Appearance
A manufacturer of aerospace structural components for a single-aisle commercial aircraft program was gun drilling Ø12 mm × 480 mm deep (L/D = 40:1) bores in 7075-T7351 aluminum plate for wing spar structural fittings. The production volume was 60,000 parts per year across eight four-spindle gun drilling machines. The existing process used standard C2 grade carbide gun drills with TiAlN coating, 135° point angle, +6° rake angle, and 10 µm edge hone, operating at Vc = 180 m/min (4,780 RPM), f = 0.040 mm/rev, with semi-synthetic coolant at 6% concentration and 60 bar pressure. The chip form was long, stringy, continuous ribbon chips (200–500 mm length, 2–3 mm width) that tended to pack in the gun drill flute, causing chip jamming that triggered machine spindle overload alarms every 15–20 cycles. The forced machine stops for chip clearance (requiring manual removal of the packed chips from the flute) added 3–5 minutes of unplanned downtime per alarm. Tool life averaged 180 m per edge, limited by abrasive wear from the aluminum oxide layer on the 7075 surface. Scrap from bore surface tearing (caused by re-cutting of packed chips in the flute) was 1.5%. A process change evaluated three variables: coating (TiAlN, DLC, uncoated polished), flute surface finish (standard Ra 0.3 µm, polished Ra < 0.1 µm), and chip breaker geometry (standard flat cutting edge, integrated chip breaker with 0.3 mm step and 0.5 mm width). The optimal combination was DLC-coated carbide (reducing friction coefficient in the flute from 0.5 to 0.15), polished flute (Ra < 0.1 µm), and a chip breaker ground into the cutting edge (0.3 mm deep, 0.5 mm wide, positioned 0.8 mm from the outer corner). The chip breaker modified the chip form from continuous ribbons (200–500 mm) to short helical chips (20–50 mm) that evacuated reliably through the flute. Coolant pressure was increased to 80 bar to improve chip transport velocity. The scrap rate dropped to 0.05%, tool life increased to 420 m per edge (2.3×), and machine alarms from chip packing were eliminated.
Aluminum Alloy Metallurgy and Chip Formation
Alloy Characteristics
| Alloy | Temper | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HB) | Thermal Conductivity (W/m·K) | Machinability Rating |
|---|---|---|---|---|---|---|---|
| 7075 | T6/T651 | 570–600 | 500–540 | 11 | 150–175 | 130–155 | Fair to good |
| 7075 | T73/T7351 | 500–540 | 430–470 | 12 | 140–160 | 130–155 | Good |
| 2024 | T3/T351 | 470–500 | 340–380 | 18 | 120–140 | 120–150 | Good |
| 2024 | T4 | 470–500 | 325–375 | 20 | 120 | 120–150 | Good |
| 2024 | T6 | 500–540 | 430–470 | 10 | 140 | 120–150 | Good |
| 6061 | T6/T651 | 310–350 | 275–310 | 17 | 95–105 | 160–180 | Excellent |
| 7050 | T7451 | 510–550 | 460–500 | 11 | 145–165 | 130–155 | Fair to good |
| 2024 | T851 | 485–520 | 405–460 | 6 | 140 | 120–150 | Good |
Chip Form and Control
The primary challenge in deep hole drilling of high-strength aluminum alloys is chip form control. Aluminum produces long, continuous chips at the cutting speeds used in production gun drilling (100–300 m/min). The chip form is determined by the alloy composition, temper condition, cutting speed, feed rate, tool geometry, and the presence or absence of a chip breaker.
Chip breaker geometry is the most effective tool for chip form control in aluminum deep hole drilling. A chip breaker is a step or groove ground into the cutting edge that mechanically curls the chip to a smaller radius, causing it to break under its own bending stress. Effective chip breaker parameters for aluminum gun drilling: step depth of 0.2–0.5 mm; step width of 0.3–0.8 mm; distance from outer corner of 0.5–1.5 mm; and positive rake angle of +6 to +12° to promote chip flow into the breaker.
Tool Geometry Recommendations
| Parameter | 7075-T6/T73 | 2024-T3/T351 | 6061-T6/T651 | Rationale |
|---|---|---|---|---|
| Point angle | 130–140° | 130–140° | 130–140° | Flatter angle reduces chip width and improves chip breaking in high-strength aluminum |
| Rake angle | +6 to +12° | +6 to +12° | +8 to +14° | Positive rake reduces cutting forces and promotes chip flow; higher rake for softer 6061 |
| Relief angle | 8–12° | 8–12° | 10–14° | Adequate clearance prevents rubbing on the bore surface |
| Edge hone | 5–15 µm | 5–15 µm | <10 µm | Sharp edge required for aluminum; excessive hone increases cutting forces and BUE tendency |
| Chip breaker | 0.2–0.4 mm depth, 0.5–0.8 mm width | Same | 0.3–0.5 mm depth, 0.5–1.0 mm width | Deeper breaker for softer alloys |
| Flute surface finish | Ra < 0.1 µm (polished) | Ra < 0.1 µm (polished) | Ra < 0.15 µm (polished) | Polished flute reduces chip adhesion and friction |
| Coating | DLC or uncoated polished | DLC or uncoated polished | Uncoated polished | DLC provides lowest friction; uncoated polished is cost-effective |
Cutting Parameters for Deep Hole Drilling
Gun Drilling Parameters
| Alloy | Temper | Ø (mm) | Vc (m/min) | f (mm/rev) | Coolant Pressure (bar) | Expected Ra (µm) | Expected Tool Life (m/edge) |
|---|---|---|---|---|---|---|---|
| 7075 | T6/T651 | 4–12 | 100–200 | 0.025–0.060 | 60–100 | 0.4–1.2 | 150–400 |
| 7075 | T6/T651 | 12–25 | 120–250 | 0.030–0.080 | 50–80 | 0.5–1.5 | 200–500 |
| 7075 | T73/T7351 | 4–12 | 100–180 | 0.025–0.060 | 60–100 | 0.4–1.2 | 200–500 |
| 7075 | T73/T7351 | 12–25 | 120–220 | 0.030–0.080 | 50–80 | 0.5–1.5 | 250–600 |
| 2024 | T3/T351 | 4–12 | 80–180 | 0.020–0.050 | 60–100 | 0.5–1.5 | 150–350 |
| 2024 | T3/T351 | 12–25 | 100–200 | 0.025–0.070 | 50–80 | 0.6–1.8 | 180–400 |
| 2024 | T6/T851 | 4–12 | 100–200 | 0.025–0.055 | 60–100 | 0.4–1.2 | 200–450 |
| 6061 | T6/T651 | 4–12 | 100–250 | 0.030–0.070 | 40–80 | 0.5–1.5 | 300–700 |
| 6061 | T6/T651 | 12–25 | 120–300 | 0.035–0.100 | 40–70 | 0.6–2.0 | 400–1,000 |
| 7050 | T7451 | 4–12 | 100–180 | 0.025–0.055 | 60–100 | 0.4–1.2 | 150–350 |
| 7050 | T7451 | 12–25 | 120–200 | 0.030–0.070 | 50–80 | 0.5–1.5 | 200–400 |
BTA Drilling Parameters (for larger bores in aluminum)
| Alloy | Ø (mm) | Vc (m/min) | f (mm/rev) | Coolant Pressure (bar) | Expected Ra (µm) | Notes |
|---|---|---|---|---|---|---|
| 7075-T6 | 25–80 | 150–350 | 0.20–0.50 | 20–50 | 1.0–3.0 | High feed rates achievable; chip breaker inserts recommended |
| 2024-T3 | 25–80 | 130–300 | 0.15–0.45 | 20–50 | 1.0–3.5 | Lower speeds for 2024-T3 due to higher ductility |
| 6061-T6 | 25–80 | 180–400 | 0.25–0.60 | 20–50 | 1.0–3.0 | Best machinability of all aluminum alloys |
| 7050-T7451 | 25–80 | 140–300 | 0.18–0.50 | 20–50 | 1.0–3.5 | Similar to 7075 in machinability |
Coolant Selection for Aluminum
Aluminum requires specific coolant considerations that differ from steel or titanium:
| Coolant Type | Suitability | Concentration | Advantages | Disadvantages |
|---|---|---|---|---|
| Semi-synthetic | Excellent | 5–8% | Good cooling, low residue, good surface finish | Lower lubricity than soluble oil |
| Soluble oil | Good | 5–7% | Good lubricity, excellent chip flushing | Higher residue, can stain aluminum |
| Straight oil (light mineral) | Excellent | 100% (neat) | Best surface finish, no staining, long sump life | High cost, fire risk, disposal cost |
| Minimum quantity lubrication | Limited | N/A (0.05 L/h) | Low cost per part, dry chips, no disposal | Insufficient cooling for deep hole drilling L/D > 15:1 |
For aerospace aluminum drilling, semi-synthetic coolant at 6–8% concentration is the most common choice — it provides adequate cooling and lubricity for the moderate cutting forces in aluminum, leaves minimal residue that could interfere with subsequent anodizing or painting, and has good resistance to bacterial growth. Coolant pressure requirements for aluminum are lower than for steel or titanium: 40–80 bar for gun drilling and 20–50 bar for BTA drilling, because the chips are lighter and more easily transported by the coolant flow. Coolant filtration to 30–50 µm is adequate for aluminum — tighter filtration provides minimal tool life benefit because aluminum chips are soft and non-abrasive.
FAQ
What is the best tool coating for gun drilling 7075 aluminum?
Diamond-like carbon (DLC) coating is the best tool coating for gun drilling 7075 aluminum. DLC provides three advantages specific to aluminum: the lowest coefficient of friction of any coating (0.1–0.2 versus 0.5–0.6 for TiAlN), which minimizes chip adhesion to the flute surface and prevents chip packing — the primary failure mode in aluminum gun drilling; high hardness (3,000–5,000 HV), which resists the abrasive wear from the aluminum oxide layer on 7075 surfaces — the secondary wear mechanism; and chemical inertness — DLC does not react with aluminum at cutting temperatures, preventing built-up edge formation. Uncoated polished carbide tools are a cost-effective alternative for lower-volume production or less demanding applications, providing approximately 60–70% of DLC tool life. TiAlN and AlCrN coatings are not recommended for aluminum because they have higher friction coefficients and can undergo chemical reaction with aluminum at the cutting edge, promoting BUE formation.
Why is chip breaking more difficult in 2024-T3 than in 7075-T6?
Chip breaking is more difficult in 2024-T3 than in 7075-T6 because 2024-T3 has higher elongation (18% versus 11% for 7075-T6) — the more ductile material produces longer, more continuous chips that resist bending fracture. The higher ductility means that the chip must be curled to a smaller radius before it reaches the fracture strain required for breaking. Effective chip breaking in 2024-T3 requires a deeper chip breaker step (0.3–0.5 mm versus 0.2–0.4 mm for 7075), a narrower chip breaker width (0.4–0.6 mm), and a more positive rake angle (+8 to +12°). In production, 2024-T3 may also benefit from a peck cycle (shallow peck depth of 3–5× diameter with partial retraction) to mechanically break the chip if the chip breaker alone is insufficient. The temper condition significantly affects chip breaking — 2024-T851 (artificially aged, lower elongation of 6%) produces chips that break more readily than 2024-T3 (naturally aged, higher elongation of 18%).
What coolant pressure is required for gun drilling aluminum?
Coolant pressure requirements for gun drilling aluminum are lower than for steel or titanium — 40–80 bar is typically sufficient for diameters of 4–25 mm at L/D ratios up to 100:1. The lower pressure requirement is due to the low density of aluminum chips (2.7 g/cm³ versus 7.8 g/cm³ for steel) and the smooth, burnished surface of aluminum chips that reduces friction in the flute. For aluminum chip evacuation, coolant velocity at the flute exit should be at least 5 m/s. At 60 bar pressure through a 2.5 mm diameter coolant channel (typical for a Ø12 mm gun drill), the coolant velocity at the channel exit is approximately 40–50 m/s, which provides ample chip transport capacity. Increasing pressure beyond 80 bar provides diminishing returns in aluminum — the incremental improvement in chip evacuation is negligible, and the higher pressure can cause chip re-circulation at the bore entry. The recommended minimum pressure for gun drilling aluminum is 40 bar for diameters above 6 mm and 60 bar for diameters below 6 mm.
How does the surface finish of gun-drilled aluminum compare to BTA-drilled aluminum?
Gun drilling produces consistently better surface finish in aluminum (Ra 0.4–1.8 µm) than BTA drilling (Ra 1.0–3.5 µm). The difference is attributable to three factors: the single-point cutting action of the gun drill produces a clean, continuous feed mark pattern, while the multi-insert BTA head produces overlapping feed marks from multiple insert cutting edges; the gun drill guide pads burnish a larger arc of the bore surface (approximately 270° of the circumference) compared to BTA guide pads (approximately 180°), providing more complete surface smoothing; and the gun drill's lower feed rate (0.025–0.080 mm/rev versus 0.15–0.60 mm/rev for BTA) produces proportionally smaller feed marks. For applications requiring the best possible as-drilled surface finish in aluminum (such as valve bores, seal surfaces, or bearing fits), gun drilling is the preferred method. For larger diameters where BTA is the only practical method, the as-drilled finish typically requires a subsequent finishing operation (skiving and burnishing, honing, or reaming) to meet surface finish requirements below Ra 0.8 µm.
Can aluminum alloys be deep hole drilled with MQL (minimum quantity lubrication)?
MQL can be used for deep hole drilling of aluminum at L/D ratios below approximately 15:1, but is generally not recommended for L/D > 15:1 because the mist lubrication provides insufficient cooling for the extended cutting time, and the reduced coolant flow is inadequate for chip evacuation from deep holes. For L/D > 20:1 in aluminum, flood coolant at 40–80 bar is required — the coolant must fill the flute or chip passage to transport chips from the cutting edge to the bore exit. MQL may be viable for short deep holes (L/D 10:1–15:1) in aluminum where chip evacuation is assisted by gravity (vertical drilling), the hole diameter is above 10 mm (providing adequate chip passage area), and the chip breaker produces short chips (<10 mm) that do not require high coolant velocity for evacuation. For all aluminum deep hole drilling above 15:1 L/D, flood coolant is the standard recommendation.
Disclaimer: The process parameters, tool selection recommendations, and performance data presented in this article are based on published technical literature, tooling manufacturer specifications, and industry-reported experience with deep hole drilling of high-strength aluminum alloys. Actual results depend on specific alloy composition and temper, machine tool rigidity, coolant system capability, and tooling quality. The cutting parameters provided should be used as starting recommendations and verified through process development trials for each specific application. No guarantee of specific tool life, bore quality, or process stability is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.