Appearance
Gun drill walking at hole entry is almost always caused by inadequate support at the point of entry. The drill tip deflects before it establishes full diameter contact, producing a misaligned, oversized, or angled hole entry. Proper starting bushings, lead holes, and feed rate control prevent the vast majority of walking problems.
How Gun Drill Walking Occurs
The Mechanics of Entry Deflection
When a gun drill first contacts the workpiece, only a portion of the cutting edge engages. Unlike a twist drill with symmetrical cutting edges, a gun drill has an asymmetrical cutting face. This creates an unbalanced radial force that must be counteracted by the guide bushing and the drill's tracking pad.
| Phase | What Happens | Risk of Walking |
|---|---|---|
| Initial contact | One side of the drill tip engages first | High — unbalanced cutting force |
| Partial engagement | The drill enters the workpiece at an angle | Maximum — insufficient guidance |
| Full diameter | Both cutting edges and tracking pads engaged | Low — drill stabilizes |
| Deep penetration | Drill follows established path | Minimal — if entry was correct |
Tip: The first 2–5 mm of penetration determines the entire hole path. If the drill walks at entry, the error is carried through the full hole depth. Fix the entry, and you fix the hole.
Starting Bushing Requirements
Bushing Fit and Condition
| Parameter | Requirement | Consequence of Error |
|---|---|---|
| Bushing bore diameter | Drill diameter + 0.01–0.02 mm | Too tight: drill binds. Too loose: drill walks |
| Bushing length | Minimum 2× drill diameter | Short bushing provides insufficient guidance |
| Bushing concentricity | Within 0.005 mm TIR to spindle | Misaligned bushing steers the drill |
| Bushing hardness | Minimum 60 HRC | Soft bushing wears quickly, increases clearance |
| Bushing wear limit | Max 0.02 mm oversize | Worn bushing allows drill movement at entry |
Bushing Types for Entry Support
| Bushing Type | Best For | Walking Prevention Level |
|---|---|---|
| Fixed (press-fit) | Precision drilling, small diameters | Highest |
| Replaceable (screw-in) | Production with frequent diameter changes | High |
| Rotating bushing | High spindle speeds | Medium — clearance needed for rotation |
| Lead-hole only | Emergency, low-precision work | Lowest — not recommended |
Warning: A worn or oversized starting bushing is the single most common cause of gun drill walking. Check bushing bore diameter with a plug gauge or inside micrometer every 200–500 holes. Replace the bushing when clearance exceeds 0.02 mm.
Lead Hole Preparation
Lead Hole Specifications
| Gun Drill Diameter | Lead Hole Diameter | Lead Hole Depth | Lead Hole Concentricity |
|---|---|---|---|
| 3–10 mm | Drill Ø + 0.02–0.05 mm | 3–5 mm | Within 0.01 mm TIR |
| 10–25 mm | Drill Ø + 0.03–0.08 mm | 5–10 mm | Within 0.015 mm TIR |
| 25–50 mm | Drill Ø + 0.05–0.10 mm | 8–15 mm | Within 0.02 mm TIR |
| Above 50 mm | Drill Ø + 0.08–0.15 mm | 10–20 mm | Within 0.03 mm TIR |
Lead Hole Preparation Methods
| Method | Accuracy | Application |
|---|---|---|
| Short drill + boring | Best | Precision drilling, tight tolerances |
| Spot face + center drill | Good | Production, moderate tolerances |
| Pre-drilled starter hole | Good | When part geometry requires it |
| No lead hole (direct bushing) | Fair | Coarse tolerances, high-production only |
Tip: When possible, use the gun drill machine itself to cut the lead hole. Set up a short, rigid tool (center drill or spot face) in the same spindle, cut the lead hole, then switch to the gun drill. This guarantees concentricity between the lead hole and the spindle axis.
Feed Rate Optimization at Entry
Recommended Entry Feed Rates
| Condition | Entry Feed Rate (vs normal) | Feed Distance | Rationale |
|---|---|---|---|
| Normal entry | 30–50% of production feed | First 2–5 mm | Allow drill to stabilize |
| Hard material (> 40 HRC) | 20–30% of production feed | First 3–8 mm | Reduce shock loading |
| Soft material (aluminum, brass) | 40–60% of production feed | First 2–4 mm | Prevent dig-in |
| Large diameter (> 25 mm) | 25–40% of production feed | First 5–10 mm | Reduce unbalanced forces |
| Interrupted entry (cross hole) | 10–20% of production feed | Until full engagement | Minimize impact deflection |
Feed Ramp Strategy
| Segment | Feed Rate | Distance | Purpose |
|---|---|---|---|
| Approach | 50 mm/min rapid | Until contact | Fast positioning |
| Entry | 30–50% of target | 2–5 mm | Controlled engagement |
| Transition | 60–80% of target | 5–15 mm | Gradual increase |
| Full production | 100% of target | Remaining depth | Stable drilling |
Warning: Do not use peck cycle at entry. Each retract and re-engagement creates a new entry condition that can cause the drill to walk. For deep hole drilling, the entry feed should be continuous from initial contact through full engagement.
Machine Alignment Factors
| Alignment Factor | Effect on Walking | Tolerance |
|---|---|---|
| Spindle-to-bushing concentricity | Drill steered off-center at entry | 0.01 mm TIR max |
| Guide bushing square to spindle axis | Drill enters at angle | 0.005 mm over bushing length |
| Workpiece surface square to drill axis | Uneven initial contact | 0.05 mm over 25 mm |
| Steady rest alignment | Additional support misalignment | 0.02 mm TIR |
| Coolant pressure at entry | Chip clearance affects stability | Within ±5% of specification |
Common Walking Patterns and Root Causes
| Walking Pattern | Visual Symptom | Most Likely Root Cause |
|---|---|---|
| Bell-mouth entry | Hole entry wider than rest of hole | Worn starting bushing |
| Offset entry | Hole starts off-center, then straightens | Bushing misaligned to spindle |
| Angled entry | Hole enters at angle, continues straight | Workpiece face not square to spindle |
| Spiral entry mark | Spiral pattern at hole entry | Chip packing in bushing clearance |
| Intermittent walking | Walking that starts and stops | Material hardness variation at surface |
Corrective Actions for Existing Walking
When Walking Has Already Occurred
| Situation | Corrective Action | Success Rate |
|---|---|---|
| Minor entry oversize | Accept if within print tolerance | — |
| Entry can be counterbored | Counterbore to larger diameter | High — removes walked section |
| Walking within first 5 mm | Accept if functional — difficult to correct | — |
| Angled entry affecting function | Scrap part or salvage by machining | Depends on remaining wall thickness |
| Consistent walking on every part | Fix root cause (bushing, alignment, feed) | High |
Permanent Fixes
| Fix | Effort | Effect |
|---|---|---|
| Replace starting bushing | Low | Eliminates most walking problems |
| Re-align bushing holder | Medium | Fixes offset entry |
| Reduce entry feed rate | Low | Improves entry stability |
| Add lead hole operation | Medium | Provides guided entry |
| Square workpiece face | Low | Eliminates angled entry cause |
| Check coolant flow at entry | Low | Ensures chip clearance |
FAQ
What causes a gun drill to walk at hole entry?
Inadequate support at the entry point. The most common causes are a worn or oversized starting bushing, misalignment between the bushing and spindle, excessive entry feed rate, a workpiece face that is not square to the drill axis, or insufficient lead hole depth.
How do I prevent gun drill walking?
Use a properly sized starting bushing (drill Ø + 0.01–0.02 mm clearance), align the bushing to the spindle within 0.01 mm TIR, reduce feed rate to 30–50% of production feed for the first 2–5 mm of penetration, and ensure the workpiece face is square to the drill axis.
Can a worn starting bushing cause walking?
Yes — this is the most common cause. As the bushing wears, the clearance between the drill and bushing bore increases. The drill can tilt within the bushing clearance, causing the tip to enter the workpiece off-center. Replace bushings when clearance exceeds 0.02 mm.
Is walking worse with small-diameter gun drills?
Yes. Small drills (under 6 mm) are less rigid and deflect more easily. The clearance in the bushing represents a larger percentage of the drill diameter, and the unbalanced cutting forces are more significant relative to the drill's stiffness. Smaller drills need tighter bushing tolerances and lower entry feed rates.
Can I fix a hole that has walking at the entry?
If the walking is within the first few millimeters and the remainder of the hole is straight, you can counterbore the entry to remove the walked section. If the walking affects the entire hole or causes the hole to exit in the wrong position, the part may need to be scrapped.
Prevention is far more effective than correction for gun drill walking. Invest in quality starting bushings, maintain alignment tolerances, and control entry feed rates. This article reflects industry practice as of 2026.