3D Printing Tolerances: Why the CAD Dimension Isn't Everything
Two parts with exactly the same nominal dimension usually don't simply slot into each other in 3D printing. Printer, material, orientation, geometry and slicer settings all cause the real part to deviate slightly from the model. That's why a functional design needs built-in clearance, and a test print for critical fits.
There is no universal tolerance value for every 3D printer and material.
What does tolerance mean?
In the technical sense, a tolerance describes the permitted range within which a real dimension may deviate from the target value. In 3D printing, there's an additional question: how much built-in clearance do I need between two parts so they behave the way I want?
Why two identical dimensions don't automatically fit
Example: a peg at 20.00 mm and an opening at 20.00 mm. Identical in theory. In practice, the following factors (among others) come into play, and the result can end up too tight.
- material flow
- line width
- rounding
- shrinkage
- first layer
- elephant foot
- print orientation
- geometry resolution
- material properties
Which factors affect the fit?
Prusa explicitly names the following influencing factors. That's why a single number like "always 0.2 mm" isn't a reliable universal rule.
- model size
- horizontal or vertical orientation
- geometry
- calibration
- settings
- material properties
A practical starting range
As a starting point, not a guarantee, a clearance in the range of a few tenths of a millimeter can work on a well-calibrated FDM printer. Prusa names roughly 0.3 mm as one possible starting point for moving parts, while stressing that no universal value exists. Vuolti generators therefore make clearance explicitly adjustable, never present a single value as correct, and recommend a test print for critical parts.
Per side or total?
A common source of error: if an opening should be 0.4 mm larger than a peg, it must be clear whether that means 0.4 mm total difference, or 0.4 mm per side — i.e. 0.8 mm total difference. Vuolti generators show this distinction explicitly, so it is always clear which one a given clearance value refers to.
The best test is a small one
Instead of printing a multi-hour holder in full, a small preliminary test pays off.
- isolate the relevant fit zone
- print a short test piece
- check the real fit
- adjust the clearance
- print the final part