Why Interference Fit Calculations Matter
Press-fit locating pins (m6 tolerance) rely on interference between the pin outer diameter and the plate bore to create a permanent, vibration-resistant datum point. Too little interference → the pin loosens under cyclic loading. Too much → the bore deforms or cracks. This guide provides the formulas to get it right.
Interference Range by Diameter
| Pin Diameter | m6 Range (µm) | H7 Range (µm) | Interference Min (µm) | Interference Max (µm) |
|---|---|---|---|---|
| Ø6 | +12 to +20 | 0 to +12 | 0 | +20 |
| Ø10 | +15 to +24 | 0 to +15 | 0 | +24 |
| Ø16 | +18 to +29 | 0 to +18 | 0 | +29 |
| Ø20 | +21 to +33 | 0 to +21 | 0 | +33 |
| Ø25 | +21 to +33 | 0 to +21 | 0 | +33 |
Note: At minimum material condition (pin at low m6, bore at high H7), interference can be zero — essentially a transition fit. For guaranteed interference, specify tighter bore tolerances (H6) or select pin blanks at mid-to-high m6 range.
Press-In Force Formula
The axial force required to press a pin into a bore is:
F = π × d × L × µ × p
- F = Press-in force (N)
- d = Nominal pin diameter (mm)
- L = Engagement length (mm)
- µ = Friction coefficient (0.12-0.18 for steel-on-steel, dry)
- p = Contact pressure = δ × E / (2 × d) for a solid pin in a thick plate
- δ = Diametral interference (mm)
- E = Young's modulus (~210 GPa for steel)
Worked Example: Ø10 m6 Pin
Given: Ø10 m6 pin (actual diameter 10.020 mm) in H7 bore (actual diameter 10.005 mm). Engagement length L = 15 mm.
- Interference: δ = 10.020 - 10.005 = 0.015 mm
- Contact pressure: p = 0.015 × 210,000 / (2 × 10) = 157.5 MPa
- Press-in force: F = π × 10 × 15 × 0.15 × 157.5 = 11,130 N ≈ 1.1 kN
A standard arbor press (1-3 ton capacity) handles this easily. For larger pins (Ø20+) with longer engagement, hydraulic pressing may be required.
Bore Stress Check
The bore wall experiences hoop stress from the interference fit. For a pin in a finite plate:
σ_hoop = p × (D² + d²) / (D² - d²)
Where D = outer diameter of the effective plate annulus (typically 2-3× pin diameter). Keep σ_hoop below 60% of plate material yield strength to prevent bore deformation. For S45C plates (σ_y ≈ 360 MPa), maximum allowable hoop stress is 216 MPa.
Practical Tips
- Chamfer the bore entry: A 0.5 mm × 45° chamfer guides the pin and prevents bore edge chipping
- Clean both surfaces: Any debris between pin and bore increases effective interference and can crack the bore
- Press speed: Slow, steady pressing (5-10 mm/s) gives better results than impact insertion
- Temperature assist: For tight fits, cooling the pin in dry ice (-78°C) reduces its diameter by ~0.01 mm per 100°C drop, enabling easier insertion