Tolerances and Specifications
Engineering tolerances, minimum wall thickness, thread standards, maximum dimensions and surface-finish grades across CNC machining, sheet metal fabrication and plastic moulding. The engineering limits we apply across all production lanes.
CNC, sheet metal and moulding
Standard tolerances and tighter grades
Thread and surface-finish rules
How to Use These Specifications
How to read the limits below, what they assume, and where you can tighten without breaking the budget.
The values below are the working tolerances and limits we apply on standard orders across CNC machining, sheet metal fabrication and plastic moulding. Tighter tolerances and finer finishes are routinely available on request; what follows is the contractual baseline.
How tolerances are quoted
Tolerances are quoted as a lower and upper bound against the nominal dimension and have to be specified on the drawing. Anything not called out will be produced to ISO 2768-m unless the part file specifies otherwise. The same rule applies to surface roughness and to plastics moulded parts.
General Machining Tolerances
The default tolerances on CNC turned and milled features. International tolerance grades follow ISO 286.
| Nominal dimension (mm) | Fine | Standard (ISO 2768-m) | Coarse (ISO 2768-c) |
|---|---|---|---|
| 0.5 to 3 | +/-0.05 mm | +/-0.10 mm | +/-0.20 mm |
| 3 to 6 | +/-0.05 mm | +/-0.10 mm | +/-0.30 mm |
| 6 to 30 | +/-0.10 mm | +/-0.20 mm | +/-0.50 mm |
| 30 to 120 | +/-0.15 mm | +/-0.30 mm | +/-0.80 mm |
| 120 to 400 | +/-0.20 mm | +/-0.50 mm | +/-1.20 mm |
| 400 to 1000 | +/-0.30 mm | +/-0.80 mm | +/-2.00 mm |
CNC Machining – Detailed Limits
Tighter limits than the general table, organised by feature type.
| Feature | Default | Tightest typical |
|---|---|---|
| Linear milling (steel/aluminium) | +/-0.05 mm | +/-0.025 mm |
| Linear milling (engineering plastic) | +/-0.10 mm | +/-0.05 mm |
| Bore diameter | +/-0.05 mm | +/-0.0125 mm |
| Shaft diameter | +/-0.025 mm | +/-0.0125 mm |
| Position tolerance | +/-0.05 mm | +/-0.020 mm |
| Flatness on 50 mm | 0.05 mm | 0.025 mm |
| Parallelism | +/-0.05 mm | +/-0.020 mm |
| Perpendicularity | +/-0.05 mm | +/-0.025 mm |
| Surface roughness as-machined | Ra 1.6 um | Ra 0.4 um |
Sheet Metal Fabrication Limits
Deflection, bending limits and hole spacings for laser cut and bent sheet metal parts.
| Feature | Default | Tightest typical |
|---|---|---|
| Material thickness (steel) | 0.5 to 6 mm | 0.5 to 12 mm |
| Material thickness (stainless and aluminium) | 0.5 to 6 mm | 0.5 to 8 mm |
| Laser cut profile tolerance | +/-0.10 mm | +/-0.05 mm |
| Bend angle tolerance | +/-1 degree | +/-0.5 degree |
| Bend radius (steel) | 1.0 x thickness | 0.5 x thickness |
| Bend radius (stainless and aluminium) | 1.5 x thickness | 1.0 x thickness |
| Punch-to-bend edge | 2 x thickness + 1.5 mm | 1.5 x thickness + 1.0 mm |
| Hole diameter minimum | equal to thickness | 0.7 x thickness |
Injection Moulding Limits
Wall thickness, draft, gate limits and tolerance grades for injection moulded parts.
| Feature | Default | Limit |
|---|---|---|
| Minimum wall thickness (general) | 1.0 mm | 0.6 mm (engineering grades) |
| Wall thickness tolerance | +/-0.10 mm | +/-0.05 mm |
| Draft angle (vertical face) | 1 degree | 0.5 degree |
| Draft angle (textured face) | 1.5 degrees | 1 degree |
| Linear tolerance (ISO 3302-1 class M) | +/-0.20 mm at 50 mm | +0.20/-0.10 mm |
| Linear tolerance (ISO 3302-1 class F – fine) | +/-0.10 mm at 50 mm | +0.05/-0.05 mm |
| Boss diameter | 2.5 x wall | 1.5 x wall |
| Rib thickness | 0.7 x wall | 0.6 x wall |
Thread Specifications
Standard thread forms we supply in production: metric coarse, metric fine, UNC, UNF and BSP.
| Standard | Range | Notes |
|---|---|---|
| Metric coarse (M) | M1.6 to M30 | ISO 261, tolerance 6H/6g default |
| Metric fine (MF) | M3 x 0.5 to M30 x 2 | ISO 261, tolerance 6H/6g default |
| UNC | #4-40 to 1 inch | ASME B1.1 |
| UNF | #4-48 to 1 inch | ASME B1.1 |
| BSP (parallel) | G 1/8 to G 2 | ISO 228 |
| NPT | 1/16 to 4 inch | ASME B1.20.1 |
| Self-tapping | ST 2.2 to ST 8 | DIN 7970 / ISO 1478 |
| Knock-down (wood) | 3.0 to 6.5 mm | DIN 96 / 97 |
Surface Roughness (Ra)
Standard surface-finish grades across processes. Specify Ra on the drawing rather than the finish name.
| Finish source | Typical Ra (um) | Notes |
|---|---|---|
| Saw cut | 12.5 | Reference only |
| Milled as-machined | 1.6 to 3.2 | Default on machined parts |
| Milled fine | 0.8 to 1.6 | Specified critical faces |
| Milled precision | 0.4 to 0.8 | Hard-to-reach features |
| Ground | 0.2 to 0.8 | Steels and tool steels |
| Polished | 0.05 to 0.4 | Food and medical parts |
| Bead blasted | 1.6 to 3.2 | Matte appearance before finishing |
| Mirror polished | 0.025 to 0.05 | Decorative |
| As-moulded (injection) | 0.2 to 1.6 | Resin and tool dependent |
| As-extruded (aluminium) | 0.8 to 3.2 | After die draw |
Maximum Part Dimensions
Maximum envelope on common processes. Larger parts are supported on request; pricing depends on stock availability.
| Process | Maximum envelope | Notes |
|---|---|---|
| CNC milling | 1500 x 800 x 600 mm | 5-axis up to 800 mm diameter |
| CNC turning | 600 mm diameter x 1500 mm long | Including shaft work |
| Sheet metal (laser) | 3000 x 1500 mm | Standard 4 kW fibre laser |
| Sheet metal (bending) | 3000 mm bed length | Up to 250 tonne press |
| Injection moulding | 1200 x 800 x 500 mm | Up to 50 g shot weight |
| Aluminium extrusion | 300 mm circumscribed circle | Up to 9 m cut length |
| Die casting | 600 x 600 x 400 mm | Up to 5 kg shot weight |
Other Engineering Limits Worth Knowing
- Minimum feature size on CNC aluminium is 0.5 mm wall and 0.8 mm floor; thinner features need EDM or 5-axis machining.
- Minimum hole diameter on stainless steel is 1.0 x the plate thickness; smaller needs EDM, laser or water jet.
- Part features deeper than 8x the tool diameter require extended-reach tooling and review for chip evacuation.
- Cast-in inserts are recommended over tapped threads for production volumes above 500 pieces.
- Living hinges on polypropylene have a minimum thickness of 0.4 mm and a recommended flex radius of 0.5 mm.
Need a tolerance or a finish that is not on this page?
Send your drawing; we will return a quote with feasibility, lead time and cost on the tighter specification.