Free Drill Bushing Tolerance & Press Fit Calculator

Engineering Guidelines for Precision Drill Jig Bushings

Drill jig bushings are hardened steel or carbide guide sleeves used in manufacturing fixtures to locate and guide twist drills, reamers, and taps with sub-thousandth precision. Proper sizing of the reamed hole in the jig plate is critical: an oversized hole allows the bushing to spin or push out under cutting thrust, while an undersized hole will distort the bushing, crushing the inner bore and seizing the cutting tool.

1. Press Fit ('P' & 'H') vs Slip Renewable ('S' & 'F') Bushings

• Press Fit Bushings (Types P and Headed H): Designed for permanent one-step single-operation jigs. Ground deliberately oversize (typically +0.0013" to +0.0018" over nominal diameter) to achieve an interference fit when pressed into a reamed nominal hole.
• Slip Renewable Bushings (Type S): Used for multi-step progressive operations (e.g., drill then ream, or drill then tap) on the same hole. Slip bushings are never pressed directly into the jig plate; they fit with a slide fit into a permanent Master Liner Bushing (Type L) and are locked in place with a lockscrew or clamp.
• Master Liner Bushings (Type L): Permanent hardened sleeves pressed into the jig plate to provide a wear-resistant ground bore for renewable slip bushings.

2. Reamed Hole Sizing and Interference Calculations

ANSI B94.33 specifies exact tolerances for bushing outside diameters. Standard practice for steel plates is to ream the hole directly to the nominal size with a tolerance of +0.0000" / +0.0005". For aluminum plates, because aluminum has a lower modulus of elasticity (10 Mpsi vs 30 Mpsi) and higher thermal expansion, tighter reamed holes or dedicated steel liners are recommended to prevent loosening over thermal cycles.

3. Estimating Press Fit Force (Lamé's Cylinder Theory)

The force required to press a bushing into the plate depends on interference ($delta$), bushing diameter ($D$), length of engagement ($L$), material elastic moduli ($E$), and coefficient of friction ($mu$):

Always apply high-pressure anti-seize or molybdenum disulfide grease prior to pressing. Never hammer a drill bushing directly; use an arbor press with a brass or soft steel pusher pad to prevent cracking the through-hardened 62-64 HRC steel.

4. Crucial Chip Clearance Spacing

Never press a bushing flush against the workpiece unless drilling on an irregular curved surface. For general drilling in steel or aluminum, leave a chip clearance gap between the bushing bottom and the workpiece equal to 1.0 to 1.5 times the drill diameter. This allows spiral swarf to escape freely into the jig evacuation channels without galling the bushing bore.