Appearance
Aluminium is drilled at 150 m/min while steel is drilled at 60 — but aluminium breaks drills more often. The chip is the problem. A steel chip from a BTA drill comes out as small, broken segments that fall away cleanly. An aluminium chip comes out as a continuous ribbon metres long, travelling at the speed of the cutting edge, carrying the heat of deformation with it. If that ribbon catches on anything — a flute wall, a guide pad, a chip wiper — it packs instantly. The coolant pressure spikes. The torque rises. The drill seizes. The operator who learned deep hole drilling on steel must unlearn everything when switching to aluminium. The material is softer, but the process is harder to control. The difference is not in the cutting forces — they are three times lower than steel. The difference is in the chip that does not break, the built-up edge that forms in seconds, and the burr that grows at breakthrough like a flower unfolding. Every aluminium deep hole drilling operation is a battle against ductility, and the outcome is determined by chip control.
Automotive and Aerospace Applications
| Component | Material | Bore Diameter | Length | Industry |
|---|---|---|---|---|
| Engine block oil gallery | Cast aluminium (A319, A356) | 10–20 mm | 300–800 mm | Automotive |
| Piston oil cooling gallery | Wrought 4032 or 2618 | 3–8 mm | 50–150 mm | Automotive |
| Airframe structural member | 7075-T6, 2024-T3 | 6–25 mm | 200–2,000 mm | Aerospace |
| Landing gear component (aluminium) | 7075-T73, 7050 | 10–50 mm | 300–1,500 mm | Aerospace |
| Fuel system component | 6061-T6, 2024 | 4–20 mm | 100–1,000 mm | Aerospace |
| Hydraulic manifold (aircraft) | 7075-T6 | 6–30 mm | 100–500 mm | Aerospace |
| Wing actuation component | 7075-T6, 2024 | 8–40 mm | 200–800 mm | Aerospace |
| Helicopter gearbox housing | Aluminium casting | 5–50 mm | 100–600 mm | Aerospace |
Aluminium Alloy Families for Deep Hole Drilling
Wrought Alloys (Highest Volume for Deep Hole Drilling)
| Alloy | Temper | Tensile Strength | Hardness | Machinability | Primary Application |
|---|---|---|---|---|---|
| 6061 | T6 | 310 MPa | 95 HB | Excellent — good chip breaking | General structural, fuel systems |
| 2024 | T351 | 470 MPa | 120 HB | Good — moderate chip control | Airframe structures |
| 7075 | T6 / T73 | 570 MPa | 150 HB | Good — higher forces, better chip breaking | High-strength structural, landing gear |
| 7050 | T7451 | 510 MPa | 135 HB | Good — similar to 7075 | Thick-section airframe |
| 2011 | T3 | 310 MPa | 100 HB | Excellent — free-machining | High-production screw machine parts |
| 6262 | T9 | 310 MPa | 120 HB | Excellent — free-machining | Hydraulic components |
Cast Alloys (Automotive Engine Components)
| Alloy | Tensile Strength | Hardness | Machinability | Application |
|---|---|---|---|---|
| A319 (Al-Si-Cu) | 234 MPa | 85 HB | Good | Engine blocks |
| A356 (Al-Si-Mg) | 228 MPa | 80 HB | Good | Cylinder heads, structural castings |
| 390 (Al-Si-Cu-Mg) | 280 MPa | 120 HB | Fair — high silicon causes abrasive wear | Engine blocks (high-silicon) |
Drilling Behaviour Comparison
| Alloy | Chip Formation | BUE Tendency | Burr Tendency | Feed Range |
|---|---|---|---|---|
| 6061-T6 | Stringy, continuous | High | High | 0.04–0.15 mm/rev |
| 2024-T351 | Moderate chip breaking | Moderate | Moderate | 0.03–0.12 mm/rev |
| 7075-T6 | Better chip breaking | Low-moderate | Low-moderate | 0.03–0.12 mm/rev |
| 2011-T3 | Excellent — free-cutting | Very low | Low | 0.05–0.20 mm/rev |
| A356 cast | Moderate — some silicon abrasion | Moderate | Moderate | 0.04–0.15 mm/rev |
Gun Drilling Parameters
Cutting Parameters by Alloy
| Parameter | 6061-T6 | 2024-T351 | 7075-T6 | 7050-T7451 | A356 Cast |
|---|---|---|---|---|---|
| Cutting speed | 120–200 m/min | 80–150 m/min | 80–150 m/min | 80–150 m/min | 100–180 m/min |
| Feed rate | 0.04–0.15 mm/rev | 0.03–0.12 mm/rev | 0.03–0.12 mm/rev | 0.03–0.10 mm/rev | 0.04–0.15 mm/rev |
| Coolant pressure | 40–80 bar | 40–80 bar | 40–80 bar | 40–80 bar | 40–80 bar |
| Coolant type | EP oil or MQL | EP oil | EP oil | EP oil | EP oil or MQL |
| Tool grade | K10–K15, uncoated or DLC | K10–K15, TiAlN | K10–K15, TiAlN or DLC | K10–K15, TiAlN | K15–K20, PCD (high Si) |
| Expected surface finish | Ra 0.2–0.6 µm | Ra 0.3–0.8 µm | Ra 0.2–0.6 µm | Ra 0.2–0.6 µm | Ra 0.4–1.0 µm |
Effect of Parameters on Straightness
Research on 7075-T6 aluminium (Zhao et al., 2021) established the following relationships for gun-drilled straightness:
| Parameter | Effect on Straightness | Recommendation |
|---|---|---|
| Cutting speed | Higher speed improves straightness up to optimum ~120 m/min | 100–120 m/min |
| Feed rate | Lower feed produces better straightness | 0.03–0.06 mm/rev for critical straightness |
| Coolant pressure | Higher pressure improves chip evacuation and reduces deviation | 60–80 bar |
| Counter-rotation | Reduces deviation significantly | Workpiece rotation opposite to drill |
Machine Requirements
| Parameter | Recommendation |
|---|---|
| Spindle speed | 5,000–20,000 rpm (higher for small diameters) |
| Feed resolution | 0.001 mm |
| Coolant filtration | ≤ 30 µm (to prevent nozzle blockage in small-diameter drills) |
| Guide bush | Required for diameters < 10 mm |
| Counter-rotation capability | Recommended for straightness-critical applications |
BTA Drilling of Aluminium
For larger diameter bores (20–200 mm) in aluminium, BTA drilling is the preferred method.
BTA Parameters by Alloy
| Parameter | 6061-T6 | 7075-T6 | A356 Cast |
|---|---|---|---|
| Cutting speed | 150–250 m/min | 100–180 m/min | 120–200 m/min |
| Feed rate | 0.10–0.30 mm/rev | 0.08–0.20 mm/rev | 0.10–0.25 mm/rev |
| Coolant flow | 100–300 L/min | 100–250 L/min | 100–250 L/min |
| Coolant pressure | 20–40 bar | 20–40 bar | 20–40 bar |
| Insert grade | K10, uncoated or polished | K10–K15, TiAlN | K15–K20, PCD or polished |
| Achievable tolerance | H8–H9 | H8–H9 | H9–H10 |
| Achievable surface finish | Ra 0.4–1.0 µm | Ra 0.4–1.0 µm | Ra 0.6–1.6 µm |
Guide Pad Considerations
Aluminium requires specific guide pad materials and geometry:
| Guide Pad Feature | Steel Drilling | Aluminium Drilling |
|---|---|---|
| Pad material | Carbide or HSS | Carbide with polished surface |
| Pad coating | TiAlN or uncoated | DLC or uncoated polished |
| Pad geometry | Standard land width | Narrower land width to reduce friction |
| Coolant groove | Standard | Enhanced — prevents chip packing between pad and bore |
Chip Control Strategy
Chip control is the single most important factor in aluminium deep hole drilling.
Chip Formation by Alloy
| Alloy | Chip Type | Chip Breaking Strategy |
|---|---|---|
| 6061-T6 | Long, stringy, continuous | Chip breaker geometry essential; higher feed helps |
| 2024-T351 | Moderate curl, occasional breaking | Moderate chip breaker; optimised speed-feed combination |
| 7075-T6 | Shorter chips, better breaking | Standard chip breaker; lower feed acceptable |
| 2011-T3 | Small, broken chips | Minimal chip breaker needed |
| A356 cast | Moderate, some powder from silicon | Standard chip breaker |
Chip Breaker Design
| Chip Breaker Feature | Effect |
|---|---|
| Stepped rake face | Creates mechanical weak point in chip at regular intervals |
| Grooved insert geometry | Forces chip curl radius below critical value for fracture |
| Polished flute surface | Reduces friction on chip, prevents packing |
| Optimum feed-speed combination | Matches chip thickness to breaker geometry |
The fundamental relationship for chip breaking in aluminium is:
- Chip thickness (h) = feed per revolution (f) × sin(κ), where κ = cutting edge angle
- A minimum chip thickness of 0.03–0.05 mm is required for reliable chip breaking in most aluminium alloys
- Below this threshold, the chip becomes too thin to break and forms a continuous ribbon
Feed and Speed Optimisation
Research on gun drilling of 7075-T6 established that chip control is optimised at the following parameter combination:
| Parameter | Optimised Value |
|---|---|
| Cutting speed | 100–120 m/min |
| Feed rate | 0.06–0.10 mm/rev |
| Chip thickness | 0.03–0.05 mm |
| Coolant pressure | 60–80 bar |
At lower feed rates (< 0.04 mm/rev), the chip becomes too thin and continuous, increasing the risk of packing. At higher feed rates (> 0.15 mm/rev), cutting forces increase and surface finish degrades.
Coolant and Lubrication
Coolant Strategies Compared
| Method | Typical Application | Advantages | Disadvantages |
|---|---|---|---|
| Flood coolant (emulsion) | General production | Good cooling, low cost | Chip evacuation limited, messy |
| High-pressure oil (EP) | Gun drilling | Excellent lubrication, chip evacuation | Higher cost, requires filtration |
| MQL (minimum quantity lubrication) | Automotive production, dry preferred | Near-dry, low cost, clean parts | Limited cooling at high speeds |
| Dry | Aerospace assembly drilling | No residue, no cleaning | Highest BUE risk |
MQL in Automotive Aluminium Deep Hole Drilling
Minimum Quantity Lubrication has been validated for deep hole drilling of aluminium engine blocks. Research by Hussain et al. (2008) on main oil gallery holes in cast aluminium cylinder blocks demonstrated:
| Parameter | MQL Performance |
|---|---|
| Surface finish | Equivalent to wet drilling |
| True position | Within specification |
| Roundness | Within specification |
| Straightness | Within specification |
| Coolant consumption | < 50 mL/hour vs. 50+ L/minute for flood |
| Part cleanliness | Significantly better — no coolant residue |
Coolant Pressure Requirements
| Bore Diameter | Minimum Coolant Pressure | Recommended Pressure |
|---|---|---|
| < 5 mm | 60 bar | 80–120 bar |
| 5–15 mm | 40 bar | 60–80 bar |
| 15–50 mm | 20 bar | 40–60 bar |
| > 50 mm (BTA) | 10 bar | 20–40 bar |
Quality Requirements
Achievable Quality
| Parameter | Gun Drilling (Aluminium) | BTA Drilling (Aluminium) |
|---|---|---|
| Diameter tolerance | H8–H9 | H8–H10 |
| Roundness | 0.005–0.020 mm | 0.010–0.050 mm |
| Straightness | 0.005–0.020 mm / 100 mm | 0.010–0.050 mm / 100 mm |
| Surface finish (Ra) | 0.2–0.8 µm | 0.4–1.6 µm |
Burr Control
Burr formation at drill entry and exit is a significant quality issue in aluminium deep hole drilling, particularly in aerospace:
| Burr Type | Typical Height (Al 7075) | Mitigation |
|---|---|---|
| Entry burr | 0.05–0.20 mm | Use guide bush, reduce entry feed |
| Exit burr (drill breakthrough) | 0.10–1.00 mm | Reduce feed in last 3–5 mm, use support |
| Exit burr (BTA) | 0.05–0.50 mm | Controlled breakthrough, wiper inserts |
TIP
For aerospace aluminium components, exit burr control often determines the cycle time, not the drilling speed. A burr that exceeds 0.2 mm must be removed manually, adding 5–15 minutes per hole and introducing the risk of bore surface damage. The most cost-effective approach is to reduce feed to 50% in the last 5 mm of the bore, producing a burr that can be left as-drilled or removed with a single pass of a deburring tool. In production, this controlled breakthrough strategy has been shown to reduce burr height by 60–80% compared to constant-feed drilling.
Comparison: Aluminium vs. Steel Deep Hole Drilling
| Factor | Aluminium (6061/7075) | Steel (4140/4340) |
|---|---|---|
| Cutting speed | 80–200 m/min | 60–90 m/min |
| Feed rate | 0.03–0.20 mm/rev | 0.02–0.08 mm/rev |
| Chip formation | Continuous, stringy, ductile | Short, segmented, brittle |
| Chip control approach | Chip breaker geometry essential | Natural breaking |
| Built-up edge risk | High | Low |
| Coolant pressure required | Lower (40–80 bar) | Higher (80–150 bar) |
| Tool wear rate | Low | Moderate |
| Surface finish achievable | Ra 0.2–0.6 µm | Ra 0.4–0.8 µm |
| Burr risk | High | Low |
| Main failure mode | Chip packing / BUE | Tool wear / breakage |
Common Defects and Troubleshooting
| Defect | Cause | Corrective Action |
|---|---|---|
| Chip packing / drill seizure | Stringy chip jams in flute | Increase coolant pressure, add chip breaker, reduce feed |
| Built-up edge on drill | Low cutting speed, inadequate lubrication | Increase speed to 120+ m/min, use DLC coating |
| Oversize bore (entry) | Drill vibration at start | Improve guide bush fit, counter-rotate workpiece |
| Exit burr too large | Feed too high at breakthrough | Reduce feed in last 5 mm to 50% |
| Surface tearing | BUE on cutting edge | Change coating to DLC, increase speed |
| Spiral marks on bore | Chip rubbing between drill and bore | Increase coolant flow, check chip breaker |
| Diameter taper (entry larger) | Drill deflection | Use stiffer drill shank, reduce feed |
| Rough surface in high-silicon cast | Silicon particle abrasion | Switch to PCD-tipped tool |
FAQ
Q: Which aluminium alloy is easiest to deep hole drill? 2011-T3 (free-machining aluminium) is the easiest — it produces small, broken chips with minimal BUE. 6061-T6 is the most common general-purpose alloy and drills well with proper chip control. 7075-T6 requires more attention to parameters but produces good results.
Q: What is the biggest challenge in deep hole drilling of aluminium? Chip control. Aluminium produces long, ductile, continuous chips that easily pack in the drill flute. Chip breaker geometry on the cutting edge, optimised feed-speed combinations, and adequate coolant pressure are essential to produce chips that break and evacuate cleanly.
Q: What coolant is recommended for gun drilling aluminium? Extreme-pressure oil at 40–80 bar is standard for gun drilling. For aerospace components where oil residue is unacceptable, MQL (minimum quantity lubrication) or water-miscible coolants are used. Dry drilling is practical for shallow holes (< 10× diameter) but risky for deep holes.
Q: What cutting speed is recommended for 7075-T6 aluminium? 80–150 m/min is the standard range. The optimal speed for straightness and chip control is approximately 100–120 m/min. Higher speeds improve surface finish but increase heat generation. Above 200 m/min, the risk of built-up edge increases.
Q: What is the best coating for gun drills in aluminium? DLC (diamond-like carbon) coating provides the lowest friction and best resistance to built-up edge formation. For high-silicon cast aluminium alloys (> 12% Si), PCD (polycrystalline diamond) tipped tools are recommended due to the abrasive wear from silicon particles.
Q: How does deep hole drilling of aluminium differ from steel? Aluminium requires 2–3× higher cutting speed but produces lower cutting forces. The main challenge shifts from tool wear (steel) to chip control and BUE (aluminium). Coolant pressure requirements are lower, but burr control is more critical. Tool life is typically much longer in aluminium.
Q: Can BTA drilling be used for aluminium? Yes. BTA drilling is used for larger diameter bores (20–200 mm) in aluminium, particularly in aerospace structural components. Cutting speeds of 100–250 m/min are achievable with appropriate insert grades and chip breaker geometry.
Q: What surface finish can be achieved gun drilling aluminium? Ra 0.2–0.6 µm is typical for gun-drilled aluminium. The material's low hardness and good machinability allow excellent surface finish when parameters are optimised. BTA drilling typically achieves Ra 0.4–1.6 µm depending on the alloy and feed rate.
Q: How do you prevent burrs in aluminium deep hole drilling? Reduce feed to 50% of the drilling feed for the last 3–5 mm before breakthrough. Use a guide bush at entry to minimise entry burr. For thin-walled components, a sacrificial backing plate at the exit face eliminates exit burr formation. In production, controlled breakthrough is the most effective single measure.
Q: What are the main automotive applications for deep hole drilling of aluminium? Engine block main oil galleries (gun-drilled 10–20 mm × 300–800 mm in cast aluminium), piston oil cooling galleries, transmission valve body bores, and hydraulic manifold passages. The automotive industry increasingly uses MQL or near-dry drilling to eliminate coolant residue and reduce environmental costs.