A drawing built for a milled part, handed to an additive shop unchanged, usually produces one of two outcomes: a quote that is far too high because every dimension was toleranced as if it were machined, or a part that fails inspection because the tolerances were never achievable in the first place. Geometric Dimensioning and Tolerancing (GD&T) still applies to printed parts – it is still the clearest way to say what actually matters – but a good printed-part drawing looks different from a machined one. It tolerances less, tolerates more, and adds information a machinist never needs: how the part is oriented in the build.
Tolerance to the process, not the habit
The first discipline is to stop over-tolerancing. As-printed additive accuracy is far looser than machining, and it is not uniform – accuracy in the build plane (X–Y) differs from accuracy along the build direction (Z), and small features hold tighter than large spans. Reserve tight tolerances for the few features that genuinely mate, seal, or locate, and let everything else fall under a generous general tolerance block. Where a feature truly needs precision the printer cannot deliver, the honest answer is a secondary machining operation on that feature, called out explicitly. For the numbers each process actually holds, see our tolerances guide; for how orientation and process interact, the process selection guide helps.
| GD&T control | How well AM holds it | Guidance for the drawing |
|---|---|---|
| Linear dimension, X–Y | Best case for the process | Fine for most mating features; still looser than machining |
| Linear dimension, Z (build axis) | Often looser, layer-dependent | Give it more tolerance, or machine the critical face |
| Flatness | Fights warping on large faces | Loosen on big flat areas; add ribs or machine if it must be flat |
| Position of holes | Reasonable within a plane | Usable, but drill or ream if the fit is tight |
| Profile of a surface | Practical for organic shapes | Good general control for curved and freeform faces |
| Perpendicularity / parallelism | Harder across the build direction | Reference a printed datum carefully; machine when critical |
Datums that survive printing
Datum choice makes or breaks a printed-part drawing. A datum has to be a surface that both prints repeatably and can be located during inspection and any downstream machining. Some surfaces fail that test: the first-layer face often shows “elephant’s foot” spread, down-facing surfaces built on support come out rough and inaccurate, and any warped face drifts. Prefer datums on well-defined, stable geometry – a solid vertical wall, a deliberate boss, or best of all a face that will be machined flat after printing, which gives you a precise, repeatable reference. Choose the datum reference frame with the whole workflow in mind, so the same features that locate the part in inspection can also fixture it for any secondary operation.
Never place a primary datum on an unmachined first-layer face or a support-facing surface. Put datums on printed features that are stable and reachable, or on a face you machine after the build to create a true reference.
Put build orientation on the drawing
This is the callout a machined drawing never carries and a printed one needs. Because accuracy and strength are direction-dependent, the same geometry printed on its side behaves differently from the same part printed upright – layer lines become the plane of weakness, and Z-axis dimensions shift. If orientation matters to the part’s function or its tolerances, say so: add a build-orientation note or an axis indicator so the shop cannot silently reorient the part to save time and quietly change its properties. At minimum, flag the load direction and any surface whose finish or accuracy is critical, so orientation is chosen deliberately rather than for throughput alone.
Add a small build-orientation flag and a short note – for example, an up-arrow marking +Z and “critical face A to be built upward”. It costs one symbol on the drawing and prevents the single most common cause of a printed part failing to match intent.
Inspection callouts, and why the drawing differs
Specify how critical features are inspected, because as-printed surfaces make measurement harder: a rough or textured face gives noisy readings, so note where measurement is taken and, if needed, that a feature is measured after finishing rather than as-printed. Step back and the pattern is clear – a printed-part drawing carries fewer tight tolerances, a generous general block, datums chosen to survive the process, an explicit build orientation, and clear separation between as-printed and machined features. It communicates not just the final geometry but an awareness of how that geometry comes to exist. That is the mindset difference between drawing for a subtractive process and drawing for an additive one.
Reading a process well enough to tolerance it correctly is core DfAM. Our courses build that judgement on real drawings and real parts.
Take Foundations free See Pro pricingPrinted-part drawing checklist
- Apply tight tolerances only to features that mate, seal, or locate; put everything else under a generous general block.
- Separate X–Y and Z expectations, and give the build direction more tolerance or a machining callout.
- Choose datums on stable, reachable printed features – or on faces machined flat after the build.
- Keep primary datums off first-layer and support-facing surfaces.
- Add a build-orientation flag and note whenever orientation affects fit, finish, or strength.
- Call out any feature that will be finish-machined, and distinguish as-printed from machined dimensions.
- Specify inspection method and where each critical feature is measured, before or after finishing.
- Validate tight callouts with a test coupon and the shop’s stated capability before releasing the drawing.