Tolerance expectations that are acceptable in CNC machining often set a 3D printing project up for failure. Engineers frequently over-specify tolerances on 3D printed parts — adding cost and lead time — or under-specify them and receive parts that don't assemble. At Layer X we measure every batch against customer-specified GD&T callouts. Here is what each process genuinely delivers.
FDM (Fused Deposition Modelling) Tolerances
FDM is the most variable process. Layer height, nozzle diameter, print temperature, and part orientation all influence dimensional output. As a baseline, expect ±0.3 mm on any dimension below 100 mm, and ±0.5% on dimensions above 100 mm. Hole diameters print 0.1–0.3 mm undersize due to filament shrinkage — design holes at +0.2 mm oversize or drill to final diameter.
Engineering materials (PC, PA-CF, PETG) add complexity. Carbon-fibre filled filaments are stiffer but exhibit greater thermal warping in large parts. Layer X's enclosed Markforged-compatible printers hold ±0.2 mm routinely with Onyx (PA-CF) up to 200 mm build length.
SLA (Stereolithography) Tolerances
SLA is the most accurate polymer process. UV laser spot diameters of 0.1–0.14 mm combined with layer heights of 0.025–0.1 mm produce parts with detail resolution routinely below 0.1 mm. Dimensional tolerance: ±0.1 mm standard, ±0.05 mm in fine-detail dental and jewellery modes. Cure shrinkage is managed by calibration compensation — the slicer scales slightly to account for known contraction.
Important: SLA parts continue to cure under ambient UV. Store in opaque packaging. Parts may shrink a further 0.1–0.2% over 48 hours post-print. For critical-fit gauge blocks or master patterns, let parts stabilise for 24 hours before measuring.
SLS (Selective Laser Sintering) Tolerances
SLS is the preferred process when dimensional accuracy and mechanical performance must coexist. PA12 nylon parts hold ±0.2 mm up to 100 mm, ±0.2% beyond 100 mm. Because no supports are used, complex internal geometry retains accuracy with no post-processing distortion from support removal.
Consistent batch-to-batch accuracy is high — SLS is used for end-use production parts (not just prototypes) precisely because tolerances are repeatable across dozens of builds.
DMLS (Direct Metal Laser Sintering) Tolerances
DMLS metal parts are quoted at ±0.1 mm on critical features with machining, ±0.2–0.3 mm as-printed. Thermal residual stresses cause warping in large flat parts — stress relief annealing and strategic part orientation are essential for flatness below 0.3 mm/100 mm. After heat treatment and surface grinding of datum faces, Layer X routinely achieves ±0.05 mm on sealing faces for Ti-6Al-4V hydraulic components.
Tolerance Comparison Table
| Process | Standard tolerance | Fine tolerance (with care) | Min feature |
|---|---|---|---|
| FDM (PLA/PETG) | ±0.3 mm | ±0.2 mm | 0.8 mm wall |
| FDM (PA-CF/PC) | ±0.3 mm | ±0.2 mm | 1.0 mm wall |
| SLA (standard resin) | ±0.1 mm | ±0.05 mm | 0.2 mm feature |
| SLS (PA12 nylon) | ±0.2 mm | ±0.15 mm | 0.6 mm wall |
| DMLS (Ti, SS, Inconel) | ±0.2 mm as-printed | ±0.05 mm machined | 0.4 mm feature |
Designing for the Process
- Clearance fits: Use 0.3–0.5 mm bilateral clearance for snap fits in FDM; 0.1–0.2 mm in SLS and SLA.
- Press fits: SLS and SLA hold press-fit shafts reliably; FDM is too variable for consistent interference fits.
- Thread inserts: Always use heat-set brass inserts in FDM rather than self-tapping into plastic walls. M3–M6 inserts are standard at Layer X.
- Orientation strategy: X-Y plane features are always more accurate than Z-direction features — design critical holes in the X-Y plane.
Need certified dimensional inspection? Layer X provides CMM and optical scanning reports for DMLS and SLS production orders. Request a tolerance consultation before your next job.