Chip control is the single most critical operational factor in deep hole drilling. Unlike conventional machining where chips fall away freely, deep hole drilling is an enclosed process — chips must travel the entire length of the hole to exit. When chip evacuation fails, the result is tool breakage within seconds.
This guide covers chip formation mechanics, breaking strategies, evacuation parameters, and a systematic troubleshooting approach.
1. Ideal Chip Shapes
The target chip shape for deep hole drilling is small, segmented, and predictable:
| Chip Shape | What It Indicates | Action Required |
|---|---|---|
| C-shaped or conical (ideal) | Stable cutting, good evacuation | Maintain parameters |
| Sixes and nines (figure-6/9) | Good chip breaking, reliable | Maintain parameters |
| Continuous ribbon | Insufficient chip breaking | Increase feed, check chip breaker |
| Stringy, thin | Feed too low (< 0.05 mm/rev) | Increase feed rate |
| Powder or fan-shaped | Excessive coolant pressure | Reduce pressure |
| Mixed / irregular | Unstable cutting conditions | Check tool wear, rigidity |
Sources: Allied Machine, “Breaking It Down Chip by Chip”; HNCarbide chip control guide.
2. Chip Breaking Mechanisms
Mechanical Chip Breaking
Chip breakers on the cutting edge thin the chip width so it curls and fractures. The key parameters:
| Parameter | Effect |
|---|---|
| Chip breaker depth | Deeper = shorter chips; too deep = powder chips |
| Chip breaker radius | Smaller radius = tighter curl = easier fracture |
| Feed rate | Higher feed = thicker chips = easier fracture |
| Cutting speed | Higher speed = more heat = more ductile = harder to break |
Optimal Parameter Window for Steel
| Parameter | Recommended Range |
|---|---|
| Cutting speed | 50–80 m/min |
| Feed rate | 0.08–0.15 mm/rev (critical — below 0.05 causes stringy chips) |
Feed rates below 0.05 mm/rev produce chips too thin to curl and fracture, leading to clogging.
Chip Deformation Ratio
[ \text{Deformation ratio} = \frac{\text{deformed chip thickness}}{\text{undeformed chip thickness (feed)}} ]
| Material | Typical Ratio |
|---|---|
| Most steels | 2–3:1 |
| Stainless 304/316 | 5–10:1 |
| Pure titanium | 5–10:1 |
Higher ratio = more difficult chip breaking. Materials with high deformation ratios require higher feed rates and specialized chip breaker geometries.
3. Coolant Pressure and Chip Evacuation
Testing on stainless steel 304 (Ø12 mm, L/D=15) demonstrates the critical relationship:
| Coolant Pressure | Chip Morphology | Stability | Surface Finish |
|---|---|---|---|
| 20 bar | Long stringy chips | Poor | Ra 2.5 μm |
| 50 bar | Uniform C-shaped | Stable | Ra 1.6 μm |
| 80 bar | Powder / fragmented | Unstable | Ra 3.2 μm |
Source: HNCarbide chip control guide.
Key finding: There is an optimal coolant pressure window. Too low → chips not evacuated. Too high → chips over-fragment into powder that packs between the tool and bore wall. The optimal window depends on material, diameter, and L/D ratio.
Coolant Volume vs. Pressure
| Factor | Role |
|---|---|
| Volume | Provides kinetic energy to transport chips |
| Pressure | Force delivering volume through the tool |
| Rule | Small diameters need high pressure; large diameters need high volume |
See Coolant Pressure and Flow Rate Guide for detailed calculations.
4. Chip Evacuation by Drilling Method
| Method | Chip Path | Evacuation Area | Min. Coolant Velocity |
|---|---|---|---|
| Gun drilling | External V-groove | 22–26% of hole area | 5–12 m/s |
| BTA drilling | Internal hollow tube | > 60% of hole area | 3–6 m/s |
| Ejector drilling | Internal tube (Venturi) | 35–40% of hole area | 4–8 m/s |
For method-specific details, see The Four Deep Hole Drilling Methods Explained.
5. Material-Specific Chip Control Strategies
| Material | Challenge | Strategy |
|---|---|---|
| Low-carbon steel | Gummy, long chips | Higher speed + higher feed to promote thermal softening |
| Stainless 304/316 | Work-hardens, high deformation ratio | Moderate speed (30–50 m/min), feed > 0.08 mm/rev, never dwell |
| Aluminum | Built-up edge, chip welding | AlCrN coating (not TiAlN), polished flutes, high coolant flow |
| Inconel / superalloys | Stringy, work-hardens | Low speed (10–25 m/min), positive feed, high coolant pressure |
| Cast iron | Abrasive, powder-like chips | TiCN coating, adequate coolant to prevent dust re-cutting |
6. Troubleshooting Chip Problems
| Symptom | Cause | Fix |
|---|---|---|
| Sudden change to longer chips | Tool wear progressing | Replace tool |
| Chips packing in flutes | Insufficient coolant volume | Increase flow rate |
| Torque spikes during drilling | Intermittent chip clogging | Increase pressure, check chip breaker |
| Powder chips at exit | Coolant pressure too high | Reduce pressure |
| Chips welding to cutting edge | BUE from low speed | Increase speed, check coolant concentration |
| Surface scratches in bore | Chips dragging during evacuation | Improve chip breaking, check chip breaker |
For a comprehensive troubleshooting reference, see the Deep Hole Drilling Troubleshooting Guide.
7. Systematic Approach to Chip Control
- Examine chips first — shape, size, consistency are the most accessible diagnostics
- Verify coolant delivery — pressure, volume, filtration, temperature
- Check cutting parameters — feed rate ≥ 0.05 mm/rev minimum
- Assess tool condition — wear, chipping, BUE, chip breaker condition
- Evaluate rigidity — clamping length, L/D ratio, anti-vibration needs
- Monitor load meter — steady = stable evacuation; fluctuating = intermittent clogging
Key Sources
- Allied Machine, “Breaking It Down Chip by Chip: Five Things to Know About Chip Formation” — chip shape analysis
- HNCarbide, “Why Deep Hole Drilling Still Fails Even with High-Pressure Coolant” — coolant pressure window research
- CTE Magazine, “5 Things to Know About Chip Formation” — chip mechanics
- Sandvik Coromant, drilling tips — material-specific recommendations
- ISCAR Drilling Handbook — coolant delivery methods
- Sciencedirect, LFVAD research (nickel superalloy) — advanced chip breaking