Micro CNC Basics Explain the Limits of Tiny Machining

• 1
  What micro CNC removes, and at what scale
• 2
  How a micro CNC machine keeps a Ø0.2 mm tool alive
• 3
  Workholding and zero point on small parts
• 4
  Which materials suit micro milling
• 5
  Where micro CNC stops making sense
• 6
  Micro CNC compared with other micro processes

Micro CNC uses the same subtractive logic as any machining center: a rotating cutter follows a programmed path and shears material away. The difference is scale. Where a general shop runs a Ø10 mm end mill, micro work runs Ø0.1 mm to Ø1 mm cutters, and the chip load per tooth drops to a few microns.

That change forces everything else to change. Spindle speed climbs into the 20,000–60,000 rpm range so the cutting edge keeps enough surface speed to shear metal instead of rubbing it. Feed rates fall to 20–200 mm/min. A single pass may remove 5–20 μm of radial depth.

The part itself is often smaller than a coin. Typical features sit between 10 μm and 1 mm: slots narrower than a human hair, walls 30–80 μm thick, holes Ø0.1–0.5 mm, and surface finishes down to Ra 0.2–0.8 μm on the fine end. These numbers are where the process earns its place, and also where it becomes fragile.

Dimensions are measured in microns, not thousandths of an inch, though both appear on drawings. GreatLight holds ±0.005 mm (±0.0002 in) on qualified micro work, with 100% inspection before shipment.

A Ø0.2 mm carbide end mill is roughly the stiffness of a sewing needle. Push it 0.01 mm too hard and it snaps or deflects past the tolerance band. The machine has to control position, spindle runout and thermal drift at a level a standard VMC cannot reach.

Spindle runout is the first limit. If the tool holder has 3 μm of runout, one flute does all the cutting and wears twice as fast. Micro spindles are balanced and measured so total indicated runout stays under about 1–2 μm at the tool tip. Tool holders are shrink-fit or high-precision collets, not general-purpose chucks.

The second limit is the minimum chip. Every material has a thickness below which the edge cannot cleanly shear the workpiece. For aluminum that threshold is around 1–2 μm per tooth; for stainless and titanium it is higher. Program the feed below it and the tool rubs, work-hardens the surface, and dulls in minutes.

The third limit is thermal. A small tool in a small cut generates little heat, but the part is also small, so a few watts raise its temperature fast. Coolant delivery, often through-spindle air or oil mist, matters more than flood coolant on micro features.

A micro part is usually cut from a larger blank and then released. Holding it directly is difficult, so the first operation machines the part profile and the second operation separates it. Soft jaws, vacuum chucks and fixture plates with machined pockets keep the blank rigid without crushing thin walls.

Zero point matters as much as the cutter. On a 0.4 mm wide slot, a 0.05 mm error in the work offset is a visible shift. Probes and tool setters are used to re-establish the datum after every tool change, and the offset is verified against a test cut before the finishing pass.

Thin walls deflect. A 50 μm wall will bend under normal clamping force, so support material or sacrificial webs are left in place until the final light passes. This is why micro parts often look like they are held by a frame in the drawing but arrive loose in the box.

Burrs behave differently at this scale too. A burr the size of a grain of sand is a functional defect on a microfluidic channel. Deburring is done with fine abrasive flow, electropolishing or careful hand work under magnification, not with a standard scraper.

Aluminum is the easiest starting point. Grades 6061, 6061-T6, 2024, 5083 and 7075 all machine well at micro scale, hold a sharp edge and tolerate high spindle speeds. Most micro prototype work in electronics and robotics starts here because the material is forgiving and the chips clear easily.

Copper and brass are common in microfluidics, RF housings and connectors. C101, C110 and C36000 cut cleanly and give good surface finish, though copper is gummy and needs sharp tooling and generous feed to avoid smearing. Beryllium copper machines well but requires dust control.

Stainless 303, 304, 316L and 17-4PH are used for medical and food-contact parts. They work-hard-enable quickly at micro depths, so feeds must stay above the minimum chip and tools are changed before they dull. Titanium TC4 (Ti-6Al-4V) and Inconel are possible but slow, with heavy tool wear and higher cost per part.

Plastics such as POM, PEEK, PMMA and PC machine fast but move with temperature. A 0.1 mm feature in PEEK can grow 10 μm from a 20 °C temperature swing, so roughing and finishing are often separated by a cool-down.

Micro machining is not the answer for every small feature. When the geometry is a deep, narrow channel or an array of identical micro features, other processes win. Photochemical etching, laser cutting and EDM can produce shapes a rotating cutter cannot reach, and they do it without tool deflection.

Aspect ratio is the clearest limit. A Ø0.2 mm end mill cutting 1.0 mm deep is already at 5:1. Push past roughly 6:1 and the tool bends, the wall tapers, and the bottom of the slot is undersize. If the design needs a 10:1 slot, micro milling is the wrong process.

Hardness matters too. Above roughly 45 HRC, carbide micro tools wear quickly and the cost per part climbs. Hardened tool steel or ceramics are usually ground or EDM'd instead. GreatLight machines tool steel and 17-4PH in the annealed or solution-treated state, then the part is heat treated afterward.

Volume is the last filter. For one prototype, micro milling gives full material choice and fast turnaround. For 100,000 identical micro features, stamping, injection molding or etching will beat it on unit cost. The crossover usually sits somewhere in the low thousands of parts.