Why 3D printed holes often come out too small
Internal holes can print smaller than their CAD dimensions for several reasons, including extrusion behaviour, orientation, geometry and layer shape. For screw clearance holes, there is an additional issue: the correct design diameter is already larger than the screw's nominal diameter. First identify whether the CAD hole is too small by design or the printer is reproducing it smaller than intended.
M4 does not mean a 4.0 mm clearance hole
A clearance hole for an M4 screw is intentionally larger than 4 mm.
Separate standard clearance from FDM compensation
Use: standard clearance diameter + your print-process allowance = CAD hole diameter.
Horizontal holes
Horizontally printed holes can distort because their upper region behaves like an overhang. Possible solutions:
- change orientation
- use suitable self-supporting geometry where appropriate
- drill to final size
- use support if necessary
Check over-extrusion
If other inside dimensions are also too tight and surfaces show excess material, flow may be contributing.
Do not compensate one feature by changing the entire printer scale
If only one hole geometry is wrong, do not immediately change global scaling.
Calculate a screw hole
Calculate the correct model dimension directly instead of enlarging every hole by guesswork.
Calculate a screw hole