Design · 5 min read
Designing tolerances for FDM parts
How accurate FDM printing is, why it varies, and how to design clearances and fits that work.
FDM is a dimensionally capable process, but not a precision machining process. Designing with that in mind avoids most fit problems.
Why dimensions vary - Plastic shrinks as it cools - Extrusion width and layer height limit fine detail - Vertical (Z) and horizontal (XY) accuracy behave differently - Larger parts accumulate more variation than small ones
Practical approach Rather than relying on one published accuracy figure, design clearances into mating features and test the critical ones. A small test coupon of a hole, peg or slot is cheap and tells you exactly how a fit will behave with a given material and printer.
Starting points for fits - Sliding or removable fits need noticeable clearance between parts - Press fits are sensitive and usually need a test print to tune - Holes tend to print undersized; plan to adjust in CAD or drill to size - Threads: for repeated assembly, use heat-set inserts or captive nuts instead of printed threads
Tell us what's critical Mark the dimensions that matter. We'll choose orientation to protect them and flag any that FDM can't reliably hold.
What we won't do We don't promise industrial or certified tolerances for FDM parts. If your application needs guaranteed tolerances, we'll say so up front.
General guidance for FDM printing. Results depend on your geometry, material and use — ask us about your specific part.