Specifying Tolerances for 3D Printing Correctly
In additive manufacturing, a recurring question arises: which tolerance standard is suitable for 3D-printed parts? The two most relevant standards in the FDM domain are ISO 2768-1 Class M (medium tolerance class) and the newer ISO 20457:2021-06 TG4 (specifically developed for additive manufacturing). This article compares both approaches.
ISO 2768-1 Class M — The Classic Standard
ISO 2768-1 defines general tolerances for linear and angular dimensions. Class M (medium) is the most commonly referenced class in manufacturing engineering.
Tolerance Values Class M (Excerpt)
| Nominal Dimension Range | Tolerance |
|---|---|
| 0.5–3 mm | ±0.1 mm |
| 3–6 mm | ±0.1 mm |
| 6–30 mm | ±0.2 mm |
| 30–120 mm | ±0.3 mm |
| 120–400 mm | ±0.5 mm |
| 400–1000 mm | ±0.8 mm |
Strengths
- Universally known and established in industry
- Compatible with conventional manufacturing processes
- Easy to reference in technical drawings
Limitations in 3D Printing
- Not developed for additive processes
- Does not account for process-specific effects such as shrinkage, warping, or anisotropy
- Tightly toleranced dimensions (below 6 mm) are difficult to reliably achieve in FDM printing
ISO 20457:2021-06 TG4 — The Standard for Additive Manufacturing
ISO 20457 was developed specifically for additive manufacturing and takes into account the process-specific characteristics. TG4 is one of the defined tolerance groups.
Key Features
- Process-appropriate — Tolerances are based on the actual capabilities of additive processes
- Direction-dependent — Accounts for anisotropy (different accuracy in X/Y vs. Z)
- Material-specific — Different tolerances depending on material and process
- Practical — Tolerance values reflect realistic manufacturing results
Tolerance Values TG4 (Typical for FDM)
| Nominal Dimension Range | Tolerance |
|---|---|
| 0–30 mm | ±0.3 mm |
| 30–120 mm | ±0.5 mm |
| 120–400 mm | ±0.8 mm |
| 400–1000 mm | ±1.2 mm |
Direct Comparison
| Criterion | ISO 2768-1 Class M | ISO 20457 TG4 |
|---|---|---|
| Target process | Conventional | Additive |
| Accuracy small dimensions | ±0.1 mm | ±0.3 mm |
| Accuracy large dimensions | ±0.8 mm | ±1.2 mm |
| Anisotropy considered | No | Yes |
| Industry acceptance | Very high | Growing |
| FDM practicality | Limited | High |
Our Recommendation
For FDM 3D printing, we recommend ISO 20457 TG4 as the reference standard, as it realistically reflects the actual capabilities of the process. When ISO 2768-1 Class M is required by the client, we review in advance which dimensions require this tolerance and optimise print orientation and parameters accordingly.
As a general rule: the tighter the tolerance, the more important the coordination between design and manufacturing becomes. Get in touch with us early — we are happy to advise you on optimal tolerancing for your 3D-printed parts.
Contact: info@reents3d.de or +49 40 56118777.
