Engineering Tolerances for CNC, Sheet Metal and 3D Printed Parts
Tolerance is the silent specification that determines whether a part fits, functions and looks right. At SkillsProto, we default to ISO 2768-m (medium) for general features and tighten critical dimensions to ±0.05 mm or tighter where geometry allows.
If you are quoting a part without a tolerance callout, ISO 2768-m applies. Tell us on your RFQ which dimensions are critical and we will hold them.
ISO 2768 tolerance classes
ISO 2768 defines four tolerance classes:
| Class | Description | Typical Use |
|---|---|---|
| ISO 2768-f (fine) | Tighter tolerances | Precision fits, gauges |
| ISO 2768-m (medium) | Our default | General engineering, most parts |
| ISO 2768-c (coarse) | Looser tolerances | Sheet metal, non-critical features |
| ISO 2768-v (very coarse) | Very loose tolerances | Cast surfaces, weldments |
ISO 2768-m tolerance examples (linear dimensions in mm)
| Length Range | Tolerance Class m (±) |
|---|---|
| 0.5 – 3 | 0.1 mm |
| 3 – 6 | 0.1 mm |
| 6 – 30 | 0.2 mm |
| 30 – 120 | 0.3 mm |
| 120 – 400 | 0.5 mm |
| 400 – 1000 | 0.8 mm |
| 1000 – 2000 | 1.2 mm |
Tightest tolerances by process
| Process | Typical Achievable | Tightest Achievable (geometry permitting) |
|---|---|---|
| CNC machining (milling) | ±0.05 mm | ±0.02 mm |
| CNC turning | ±0.05 mm | ±0.02 mm |
| Sheet metal (laser cut) | ±0.1 mm | ±0.05 mm |
| Sheet metal (bent) | ±0.5 mm on overall | ±0.3 mm on overall |
| Injection moulding | ±0.1 mm | ±0.05 mm |
| Vacuum casting | ±0.2 mm | ±0.1 mm |
| Die casting | ±0.2 mm (cast) / ±0.05 mm (post-machined) | ±0.05 mm (post-machined) |
| SLA 3D printing | ±0.2 mm | ±0.1 mm |
| SLS / MJF 3D printing | ±0.3 mm | ±0.2 mm |
Tight tolerance capability
For features that must hold tighter than ISO 2768-m, we will:
- Identify the critical dimensions on your CAD or 2D drawing.
- Confirm feasibility — tighter tolerances may require special tooling, slower cycle, additional inspection.
- Quote the impact — tighter tolerances may affect lead time and unit cost.
- Inspect with calibrated CMM, optical comparator or pin gauges as required.
GD&T callouts
If your 2D drawing includes GD&T (Geometric Dimensioning & Tolerancing) callouts — flatness, perpendicularity, position, runout — we will quote to those. Send your drawing with the CAD and we will return a quote that respects your GD&T.
Frequently Asked Questions
Q1: What happens if I do not specify a tolerance? A: ISO 2768-m applies. If you need tighter, add a tolerance callout to your drawing or note on your RFQ.
Q2: Can you hold ±0.02 mm? A: Yes on geometry that allows it (single feature, accessible, machineable). It will increase cost and lead time.
Q3: How do you measure tight tolerance parts? A: Calibrated CMM, optical comparator, micrometers, pin gauges — whichever is appropriate for the feature.