Tolerance questions kill more additive projects than strength ever does. The honest answer is process-specific: a printed part is a near-net part, and the discipline is knowing what your process holds naturally, what needs compensation, and what needs a machining step you planned from the start.
Realistic as-printed tolerances
| Process | Typical general tolerance | Practical floor (tuned) | Notes |
|---|---|---|---|
| FDM / FFF | ± 0.5% (min ± 0.3 mm) | ± 0.15 mm on small parts | XY better than Z; material shrink dominates |
| SLA / DLP | ± 0.2% (min ± 0.1 mm) | ± 0.05 mm | Excellent detail; watch post-cure warp |
| SLS / MJF | ± 0.3% (min ± 0.3 mm) | ± 0.2 mm | Thermal spread across the build; nesting matters |
| Metal LPBF | ± 0.2 mm + machining | ± 0.1 mm small features | Critical interfaces get machined — always |
| Binder jetting (sintered) | ± 1–2% pre-comp | ± 0.5% with shrink comp | Sinter shrink is large and mostly predictable |
Never put a blanket ±0.1 mm on a printed-part drawing. Tolerance the few surfaces that matter, leave the rest at process-capable general tolerance, and machine what the process can't hold.
Holes shrink. Compensate or drill.
Printed holes come out undersized on nearly every process — melt squeeze-out, cure bleed, or powder adhesion all bias material inward. On FDM expect 0.1–0.4 mm under nominal depending on diameter and material.
- Functional holes: model at nominal + compensation and validate, or model undersize and drill/ream to size — drilling a printed pilot is often cheaper than tuning.
- Threads: below M5, use heat-set inserts (FDM) or tapped inserts rather than printed threads. Printed threads work from ~M6 up with 0.2–0.4 mm clearance offsets.
- Press fits: design the boss to be drilled or reamed; as-printed press fits drift batch to batch.
Clearances for mating printed parts
| Fit | FDM clearance per side | SLS/MJF per side |
|---|---|---|
| Free running / loose | 0.4–0.5 mm | 0.3–0.4 mm |
| Sliding | 0.25–0.3 mm | 0.2 mm |
| Snug (assembled by hand) | 0.15–0.2 mm | 0.1–0.15 mm |
| Press / interference | Drill or machine it | Machine it |
Print a fit ladder — the same boss/bore pair stepped in 0.05 mm increments — once per machine + material, and pin the results next to the printer. Ten minutes of coupon beats a week of arguing with the datasheet.
Our free AM Foundations course teaches the process-selection filters behind these numbers, and the Pro FDM course has a full module on tolerances, inspection, and release evidence.
Start Foundations — free See Pro pricingTolerance checklist
- Identify the 3–5 surfaces that actually need tolerance; leave the rest general.
- Apply process-realistic values from qualification coupons, not datasheets.
- Compensate or post-drill every functional hole.
- Plan machining stock (0.5–1 mm) and datums on metal AM interfaces from day one.
- Write the inspection method on the drawing — a tolerance without a measurement plan is a wish.