Engineering reference data
ISO 2768 general tolerances, surface roughness conversion, metric tap drill sizes, ISO 286 fits and hardness equivalents — the tables we use on the shop floor, in one place.
ISO 2768-1 — general linear tolerances
Applies to any dimension on your drawing that has no individual tolerance. Unless you state otherwise we machine to class m (medium).
| Nominal size (mm) | f — fine | m — medium | c — coarse | v — very coarse |
|---|---|---|---|---|
| 0.5 – 3 | ±0.05 | ±0.1 | ±0.2 | — |
| 3 – 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| 6 – 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| 30 – 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| 120 – 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| 400 – 1000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
| 1000 – 2000 | ±0.5 | ±1.2 | ±3.0 | ±6.0 |
| 2000 – 4000 | — | ±2.0 | ±4.0 | ±8.0 |
ISO 2768-1 — chamfers and external radii
| Nominal size (mm) | f — fine | m — medium | c — coarse | v — very coarse |
|---|---|---|---|---|
| 0.5 – 3 | ±0.2 | ±0.2 | ±0.4 | ±0.4 |
| 3 – 6 | ±0.5 | ±0.5 | ±1.0 | ±1.0 |
| over 6 | ±1.0 | ±1.0 | ±2.0 | ±2.0 |
ISO 2768-2 — general geometric tolerances
Applies when a feature carries no explicit geometric callout. Class K is our default.
| Characteristic | Nominal size (mm) | H | K | L |
|---|---|---|---|---|
| Flatness / straightness | up to 10 | 0.02 | 0.05 | 0.1 |
| Flatness / straightness | 10 – 30 | 0.05 | 0.1 | 0.2 |
| Flatness / straightness | 30 – 100 | 0.1 | 0.2 | 0.4 |
| Flatness / straightness | 100 – 300 | 0.2 | 0.4 | 0.8 |
| Perpendicularity | up to 100 | 0.2 | 0.4 | 0.6 |
| Perpendicularity | 100 – 300 | 0.3 | 0.6 | 1.0 |
| Symmetry | up to 100 | 0.5 | 0.6 | 0.6 |
| Circular run-out | all sizes | 0.1 | 0.2 | 0.5 |
Surface roughness conversion
Our standard as-machined finish is Ra 1.6 (N7). Anything finer than Ra 0.8 requires a separate finishing operation.
| Grade | Ra (µm) | Ra (µin) | Rz (µm) | Typical process | We supply |
|---|---|---|---|---|---|
| N10 | 12.5 | 500 | 50 | Rough milling, sawing | On request |
| N9 | 6.3 | 250 | 25 | Milling, turning — roughing pass | On request |
| N8 | 3.2 | 125 | 12.5 | Standard machined finish | Standard |
| N7 | 1.6 | 63 | 6.3 | Finish milling / turning — our default | Standard |
| N6 | 0.8 | 32 | 3.2 | Fine finish pass, reaming | Standard |
| N5 | 0.4 | 16 | 1.6 | Grinding | Standard |
| N4 | 0.2 | 8 | 0.8 | Fine grinding, honing | Standard |
| N3 | 0.1 | 4 | 0.4 | Lapping, superfinishing | On request |
Metric coarse thread reference
Tap drill sizes give roughly 70 % thread engagement, which is the industry norm for steel and aluminium.
| Thread | Pitch (mm) | Tap drill Ø | Clearance hole Ø | Min. blind hole depth |
|---|---|---|---|---|
| M2 | 0.40 | 1.60 | 2.4 | 3.0 |
| M2.5 | 0.45 | 2.05 | 2.9 | 3.8 |
| M3 | 0.50 | 2.50 | 3.4 | 4.5 |
| M4 | 0.70 | 3.30 | 4.5 | 6.0 |
| M5 | 0.80 | 4.20 | 5.5 | 7.5 |
| M6 | 1.00 | 5.00 | 6.6 | 9.0 |
| M8 | 1.25 | 6.80 | 9.0 | 12.0 |
| M10 | 1.50 | 8.50 | 11.0 | 15.0 |
| M12 | 1.75 | 10.20 | 13.5 | 18.0 |
| M16 | 2.00 | 14.00 | 17.5 | 24.0 |
| M20 | 2.50 | 17.50 | 22.0 | 30.0 |
| M24 | 3.00 | 21.00 | 26.0 | 36.0 |
Common ISO 286 fits
Hole-basis system. The hole is held at H7 and the shaft tolerance sets the character of the fit.
| Fit | Type | Behaviour | Typical use |
|---|---|---|---|
| H7/g6 | Clearance — sliding | Small guaranteed clearance. Parts slide freely but without perceptible play. | Guide shafts, locating pins that must be removable |
| H7/h6 | Clearance — location | Zero to minimal clearance. Assembles by hand, locates accurately. | Bearing housings, covers, spigot joints |
| H7/k6 | Transition | May end up with slight clearance or slight interference. Needs light force to assemble. | Gears and pulleys that are also keyed |
| H7/p6 | Interference — press | Guaranteed interference. Requires an arbor press or thermal fitting. | Bushings, permanent dowel pins, bearing inner races |
Hardness conversion and machinability
Conversions are approximate and valid for steels. Above 45 HRC we quote grinding or EDM rather than milling.
| HRC | HB | HV | Approx. tensile | Notes |
|---|---|---|---|---|
| — | 120 | 126 | 410 MPa | Mild steel, annealed |
| 20 | 226 | 238 | 770 MPa | Normalised carbon steel |
| 30 | 286 | 302 | 980 MPa | Quenched and tempered alloy steel |
| 40 | 371 | 392 | 1290 MPa | Upper limit for routine carbide milling |
| 45 | 421 | 446 | 1480 MPa | Hard milling with ceramic or CBN tooling |
| 50 | 481 | 513 | 1740 MPa | Grinding or EDM territory |
| 55 | 560 | 595 | 2050 MPa | Tool steel, hardened — EDM only |
| 60 | — | 697 | 2400 MPa | Fully hardened tool steel |
CAD formats we accept
Send a 3D model for geometry and a PDF drawing for tolerances, finishes and GD&T. Either alone leaves gaps we have to ask about.
| Extension | Format | Notes |
|---|---|---|
| .STEP / .STPPreferred | ISO 10303 | Preferred for quoting. Neutral solid model, reads identically in every CAM system. |
| .X_T / .X_BPreferred | Parasolid | Equally good. Preserves exact geometry with no tessellation error. |
| .IGS / .IGES | IGES | Accepted, but surfaces sometimes arrive unstitched and need repair before programming. |
| .SLDPRTPreferred | SolidWorks | Native SolidWorks part. We read it directly, no conversion needed. |
| .DWG / .DXF | AutoCAD | For 2D drawings and flat profiles. Send alongside a 3D model, not instead of one. |
| .PDFPreferred | Drawing | The dimensioned drawing. Always send this if you have tolerances, finishes or GD&T to convey. |
| .STL | Tessellated mesh | Accepted for reference only. A mesh has no exact surfaces, so we cannot hold tight tolerances from it. |
Files encrypted in transit and at rest
Need these tolerances held on a real part?
Upload your drawing and we will confirm in writing which dimensions we can hold, on which equipment, and what the inspection report will contain.