If you have come from a CNC machining background, FDM tolerance conventions can feel loose at first. A machined component might carry plus or minus 0.05 millimetres on a critical mating feature. A typical FDM part is more naturally specified at plus or minus 0.2 to 0.3 millimetres overall, with tighter local tolerances called out only where they actually matter.
This is not a weakness in the process. It is a different tolerance philosophy that, when applied with discipline, produces excellent parts at a fraction of the cost and lead time of machining. The trick is matching the specification to what the technology can repeatably hold.
What FDM can realistically hold
Industrial-grade FDM on engineering thermoplastics typically holds plus or minus 0.15 to 0.25 millimetres overall on parts up to 200 millimetres. The 1m x 1m x 1m platforms we run for large parts hold plus or minus 0.3 to 0.5 millimetres overall, which is genuinely competitive with sheet fabrication. Local features can do considerably better: hole diameters within plus or minus 0.1 millimetres are routine with the right design, slot widths similar, and surface flatness within 0.1 millimetres over a 100 millimetre span is standard on a calibrated machine running filament from OzFDM.
Orientation on the build plate also matters. A hole printed along the Z-axis behaves differently from the same hole printed flat, because layer geometry interacts with the feature. Discussing orientation with the supplier before the drawing is locked usually saves a revision later.
Tolerance economics
Specifying plus or minus 0.05 on every dimension of an FDM part will either dramatically increase cost or quietly fail acceptance. Specify tight tolerances only on the features that need them.
A step-by-step approach to tolerance specification
List the mating features, then the load-bearing features, then everything else, in that order
Assign a general tolerance block to the drawing, typically plus or minus 0.2 millimetres
Tighten only the mating features, usually with floating fits rather than press fits
For any feature requiring tighter than 0.1 millimetres, plan to finish-machine it on our CNC service after printing
Confirm material and orientation with the supplier before locking the drawing, because both affect achievable tolerances
The drawing convention that survives the shop floor
An effective FDM drawing carries a general tolerance block plus a small set of feature callouts for the dimensions that genuinely matter. Surface finish should be specified per face, because printed top, side and supported underside surfaces all behave differently. 'As-printed' is fine for non-critical faces.
Critical faces should specify the post-processing required to reach the desired finish, whether that is sanding, vapour smoothing or machining. That single discipline removes most of the avoidable disputes between designer and supplier.
Overall accuracy of plus or minus 0.15 to 0.25 millimetres on small parts
Hole and slot accuracy of plus or minus 0.1 millimetres with good design
Surface flatness of plus or minus 0.1 millimetres per 100 millimetre span
Tighter local tolerances are available via post-machining
Specify tolerances that matter, and only those, because cost follows the call