Appearance
A gun drill with a perfectly ground tip and a correctly set coolant system will still produce a bent hole if the starting bush clearance is wrong. A 6 mm gun drill running in a starting bush with 0.03 mm excess clearance will deflect by up to 0.15 mm at the entry point — enough to cause the drill to walk off-center and produce a hole that misses the target location. The starting bush is the first point of contact between the machine and the drill, and the clearance between the bush bore and the drill diameter determines whether the entry is accurate or compromised.
Starting Bush Types and Configurations
Bushing Type Comparison
| Type | Design | Typical L/D Ratio | Clearance Range (mm) | Application | Service Life |
|---|---|---|---|---|---|
| Stationary (press-fit) | Fixed bushing pressed into bushing holder | 2:1–4:1 | 0.005–0.015 | Standard gun drilling — general production | High — no moving parts |
| Stationary (screw-mounted) | Replaceable bushing with flange and screws | 2:1–4:1 | 0.005–0.020 | Quick-change applications — multiple diameters | High — easy replacement |
| Rotating | Bush rotates with drill through drive pins | 2:1–5:1 | 0.010–0.025 | Deep holes — high RPM — reduced friction | Moderate — bearing wear |
| Adjustable | Split bushing with clamping adjustment | 3:1–5:1 | Adjustable 0.005–0.030 | Variable diameter — prototype — low-volume | Moderate — wear at split line |
| Coolant-fed | Integrated coolant inlet ports | 2:1–4:1 | 0.008–0.025 | High-pressure coolant — deep holes | Moderate — coolant erosion |
| Carbide-lined | Carbide insert bonded to steel body | 2:1–5:1 | 0.005–0.015 | High-volume production — abrasive materials | Very high — 5–10× steel life |
Bushing Material Properties
| Material | Hardness | Wear Resistance | Galling Resistance | Cost | Typical Application |
|---|---|---|---|---|---|
| Tool steel (A2, D2) | 58–62 HRC | Good | Moderate | Low | General production — standard steels |
| High-speed steel (M2, T15) | 62–66 HRC | Very good | Moderate | Moderate | Higher wear resistance — longer intervals between changes |
| Carbide (WC-Co) | 88–92 HRA | Excellent | Good | High | High-volume — abrasive materials — extended life |
| Bronze (leaded or phosphor) | 80–100 HB | Low | Excellent | Low | Soft materials — non-galling — low RPM applications |
| Ceramic (alumina or zirconia) | 90+ HRA | Excellent | Excellent | Very high | Specialized — high-temperature — corrosive environments |
Clearance and Sizing Specifications
Recommended Bore Clearance by Drill Diameter
| Drill Diameter (mm) | Min Clearance (mm) | Recommended Clearance (mm) | Max Clearance (mm) | Tolerance Band |
|---|---|---|---|---|
| 2–4 | 0.003 | 0.005–0.008 | 0.010 | ISO H5/g5 |
| 4–8 | 0.005 | 0.008–0.012 | 0.015 | ISO H5/g5 |
| 8–15 | 0.008 | 0.010–0.015 | 0.020 | ISO H6/g5 |
| 15–25 | 0.010 | 0.012–0.018 | 0.025 | ISO H6/g6 |
| 25–40 | 0.012 | 0.015–0.020 | 0.030 | ISO H6/g6 |
Bushing Length Recommendations
| Application | Material Group | L/D Ratio | Example Length (for 10 mm drill) | Purpose |
|---|---|---|---|---|
| Standard — short holes | Steel — cast iron | 2:1 | 20 mm | General guidance — shallow holes |
| Deep holes | Steel — alloy | 3:1 | 30 mm | Better entry stability — longer guidance |
| Precision — aerospace | Titanium — Inconel | 4:1 | 40 mm | Maximum straightness — critical applications |
| Small diameter (<5 mm) | All materials | 3:1–5:1 | — | Additional stiffness for small drills |
| Soft materials | Aluminum — brass | 2:1–3:1 | 20–30 mm | Sufficient guidance — reduced galling risk |
FAQ
What is the correct clearance between the gun drill and the starting bush?
The correct clearance between the gun drill and the starting bush bore is typically 0.005–0.020 mm depending on drill diameter — smaller drills require tighter clearances. For a 6 mm gun drill, 0.008–0.012 mm clearance is recommended; for a 20 mm gun drill, 0.012–0.018 mm. Excessive clearance allows the drill to tilt inside the bush, causing entry misalignment and hole position error. Insufficient clearance causes binding, overheating, and galling between the drill shank and the bush bore — particularly in long drilling cycles where thermal expansion can close the clearance gap completely. The clearance should be measured at operating temperature, since both the steel bush and the carbide drill head expand during drilling.
How do I know when a starting bush needs replacement?
A starting bush should be replaced when the bore diameter has worn by more than 0.02 mm from the original size — typically corresponding to 0.005–0.010 mm wear in the normal wear area. Visual indicators for replacement include visible bell-mouthing at the entry end (caused by chip abrasion during drill entry), longitudinal scoring or scratching on the bore surface, measurable ovality exceeding 0.005 mm (caused by uneven loading), and a dull or polished appearance on the carbide drill shank at the bush contact area — indicating excessive friction. Many production facilities establish a fixed replacement interval based on operating hours or holes drilled, such as replacing the bush every 5,000–20,000 holes depending on the material and operating conditions.
What causes bell-mouthing in starting bushes?
Bell-mouthing — wear that creates a tapered or flared opening at the entry end of the bush — is caused primarily by chip abrasion as chips are forced back through the bush during drill entry and withdrawal. The combination of high coolant pressure, chip particles (which may be harder than the bush material), and the reversing motion of the drill creates a wear pattern that widens the entry end preferentially. Contributing factors include insufficient coolant flow (allowing chips to accumulate at the bush entry), incorrect chip breaker geometry producing large chips that scrape the bush bore, soft bush material that wears rapidly under abrasive chip flow, and misalignment between the bush and the drill spindle that causes one-sided loading. Prevention includes using carbide or carbide-lined bushes for abrasive applications, ensuring adequate coolant flow through the bush, and maintaining proper drill geometry for consistent chip formation.
Can the same starting bush be used for different drill diameters?
Using the same starting bush for different drill diameters is not recommended for precision gun drilling. Each drill diameter requires its specific clearance — using a 10 mm drill in a bush sized for 12 mm produces excessive clearance that degrades hole straightness and position accuracy. However, for non-critical applications or when making small diameter adjustments (less than 0.5 mm difference), a bush can sometimes accommodate multiple sizes if the clearance remains within acceptable limits. Quick-change bushing systems are the practical solution for multi-diameter operations — the bush holder remains fixed in the machine while the replaceable bush insert is swapped to match each drill diameter, typically requiring less than 30 seconds for the change.
What is the correct alignment procedure for mounting a starting bush?
The correct alignment procedure starts with indicating the bush holder bore to within 0.01 mm of the spindle centerline at the bush location, using a coaxial indicator or test bar. The bush holder face must be perpendicular to the spindle axis within 0.005 mm over the holder diameter. Once the holder is aligned and locked, the bush is installed and its bore is checked with a plug gauge or internal micrometer to confirm the correct diameter and verify that installation did not distort the bore. A test bar of the same diameter as the gun drill should pass freely through the bush and the spindle collet with the machine in the Z-home position. Final verification includes running a test part and checking the entry diameter and hole position against specification. Alignment should be rechecked whenever the bush or holder is changed and periodically as part of scheduled machine maintenance.
Disclaimer: The bushing specifications, clearance ranges, and alignment procedures provided in this article are general guidelines based on industry-standard practices for gun drilling starting bush selection. Actual requirements vary by machine configuration, workpiece material, coolant type, and specific drilling application. Bushing selection should be verified through application testing and consultation with the machine manufacturer and bushing supplier. The authors and publisher assume no liability for any damages or losses arising from the use of this information — always follow original equipment manufacturer guidelines for your specific equipment. Content is for informational purposes only and does not constitute professional engineering advice. Verify all parameters with qualified personnel before implementation as of 2026.