Appearance
An aerospace machining shop wins a contract to deep-drill 5,000 aluminium alloy 7075-T6 landing gear components per year. The existing gun drilling process uses parameters developed for 4140 steel, resulting in poor surface finish, built-up edge on the tool, and chip evacuation problems in deep holes over 20× diameter. The process engineering team develops material-specific parameters: carbide gun drills at 350–450 SFM cutting speed, 0.003–0.005 IPR feed rate, high-pressure coolant at 500–1,000 PSI with oil-based coolant, and optimised point geometry at 130° point angle with DLC coating to eliminate BUE. The new process reduces surface roughness to Ra 0.8 µm, eliminates built-up edge, and achieves 300 holes per tool regrind.
Aluminium Alloy Properties for Deep Hole Drilling
| Property | 6061-T6 | 7075-T6 | 2024-T3/T4 | Impact on Drilling |
|---|---|---|---|---|
| Hardness (HB) | 95 | 150 | 120 | Higher hardness increases tool wear |
| Tensile strength (MPa) | 310 | 570 | 470 | Affects cutting force and power requirement |
| Elongation (%) | 12–17 | 10–12 | 10–15 | Higher elongation = stringier chips |
| Thermal conductivity (W/m·K) | 167 | 130 | 121 | Affects heat dissipation at cutting zone |
| Machinability rating | Excellent | Good | Good | 7075 requires lower speeds than 6061 |
| Chip formation tendency | Continuous, stringy | More segmented | Continuous | Stringy chips challenge evacuation |
| BUE tendency | Moderate | Low-moderate | Moderate | Requires sharp tools and proper coolant |
Cutting Parameter Recommendations
| Parameter | 6061-T6 | 7075-T6 | 2024-T3/T4 |
|---|---|---|---|
| Cutting speed — carbide gun drill (SFM) | 400–550 | 350–500 | 350–450 |
| Cutting speed — HSS gun drill (SFM) | 200–250 | 150–200 | 175–225 |
| Feed rate (mm/rev) for 6 mm diameter | 0.10–0.15 | 0.08–0.13 | 0.08–0.13 |
| Feed rate (mm/rev) for 10 mm diameter | 0.13–0.18 | 0.10–0.15 | 0.10–0.15 |
| Feed rate (mm/rev) for 20 mm diameter | 0.18–0.25 | 0.15–0.20 | 0.15–0.20 |
| Coolant pressure (bar) | 35–70 | 35–70 | 35–70 |
| Point angle (°) | 118–130 | 118–130 | 118–130 |
| Helix angle | 30–35 | 25–30 | 25–30 |
| Expected surface finish Ra (µm) | 0.4–0.8 | 0.4–0.8 | 0.4–1.2 |
TIP
Aluminium alloys are among the most drillable materials for deep hole drilling, but they present unique challenges: built-up edge (BUE) from the adhesive nature of aluminium, stringy chip formation that can clog evacuation passages, and the need for higher cutting speeds than steel. The single most important factor for successful deep hole drilling in aluminium is maintaining a sharp cutting edge with adequate coolant flow. A dull tool or insufficient coolant will immediately produce BUE, degrading surface finish and increasing torque. For production deep hole drilling in aluminium, always use carbide gun drills with DLC (diamond-like carbon) or AlCrN coating to reduce aluminium adhesion. Run at the highest practical speed within the recommended range to keep the shear zone warm enough to prevent BUE formation — a cutting speed below 200 SFM with carbide is the most common cause of BUE in 7075.
Feed Rate by Drill Diameter
| Drill Diameter (mm) | Feed Rate Range (mm/rev) — 6061 | Feed Rate Range (mm/rev) — 7075 | Feed Rate Range (mm/rev) — 2024 |
|---|---|---|---|
| 3 | 0.04–0.08 | 0.03–0.07 | 0.03–0.07 |
| 6 | 0.10–0.15 | 0.08–0.13 | 0.08–0.13 |
| 8 | 0.12–0.17 | 0.09–0.14 | 0.09–0.14 |
| 10 | 0.13–0.18 | 0.10–0.15 | 0.10–0.15 |
| 12 | 0.15–0.20 | 0.12–0.17 | 0.12–0.17 |
| 16 | 0.17–0.23 | 0.13–0.18 | 0.13–0.18 |
| 20 | 0.18–0.25 | 0.15–0.20 | 0.15–0.20 |
| 25 | 0.20–0.28 | 0.16–0.22 | 0.16–0.22 |
| 32 | 0.22–0.30 | 0.18–0.24 | 0.18–0.24 |
| 40 | 0.25–0.35 | 0.20–0.28 | 0.20–0.28 |
Tool Geometry for Aluminium
| Geometry Parameter | Recommended Value | Rationale |
|---|---|---|
| Point angle | 118–130° | Lower angle reduces thrust; 130° gives best surface finish in 7075 |
| Inner cutting edge angle (φ) | 20° | Reduces BUE tendency in soft/gummy aluminium |
| Outer cutting edge angle (φ₁) | 15° | Matched with inner angle for smooth chip flow |
| Guide hole profile (ψ) | 120° or 180° | Smoothest insertion for φ/φ₁ = 20/15 combination |
| Rake angle | 6–12° positive | High positive rake reduces cutting forces |
| Relief / clearance angle | 8–12° | Prevents rubbing on the aluminium surface |
| Tip displacement (m) | 0.25 × D | Standard gun drill geometry |
| Coating | DLC or AlCrN | Prevents aluminium adhesion (BUE) |
| Guide bushing tolerance | G6 | ISCAR standard for consistent accuracy |
Coolant Selection and Parameters
| Coolant Type | Suitability | Pressure Required | Filtration | Disposal |
|---|---|---|---|---|
| Neat oil (mineral) | Excellent | 35–70 bar | 10–20 µm | Standard oil disposal |
| Semi-synthetic (emulsion) | Good | 35–70 bar | 20–40 µm | Emulsion treatment |
| MQL (oil mist) | Good (shorter holes) | 5–10 bar (air) | N/A | Minimal |
| Cryogenic CO₂ | Excellent (specialised) | 15–30 bar | N/A | Vent to atmosphere |
WARNING
Built-up edge (BUE) is the most common failure mode in deep hole drilling of aluminium alloys. It occurs when aluminium adheres to the cutting edge, altering the effective tool geometry and increasing cutting forces. BUE degrades surface finish, causes oversize holes, and can lead to catastrophic tool failure when the built-up material breaks off, taking carbide particles with it. The three primary factors controlling BUE are: (1) cutting speed — maintain minimum 250 SFM for carbide to keep the shear zone warm; (2) coolant — use high-pressure oil-based coolant or MQL; never dry drill aluminium deep holes; (3) coating — DLC-coated tools reduce aluminium adhesion by 60–80% compared to uncoated carbide. When you see a rough, torn surface finish on the aluminium hole wall, BUE is already present — stop and replace or regrind the tool immediately.
Surface Finish Expectations
| Condition | Ra (µm) | Rz (µm) | Application Suitability |
|---|---|---|---|
| Optimised carbide gun drill, new tool | 0.4–0.8 | 3–6 | Aerospace, hydraulic components |
| Production gun drill, mid-life | 0.8–1.6 | 6–12 | General engineering |
| Worn gun drill or poor parameters | 1.6–3.2 | 12–20 | Unacceptable for most applications |
| BTA drilling, new head | 1.6–3.2 | 12–20 | Acceptable with secondary finishing |
| With BUE present | > 3.2 | > 25 | Reject — requires tool change |
Chip Control in Aluminium Deep Hole Drilling
| Chip Type | Appearance | Cause | Risk Level | Corrective Action |
|---|---|---|---|---|
| Segmental (ideal) | Small disconnected segments | Correct speed and feed | Low | Maintain parameters |
| Long stringy | Continuous ribbon > 50 mm | Feed too low, sharp edge | Medium | Increase feed 15–25% |
| Needle / splinter | Sharp fragments | Feed too high | Medium | Reduce feed |
| Powder / dust | Fine particles | Speed too high, tool worn | High | Reduce speed, inspect tool |
| BUE chips | Irregular with smeared aluminium | BUE on cutting edge | Critical | Replace or regrind tool |
| Burned / discoloured | Brown/blue tint | Speed too high, coolant insufficient | Critical | Reduce speed, increase coolant |
Troubleshooting Aluminium Deep Hole Drilling
| Symptom | Likely Cause | Solution |
|---|---|---|
| Built-up edge on tool | Cutting speed too low (< 200 SFM with carbide) | Increase speed to 350–450 SFM; switch to DLC-coated tool |
| Poor surface finish (Ra > 1.6) | BUE, worn tool, or inadequate coolant | Check and replace tool; verify coolant pressure and filtration |
| Chip clogging / jamming | Feed too low producing stringy chips | Increase feed rate by 15–25%; check coolant flow |
| Oversize hole (> +0.05 mm) | Tool wear, BUE, or misalignment | Replace tool; check guide bushing wear; verify spindle alignment |
| Hole deviation | Misalignment, worn guide pads | Check guide bush-spindle alignment; replace guide pads |
| Tool breakage | Chip packing at entry or mid-hole | Increase coolant pressure; verify pilot hole; adjust peck cycle |
| Torque spike / spindle overload | Chip packing or material hard spot | Stop immediately, retract, clear chips; check material batch |
| Coolant pressure fluctuation | Clogged filter, worn pump, or leak | Inspect filtration; service pump; check seal locations |
| Excessive burr at hole exit | Dull tool, too high feed | Replace tool; reduce feed 20% for last 3 mm |
FAQ
What cutting speed is recommended for gun drilling 7075 aluminium?
For carbide gun drills in 7075-T6, recommended cutting speed is 350–500 SFM (105–150 m/min). For HSS gun drills, reduce to 150–200 SFM (45–60 m/min). 7075 is harder than 6061, so use the lower half of the carbide speed range for longer tool life. Maintain minimum 250 SFM with carbide to prevent built-up edge — running too slow is worse than running too fast for aluminium deep hole drilling.
What feed rate should be used for deep hole drilling aluminium 6061?
For 6061-T6 with carbide gun drills, recommended feed rate depends on hole diameter: 0.10–0.15 mm/rev for 6 mm diameter, 0.13–0.18 mm/rev for 10 mm, 0.18–0.25 mm/rev for 20 mm. The general rule is feed per revolution ≈ D/80 to D/100 where D is drill diameter in mm. For deep holes exceeding 30× diameter, use the conservative end of the range.
What coolant is best for deep hole drilling aluminium alloys?
High-pressure oil-based coolant (neat oil) at 35–70 bar (500–1,000 PSI) is the best choice for deep hole drilling aluminium. Water-miscible semi-synthetic coolants at 8–12% concentration are also effective and more economical. The coolant must have excellent lubricity to prevent built-up edge formation. Filtration to 10–20 µm is essential — unfiltered coolant allows swarf to recirculate and damage the guide pads and cutting edge. MQL (minimum quantity lubrication) is viable for shallower holes under 15× diameter but does not provide sufficient chip evacuation for deeper holes.
How do you prevent built-up edge when deep drilling aluminium?
Built-up edge is prevented by: (1) maintaining cutting speed above 250 SFM (75 m/min) for carbide tools — higher speeds increase shear zone temperature, reducing aluminium adhesion; (2) using DLC (diamond-like carbon) or AlCrN-coated carbide tools — coatings reduce aluminium adhesion by 60–80%; (3) ensuring adequate coolant flow with high lubricity — oil-based coolant is superior to emulsion; (4) maintaining a sharp cutting edge with positive rake geometry (6–12°); (5) using a point angle of at least 130° which reduces contact pressure on the cutting edge. When BUE is detected (rough surface finish, torque increase), replace the tool immediately.
What surface finish can be expected when gun drilling aluminium?
With an optimised carbide gun drill in good condition, surface finish of Ra 0.4–0.8 µm is achievable in 6061 and 7075 aluminium. This is superior to typical gun-drilled steel (Ra 0.8–1.6 µm) due to aluminium's lower hardness and the higher cutting speeds used. The finish depends primarily on tool condition and coolant performance. Worn tools or inadequate coolant will rapidly degrade surface finish to Ra 1.6–3.2 µm. The presence of built-up edge is immediately visible as rough, torn surface finish above Ra 3.2 µm.
What gun drill geometry is best for aluminium?
For aluminium, use a gun drill with φ/φ₁ = 20/15° cutting edge angles (lower than the standard 40/30° used for steel), a positive rake angle of 6–12°, DLC coating, and a point angle of 118–130°. The 20/15° angle combination reduces the tendency for built-up edge in soft/gummy aluminium. The guide hole profile angle (ψ) should be 120° or 180° for smoothest tool insertion. Tip displacement should be 0.25 × diameter — standard gun drill design.
Can BTA drilling be used for aluminium?
Yes, BTA drilling works well for aluminium at diameters above 12 mm. Recommended cutting speed is 80–160 m/min, feed rate 0.06–0.25 mm/rev depending on diameter. Coolant pressure requirements are lower than for steel — typically 20–50 bar. BTA offers faster material removal than gun drilling for large diameters, but surface finish is typically poorer (Ra 1.6–3.2 µm vs 0.4–0.8 µm for gun drilling). The external coolant supply of BTA systems provides excellent chip evacuation for aluminium's stringy chips. Guide bush tolerance should be G6 per ISCAR standards.
What is the difference in drilling 6061 vs 7075 aluminium?
7075-T6 is approximately 60% harder and 80% stronger than 6061-T6, requiring 15–20% lower cutting speeds and 10–15% lower feed rates. 7075 produces more segmented chips that evacuate more easily, while 6061 tends to produce stringy continuous chips that require higher coolant flow for evacuation. Surface finish is similar between the two alloys at optimised parameters. Tool life is approximately 20–30% shorter in 7075 due to higher hardness. 7075 has a lower tendency for built-up edge than 6061.
When should peck drilling be used for aluminium deep holes?
Peck drilling should be used for aluminium deep holes when: (1) the hole depth exceeds 30× diameter; (2) coolant pressure is below 35 bar; (3) the drill diameter is small (< 6 mm) where chip evacuation clearance is limited; (4) drilling 6061 or other high-elongation alloys prone to stringy chips. Recommended peck depth is 2–5× diameter with full retraction to clear chips. For production deep hole drilling with adequate coolant pressure (70 bar+), continuous drilling without pecking is feasible in aluminium — but always validate with chip inspection.
What is the most common mistake in deep hole drilling aluminium?
The most common mistake is running the cutting speed too low. Many operators transfer parameters from steel drilling (80–120 SFM) to aluminium, producing severe built-up edge that destroys the tool and ruins surface finish within the first few holes. Aluminium requires 3–4× higher cutting speed than steel when using carbide tools. The second most common mistake is using inadequate coolant pressure — aluminium stringy chips require strong hydraulic force for evacuation, and coolant pressure below 35 bar will result in chip packing and tool breakage in holes exceeding 15× diameter.
Summary
Deep hole drilling of aluminium alloys 7075, 6061, and 2024 requires fundamentally different parameters than steel drilling — higher cutting speeds (350–550 SFM for carbide), moderate feed rates (0.08–0.25 mm/rev depending on diameter), and high-pressure coolant (35–70 bar) with excellent lubricity to prevent built-up edge. BUE is the primary failure mode and is controlled by maintaining cutting speed above 250 SFM, using DLC-coated tools, and ensuring adequate coolant flow. 7075 is harder and stronger than 6061, requiring 15–20% lower speeds and producing more segmented chips, while 6061 produces stringier chips that demand higher coolant flow for evacuation. Surface finish of Ra 0.4–0.8 µm is achievable with optimised parameters — superior to typical steel gun drilling. Tool geometry should be tailored for aluminium with φ/φ₁ = 20/15° cutting edge angles, positive rake, and DLC coating. Coolant must be filtered to 10–20 µm and delivered at sufficient pressure to evacuate chips continuously through the drill flute or BTA chip tube. With correct parameters, carbide gun drills achieve 200–400 holes per regrind in aluminium alloys, making deep hole drilling of these materials highly productive and economical.