Printed parts are accurate enough for most geometry and are not accurate in the way a machined part is accurate. Tolerance varies with process, with the direction the feature lies in relative to the build, with part size, and with what happens after the build. A drawing that treats a printed part like a machined one will produce parts that fail inspection and work perfectly.
Why the number depends on direction
Layer based processes are more accurate within a layer than between layers, so a hole drilled through the build direction and a hole lying across it come out differently. Downward facing surfaces carry support scarring. The first layers are affected by adhesion and by thermal conditions at the plate. Orientation is therefore a tolerance decision and should be specified when it matters.
Typical expectations by process
Resin processes are the most dimensionally precise and the most affected by post cure shrinkage. Powder bed nylon holds shape well over a build and varies with packing density and thermal history. Filament printing is the loosest and the most operator dependent. Metal powder bed is good for general geometry and not good enough for fits, which are machined afterwards.
Designing around it
Give clearances rather than transitions: printed holes come out undersize on most processes, so either print them undersize and ream, or add clearance and use a fastener that tolerates it. Avoid press fits between two printed parts. Where two printed parts must locate accurately, put the accuracy in one machined feature or one bought component rather than in the print.
When to stop and machine
Bearing fits, sealing faces, threads that carry load, and any datum that other features are measured from. The normal industrial practice is to print the part with machining stock on those features and finish them conventionally, which is a hybrid rather than a compromise and is how most functional printed metal parts are actually made.
Questions people ask about 3d printing tolerances
What tolerance can 3D printing hold?
It depends on the process, the orientation and the part size. Resin is tightest, filament loosest, and metal powder bed is good for geometry but not for fits.
Why are my printed holes too small?
Most processes shrink holes slightly, and layer stepping reduces the effective diameter. Print undersize and ream, or design in clearance.
Does orientation affect accuracy?
Yes, significantly. Features within a layer are more accurate than features across layers, and downward faces carry support marks. Specify orientation when a feature depends on it.
Should I put tolerances on a printed part drawing?
Only where they matter, and expect the tight ones to be machined. Treating a printed part like a machined one produces failed inspections rather than better parts.