Appearance
Deep hole drilling parameters cannot be copied from conventional drilling handbooks. The extended cutting edge engagement, high coolant pressure, and self-piloting tool guidance require a different approach to speed, feed, and coolant selection.
Cutting Speed
Cutting speed in deep hole drilling is typically lower than conventional drilling due to the continuous cutting edge engagement and the challenges of heat dissipation in deep bores.
Speed by Material
| Material | Gun Drilling (m/min) | BTA Drilling (m/min) |
|---|---|---|
| Aluminum | 80 – 160 | 80 – 160 |
| Brass | 80 – 150 | 80 – 150 |
| Carbon & alloy steels | 70 – 100 | 70 – 100 |
| Cast iron (gray, ductile) | 70 – 100 | 70 – 100 |
| Stainless steel | 50 – 80 | 50 – 80 |
| Titanium alloys | 30 – 60 | 30 – 50 |
| Inconel / superalloys | 15 – 40 | 15 – 30 |
Key considerations:
- Softer materials (aluminum, brass) tolerate higher speeds but require careful chip breakage control
- Harder materials (titanium, Inconel) need lower speeds to manage heat — exceeding recommended speed causes rapid edge wear and work hardening
- Cast iron runs well at moderate speeds with the advantage of naturally broken chips
Speed Selection Strategy
Start at the lower end of the recommended range and increase in increments of 5–10 m/min while monitoring:
- Chip color and shape (blue chips indicate excessive heat)
- Coolant pressure stability (pressure drop signals chip congestion)
- Surface finish quality (degradation suggests speed is too high)
Speed vs. tool life trade-off
In deep hole drilling, a 20% reduction in cutting speed can double tool life, while a 20% increase may reduce tool life by half. The optimal speed balances productivity with acceptable tool consumption.
Feed Rate
Feed rate is the primary productivity lever in deep hole drilling. However, it is tightly constrained by chip evacuation capacity — the tool can only remove chips as fast as the coolant can flush them out.
Feed Rate by Diameter (Gun Drilling)
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) | Aluminum (mm/rev) |
|---|---|---|---|
| 3 – 4 | 0.007 – 0.013 | 0.009 – 0.038 | 0.008 – 0.050 |
| 5 – 6 | 0.016 – 0.026 | 0.018 – 0.068 | 0.020 – 0.080 |
| 8 – 9 | 0.024 – 0.036 | 0.032 – 0.096 | 0.030 – 0.120 |
| 10 – 12 | 0.030 – 0.049 | 0.050 – 0.120 | 0.035 – 0.174 |
| 16 – 18 | 0.048 – 0.079 | 0.079 – 0.170 | 0.060 – 0.200 |
| 20 – 24 | 0.060 – 0.107 | 0.106 – 0.207 | 0.070 – 0.254 |
Feed Rate by Diameter (BTA Drilling)
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) |
|---|---|---|
| 10 – 12 | 0.030 – 0.049 | 0.050 – 0.120 |
| 18 – 20 | 0.054 – 0.091 | 0.090 – 0.191 |
| 20 – 24 | 0.060 – 0.107 | 0.106 – 0.207 |
| 28 – 32 | 0.079 – 0.134 | 0.140 – 0.237 |
| 32 – 40 | 0.085 – 0.154 | 0.160 – 0.245 |
BTA drilling achieves 5–7 times higher feed rates than gun drilling at comparable diameters due to its multi-edge cutting head and more efficient internal chip evacuation.
Feed Rate Selection Strategy
Start at the lower third of the recommended range. Increase feed gradually while checking:
- Chip formation (well-broken chips indicate correct feed)
- Coolant pressure gauge (a rising trend signals chip buildup)
- Spindle load (sudden increases indicate feed overload)
Feed rate and chip breakage are linked
Insufficient feed produces thin, stringy chips that are difficult to evacuate and tend to jam the chip passage. Excessive feed produces thick chips that may not fit through the evacuation path. The correct feed produces chips that break into small, C-shaped segments.
Coolant Pressure and Flow
Coolant is the single most critical parameter in deep hole drilling — it cools the cutting zone, lubricates the guide pads, and evacuates chips. Without adequate pressure and flow, the process will fail.
Minimum Coolant Pressure by Diameter
| Diameter (mm) | Gun Drilling (bar) | BTA Drilling (bar) |
|---|---|---|
| 2 – 4 | 80 – 150 | — |
| 5 – 8 | 50 – 120 | — |
| 10 – 15 | 30 – 100 | 40 – 80 |
| 16 – 25 | 20 – 80 | 30 – 60 |
| 25 – 50 | 15 – 60 | 20 – 50 |
Coolant Flow Rate (BTA Drilling)
| Diameter (mm) | Flow Rate (L/min) |
|---|---|
| 10 – 15 | 50 – 100 |
| 16 – 25 | 100 – 200 |
| 26 – 40 | 200 – 350 |
| 40 – 60 | 300 – 450 |
Gun drilling requires higher pressure but lower flow volume. BTA drilling requires lower pressure but significantly higher flow — the annular gap between tube and bore wall presents less flow resistance.
Coolant Quality Requirements
| Parameter | Requirement | Why |
|---|---|---|
| Filtration | ≤ 20 µm | Prevents guide pad scoring and nozzle blockage |
| Concentration | 5 – 10% emulsion | Lubricity, cooling, and corrosion protection |
| Cleanliness | No tramp oil contamination | Prevents chip packing in filtration system |
Startup Procedure
- Enter pilot hole at reduced speed (200–300 rpm) with minimal feed
- Turn on coolant and wait for pressure to stabilize (2–3 seconds)
- Ramp to full speed over 2–3 seconds
- Ramp to target feed over the first 2–3× diameter of depth
- Monitor pressure — a stable reading confirms normal chip evacuation
Warning Signs
| Reading | Indication | Action |
|---|---|---|
| Pressure drop > 10% | Chip blockage or tube crack | Stop immediately, retract tool |
| Pressure rising trend | Gradual chip buildup | Increase flow, check chip shape |
| Pressure fluctuation | Unstable chip formation | Adjust feed or speed |
| Sudden pressure loss | Tool breakage or seal failure | Emergency stop |
Parameter Selection Workflow
┌─────────────────────────────┐
│ 1. Material → cutting speed │
│ (start at lower third) │
└─────────────┬───────────────┘
↓
┌─────────────────────────────┐
│ 2. Diameter → feed rate │
│ (start at lower third) │
└─────────────┬───────────────┘
↓
┌─────────────────────────────┐
│ 3. Diameter + depth → │
│ coolant pressure target │
└─────────────┬───────────────┘
↓
┌─────────────────────────────┐
│ 4. Start drilling at │
│ reduced parameters │
└─────────────┬───────────────┘
↓
┌─────────────────────────────┐
│ 5. Ramp to target over │
│ first 2–3× diameter │
└─────────────┬───────────────┘
↓
┌─────────────────────────────┐
│ 6. Monitor: pressure, │
│ chips, spindle load │
└─────────────┬───────────────┘
↓
┌─────────────────────────────┐
│ 7. Adjust if needed: │
│ ↑ speed → better finish │
│ ↑ feed → better chips │
│ ↑ coolant → evac issues │
└─────────────────────────────┘Summary
Successful deep hole drilling depends on selecting the right combination of cutting speed, feed rate, and coolant pressure — these parameters are interdependent. Changes to one parameter often require adjustments to the others.
| Parameter | Starting Point | Adjust If |
|---|---|---|
| Cutting speed | Lower third of material range | Poor finish → increase; short tool life → decrease |
| Feed rate | Lower third of diameter range | Stringy chips → increase; tool overload → decrease |
| Coolant pressure | Mid-range for diameter | Chip jamming → increase; tool breakage → check pressure |
FAQ
What happens if coolant pressure is too low?
Insufficient coolant pressure is the most common cause of tool failure in deep hole drilling. Chips are not fully evacuated, leading to packing in the chip passage, which blocks further chip flow and eventually causes the tool to seize or break. A minimum pressure of 30 bar is recommended for diameters above 15 mm, increasing to 100+ bar for diameters below 5 mm.
Why are deep hole drilling feed rates lower than conventional drilling?
Deep hole drilling feeds may appear low per revolution, but the continuous cutting action means the tool is always engaged. In conventional drilling, the tool periodically exits the hole for chip clearing (pecking). Deep hole drilling runs continuously, so the effective material removal rate is often comparable or higher despite lower instantaneous feed rates.
How do coolant pressure requirements change with depth?
Pressure requirements increase with depth due to flow friction along the chip passage. A system that delivers adequate pressure at the pump may deliver significantly less at the cutting edge. For holes exceeding 100× diameter, additional coolant boost or intermediate pump stages may be needed to maintain sufficient pressure at the cutting zone.
Can I use conventional cutting fluid for deep hole drilling?
Conventional water-soluble coolants are acceptable for general-purpose deep hole drilling (5–10% emulsion), provided filtration is adequate (≤ 20 µm). For difficult materials (stainless steel, titanium, superalloys), synthetic oils at higher concentrations (8–10%) significantly improve tool life by providing better lubricity and heat dissipation.
What is the correct sequence for setting up parameters?
Start with coolant pressure and flow, set cutting speed next, then feed rate last. Coolant is the foundation — without adequate chip evacuation, no combination of speed and feed will work. Once coolant is verified, set speed based on material, then adjust feed based on chip formation observation during the first few holes.
Parameters are starting recommendations. Actual values depend on machine condition, coolant system capacity, workpiece material, and specific tooling geometry. Always consult your tool supplier for application-specific data.