Micro and Precision Machining: Machines, Tooling and Proof
Micro and precision parts need a machine built for small tools and thermal stability, tooling with tiny runout, and metrology that can resolve the tolerance. When the tolerance is a few micrometers, the measurement plan matters as much as the machine.
Two different problems: small features and tight tolerances
Micro machining means small features, often cut with tools under about 1 mm in diameter. Precision machining means tight tolerances on parts of any size. Many aerospace parts need both: fuel and propellant control orifices, valve spools and seats, connector contacts, sensor housings, optical mounts and RF waveguide details.
The two problems call for different checks. Small features stress the tool and spindle. Tight tolerances stress temperature control and measurement. A shop can be good at one and weak at the other, so ask about each. For the full range of machine classes see the machines hub.
Machines used for micro and precision work
| Machine | Typical parts | Why it fits |
|---|---|---|
| Swiss-type lathe | Pins, contacts, small shafts, fittings | Guide bushing supports slender stock at the cut |
| High speed precision mill (3 or 5-axis) | Small housings, manifolds, optical and RF details | High spindle speed for small tools, thermally stable frame |
| Wire EDM | Sharp internal corners, thin slots, hardened details | No cutting force, so thin features do not deflect |
| Sinker and small-hole EDM | Fine holes, cavities in hard alloys | Reaches shapes and materials tools cannot |
| Jig grinder and precision cylindrical grinder | Bearing fits, gage pins, hardened bores | Best size control and finish on hardened steel |
Two published examples show what the top end of this class claims. KERN describes microprecision as workpiece accuracy under 10 µm, ultraprecision under 3 µm and nanoprecision under 1 µm. Its 2013 press release for the KERN Micro quoted positioning accuracy down to plus or minus 0.5 µm and part accuracy down to plus or minus 2 µm. A dealer listing for a Citizen L20 Swiss lathe gives a 20 mm maximum machining diameter. Use figures like these to calibrate expectations, then ask for test cut or inspection data from the shop's own machine.
Process detail is in turning and Swiss and EDM and precision grinding.
Tooling and spindle runout
Small tools fail by breaking, not by wearing, and runout is what breaks them. Harvey Tool, for example, catalogs square-end miniature end mills with cutter diameters as small as 0.001 in. At that scale, a spindle or holder with a few tenths of runout loads one flute far more than the other.
- Spindle speed. Small diameters need very high rpm to reach a sensible surface speed. A shop cutting 0.010 in features on a 10,000 rpm machine is working well below the tool's design speed and will push feed per tooth too low or too high.
- Holders. Shrink fit and precision collet holders keep runout low. Ask what the shop uses for tools under 1 mm.
- Tool length setting. Non-contact laser tool setters measure small tools without breaking them, and can check for breakage between operations.
- Chip load. Micro cutting runs at chip loads close to the tool's edge radius; too light a chip rubs and work hardens the surface, which is a real problem in stainless and nickel alloys.
Thermal and environmental control
At micrometer tolerances, temperature is the largest error source. ISO 1 sets 20 °C as the reference temperature for all dimensional specifications, and aluminum grows about 23.6 µm per meter per °C. A 50 mm aluminum feature 2 °C warm is about 2.4 µm long, which can be most of an ultraprecision tolerance.
What to look for: a temperature controlled room for the machine as well as for inspection, warm-up cycles before finish passes, coolant temperature control, and parts soaked before measuring. Vibration matters too; precision machines are often isolated from forklifts and presses.
Burrs, edges and surface integrity
On micro features, a burr can be a large fraction of the feature. Burrs in fluid passages break loose and contaminate valves and injectors. Specify edge condition explicitly: a maximum burr height or edge break, and how it will be verified.
Deburring options each have tradeoffs. Hand deburring under magnification is flexible but variable. Thermal and electrochemical deburring reach internal passages but need process control and may need customer approval. Abrasive flow polishes passages but changes dimensions. For EDM surfaces, the recast layer may need removal or a limit on its depth, especially on fatigue loaded parts. For parts that will be precision or oxygen cleaned, note that cleaning cannot fix a burr; see precision and oxygen cleaning.
Measuring small and tight features
The rule is simple: the measurement must be much better than the tolerance. ANSI/NCSL Z540.3 limits the probability of false accept to 2 percent when a measurement claims compliance, and accepts a test uncertainty ratio of 4:1 or better where that probability cannot practically be estimated.
| Feature | Common methods | Notes |
|---|---|---|
| Small holes and orifices | Gage pins, air gaging, optical or vision systems, flow testing | Flow test confirms function when geometry alone is not enough |
| Tight bores and diameters | Air gages, bore gages set to masters, CMM with small styli | Stylus size limits access to small bores |
| Small profiles and edges | Vision measuring machines, optical comparators | Edge detection depends on lighting and surface |
| Surface finish | Contact profilometer or optical profiler | State the parameter (Ra, Rz) and the governing standard, such as ASME B46.1 |
| 3D features | CMM verified to ISO 10360 | Measure at controlled temperature |
For tolerances of a few micrometers, ask the shop to state measurement uncertainty for each critical feature. More detail is in inspection and metrology.
Design rules that make precision parts cheaper
- Put the tight tolerance only where function needs it. A precise bore with loose surrounding geometry costs far less than a uniformly tight part.
- Give internal corners a radius the tool can make; sharp internal corners force EDM.
- Keep small-tool depth to a few diameters where you can. Deep, narrow features need long-reach tools that deflect and break.
- Make datums measurable: flat, accessible faces and bores, not edges and small radii.
- Say whether size applies before or after plating, anodize or passivation. A coating can consume a micrometer tolerance on its own; see surface finishing.
Callouts and paperwork
For micro and precision parts, the drawing and purchase order should state: the GD&T standard and revision, measurement temperature if not standard, edge and burr limits with magnification for inspection, surface finish parameters, cleanliness level and packaging if the part is going into a fluid or optical system, and the inspection evidence required, such as an AS9102 first article with CMM or vision reports. Ask for calibration records for the gages used on critical features.
Workholding for small and delicate parts
Small parts are hard to hold without distorting them, and distortion that springs back after release is invisible until inspection. Collets and soft jaws machined to the part shape spread clamping force. Vacuum fixtures suit thin flat parts but need a sealed surface. Some shops machine small parts from a larger blank, leaving tabs or a frame, and cut them free in a final operation so the part is never clamped directly. Others use low melting point alloys or adhesives to support thin walls during cutting, which then need complete removal and possibly cleaning verification. Ask the supplier how the part will be held for its tightest features, and whether those features are measured clamped or free.
Quote a precision part
Send the model, drawing and the three tightest characteristics when you request a quote. We match the part to qualified suppliers whose machines, tooling and measurement equipment fit the tolerance, flow down your requirements and check the inspection evidence before delivery. A mutual NDA is in place before files are shared.
Questions
What is the smallest end mill that is commercially available?
Catalog tools go very small. Harvey Tool, for example, lists square-end miniature end mills with cutter diameters down to 0.001 in. Whether a shop can use them productively depends on spindle speed, runout, tool setting and machine stability more than on the tool. Ask the supplier about their experience with the tool sizes your part needs, and design features to use the largest tool the geometry allows.
How do I know if a supplier can measure a tolerance of a few micrometers?
Ask for the measurement method and its uncertainty for each critical feature, plus calibration records for the gages and CMM. ANSI/NCSL Z540.3 limits false accept probability to 2 percent and accepts a 4:1 test uncertainty ratio where that cannot be estimated. If the stated uncertainty is a large share of the tolerance, the part cannot be reliably accepted, however good the machine is.
Why do micro features in stainless or nickel alloys cause so much trouble?
These alloys work harden. Micro tools often run at chip loads close to their edge radius, so a light or interrupted cut rubs instead of cutting and hardens the surface for the next pass. Tools then wear fast or snap. The fixes are sharp tools, correct speed, steady chip load and rigid setups, which is why spindle speed and runout matter so much in micro work.
Is wire EDM better than milling for small precise features?
It depends on the feature. Wire EDM has no cutting force, so thin walls and narrow slots do not deflect, and it cuts hardened material and sharp internal corners that a tool cannot. It leaves a recast layer, cuts only through features and is slower. Milling is faster for pockets and 3D shapes. Many precision parts use both, milled first and EDM finished.
Should I call out size before or after coating on precision parts?
Always say which. A coating such as anodize, plating or chem film adds or converts material, and on a part with a tolerance of a few micrometers that change can be the entire band. State whether dimensions apply before or after finishing, mask features that must stay bare, and confirm the finisher's thickness control on the specific process.
Related
Sources
- Cutting Tool Engineering, micromachining machine article with KERN precision classes
- KERN Precision press release, KERN Micro 5-axis machine tool (GlobeNewswire, 2013)
- Exapro dealer listing, Citizen Cincom L20 VII Swiss-type lathe specifications
- Harvey Tool, miniature end mills (cutter diameters from .001 in)
- ISO 1:2022, standard reference temperature for geometrical product specification (ISO catalogue via iTeh)
- AMESWeb, linear thermal expansion coefficients of metals
- NCSLI Measure journal, ANSI/NCSL Z540.3 false accept and TUR discussion
- NASA Technical Reports Server, measurement decision risk and Z540.3 (NTRS 20130012068)
- ASME B46.1, Surface Texture (Surface Roughness, Waviness and Lay)
Ready to
source it?
A person replies within 1 business day. NDA first.
Request a quote