more rubbing than shearing.
In ordinary machining the chip is many times thicker than the edge radius, so the edge is effectively sharp and shears cleanly. As the chip thins toward the size of the edge radius, more of the cutting happens against the rounded edge, which ploughs and rubs rather than shears (the hero) — and rubbing wastes energy. So the specific cutting energy of §2 on the power page is not truly constant: it climbs steeply for very thin chips. The practical effect is that micro-cuts generate more heat and force per unit of metal removed than their size suggests, and that there is a lower limit below which the tool stops cutting altogether (§3). The size effect is why you cannot simply scale a normal cut down and expect it to behave.
Contents§3Minimum chip thickness
Below a certain chip thickness the tool ceases to cut and merely ploughs the surface — the minimum chip thickness, set by the edge radius.
The minimum chip thickness is a fraction — very roughly a third — of the edge radius. For a tool with a 2 µm edge radius, that is about 2 × 0.3 = 0.6 µm: feed the tool less than this per edge and it stops making chips, smearing and rubbing the surface instead, which work-hardens it and wears the tool without removing metal. This sets a hard floor under the feed per tooth in micromachining and, with it, a floor under how fine a cut can be taken. It also means the feed per tooth cannot simply be scaled down with the tool — it must stay above the minimum chip thickness the edge radius allows, which is why micro-tools are made with the keenest, smallest-radius edges achievable.
§4Very high spindle speeds
A useful cutting speed on a tiny diameter demands a colossal spindle speed — the single most conspicuous requirement of micromachining.
The same N = 1000 V/(π D) that sets any spindle speed turns brutal at small diameters. To cut at even a modest 50 m/min with a 0.1 mm micro-endmill needs N = 1000 × 50/(π × 0.1) = 159 000 rev/min — far beyond an ordinary machine, whose few thousand rev/min would leave the tiny tool crawling at a fraction of its proper cutting speed and rubbing rather than cutting. This is why micromachining depends on ultra-high-speed spindles — tens to hundreds of thousands of rev/min, often air- or magnetically-borne — and why simply fitting a small tool to a normal machine does not work: without the speed, the cutting speed collapses and the size effect takes over. Small tools need fast spindles, in direct proportion to how small they are.
§5Fragility and deflection
A micro-tool is extraordinarily weak, because bending stiffness falls with the fourth power of diameter — the property that most limits how micromachining is done.
Halve a tool’s diameter and it becomes sixteen times less stiff, since stiffness scales as the fourth power of diameter; a 0.1 mm tool compared with a 10 mm one is on the order of a hundred million times less stiff. Two consequences follow. The tool deflects under even tiny cutting forces, so it wanders off the intended path and the cut loses accuracy unless the force is kept minute. And it breaks at the smallest overload — a momentary chip jam, a hard spot, a careless feed — with no warning, because there is no reserve of strength. Micromachining therefore runs at very light forces, tiny depths and feeds, scrupulous chip clearance and often no cutting fluid pressure that could deflect the tool, all to keep the fragile edge intact. The whole method is shaped by the tool’s weakness.
Contents§6Making it work
Successful micromachining is a matter of matching the machine and the method to the scale — speed, stability and cleanliness above all.
It takes an ultra-high-speed spindle to reach cutting speed on tiny diameters (§4), running with very low runout, since a micron of runout is a large fraction of the tool’s size. It takes a stiff, precise, vibration-isolated machine, because deflection and chatter that are trivial at normal scale are ruinous here. It takes feeds kept above the minimum chip thickness yet light enough not to snap the tool (§3, §5), and reliable chip clearance so swarf cannot jam the tiny flutes. And it takes keen-edged tools — the smallest edge radius achievable, in fine-grain carbide or diamond — to hold the size effect at bay. Given all that, micromachining makes features no other cutting process can; without it, the tiny tool simply rubs, wanders and breaks.
Contents§7Quick reference
The working core of the page on one card rack.
Regime
edge radius not negligible
once-ignored effects dominate
Size effect
thin chips → more energy
ploughing not shearing
Min chip
~⅓ of edge radius
2 µm edge → ~0.6 µm
Speed
0.1 mm @ 50 m/min → 159 000 rpm
Fragility
stiffness ∝ d⁴
light forces, no reserve