The Drill Wander Problem
When a drill bit contacts a workpiece without guidance, the point wanders across the surface before engaging. This wander creates hole position errors of ±0.15 to ±0.3 mm depending on drill diameter, material hardness, and spindle rigidity. In high-volume production, this translates to 5-8% rejection rates at assembly.
How Bushings Eliminate Wander
A drill bushing constrains the drill bit within a hardened bore (typically 62 HRC, H7 tolerance) during the critical first 2-3 mm of engagement. The bushing bore-to-drill clearance is controlled to 0.005-0.015 mm, which physically prevents lateral wander while allowing rotation and axial feed.
- Bore hardness (62 HRC): Prevents the drill from wearing the bushing bore. Unhardened bushings degrade after 500-1,000 holes.
- H7 bore tolerance: Provides consistent clearance regardless of drill diameter within the standard tolerance band.
- Bore length: Minimum bore length = 1.5× drill diameter for effective guidance. Longer bores provide more support but increase drill friction.
Accuracy Improvement Data
| Drill Diameter | Without Bushing | With Bushing | Improvement |
|---|---|---|---|
| Ø3 mm | ±0.15 mm | ±0.02 mm | 7.5× |
| Ø6 mm | ±0.20 mm | ±0.02 mm | 10× |
| Ø10 mm | ±0.25 mm | ±0.03 mm | 8× |
| Ø16 mm | ±0.30 mm | ±0.04 mm | 7.5× |
Bushing Types for Different Operations
- Type P (press-fit): Permanent installation. Best accuracy. Used when the jig always drills the same hole size.
- Type S (slip-renewable): Removable bushing in a permanent liner. Allows tool changes (drill → reamer → tap) without removing the jig from the workpiece.
- Type HL (headed-liner): Flanged liner bushing that receives slip bushings. Prevents the liner from pushing through the jig plate.
Design Considerations
- Bushing-to-workpiece gap: Keep ≤2× drill diameter. Larger gaps reduce guidance effectiveness.
- Chip clearance: For through-drilling, provide chip clearance holes in the jig plate below the bushing. For blind holes, use peck drilling cycles.
- Coolant delivery: Grooved bushings allow coolant to reach the cutting zone. Essential for deep hole drilling (>5× diameter).