Peck drilling is a technique that every machinist knows from conventional drilling: drill a little, retract, clear chips, drill again. In deep hole drilling, this approach can do more harm than good. The re-engagement after each peck creates an entry condition that gun drills and BTA drills are not designed to handle.
Conventional Peck Drilling vs Deep Hole Drilling
Key Differences
| Aspect | Conventional Drilling | Deep Hole Drilling |
|---|
| Cutting edge geometry | Symmetrical (self-centering) | Asymmetrical (needs bushing guidance) |
| Chip evacuation | Flutes on drill | Coolant pressure through annular gap or drill tube |
| Re-entry after peck | Drill follows existing hole | Drill can walk, chip can jam at re-entry |
| Typical peck depth | 2–5× diameter | Not recommended for gun drilling |
| Chip clearance method | Mechanical flutes | Coolant pressure |
| Re-entry risk | Low | High — risk of drill walking or damage |
Tip: If you find yourself needing to peck in gun drilling, something else is wrong. Gun drills are designed to drill continuously at depth-to-diameter ratios of 100:1 or more without pecking. The need to peck indicates insufficient coolant pressure, incorrect chip breaking, or a parameter problem.
Gun Drilling: When to Never Peck
Why Pecking Is Harmful in Gun Drilling
| Problem | Mechanism | Consequence |
|---|
| Drill walking | At re-entry, the drill tip re-engages without bushing support | Crooked hole, oversize entry |
| Chip jamming | Chips settle in bore during retraction, get packed at re-entry | Blocked annular gap, tool breakage |
| Surface damage | Guide pads lose contact with bore, re-engage at different orientation | Scoring, burnishing marks |
| Coolant disruption | Coolant flow interrupted, chips not flushed continuously | Pressure fluctuation, inconsistent cutting |
| Thermal cycling | Cutting edge cools during retraction, heats on re-entry | Thermal fatigue cracking on carbide |
Acceptable Exceptions for Pecking in Gun Drilling
| Exception | Condition | Procedure |
|---|
| Extreme depth (> 200:1 ratio) | Coolant pressure insufficient at full depth | Peck only at pre-planned depths, with reduced entry feed |
| Interrupted cuts (cross holes) | Risk of chip accumulation beyond cross hole | Retract before cross hole, clear chips, reduce feed through cross hole |
| Machine limitation | Machine lacks adequate coolant pressure | Minimize pecking, use longest peck depth possible |
If You Must Peck in Gun Drilling
| Parameter | Recommendation | Reason |
|---|
| Peck depth | Maximum possible (minimize re-entries) | Each re-entry is a risk event |
| Retraction distance | Retract fully from hole | Clear all chips |
| Re-entry feed rate | 30–50% of production feed (same as initial entry) | Prevent walking on re-entry |
| Re-entry delay | 1–2 second dwell | Let coolant stabilize |
| Maximum peck cycles | 3–5 cycles max | After that, the process needs a fundamental fix |
Warning: Pecking in gun drilling should be a last resort, not a planned strategy. Each peck cycle creates a new entry condition inside the bore, and the drill must re-establish its cutting position without the support of the starting bushing. If you need more than 3 peck cycles to complete a hole, stop and fix the root cause (coolant pressure, chip breaking, or parameters).
BTA Drilling: When Pecking May Be Necessary
Acceptable Pecking Scenarios in BTA
| Scenario | Why Pecking Helps | Risk Level |
|---|
| Very deep holes (> 80:1 ratio) | Coolant may not clear chips at full depth | Moderate |
| Ductile materials (aluminum, low-carbon steel) | Chips tend to string and accumulate | Moderate |
| Partially blocked drill tube | Pressure drop indicates restriction | High — fix blockage first |
| Machine with limited coolant capacity | Pump cannot maintain flow at depth | Low — design limitation |
| Unstable chip formation | Chips inconsistent, some large | Moderate |
BTA Pecking Procedure
| Step | Action | Detail |
|---|
| 1 | Drill to planned peck depth | Typically 50–150 mm depending on diameter |
| 2 | Stop feed, continue coolant flow | Flush chips without cutting |
| 3 | Dwell 2–5 seconds | Allow chips to clear through drill tube |
| 4 | Retract at rapid speed | Keep coolant flowing during retraction |
| 5 | Fully withdraw from hole | Clear all chips |
| 6 | Re-enter at reduced feed | 50–70% of production feed until full depth |
| 7 | Resume production feed | Continue to next peck depth |
BTA Peck Depth Guidelines
| Hole Diameter | Suggested Peck Depth | Maximum Peck Depth |
|---|
| 20–30 mm | 50–80 mm | 100 mm |
| 30–50 mm | 80–120 mm | 150 mm |
| 50–80 mm | 100–150 mm | 200 mm |
| 80–120 mm | 150–200 mm | 300 mm |
Alternatives to Pecking
Better Solutions for Chip Management
| Problem | Alternative to Pecking | Why It Is Better |
|---|
| Chips not clearing | Increase coolant pressure | Continuous process, no re-entry risk |
| Stringy chips | Improve chip breaking (feed, geometry) | Fixes root cause |
| Chip accumulation | Increase coolant flow (larger pump) | Continuous evacuation |
| Pressure drop at depth | Install pressure booster or larger pump | Maintains pressure at all depths |
| Chip shape inconsistent | Optimize feed rate for consistent chip formation | Eliminates the need to peck |
| Drill tube partially blocked | Clean drill tube, check for internal obstructions | Restores full flow |
Continuous Chip Evacuation Methods
| Method | How It Works | Application |
|---|
| Through-spindle coolant (gun drilling) | Coolant travels through drill, exits at tip, returns through annular gap | Gun drilling — standard |
| Reverse flow (BTA) | Coolant through annular gap, chips return through drill tube | BTA drilling — standard |
| Ejector system | Venturi effect draws chips out | Ejector drilling |
| Mist cooling | Air/oil mist for micro-drilling | Small diameter, shallow holes |
| High-pressure booster | Increases pressure at depth | All methods, extreme depth |
Process Design Considerations
Designing for Continuous Drilling
| Design Factor | Target | Benefit |
|---|
| Coolant pump pressure | 1.5× calculated requirement at max depth | Margin for chip evacuation |
| Chip breaker geometry | Optimized for the material | Consistent chip breaking without pecking |
| Feed rate | Above minimum chip-breaking threshold | Prevents stringy chips |
| Guide bushing condition | Within 0.01 mm of nominal | Stable drill guidance for the full hole |
| Material selection | Uniform hardness | Consistent chip formation |
Machine Selection for Continuous vs Peck Drilling
| Machine Feature | Continuous Drilling | Supports Peck Drilling |
|---|
| Coolant pump | High pressure, adequate flow | Same |
| Coolant tank capacity | Large enough for full-depth flow | Same |
| CNC control | Standard | Peck cycle programming capability |
| Spindle brake | Standard | Required for stopped-spindle peck |
| Chip handling | Standard chip conveyor | Larger chip bin for batch discharge |
FAQ
Should I use peck cycle in gun drilling?
No — pecking should be avoided in gun drilling. Gun drills are designed for continuous drilling and rely on constant coolant flow for chip evacuation and guide pad lubrication. Each peck creates a re-entry condition that risks drill walking, chip jamming, and surface damage. If you need to peck, fix the root cause instead.
When is pecking acceptable in deep hole drilling?
Pecking is occasionally acceptable in BTA drilling when drilling very deep holes (over 80:1 ratio), machining ductile materials that produce stringy chips, or when the machine's coolant system cannot maintain adequate pressure at full depth. BTA pecking is less risky than gun drill pecking because the chip evacuation path (through the drill tube) is not disrupted by retraction.
How do I clear chips without pecking?
Increase coolant pressure to maintain chip transport velocity at depth, optimize chip breaker geometry for the material being drilled, and maintain feed rate above the minimum chip-breaking threshold. These three adjustments eliminate the need for pecking in most applications.
What happens if chips accumulate in the bore without pecking?
Chips that accumulate in the bore reduce the annular gap, causing coolant pressure to rise. If chips continue to accumulate, the gap becomes blocked entirely, coolant flow stops, and the drill either jams or breaks. Chip accumulation is dangerous — if you suspect it, retract and inspect rather than continuing to feed.
Can pecking cause drill breakage?
Yes — especially in gun drilling. When a gun drill retracts and re-enters, chips that settled in the bore during retraction get packed at the drill tip. This packed chip mass blocks coolant flow and causes immediate overheating and breakage. Pecking-related breakage is one of the most common failure modes when shops try to adapt conventional drilling practices to deep hole work.
Continuous drilling is the goal for deep hole drilling. Design the process to eliminate the need for pecking rather than planning for it. This article reflects industry practice as of 2026.
