Micro Milling in Practice: Tool Diameter, ISO 21920 Callouts, and the Decision to Switch

Micro milling is CNC milling with cutters at or below 1 mm diameter, cutting features a standard end mill cannot reach without breaking or deflecting past tolerance. The switch pays off when a feature drops below 0.5 mm, a 2 mm end mill leaves witness marks the drawing does not allow, or the Ra callout tightens below Ra 0.4 µm on a face too small for grinding. None of those thresholds comes from an ISO standard. ISO 21920-1:2021 and ISO 21920-2:2021 govern how the drawing expresses surface texture, not what a machine can deliver. The sections below cover when to switch, the Ra callout syntax ISO 21920-1 accepts, and where micro EDM or micro laser takes the job instead.

1. When conventional CNC milling stops working

Three signals push a part off the conventional CNC mill and onto a micro-milling setup. None of them is the part count on the PO. All three surface during quoting or during the first cut, and the pattern repeats across medical, microfluidic, and micro-connector jobs.

1.1 Feature size dropping below what a 2 mm end mill can reach

A 2 mm end mill leaves an internal corner radius of about 1 mm. Once the drawing calls for an internal radius under 1 mm, the cutter has to shrink. Under R 0.5 mm, the cutter drops to 1 mm diameter or below. Under R 0.25 mm, the cutter sits at 0.5 mm diameter or smaller, and the operation is micro milling by any working definition.

Two features force the decision on most micro parts machining work we quote at EPOC CRAFTER: microfluidic chip channels at 200 to 400 µm width, and thread reliefs on bone screws at 0.2 to 0.4 mm. Neither can be roughed on a 2 mm cutter without losing the geometry the drawing needs.

Shank diameter is not cutter diameter. A 0.5 mm cutter often sits on a 3 mm shank with a step-down, so reach and stickout drive whether the tool survives the cut. That distinction shows up in DFM review, which the CNC machining capabilities page covers in more detail.

1.2 Deflection and runout dominating over programmed feed

On a 0.5 mm end mill, tool deflection and spindle runout dominate the cutting geometry once feed exceeds a few micrometers per tooth. The programmed feed from CAM stops matching the chip thickness the tool sees. Spindle runout at 2 µm on a 25,000 RPM spindle becomes the variable that decides finish, because one flute is doing most of the cutting while the other loafs.

The signal on the shop floor is a finish worse than the CAM simulation. If the toolpath shows Ra 0.3 µm and the profilometer reads Ra 0.9 µm on the first article, the cause is almost always deflection plus runout, not the surface texture callout.

Micro-milling spindles run 40,000 to 120,000 RPM with runout at or below 1 µm. Feed per tooth drops to 1 to 3 µm. Those values come from spindle datasheets and shop practice, not from ISO 21920.

1.3 Ra callout below Ra 0.4 µm on a non-grindable surface

Ra 0.4 µm is about as tight as a well-tuned 3-axis milling operation reaches on 6061-T6 aluminum with a sharp carbide cutter at moderate feed. Push the callout below Ra 0.4 µm and one of three routes has to change: the surface goes to grinding (flat or cylindrical only), it goes to lapping (flat only), or the operation moves to a micro-milling setup with fine-grain carbide tooling and step-downs at 5 to 10 µm.

A sealing face on a titanium bone screw, 3 mm by 4 mm at Ra 0.2 µm, is the standard trigger for the third route. Too small for a grinding wheel to enter. Wrong geometry for lapping. Micro milling or micro EDM covers it, and the Ra callout syntax on the drawing decides how the shop plans the pass. Practical Ra ranges each CNC process holds are laid out in the tolerance and standards guide.

2. What micro milling means on the shop floor

The industry does not have one boundary between conventional milling and micro milling. What shops call micro milling, or micro CNC machining more broadly, sits inside three overlapping ranges. The definition matters at quoting time because tool life, spindle choice, and inspection method all shift once a job crosses into it.

Cutter diameter: 1 mm and below. The number comes from tooling catalogs and machining literature, not from ISO. Cutters go down to 0.1 mm on standard catalog offerings and to 0.05 mm on specialty grades.

Feature size: 0.5 mm and smaller. A slot 300 µm wide, a rib 200 µm thick, or a hole 400 µm in diameter all fall in this range. The line is soft. A 0.6 mm feature machined on a 0.4 mm micro milling cutter still uses micro-milling practice.

Chip thickness per tooth: 1 to 5 µm. The value sits at or below the cutting edge radius on most micro cutters, which is where the size effect kicks in. Below the edge radius, cutting turns into plowing, cutting forces rise sharply, and Ra worsens instead of improving with slower feed. Academic literature on the micro milling process documents the effect and the profilometer confirms it.

The three ranges do not always align. A 0.3 mm feature can come off a 1.2 mm cutter if the shop programs a small step-down and accepts a longer cycle. The shop makes the call on cutter, spindle, and inspection method together.

3. Reading ISO 21920-1:2021 correctly on a micro-milled drawing

ISO 21920-1:2021 replaced ISO 1302:2002 for surface texture indication on technical drawings. Every micro-milled part quoted in 2026 should be running the new callout syntax, and most drawings we receive at EPOC CRAFTER still are not. The three points below cover the calls that trip up buyers and design engineers most.

3.1 Ra 0,4 means upper limit, not nominal

Under ISO 21920-1:2021 Clause 7.5, a specification value written by itself carries an upper limit tolerance type by default. Ra 0,4 on a drawing means the arithmetic mean height, evaluated per ISO 21920-2:2021, must not exceed 0.4 µm. It does not mean “aim for 0.4”. It does not mean “within some window around 0.4”. The upper limit is the acceptance rule.

The comma is not a typo. ISO drawings use a comma as the decimal separator: Ra 0,4 and Ra 0.4 are the same value.

The default acceptance rule under ISO 21920-1:2021 is Tmax. Every measurement across the surface must satisfy the callout. This is a change from ISO 1302:2002, which used a 16 % rule allowing a fraction of measurements to exceed the value. If your drawing still carries the older rule, the inspection sheet needs to say so.

3.2 Ra [0,1; 0,4] is the correct bilateral callout syntax

When a surface needs both a floor and a ceiling on roughness, ISO 21920-1:2021 Clause 9.6 accepts the interval syntax: parameter symbol, opening bracket, lower value, semicolon, upper value, closing bracket. Ra [0,1; 0,4] means the measured Ra must sit at or above 0.1 µm and at or below 0.4 µm.

The bilateral callout matters on functional surfaces where too smooth causes a different failure than too rough. Common cases at EPOC CRAFTER:

• Bone screw thread flanks. Too rough, and the screw galls in the tap. Too smooth, and the screw backs out under cyclic load because the surface does not grip bone.
• Bearing seat inner diameters. Too rough, and the outer race scores during press-fit. Too smooth, and the retained oil film breaks down under start-stop cycles.
• Adhesive bond surfaces on device housings. Too rough, and the bond line pulls off at the adhesive. Too smooth, and the bond fails at the substrate.

Drawings that write “Ra 0.1 to 0.4” in freehand text carry the intent but not the standard-compliant call. On regulated medical or aerospace work, the callout syntax matters as much as the value.

3.3 What ISO 1302:2002 drawings still in your files need updated

ISO 1302:2002 is withdrawn but still lives in a large share of active drawings, especially on parts released before 2022 and never revised. Migration to ISO 21920-1:2021 is not a symbol swap.

What carries over: the Ra parameter itself, the value in micrometers, the general position on the surface texture symbol, and the material removal requirement bar over the tick.

What does not carry over: the default acceptance rule (16 % rule under ISO 1302:2002 becomes Tmax), and the machining allowance callout. ISO 21920-1:2021 no longer defines machining allowance on the surface texture symbol. Drawings that show a machining allowance value should move that dimension to the drawing body under GD&T.

For any drawing older than 2022 still shipping to production, add a general note on the acceptance rule expected and update the symbol block on the next revision. The same standard-year migration is playing out on general geometric tolerance callouts, which we cover in how ISO 2768 Edition 2 changed the CNC drawing default.

4. What ISO 21920-2:2021 changed about Ra, Rp, Rv, Rz

ISO 21920-2:2021 replaced ISO 4287:1997 for surface texture parameter definitions. The parameter symbols look identical on a drawing. The math behind them shifted. Anyone reading a micro-milled inspection report against a pre-2022 material data sheet needs to know where the definitions moved.

4.1 Ra and Rq stayed on the evaluation length

Ra is the arithmetic mean of the absolute ordinate values on the R-profile, evaluated over the evaluation length le. Rq is the root-mean-square of the same ordinate values on the same length. Both definitions carried over from ISO 4287 without a functional change.

The R-profile is the roughness profile after the S-filter and L-filter separate it from waviness. What ISO 21920-2:2021 renamed is the filter language. The old cutoff λc is now the nesting index. The old sampling length lr is now the section length lsc. The physical operation is the same. The vocabulary on the inspection report is different.

For a Ra 0.4 µm callout, the default nesting index is 0.8 mm, and the evaluation length runs five section lengths (le = 4 mm).

4.2 Rp, Rv, Rz moved to the section length

The shift most likely to catch an experienced reader sits here. Under ISO 4287:1997, Rp was the maximum peak height of the entire evaluation length. Under ISO 21920-2:2021, Rp is the mean of the maximum peak heights, one per section length, averaged across the evaluation length. Rv and Rz follow the same rule.

The consequence at the profilometer: two surfaces with identical old-Rp values can report different new-Rp values, because averaging five section peaks smooths over a single tall spike. The single tallest peak on the whole evaluation length is now called Rpt, not Rp. On a micro-milled surface where one deflection event might produce a single 1.5 µm spike inside an otherwise Rp 0.3 µm surface, the old Rp read 1.5 and the new Rp reads closer to 0.5.

For inspection acceptance on medical parts, name the parameter version explicitly on the drawing. First-article reports at EPOC CRAFTER cite the standard year against every non-Ra parameter to keep this from surfacing as a rejection later.

5. Micro milling vs micro EDM vs micro laser

Within micromachining, three processes cover most micro-feature work. Each wins on a different feature type, and the drawing makes the call clearer than the material does. Values in the table are typical operating ranges from process literature and shop practice, not ISO-defined capability limits.

Feature or requirement	Micro milling	Micro EDM	Micro laser
Smallest cutter / electrode / spot	0.05 mm cutter	0.02 mm electrode	0.010 mm spot
Internal sharp corner radius	Limited by cutter (~0.025 mm min)	Sharp, near-zero	Sharp, near-zero
Aspect ratio (depth to width)	3:1 typical, 5:1 on stiff cutters	10:1 and beyond on wire and sinker	10:1 on ablation, lower on through-cutting
Surface texture reachable	Ra 0.1 to 0.4 µm on 6061-T6 and 316L	Ra 0.2 to 0.8 µm with recast layer	Ra 0.5 to 2 µm with heat-affected zone
Heat-affected zone	None. Cold cutting	Recast layer 2 to 10 µm, must be removed for medical	Heat-affected zone 5 to 50 µm depending on pulse
Material restriction	Any machinable material	Conductive materials only	Most materials, absorbs at specific wavelength
Setup time	Long. Fixture, probe, tool change	Long. Electrode manufacture is its own job	Short. Fixture and focus, then cut
Cost per part at 100 pieces	Medium	High	Low on 2D, high on 3D
Where it wins	Prismatic pockets, closed channels, sealing faces, threaded bosses	Sharp inside corners, deep narrow slots, hardened tool steel	Thin-walled cuts, high-throughput profiles, marking

Reading the table by feature type gives the decision:

• Prismatic pocket with a sealing floor: micro milling. The floor takes the Ra callout, and the process leaves no recast to remove.
• Sharp internal corner deeper than 3 mm at R 0.05 mm: micro EDM. The cutter that could reach it does not survive.
• Through-cut profile in 0.1 mm stainless foil at 500 parts per hour: fiber laser. The heat-affected zone at the edge is acceptable, and throughput dominates.
• Threaded 0.3 mm bore in Ti-6Al-4V: micro milling. Neither EDM nor laser cuts threads.
• Blind hole 0.05 mm diameter, 0.4 mm deep in hardened 440C: micro EDM. Cutter breaks. Laser cannot reach the aspect ratio.

The recast layer on micro EDM matters more than the callout suggests. On implantable medical parts, it needs a secondary etch or micro-polish, and the cost of that step pushes many marginal EDM jobs back to milling. On tool-steel dies, EDM wins on cost. Material decides half the call, feature the other half.

6. Medical part examples where the drawing decides the process

Medical devices generate more medical micro machining work than any other category we quote at EPOC CRAFTER. Three part families show up repeatedly, and in each one the drawing callouts, not the material choice, drive whether micro milling is the right process for medical device micromachining.

Bone screw thread geometry. A 2 mm outer diameter cortical screw in Ti-6Al-4V ELI carries a thread pitch around 0.5 mm and a thread relief at 0.2 to 0.3 mm. The relief is where the callout gets tight. Ra on the flank sits at Ra 0.4 µm upper limit or a bilateral Ra [0,2; 0,6] depending on the coating spec. Rolling can form the thread on stock stainless screws. On titanium implants with a variable thread lead, micro milling on a Swiss-type lathe with live tooling takes the job.

Microfluidic chip channels. A diagnostic chip in PMMA or PEEK carries channels 100 to 400 µm wide, 50 to 200 µm deep, with a floor Ra callout at Ra 0.2 µm upper limit because any surface texture scatters the optical read signal. Micro milling with a 0.2 mm two-flute cutter at 100,000 RPM covers it. Laser ablation reaches the width but leaves an oxidized wall that fails the optical spec. Injection molding takes over above 10,000 chips per year, and the micro-milled part validates the mold geometry first.

Micro-connector sealing faces. A miniature waterproof connector for a wearable device carries an O-ring groove 0.4 mm wide, 0.3 mm deep, machined into a 6061-T6 or 316L housing. The groove floor Ra sits at Ra 0.4 µm upper limit because O-ring compression depends on surface texture, and the wall verticality carries a straightness callout under 5 µm. A 0.3 mm end mill on a rigid setup covers it. The raised sealing face itself takes a tighter Ra 0.2 µm on a separate finishing pass.

Related tolerance ranges on production medical device parts show the inspection methods we apply to each feature class. Similar sealing and groove work on non-medical wearables is covered under micro connectors and sealing faces on consumer electronics. Full material property references for Ti-6Al-4V ELI, 316L, and PEEK back the grade selections above.

7. Cost inputs you can price before quoting

Micro milling does not follow the same per-part cost curve as conventional milling. Five variables drive most of the spread between a workable quote and a rejected one. Review them against the drawing before the RFQ goes out.

• Cutter life per part. A 0.3 mm end mill running at 80,000 RPM in 316L wears measurably within tens of minutes of cut time. A single 0.2 mm carbide micro milling cutter costs 20 to 60 USD at catalog price. On a 50-piece batch that consumes three cutters, tool cost adds 1 to 4 USD per part before setup and cycle time enter the calculation.
• Setup time per Z step. A part with three Z-height planes and features at different scales carries three tool changes minimum. Each tool change plus tool length probe on a micro spindle adds 5 to 15 minutes because probe touch-off runs slower on a small tool to protect the tip.
• Inspection method. A part with a Ra 0.2 µm callout goes to a profilometer at 30 to 90 seconds per surface trace, plus fixture and repositioning. A part with a straightness callout under 5 µm goes to a CMM with a 0.3 mm ruby probe. Inspection time on micro parts can equal or exceed machining cycle time.
• First-article scrap rate. Micro-milled first articles have a higher scrap rate than conventional first articles because setup validates on the first cut, not on a dry run. Building 20 % first-article overhead into the batch quote is standard shop practice.
• Quantity transition point. Below 20 pieces, setup dominates. Above 200 pieces, fixture optimization and pallet loading start to matter more than setup. The setup-payback math is the same curve that decides when a job crosses from prototype work into low-volume production fixturing.

The one variable that does not scale linearly is Ra callout tightness. Moving from Ra 0.8 µm to Ra 0.4 µm on a micro-milled surface roughly doubles cycle time on the finish pass. Moving from Ra 0.4 µm to Ra 0.2 µm doubles it again. A drawing that calls Ra 0.2 µm on every surface, including non-mating ones, quotes at three to four times a drawing that reserves the tight callout for the surfaces that need it. The same tier-by-tier cost curve applies to dimensional tolerances, which we break down in how each precision tier actually prices out on a CNC part.

8. What to send when you RFQ a micro-milled part

An RFQ that comes back fast and accurate from a micro machining services shop carries the information needed to plan tooling and inspection. Missing any of the items below usually triggers a follow-up email that adds 24 to 48 hours to the response.

• STEP file with feature-level dimensions visible on the model tree.
• 2D drawing with GD&T and datums used for inspection.
• Surface texture callouts per ISO 21920-1:2021, with the standard year written on the drawing. If the drawing still carries ISO 1302:2002 callouts, note the acceptance rule expected (Tmax or 16 % rule).
• Material specification with grade and condition: Ti-6Al-4V ELI per ASTM F136, 316L per ASTM F138, PEEK per ASTM F2026, or the regional equivalent.
• Batch quantity and delivery target. A 5-piece first article quotes differently from a 100-piece batch of the same part.
• Inspection level required: first-article only, or in-process CMM plus profilometer per lot.
• Any regulatory context: ISO 13485 documentation, DHF traceability, material lot control.

Sending the DFM check items early shortens the quote loop. The DFM checkpoints we run before quoting cover the standard review sequence. On micro-milled parts, the review adds cutter reach clearance, tool change count, and inspection-fixture buildability. For parts that later scale to a batch, running the first pieces as a rapid prototyping run in production-intent material validates fixture and inspection routing before the batch commits.

The decision to move a part onto a micro-milling setup starts with the drawing, not the machine. Read the surface texture callout against ISO 21920-1:2021, check whether the smallest internal feature can survive a cutter that fits the reach, and price the Ra callout against the finish pass it forces. Send the drawing with the callout syntax the shop can accept without a rewrite, and the quote comes back on the first pass.