3D Printing Tolerances Explained in Plain English
· 3 min read
Tolerance is the one word in a quote that most people nod at and nobody asks about. It matters because a replacement part that is a hair too big does not go on, and one that is a hair too small rattles. Here is what the word means for FDM printed parts and how we handle it.
What tolerance is
Tolerance is how far a finished dimension is allowed to stray from the number on the model. A hole drawn at 10 mm that comes out anywhere between 9.8 and 10.2 was held to plus or minus 0.2 mm. Every process has a typical range. Machining is tight. Injection molding is tight once the mold is dialed in. FDM printing is looser, and the honest number depends on the material, the size of the feature, and how the part is oriented.
The published benchmarks
For reference, the two largest online print networks publish FDM guidance of roughly plus or minus 0.5 percent with a floor of about 0.5 mm on desktop machines, and plus or minus 0.15 to 0.3 percent with a floor of about 0.2 to 0.3 mm on industrial machines (Protolabs Network and Protolabs design guides, read October 2026). So a realistic expectation for a printed replacement part is somewhere between 0.2 and 0.5 mm depending on size, material, and machine, before any fitting work.
Why we do not quote one blanket number
A blanket tolerance number is marketing. A 200 mm housing and a 3 mm pin do not behave the same way, and a part printed in carbon fiber nylon, which barely shrinks, behaves differently from one in ABS, which shrinks noticeably. What we do instead is ask which dimensions actually matter. On most replacement parts it is two or three features. A shaft bore. A snap fit width. A bolt hole pattern. Those get the attention.
How we make the fit right
- Compensation. Holes print slightly small and outside dimensions slightly large. We adjust the model for the material so the printed result lands on the number.
- Orientation. Dimensions in the plane of the build plate come out more accurate than dimensions up the Z axis. Critical features go in the plane when the part allows it.
- Test coupons. When a press fit or a shaft fit is critical, we print a small coupon with the feature first and check it against the mating part or a gauge before printing the whole part.
- Post processing. A bore that must be exact can be printed slightly under and reamed. A thread can be printed and chased with a tap, or a metal insert can be used instead.
- Measurement. We check the dimensions that matter against the model before the part ships.
What you should tell us
Where the part has to fit and how tightly. A knob that slides onto a D shaft has one critical dimension. A housing that holds a board with four screws has a hole pattern. A gear has a bore and a tooth form. If you can send the mating part, even better, because then the test coupon is checked against reality rather than a number.
Threads deserve their own paragraph
Scanned threads are never reused. The geometry is fine and helical and small errors compound into a thread that will not turn. We identify the thread standard and model it fresh, so the printed part carries a real thread. For small or heavily used threads we recommend a brass heat set insert, which is stronger and lasts longer than any printed thread.