Press Fit

How it works

A pin or shaft is inserted into a hole that is slightly smaller in diameter. The elastic deformation of the surrounding material creates radial pressure that holds the parts together through friction. No hooks, no flexing — just tight fit.

Before:           After:

  ┌──┐  ┌────┐     ┌────────┐
  │  │  │    │     │  ████  │
  │██│→ │ ○  │  =  │  ████  │
  │  │  │    │     │  ████  │
  └──┘  └────┘     └────────┘
  pin    hole       press fit
  (larger) (smaller)

Key design parameters

Parameter	Guideline
Interference (FDM)	0.1-0.3mm on diameter
Interference (injection molded)	0.5-1.5% of hole diameter
Minimum wall around hole	2x the interference to avoid splitting
Surface finish	Smoother = more consistent fit

Design tips

• FDM needs less interference than injection molding — layer lines create surface texture that adds effective friction. Start with 0.1mm interference and test.
• Chamfer the pin end — makes alignment and insertion much easier.
• Round holes print slightly undersized on FDM — account for this in your tolerance. A “10mm” hole often prints as 9.8-9.9mm.
• Use a short interference zone — only the first few mm of insertion depth need to be tight. A long press-fit zone requires excessive force.

FDM considerations

• Hole orientation matters — holes printed vertically (along Z) are more accurate than horizontal holes (which get slight ovality from layer stacking).
• Print a tolerance test first — print a pin and hole at several interference values (0.05, 0.1, 0.15, 0.2mm) to find your printer’s sweet spot.
• Avoid press-fitting across layer lines — the radial pressure can split layers. Press-fit direction should be along the layer stack (Z-axis).

When to use

• Permanent pin/axle mounting
• Bearing seats
• Alignment dowels between mating parts
• Inserting threaded brass inserts (heat-set inserts)

Common failures

• Splitting — hole cracks from excessive interference. Cause: too much interference, not enough wall material around the hole, or wrong print orientation.
• Loose fit — parts slide out. Cause: too little interference, or material creep over time.
• Inconsistent fit — works on one print, not another. Cause: printer calibration drift, different material batches, temperature variation.