Designing screw holes for 3D printed parts
A clearance hole should be larger than the nominal screw diameter. FDM printing may require additional compensation because printed internal features can come out smaller than their CAD dimensions. Treat the standard hole size and your printer-specific allowance as two separate values.
M4 does not mean a 4.0 mm clearance hole
An M4 screw has a nominal diameter of about 4 mm. Assembly requires clearance. Standards therefore define different clearance-hole series such as:
- close
- normal
- large
ISO 273 examples
Typical normal-series values include. These are design dimensions, not FDM compensation.
- M4 → 4.5 mm
- M5 → 5.5 mm
- M6 → 6.6 mm
- M8 → 9.0 mm
Add print compensation separately
Example.
- standard hole: 4.50 mm
- your FDM allowance: +0.20 mm
- CAD hole: 4.70 mm
Edge distance
A correctly sized hole can still weaken the part if it is too close to an outside edge. Leave enough material around holes, especially in:
- mounts
- brackets
- hooks
- thin flanges
Horizontal holes
Round holes printed horizontally may become less circular because of overhangs and layer geometry. For critical holes:
- review orientation
- print a test
- consider drilling to final size