3D Printer FYITwo parts sharing the same nominal dimension are not necessarily going to fit. CAD is very accommodating about surfaces occupying exactly the same place. Plastic tends to raise an objection.
Prusa’s design guidance explicitly warns that there is no universal tolerance value: geometry, orientation, calibration, settings, and material all matter. Treat a suggested starting clearance as a test condition, not a property of the universe. Prusa Research
Decide what kind of fit you mean
A part that slides freely needs a different result from a part retained by friction. A screw clearance hole has a different job from the hole receiving a heat-set insert. Begin with the mating part and the intended behavior.
For a round pin, distinguish diametral clearance from radial clearance. If a measured pin is 5.00 mm and a measured hole is 5.20 mm, diametral clearance is 0.20 mm; clearance on each side is 0.10 mm for centered, ideal circles. These are illustrative dimensions. Real surfaces, ovality, and measurement uncertainty complicate the fit.
Writing “clearance: 0.2” without the convention is an excellent way to manufacture a disagreement.
Print the small question
Create a coupon with several labeled hole sizes bracketing the nominal dimension. Use the orientation, material, wall strategy, and slicing profile planned for the final part. Label the coupon revision and retain the profile.
Try the real mating component in each hole. Record whether it cannot enter, presses in, slides with resistance, or moves freely. If you measure the holes, record the tool and method. Small-hole measurements with ordinary calipers can be less straightforward than the number of digits on their screen suggests.
Choose the smallest clearance that repeatedly produces the intended behavior. Print another coupon before making a large batch. This is an empirical choice for this process, not a claim that the selected value will transfer unchanged to every printer and polymer.
Heat-set inserts add another contract
An M3 thread designation does not specify an insert’s outer geometry or installation hole. Use the exact insert family and the manufacturer’s drawing. CNC Kitchen provides design guidance and test parts, and specifically recommends matching test material and print settings to the eventual component. CNC Kitchen
Record the insert’s part identifier, recommended hole, length, and surrounding geometry requirements. If a dimension is missing from the source, leave it missing. Do not reverse-engineer it from a product photograph and then display it with two decimal places.
The installation process is also part of the result. A visually neat insert is not proof of pull-out strength. Structural requirements need an appropriate test of the actual joint. This guide does not assign load ratings or installation temperatures.
Keep compensation local
Suppose one feature needs adjustment. First determine whether it is a design clearance issue, a systematic dimensional error, or an orientation-specific problem. A whole-model scale change affects every feature, including the ones already correct.
A useful record contains the drawn dimension, measured outcome where available, mating-part dimension, print orientation, material, profile revision, and fit classification. Keep any computed correction beside its assumptions.
For example, a proposed hole of 5.30 mm that measures 5.10 mm has a measured difference of −0.20 mm in that observation. It does not establish a universal correction of +0.20 mm for the next hole. Repeat at the relevant geometry and size before generalizing.
The coupon is not wasted plastic. It is a cheap way to avoid producing a much larger object with the same unanswered question in it.
Next: An enclosure is a set of promises turns a successful fit into a complete housing.
Sources
- Modeling with 3D printing in mind | Prusa Knowledge Base — Prusa Research. Reviewed 15 September 2026.
- Tips & Tricks for Heat-Set Inserts used in 3D printing | CNC Kitchen — CNC Kitchen. Reviewed 15 September 2026.
Put this to work
- Fit and Fastener Workbench — Record what the file or measured part actually says.
- G-code Microscope — Record what the file or measured part actually says.