Appearance
Aluminium is the material that makes deep hole drilling look easy — until it is not. The same softness that permits cutting speeds three times those of steel also produces built-up edge at the slightest provocation. The same ductility that gives aluminium its favourable machinability rating also produces chips that weld themselves to the cutting edge if the speed drops below a critical threshold. And among aluminium alloys, the difference between 6061 and 7075 is not a matter of degree — it is a difference in kind.
Aluminium Alloy Classes for Deep Hole Drilling
Wrought Alloys: 6xxx and 7xxx Series
| Alloy | Condition | Tensile Strength (MPa) | Hardness (HB) | Machinability Rating |
|---|---|---|---|---|
| 6061 | T6 | 310 | 95 | 90% |
| 6063 | T5 | 205 | 75 | 85% |
| 6082 | T6 | 340 | 100 | 85% |
| 7075 | T6 | 570 | 150 | 60% |
| 7050 | T7451 | 510 | 135 | 65% |
| 2024 | T3 | 470 | 120 | 70% |
6061 is the general-purpose standard. Its machinability rating of 90% (relative to 1212 free-machining steel) means it is one of the most forgiving materials for deep hole drilling. Chips are typically short and C-shaped at moderate feed rates, and tool wear is minimal.
7075 is substantially stronger but harder on tooling. Its machinability rating of 60–70% reflects higher cutting forces, more abrasive wear from zinc and copper intermetallic compounds, and a tendency to produce longer, stringier chips that are more difficult to evacuate from deep holes.
Cast Aluminium Alloys
| Alloy | Silicon Content | Hardness (HB) | Application |
|---|---|---|---|
| A356 (AlSi7Mg) | 7% | 80 | General casting, automotive |
| A319 (AlSi6Cu4) | 6% | 95 | Engine blocks, cylinder heads |
| A390 (AlSi17Cu4Mg) | 17% | 120 | High-wear applications |
| ADC12 (AlSi11Cu3) | 11% | 105 | Die casting |
The distinguishing feature of cast aluminium alloys is their silicon content. Hypoeutectic alloys (below 12% Si) are machinable with standard carbide tooling. Hypereutectic alloys (A390 at 17% Si) contain primary silicon crystals that are harder than tungsten carbide — these require PCD (polycrystalline diamond) tooling for production runs.
Cutting Speed Selection
Recommended Cutting Speeds by Method
| Alloy | Gun Drill (Carbide) | BTA (Carbide) | HSS Twist Drill |
|---|---|---|---|
| 6061-T6 | 100–160 m/min | 80–140 m/min | 60–90 m/min |
| 6063-T5 | 110–160 m/min | 90–140 m/min | 60–90 m/min |
| 7075-T6 | 80–120 m/min | 70–110 m/min | 40–60 m/min |
| 2024-T3 | 90–130 m/min | 80–120 m/min | 50–70 m/min |
| A356 cast | 80–120 m/min | 70–100 m/min | 40–60 m/min |
| A390 (17% Si) | 50–80 m/min (PCD) | 40–70 m/min (PCD) | Not recommended |
Spindle Speed Calculation
n = (1000 × Vc) / (π × D)| Drill Diameter | Vc = 80 m/min | Vc = 120 m/min | Vc = 160 m/min |
|---|---|---|---|
| 5 mm | 5093 RPM | 7640 RPM | 10186 RPM |
| 10 mm | 2546 RPM | 3820 RPM | 5093 RPM |
| 20 mm | 1273 RPM | 1910 RPM | 2546 RPM |
| 30 mm | 849 RPM | 1273 RPM | 1698 RPM |
Tip: Aluminium permits high spindle speeds, but the machine spindle bearings and the workpiece setup must be capable of those speeds. Many deep hole drilling machines designed for steel are limited to 3,000–5,000 RPM, which becomes the binding constraint for small-diameter aluminium drilling. If the machine cannot reach the calculated RPM, the solution is to use the maximum available RPM and accept a lower cutting speed — but never drop below 60 m/min for 6061 or 40 m/min for 7075, as BUE formation becomes severe below these thresholds.
Feed Rate Selection
Gun Drilling Feed Rates by Diameter and Alloy
| Diameter (mm) | 6061 (mm/rev) | 7075 (mm/rev) | Cast Al (mm/rev) |
|---|---|---|---|
| 3.0–3.99 | 0.006–0.037 | 0.005–0.025 | 0.005–0.030 |
| 5.0–5.99 | 0.010–0.109 | 0.008–0.070 | 0.008–0.080 |
| 8.0–9.99 | 0.020–0.144 | 0.015–0.100 | 0.015–0.110 |
| 10.0–11.99 | 0.025–0.174 | 0.020–0.120 | 0.020–0.130 |
| 16.0–17.99 | 0.050–0.209 | 0.035–0.150 | 0.035–0.160 |
| 20.0–23.99 | 0.060–0.254 | 0.045–0.180 | 0.045–0.190 |
| 32.0–39.99 | 0.096–0.455 | 0.070–0.320 | 0.070–0.340 |
| 40–50 | 0.105–0.488 | 0.080–0.350 | 0.080–0.370 |
BTA Feed Rates by Diameter
| Diameter (mm) | 6061 (mm/rev) | 7075 (mm/rev) |
|---|---|---|
| 18–30 | 0.08–0.25 | 0.06–0.18 |
| 30–50 | 0.10–0.35 | 0.08–0.25 |
| 50–80 | 0.15–0.45 | 0.10–0.30 |
| 80–120 | 0.20–0.55 | 0.15–0.35 |
Key principle for feed rate selection in aluminium: Unlike steel, where feed rate is the primary control for chip breakage, aluminium chip formation is dominated by alloy composition. 6061 chips break readily even at moderate feeds; 7075 requires higher feeds to break chips but is constrained by surface finish requirements and guide pad loading.
Chip Control
Chip Formation by Alloy
| Alloy | Chip Type at Recommended Feed | Chip Control Strategy |
|---|---|---|
| 6061 | C-shaped and short spiral | Standard chipbreaker geometry sufficient |
| 7075 | Long continuous spiral | Requires chipbreaker or increased feed |
| 2024 | Short to medium spiral | Moderate pecking recommended |
| A356 (cast) | Short broken chips | Least problematic for evacuation |
| A390 (hypereutectic) | Fine powder-like chips | Abrasive — prioritise tool wear over chip form |
Built-Up Edge Prevention
BUE is the most common cause of surface finish degradation and hole quality problems in aluminium deep hole drilling. The mechanism is straightforward: aluminium welds to the carbide cutting edge at temperatures below a critical threshold, forming a built-up layer that periodically breaks off, taking fragments of the cutting edge with it.
| Condition | BUE Risk | Recommended Action |
|---|---|---|
| Vc > 120 m/min (6061) | Low | Maintain speed — no action needed |
| Vc 60–120 m/min (6061) | Moderate | Increase speed or increase coolant pressure |
| Vc < 60 m/min (6061) | High | Increase speed — BUE becomes severe below this threshold |
| Vc > 80 m/min (7075) | Moderate | Acceptable with good coolant coverage |
| Vc < 60 m/min (7075) | High | Increase speed — do not operate in this range |
| Coolant pressure < 50 bar | High | Increase coolant pressure regardless of cutting speed |
| Dull cutting edge (VB > 0.15 mm) | Very high | Regrind or index immediately |
Warning: BUE is not merely a surface quality issue. When the built-up edge releases, it can pull carbide fragments from the cutting edge, creating a notch that accelerates flank wear and can lead to catastrophic edge failure. If you observe surface finish degradation in aluminium, stop and inspect the cutting edge — the problem may already be structural, not cosmetic.
Coolant Strategy for Chip Evacuation
| Parameter | Recommendation |
|---|---|
| Minimum pressure (gun drilling) | 50 bar (725 PSI) |
| Recommended pressure (6061) | 50–100 bar |
| Recommended pressure (7075) | 70–140 bar |
| Coolant type | Water-soluble emulsion at 5–8% concentration, or oil-based |
| Filtration | 20 μm or better — aluminium fines can recirculate and cause scratching |
| Temperature control | Maintain below 40°C — higher temperatures increase BUE tendency |
Guide Pad Wear
Wear Comparison by Alloy
| Alloy | Relative Guide Pad Wear | Dominant Wear Mechanism |
|---|---|---|
| 6061 | 1.0 (baseline) | Adhesive — aluminium transfer to carbide pad |
| 7075 | 1.3–1.5 | Adhesive + abrasive from intermetallic compounds |
| A356 (7% Si) | 1.2–1.4 | Abrasive from silicon particles |
| A390 (17% Si) | 5–10× baseline | Severe abrasive — requires PCD pad inserts |
The adhesive wear mechanism in aluminium deep hole drilling is distinct from steel: aluminium transfers to the carbide guide pad surface, forming a built-up layer that increases friction and can score the bore wall. This is most severe in 6061 and other low-hardness alloys. Paradoxically, the harder 7075 causes less adhesive transfer but more abrasive wear.
Guide Pad Material Recommendations
| Application | Pad Material | Notes |
|---|---|---|
| 6061 production | Standard WC (K20) | Acceptable — inspect every 50–100 holes |
| 7075 production | WC with fine grain size | Better abrasion resistance |
| High-volume 6061/7075 | WC with DLC coating | Reduces adhesive transfer significantly |
| A390 or high-Si cast | PCD-tipped | Only option for production runs — WC wears too quickly |
| Small diameters (< 6 mm) | Solid carbide drill (integral) | No separate guide pad — drill body provides guidance |
Alloy-Specific Parameter Strategies
6061-T6 Strategy
6061 is the most forgiving aluminium alloy for deep hole drilling:
| Phase | Vc (m/min) | fn (mm/rev) | Coolant |
|---|---|---|---|
| Guide entry | 60–80 | 50% of target | Full pressure |
| Production | 120–160 | Per diameter table | Full pressure |
| High surface finish | 140–160 | Lower end of range | Full pressure, filtered |
Key advantages: Low cutting forces, predictable chip formation, minimal guide pad wear. 6061 can be drilled at parameters approaching those of free-machining brass.
7075-T6 Strategy
7075 requires a more conservative approach:
| Phase | Vc (m/min) | fn (mm/rev) | Coolant |
|---|---|---|---|
| Guide entry | 40–60 | 50% of target | Full pressure, 100 bar minimum |
| Production | 80–110 | Per diameter table (mid-range) | Full pressure |
| Chip control | 90–110 | Upper end of range | Full pressure — increase if chips are stringy |
Critical difference from 6061: 7075's higher hardness means that edge chipping is a real risk if feed rate is too high or if the cutting edge is dull. Monitor flank wear closely — the interval between acceptable wear and chipping is shorter than in 6061.
Cast Aluminium Strategy
For hypoeutectic cast alloys (A356, A319):
- Use carbide tooling with AlTiN coating for abrasion resistance
- Reduce cutting speeds by 20–30% relative to 6061
- Expect 1.2–1.5× guide pad wear relative to 6061
- Chip evacuation is generally good — cast alloys produce shorter chips than wrought alloys
For hypereutectic cast alloys (A390, A392):
- PCD-tipped tooling is required for production runs
- Carbide tooling will show rapid flank wear — only suitable for short runs or prototype work
- Cutting speeds must be reduced to 50–80 m/min even with PCD
- Guide pads must be PCD-tipped or will wear beyond tolerance within 10–20 holes
Parameter Troubleshooting
| Problem | 6061 | 7075 | Cast Al |
|---|---|---|---|
| Built-up edge | Increase Vc above 80 m/min | Increase Vc above 60 m/min | Less common — check coolant concentration |
| Poor surface finish | Check for BUE, reduce feed | Reduce feed, increase Vc | Check silicon particle pull-out — may need PCD |
| Oversize hole | Check guide bush clearance (may need tighter fit for Al) | Check for edge chipping | Check for abrasive wear on guide pads |
| Chip clogging | Increase feed or add chipbreaker | Increase feed — 7075 needs higher fn for chip breakage | Rare — cast Al chips are short |
| Rapid guide pad wear | Check for aluminium transfer — DLC coating helps | Check for abrasive wear — fine-grain WC recommended | Upgrade to PCD-tipped pads |
| Spindle power high | Check for BUE — power drops when BUE clears | Check feed rate — 7075 generates higher cutting forces | Check for pad galling |
FAQ
What cutting speed should I use for gun drilling 6061 aluminium?
For carbide gun drilling of 6061-T6, use 100–160 m/min (328–525 SFM). Below 80 m/min, built-up edge formation becomes severe. For 7075-T6, reduce to 80–120 m/min.
How does 6061 compare to 7075 for deep hole drilling?
6061 is significantly more machinable (90% vs 60–70% rating). It produces shorter chips, causes less guide pad wear (approximately 30% less than 7075), and permits higher cutting speeds. 7075's higher strength (570 MPa vs 310 MPa) comes at the cost of 20–30% slower speeds, more abrasive wear, and more difficult chip control.
What feed rate should I use for a 10 mm gun drill in aluminium?
For 6061, feed rate range is 0.025–0.174 mm/rev. Start at 0.08 mm/rev and increase until chip form is satisfactory. For 7075, use 0.020–0.120 mm/rev, starting at 0.06 mm/rev.
Why am I getting built-up edge when drilling aluminium?
BUE occurs when cutting speed is too low. For 6061, BUE becomes severe below 80 m/min. For 7075, below 60 m/min. Increase spindle speed, verify coolant pressure is above 50 bar, and check that the cutting edge is sharp — a dull edge accelerates BUE formation.
What coolant pressure is needed for deep hole drilling of aluminium?
Minimum 50 bar (725 PSI) for gun drilling. Recommended: 50–100 bar for 6061, 70–140 bar for 7075. For very deep holes (L/D > 50:1), increase to 100–200 bar to ensure adequate chip evacuation over the full hole length.
Do I need special tooling for cast aluminium alloys?
Hypoeutectic cast alloys (A356, A319, below 12% Si) can be machined with standard carbide tooling, preferably AlTiN-coated. Hypereutectic alloys (A390, 17% Si) require PCD-tipped tooling for production runs — primary silicon crystals in these alloys are harder than tungsten carbide.
What chip form should I target for aluminium deep hole drilling?
For 6061, short C-shaped chips (2–5 mm) are ideal. For 7075, short to medium spiral chips are typical — add a chipbreaker or increase feed rate if chips are long and continuous. Cast aluminium naturally produces short broken chips. If chips are long and stringy in any aluminium alloy, increase feed rate.
How do I prevent aluminium from galling on guide pads?
Use polished or DLC-coated carbide guide pads to reduce aluminium adhesion. Maintain coolant pressure above 50 bar to flush aluminium fines from the pad-bore interface. For 7075 in production volumes, fine-grain WC guide pads with DLC coating provide the best combination of abrasion resistance and anti-galling properties.
What tool coating is best for drilling aluminium?
Uncoated or DLC-coated carbide is generally preferred for aluminium. TiAlN and AlTiN coatings are suitable for cast aluminium (where abrasion resistance matters) but can increase friction with wrought alloys, exacerbating BUE. PCD-tipped tools are the best option for hypereutectic cast alloys and high-volume production.
What is the maximum depth-to-diameter ratio achievable in aluminium?
Aluminium can be gun drilled to L/D ratios of 100:1 or more, limited primarily by chip evacuation rather than tool rigidity. The limiting factor is coolant pressure drop over the hole length. For L/D > 50:1, expect to increase coolant pressure by 50–100% compared to shorter holes. Guide bush clearance may also need to be reduced for very deep holes to prevent drill whipping.
Conclusion
Aluminium deep hole drilling is defined less by the limits of tool strength — as it is for steel and superalloys — and more by the challenge of built-up edge formation, chip control, and guide pad adhesion. The three alloy classes covered here behave differently: 6061 is forgiving and can be drilled at speeds up to 160 m/min with standard carbide tooling; 7075 requires 20–30% lower speeds, generates more guide pad wear, and produces chips that are harder to evacuate; cast aluminium alloys with high silicon content require PCD tooling to handle the abrasive primary silicon phase. Across all aluminium alloys, the single most important parameter is cutting speed — it must be high enough (>80 m/min for 6061, >60 m/min for 7075) to prevent the built-up edge that degrades surface finish, damages the cutting edge, and ultimately causes the tool to fail. Coolant pressure is the second critical factor — aluminium fines are abrasive when recirculated, and inadequate pressure allows chip packing that can stall the tool in the bore.