Appearance
Deep hole drilling is the most coolant-intensive operation in machining. A single BTA drilling station can consume 400 litres of oil-based coolant per minute — filling a bathtub every ten seconds. Minimum Quantity Lubrication replaces this flood with a fine aerosol mist at 10–50 mL per hour. The challenge has always been chip evacuation: how to clear debris from a deep, narrow hole when there is no high-pressure liquid column pushing chips out. Recent advances in pressurised MQL delivery have solved this problem for a growing range of applications, making near-dry deep hole drilling a practical reality for gun drilling and, in limited cases, for BTA.
How MQL Works in Deep Hole Drilling
Principle
MQL delivers a precise mixture of compressed air and cutting oil to the cutting zone as a fine aerosol. The air-oil mist lubricates the cutting edge and guide pads while the compressed air provides cooling and assists chip evacuation.
| Component | Function |
|---|---|
| Compressed air supply | Carrier gas for oil mist; provides cooling and chip ejection force |
| Oil metering pump | Delivers precise oil flow (typically 10–50 mL/h) |
| Mixing chamber | Atomises oil into fine droplets (1–50 µm) |
| Delivery channels | Conducts aerosol through spindle and tool to cutting zone |
| Nozzle or tool exit | Directs mist precisely to cutting edge and guide pads |
1-Channel vs. 2-Channel Systems
| Feature | 1-Channel System | 2-Channel System |
|---|---|---|
| Mixing location | Before spindle (single aerosol line through spindle) | At the tool holder (separate air and oil lines to spindle) |
| Pressure range | Up to 16 bar | 4–10 bar |
| Oil viscosity limit | VG 20–32 | Up to VG 100 (more flexible) |
| Response time | Slower (aerosol travels through entire spindle) | Fast (~0.1 sec response) |
| Best for | Standard deep hole drilling | Applications requiring frequent start/stop |
Aerosol Characteristics
MQL aerosol droplets must be small enough to travel through long tool channels without coalescing but large enough to provide adequate lubrication:
| Droplet Size | Behaviour | Best For |
|---|---|---|
| < 5 µm | Floats in air stream, may not reach cutting zone | Low lubricity requirement |
| 5–20 µm | Optimal — follows air stream but deposits on surfaces | General deep hole MQL |
| > 20 µm | May coalesce in channels, uneven delivery | Short holes (< 5× D) |
High-Pressure MQL for Gun Drilling
The Pressure Challenge
Conventional MQL operates at 4–7 bar air pressure. For gun drilling, this is insufficient to:
- Evacuate chips from holes deeper than 20× diameter
- Overcome the pressure drop through small coolant channels in the gun drill
- Maintain adequate cooling at the cutting edge at depth
High-Pressure MQL (HP-MQL)
The breakthrough for gun drilling MQL came from combining pressure boosters with purpose-designed gun drill geometry:
| Parameter | Conventional MQL | HP-MQL |
|---|---|---|
| Air pressure | 4–7 bar | 15–25 bar |
| Oil flow rate | 10–30 mL/h | 20–50 mL/h |
| Minimum drillable diameter | 2.5 mm | 1.2 mm |
| Material removal rate | Baseline | Up to 70% higher |
| Surface finish Ra | 0.4–0.8 µm | 0.2 µm demonstrated |
How it works: A pressure booster takes the shop compressed air line (typically 6 bar) and amplifies it to 25 bar. The pressurised aerosol exits the gun drill tip and expands rapidly, creating a cooling effect that embrittles chips and causes them to break into shorter segments. The higher gas velocity also improves chip transport up the flute.
Tool Geometry Adaptations
Standard gun drills are designed for liquid coolant. For HP-MQL, the following modifications are required:
| Feature | Standard Gun Drill | HP-MQL Gun Drill |
|---|---|---|
| Coolant hole size | 0.3–0.8 mm (small) | 0.5–1.2 mm (larger) |
| Chip flute cross-section | Optimised for liquid chip transport | Wider flute for gas-chip flow |
| Cutting edge geometry | Standard K-land | Modified rake angle for chip breaking |
| Guide pad relief | Standard | Increased relief for reduced friction |
Materials Demonstrated
| Material | Diameter | Depth | Result |
|---|---|---|---|
| High-purity copper (OFHC) | 1.5 mm | 40 mm | Ra 0.2 µm, centreline deviation < 0.03 mm |
| Stainless steel 316L | 1.2 mm | 25 mm | Reliable chip evacuation, acceptable tool life |
| Low-carbon steel 1018 | 3.0 mm | 100 mm | 70% higher feed than flood coolant baseline |
| Aluminium 6061 | 4.0 mm | 120 mm | Excellent surface finish, no built-up edge |
Near-Dry BTA Drilling
Principle
BTA drilling is more challenging than gun drilling for MQL because chips exit through the inner tube rather than an external flute, requiring pneumatic transport of heavy metal chips against gravity.
System Design
Near-dry BTA replaces the flood coolant system with:
| Component | Flood BTA | Near-Dry BTA |
|---|---|---|
| Chip evacuation | Hydraulic (oil at 30–200 bar) | Pneumatic (compressed air at 4–10 bar + mist) |
| Lubrication | Oil in coolant provides lubrication | Metered MQL aerosol injected into air stream |
| Cooling | Liquid coolant absorbs heat | Air expansion + mist evaporation |
| Air flow rate | N/A | 1,000–5,000 L/min |
Chip Breaking Requirements
In near-dry BTA, chip breaking is more critical than in flood BTA because:
- Lower transport force — compressed air at 4–10 bar exerts less force on chips than hydraulic oil at 50–200 bar
- No buoyancy — chips are heavier than the air stream and tend to settle
- Chip jamming — a single long chip can block the inner tube, stopping the process
Results
| Material | Performance vs. Flood BTA |
|---|---|
| Aluminium A2017 | Comparable surface finish; chip shape control more critical |
| Grey cast iron FC200 | Good results; naturally broken chips help |
| Carbon steel S35C | Higher tool wear at guide pads; further optimisation needed |
Research on near-dry BTA has found that water-soluble cutting fluids atomise well but offer less lubrication than oil-based fluids at the guide pads, where boundary lubrication conditions dominate.
MQL Fluids for Deep Hole Drilling
Fluid Types
| Fluid Type | Lubricity | Cooling | Biodegradability | Deep Hole Performance |
|---|---|---|---|---|
| Vegetable oil (rapeseed, palm, coconut) | Good | Moderate | Excellent | Good but variable |
| Synthetic polyol ester (saturated) | Excellent | Good | Excellent | Best-in-class |
| Synthetic polyol ester (unsaturated) | Very good | Good | Excellent | Very good |
| Polyethylene glycol | Moderate | Good | Good | Limited data |
| Mineral oil with additives | Good | Moderate | Poor | Good (traditional) |
Synthetic Ester Advantage
Synthetic polyol esters form a carbon-rich lubricating film on the tool surface through strong adsorption — a mechanism not observed with vegetable oils. This film:
- Reduces friction at the cutting edge by 15–25%
- Protects guide pads from adhesive wear
- Remains stable at the elevated temperatures at the hole bottom
- Forms more readily when compressed air (oxygen) is the carrier gas — oxygen enhances ester adsorption
Selection Guidelines
| Material | Recommended Fluid | Viscosity | Reason |
|---|---|---|---|
| Low-carbon steel | Saturated polyol ester (SE-20 type) | VG 20–32 | Best film strength, low wear |
| Alloy steel | Saturated polyol ester | VG 32 | Higher viscosity for higher loads |
| Stainless steel | Synthetic ester + extreme-pressure additive | VG 20 | Needs EP additives for work hardening |
| Aluminium | Vegetable oil or synthetic ester | VG 20 | Vegetable oil cost-effective for Al |
| Copper | Saturated polyol ester | VG 20 | Excellent surface finish results |
| Cast iron | Vegetable oil | VG 20 | Low-cost, naturally broken chips |
Flow Rate Guidelines
| Diameter | Recommended Flow Rate |
|---|---|
| < 2 mm | 10–15 mL/h |
| 2–5 mm | 15–30 mL/h |
| 5–15 mm | 30–50 mL/h |
| > 15 mm | 50–100 mL/h |
Process Parameters
Parameter Adjustments for MQL
| Parameter | MQL vs. Flood Coolant | Reason |
|---|---|---|
| Cutting speed | Reduce by 10–20% | Less cooling capacity; elevated cutting temperature |
| Feed per revolution | Increase by up to 70% (gun drilling) | Improved chip breaking with aerosol expansion |
| Air pressure | 5–25 bar depending on depth | Higher pressure needed for deeper holes |
| Coolant hole size | Larger than standard | Reduce pressure drop through tool |
Parameters by Application
| Application | Speed (m/min) | Feed (mm/rev) | Air Pressure | Oil Flow | Expected Ra |
|---|---|---|---|---|---|
| Steel, Ø5 mm × 100 mm | 40–60 | 0.015–0.030 | 7–15 bar | 20–30 mL/h | 0.4–0.8 µm |
| Stainless, Ø3 mm × 75 mm | 20–35 | 0.010–0.020 | 10–20 bar | 15–25 mL/h | 0.5–1.0 µm |
| Aluminium, Ø8 mm × 200 mm | 80–150 | 0.020–0.050 | 5–10 bar | 25–40 mL/h | 0.2–0.5 µm |
| Copper, Ø1.5 mm × 40 mm | 30–50 | 0.008–0.015 | 20–25 bar | 10–15 mL/h | 0.2 µm |
| Cast iron, Ø10 mm × 150 mm | 50–80 | 0.020–0.040 | 5–7 bar | 20–30 mL/h | 0.6–1.2 µm |
Peck Cycle Considerations
For MQL deep hole drilling beyond 30× diameter, peck cycles are often necessary:
| Depth Range | Peck Strategy | Time Penalty |
|---|---|---|
| 10–30× D | Continuous drilling — no peck | None |
| 30–50× D | Peck every 10–15× D | +10–20% |
| 50–100× D | Peck every 5–10× D | +20–40% |
| > 100× D | Not recommended for MQL | N/A |
Surface Finish and Tool Life
Surface Finish Comparison
| Material | MQL | Flood Coolant | Dry |
|---|---|---|---|
| 1018 steel | Ra 0.4–0.6 µm | Ra 0.3–0.5 µm | Ra 0.8–1.5 µm |
| 316L stainless | Ra 0.5–1.0 µm | Ra 0.4–0.8 µm | Ra 1.5–3.0 µm |
| Aluminium 6061 | Ra 0.2–0.5 µm | Ra 0.2–0.4 µm | Ra 0.5–1.0 µm |
| Cast iron | Ra 0.6–1.2 µm | Ra 0.5–1.0 µm | Ra 1.0–2.0 µm |
| Copper (OFHC) | Ra 0.2 µm | Ra 0.3–0.5 µm | Ra 1.0–2.5 µm |
Tool Life Comparison
| Condition | Tool Life (relative to flood) | Notes |
|---|---|---|
| Flood coolant | 1.0× (baseline) | Standard performance |
| MQL (general) | 0.5×–1.5× | Highly dependent on material and parameters |
| MQL with synthetic ester | 0.8×–1.5× | Best MQL results; can exceed flood in some cases |
| MQL with vegetable oil | 0.4×–0.8× | Generally lower than synthetic ester |
| Dry machining | 0.05×–0.3× | Not practical for deep holes |
In controlled tests on AISI 4140 steel with through-tool MQL (5% sulfur additive), MQL produced less tool wear across 1,600 holes than either flood coolant or dry machining. The flood coolant condition started with lower wear but gradually exceeded the MQL wear after extended use.
Wear Mechanisms
| Condition | Dominant Wear Mode | Location |
|---|---|---|
| Flood coolant | Uniform flank wear + chipping | Primary cutting lip |
| MQL | Flaking on flank face | Cutting edge + guide pads |
| Dry | Catastrophic (notch wear, edge fracture) | Outer corner |
Advantages and Limitations
When MQL Excels
| Condition | Why MQL Works |
|---|---|
| Cast iron | Naturally broken chips reduce evacuation demand |
| Aluminium | Low cutting forces, good chip formation |
| Copper and brass | Low hardness, short chips |
| Moderate L/D (< 30:1) | Chip evacuation manageable with pressurised air |
| Environmental compliance | Zero coolant disposal cost, no mist collection needed |
| Process simplification | No coolant filtration, no sump maintenance |
When MQL Struggles
| Condition | Limitation |
|---|---|
| Extreme L/D (> 50:1) | Chip evacuation insufficient |
| Difficult materials (Ti, Inconel) | Insufficient cooling at cutting edge |
| Very small diameters (< 1 mm) | Aerosol delivery through tiny channels unreliable |
| High-volume production | Tool life variations less predictable than flood |
| BTA deep drilling | Pneumatic chip transport of heavy chips unreliable |
Economic Comparison (Ø5 mm × 100 mm in Steel, Batch of 1,000)
| Cost Factor | MQL | Flood Coolant |
|---|---|---|
| Coolant consumption | 0.03 L (total) | 200 L (circulating) |
| Coolant cost per hole | $0.001 | $0.02–$0.05 |
| Coolant disposal cost | None | $0.01–$0.03 per hole |
| Tooling cost per hole | $0.50–$1.00 | $0.30–$0.80 |
| Cycle time per hole | 1.5–3.0 min | 1.0–2.0 min |
| Machine hourly rate | $75–$100 | $85–$120 |
| Cost per hole (total) | $2.50–$5.50 | $2.50–$5.00 |
MQL cost per hole is comparable to flood coolant for moderate L/D ratios. The breakeven point depends primarily on coolant disposal costs and the productivity difference from reduced cutting speeds.
FAQ
Q: What is Minimum Quantity Lubrication (MQL) in deep hole drilling? MQL delivers a fine oil mist (10–50 mL/h) to the cutting zone using compressed air, replacing flood coolant systems that circulate hundreds of litres per minute. It is also called near-dry machining.
Q: What is the difference between conventional MQL and high-pressure MQL? Conventional MQL operates at 4–7 bar air pressure. High-pressure MQL uses boosters to reach 15–25 bar, enabling reliable chip evacuation in deep holes down to 1.2 mm diameter.
Q: Can MQL be used for gun drilling? Yes. High-pressure MQL (15–25 bar) with purpose-designed gun drill geometry has demonstrated reliable deep hole drilling in copper, stainless steel, aluminium, and carbon steel with surface finishes down to Ra 0.2 µm.
Q: Can MQL be used for BTA drilling? Near-dry BTA is possible but more challenging. Chip evacuation relies on compressed air at 4–10 bar with 1,000–5,000 L/min flow rate. Best results are in aluminium and cast iron; steel requires further optimisation.
Q: What oil is best for MQL deep hole drilling? Saturated synthetic polyol esters (VG 20–32) provide the best performance due to their strong lubricating film formation. Vegetable oils are cost-effective for aluminium and cast iron but generally less effective for steels.
Q: How does tool life compare between MQL and flood coolant? It depends on material and parameters. MQL with synthetic ester can achieve 80–150% of flood coolant tool life. In some cases (AISI 4140 with sulfur-additive MQL), MQL produced less tool wear than flood coolant over extended runs.
Q: What materials are best suited for MQL deep hole drilling? Cast iron (naturally broken chips), aluminium (low cutting forces), copper and brass, and carbon steels. Stainless steel is possible with specialised fluids and higher pressure. Titanium and nickel superalloys are not recommended.
Q: What is the maximum depth achievable with MQL deep hole drilling? With HP-MQL (25 bar), depths up to 50× diameter have been demonstrated. Beyond 50×, chip evacuation becomes unreliable and peck cycles add significant cycle time. For extreme L/D, flood coolant remains the standard.
Q: What are the environmental benefits of MQL? MQL eliminates coolant disposal costs, reduces fluid consumption by 99.9% compared to flood systems, requires no filtration equipment, and produces dry chips that can be recycled without cleaning.
Q: Does MQL require specialised tooling? Yes. Gun drills for MQL require larger coolant holes, modified cutting edge geometry for chip breaking, and optimised flute cross-sections. Standard gun drills designed for liquid coolant do not perform optimally with MQL.