Reference
Micro Machining Tolerances, Feature Sizes and Materials
A single page of typical values for parts under 10 mm. Use it to write a realistic drawing and RFQ. Every figure is typical, not a guarantee: the drawing, the material certificate and the matched shop set the real limit.
Tolerance Capability
What micro machining shops typically hold in production. The best values apply to selected features after DFM review.
| Characteristic | Typical | Note |
|---|---|---|
| Linear, best | ±0.005 mm | On selected features after DFM review |
| Linear, general | ±0.01 mm | What most micro shops hold in production |
| Bore / shaft fit | H6 / h5 | IT5 grade on shafts |
| Angular | ±10 arc-min | |
| Flatness | 0.005 to 0.05 mm | Depends on part size and thickness |
| Runout | 0.01 mm | Turned diameters, single setup |
| Surface roughness | Ra 0.8 μm best, 1.6 μm typical | Finer finishes need a secondary process |
ISO 286 Grades at Micro Sizes
Tolerance grades from ISO 286-1 for the size ranges that matter here, in micrometres. H6 bores and h5 shafts are common fits on micro turned parts.
| Nominal size | IT5 | IT6 | IT7 |
|---|---|---|---|
| Over 1 up to 3 mm | 4 | 6 | 10 |
| Over 3 up to 6 mm | 5 | 8 | 12 |
| Over 6 up to 10 mm | 6 | 9 | 15 |
Feature Size Limits
The smallest features micro shops typically deliver. Smaller is possible with EDM or laser processes, at a different cost.
| Feature | Typical minimum | Note |
|---|---|---|
| Overall part envelope | From 1 mm | Up to about 10 mm for this niche |
| Smallest feature | 0.3 mm | Slots, bosses, ribs |
| Drilled hole | From ⌀0.2 mm | Depth usually up to 10 x diameter |
| Wall thickness | From 0.15 mm | Material dependent |
Materials for Micro Parts
Material selection drives the tolerance ceiling on micro features. These grades are routinely stocked in bar and plate sizes suited to small parts.
| Material | Grades | Note |
|---|---|---|
| Stainless steel | 303, 304, 316, 316L | 316L is the usual choice for medical micro parts |
| Titanium | Grade 2, Grade 5 (Ti-6Al-4V) | Grade 5 ELI for implant-grade work |
| Aluminum | 6061-T6, 7075-T6 | Common for semiconductor and optics housings |
| Brass and copper | C36000, C11000 | C36000 is the easiest metal to micro turn |
| Tool steel | D2, H13, SKD11 | Hardened work goes to shops with hard milling or grinding |
| Engineering plastics | PEEK, POM, PTFE | PTFE creeps; hold tolerances loose where you can |
How Micro Features Are Inspected
A tolerance is only as good as the method that checks it. Agree the method before production. Reference temperature for measurement is 20 °C (ISO 1).
| Method | Used for | Note |
|---|---|---|
| CMM with small stylus | Bores, positions, flatness | Stylus tips down to about 0.3 mm; probing force matters on thin walls |
| Vision / optical measuring system | Profiles, small holes, slots, edges | No contact, good for soft or tiny features |
| Measuring microscope | Burrs, edge radii, small features | Fast check of edge condition |
| Pin and ring gauges | Hole and shaft size go / no-go | Pins available in fine steps for small holes |
| Profilometer | Surface roughness Ra | Stylus or optical, per ISO 21920 |
Tolerance FAQ
Background on tolerance grades: engineering tolerance.
Is ±0.005 mm achievable on every feature?
No. It is a typical best value on selected features such as a turned diameter or a bore, after DFM review. Put tight tolerances only where the function needs them and leave the rest at ±0.01 mm or a general tolerance class.
What does IT5 mean for a 4 mm shaft?
Under ISO 286, the IT5 tolerance band for sizes over 3 up to 6 mm is 5 μm. A 4 mm h5 shaft may therefore measure between 4.000 and 3.995 mm.
Why do plastics get looser tolerances?
PEEK, POM and especially PTFE move with temperature and moisture and deflect under cutting and clamping forces. Most shops quote plastics a class looser than metals for the same feature.
How are micro features measured?
With CMMs using small styli, optical or vision measuring systems, measuring microscopes and pin gauges. The measurement method should be agreed before production, especially for features under 0.5 mm.
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