The Pin-Bushing Interface
The accuracy of a fixture locating system depends entirely on the fit between the pin and its receiving bushing. Too tight → installation problems and bushing damage. Too loose → positional uncertainty that compounds across multiple fixture stations.
Tolerance Matching Table
| Pin Tolerance | Bushing Bore | Fit Type | Clearance at Ø10 | Application |
|---|---|---|---|---|
| m6 | H7 | Light interference | -0.024 to 0 mm | Permanent datum pins — pressed in, never removed |
| h7 | H7 | Transition | -0.009 to +0.015 mm | Semi-permanent — removable for maintenance |
| g6 | H7 | Clearance | +0.005 to +0.024 mm | Retractable pins, spring locators, thermal compensation |
Bushing Material Selection
The bushing must be harder than the pin to prevent bore wear. Standard pairing rules:
- Steel pin (SUJ2/SKD11, 58-62 HRC) + hardened steel bushing (60-64 HRC): Standard combination, longest service life
- Steel pin + bronze bushing (oil-impregnated): Self-lubricating, good for dirty environments but lower wear resistance
- Stainless pin (SUS304) + stainless bushing: Corrosion-resistant but both are soft — shorter life
Installation Best Practices
- Clean both surfaces before assembly — debris increases effective interference
- Chamfer bushing bore entry at 0.3-0.5 mm × 45° for guided insertion
- Apply light oil to clearance-fit pairs to prevent fretting corrosion
- Check pin diameter at 3 points (both ends + middle) before installation — out-of-round pins cause inconsistent fit
- Record actual clearance values for each pin-bushing pair in high-precision applications (inspection fixtures)
Replacement Indicators
Replace the pin or bushing when:
- Measured clearance exceeds the original specification by >50%
- Pin shows visible wear marks (diameter reduction >0.005 mm)
- Bushing bore shows scoring or galling marks
- Workpiece positioning repeatability degrades beyond acceptable tolerance