Micro CNC Machining: Feature Limits, Minimum Walls and Process Choice

A 0.4 mm wide slot, 3 mm deep, in a 6 mm titanium plate. Two shops decline to quote and the third comes back at $64 per part for a quantity of 20. The slot is not exotic geometry. It is a straight cut. What makes it hard is that the cutter needed to make it is 0.3 mm in diameter, the depth-to-diameter ratio is 10:1, and at that scale every source of error in a machine shop — runout, deflection, thermal growth, chip evacuation — gets multiplied.

Micro machining is not simply ordinary machining scaled down. The physics change. Tools break if runout exceeds a couple of microns. Chips are the same size as the coolant film. The force needed to clamp the part can be larger than the force needed to cut it. Understanding those shifts tells you what is possible, what it costs, and when a different process is a better answer.

Why the physics change at small scale

A 10 mm end mill has a core diameter of about 6 mm and can absorb a mistake. A 0.3 mm end mill has a core of about 0.18 mm — thinner than a human hair — and its stiffness scales with the fourth power of diameter. Halve the tool diameter and it becomes sixteen times less rigid.

At the same time, the tool cannot simply be spun faster to compensate. Small tools need high spindle speed, typically 40,000 to 60,000 rpm, to keep the chip load in a sensible range. At those speeds, spindle runout of 3 µm becomes a significant fraction of the feed per tooth, and one flute does most of the cutting while the others rub. Rubbing generates heat, heat softens the edge, and the tool fails.

Three consequences follow. Tool life is measured in metres of cut rather than hours. Programming has to use constant-engagement toolpaths with light radial depths, typically 10 to 20% of diameter. And the machine itself needs to be thermally stable, because a 1°C change over a 20 mm part moves dimensions by about 0.0002 mm in steel — small, but not negligible when the tolerance is ±0.005 mm.

Minimum feature size by process

No single process covers the whole micro range. The right choice depends on the feature type, the material and the quantity.

The practical lesson from this table is that micro milling owns three-dimensional geometry, and everything else owns a narrower slice. If your part is flat and 0.1 mm thick with a thousand small holes, etching will beat milling on both price and edge quality. If it is a 4 mm cube with a 0.3 mm pocket in the middle, only milling or sinker EDM will do it.

Walls, floors and aspect ratio

Minimum wall thickness is usually quoted as 0.2 mm for aluminum and 0.3 mm for stainless and titanium. Those numbers assume the wall is supported at the base and is not the full height of the part. A 0.3 mm wall that is 8 mm tall will chatter during finishing and may distort when the part is released from the stock.

The rule that governs most micro features is aspect ratio: depth divided by tool diameter. For a 0.3 mm cutter in aluminum, a 3:1 ratio is comfortable, 5:1 needs a reduced feed and a pecking strategy, and beyond 8:1 the tool will break or wander regardless of technique. In titanium and stainless, halve those numbers.

Clamping and workholding are half the cost

At this scale, workholding is often the limiting factor rather than the cutting. A 10 mm part held in a vice will deform under jaw pressure, and the deformation is comparable to the feature tolerance. Shops use vacuum chucks, wax mounting, adhesive film or a sacrificial support plate that is machined away at the end.

Deburring is the other hidden cost. A 0.1 mm burr on a 0.3 mm feature cannot be removed by hand without destroying the edge. Options are electropolishing, abrasive flow, thermal deburr or a controlled brush pass under magnification. Each is a separate operation, and each has a scrap rate.

Inspection is a design constraint

You cannot specify a tolerance you cannot measure. Micro parts run into this constantly.

A touch-trigger CMM with a 1 mm stylus cannot reliably enter a 0.8 mm slot. A 0.3 mm stylus can, but it deflects more and needs a delicate touch. Vision systems measure edges to about 2 to 5 µm but only in two dimensions, and they struggle with burrs and reflections on machined aluminum.

The accepted rule is that the measurement uncertainty should be no more than one tenth of the tolerance. For a ±0.005 mm callout, that means an uncertainty of 0.0005 mm, which is at the edge of what a good vision system or a precision CMM in a temperature-controlled room can do.

Practically, this means two things for your drawing. First, do not call ±0.005 mm on a feature that has no functional reason for it, because the inspection cost will exceed the machining cost. Second, if a feature is critical, tell the shop how it will be verified so they can plan the process around the measurement rather than discovering the problem at final inspection.

Choosing between micro processes

Start with the geometry. If the part is flat and the features are through-cuts, etching or laser will usually be cheaper and faster than milling, with no burrs and no tool wear. Etching has a further advantage: cost is driven by sheet area rather than feature count, so a part with 5,000 holes costs about the same as one with 50.

If the part is rotational, Swiss turning is the right baseline, and it can hold ±0.003 mm on a 2 mm diameter with a ground bar. Micro milling a round pin is possible but wasteful.

If the part needs pockets, steps, angled faces or mixed features on multiple faces, micro milling is the only route, and you should expect to pay for it. Typical rates for micro work run 50 to 100% above conventional machining, because cycle times are long, tools are consumable and scrap risk is high.

Frequently asked questions

What is the smallest feature micro CNC machining can produce?

With a 0.1 mm cutter, a 0.1 mm slot is achievable in aluminum, and 0.15 mm in steel, provided the depth is under about 0.5 mm. Smaller than that, the tool cannot survive the cutting forces and EDM or etching becomes the practical route.

How thin a wall can be machined?

0.2 mm in aluminum and 0.3 mm in stainless are routine when the wall is short and supported. Walls taller than about 5 mm need a support strategy, and walls under 0.15 mm are usually a different process.

Why is micro machining so expensive per part?

Cycle times are long because feeds and depths are small, tool life is short, and scrap risk is high. A part that takes 40 minutes on a micro mill may weigh two grams. Setup and inspection can also exceed the machining time.

Can micro machined parts be anodized or passivated?

Yes, but the finish thickness matters at this scale. A 10 µm anodized layer on a 0.2 mm wall is 5% of the wall thickness, and it grows in both directions. Specify the tolerance after coating and expect masking on features where growth would close a gap.