Deep Hole Drilling in Aerospace Manufacturing
Aerospace components demand the highest levels of precision, surface integrity, and reliability in deep hole drilling. A bore that fails at 10,000 cycles in an automotive part might be acceptable. In a landing gear strut or turbine shaft, the same defect could be catastrophic.
This guide covers the main aerospace applications, the materials involved, and the parameters required to meet aerospace standards.
Key Applications and Parameters
1. Landing Gear Components
Landing gear is the most demanding deep hole drilling application in aerospace. Struts, cylinders, and pistons are made from high-strength alloys — typically 300M alloy steel or titanium Ti-6Al-4V — with bore diameters ranging from 50 mm to over 200 mm and depths exceeding 2 meters.
Case study — Alta Precision landing gear struts: A UNISIG B700 Drop Bed machine drills 300M alloy steel landing gear struts with a bore diameter of 9 inches (229 mm) and depth exceeding 7 feet (2.1 meters). The machine uses counter-rotation of tool and workpiece to achieve straightness requirements, with a 124 hp tool headstock and 90 hp work headstock (UNISIG / Aero-Mag, 2024).
| Parameter | Value |
|---|---|
| Bore diameter | 50 — 230 mm |
| Bore depth | 500 — 3,000+ mm |
| L/D ratio | 10:1 — 40:1 |
| Tolerance | IT7 — IT8 |
| Surface finish | Ra 0.4 — 0.8 µm |
| Straightness | 0.05 — 0.1 mm/m |
| Material | 300M steel (52 — 56 HRC), Ti-6Al-4V |
Production parameters (BTA drilling, 300M steel):
- Cutting speed: 25 — 45 m/min
- Feed: 0.08 — 0.18 mm/rev
- Coolant: Oil-based, 30 — 50 bar, high volume (250 gpm on UNISIG machines)
- Tool: BTA single or multi-tooth head, carbide inserts, TiAlN-coated
Landing gear actuating cylinders require bottle boring — creating internal contours at depths beyond conventional boring bar reach. UNISIG’s servo-controlled bottle boring technology enables this on a single machine setup (UNISIG, 2024).
2. Turbine Shafts and Rotating Components
Gas turbine engine shafts require oil feed holes drilled axially or at angles through difficult materials like Inconel 718 and Waspaloy.
| Parameter | Value |
|---|---|
| Diameter | 3 — 20 mm |
| Depth | 100 — 800 mm |
| L/D ratio | 10:1 — 50:1 |
| Tolerance | IT7 — IT8 |
| Surface finish | Ra ≤ 0.6 µm |
| Concentricity | ≤ 0.01 mm over 20×D |
Production parameters (gun drilling, Inconel 718):
- Cutting speed: 15 — 30 m/min
- Feed: 0.008 — 0.025 mm/rev
- Coolant: 70 — 100 bar, chlorine-free EP oil
- Tool: Solid carbide gun drill, AlTiN or AlCrN-coated
Inconel 718’s low thermal conductivity (6.5 W/m·K) causes extreme heat concentration at the cutting edge. High-pressure coolant is not optional — it is required to keep the cutting zone temperature below the diffusion wear threshold (Neway Aerotech, 2024).
For turbine disk cooling holes (3 — 12 mm diameter, 15:1 L/D typical), jet-cooled drills (JCD) operating at 70 — 120 bar coolant pressure achieve dimensional accuracy of ±0.02 mm.
3. Hydraulic Valve Blocks and Manifolds
Aircraft hydraulic systems operate at 3,000 — 5,000 PSI and require precision bore intersections in valve blocks. These intersections must be burr-free to prevent valve spool sticking or contamination.
| Parameter | Value |
|---|---|
| Diameter | 3 — 25 mm |
| Depth | 50 — 400 mm |
| L/D ratio | 10:1 — 30:1 |
| Tolerance | IT7 — IT8 |
| Surface finish | Ra 0.8 — 1.6 µm |
| Hole position accuracy | ±0.05 mm |
| Material | Aluminum 7075, 17-4 PH stainless, titanium |
Production parameters (gun drilling):
- Cutting speed: 40 — 80 m/min (titanium), 80 — 150 m/min (aluminum)
- Feed: 0.02 — 0.10 mm/rev
- Coolant: 60 — 80 bar
4. Structural Components
Aircraft wing spars, ribs, and bulkheads require deep fastener holes in thick sections. Aluminum-lithium alloys (2198, 2050) are increasingly used for these components due to their weight savings.
| Parameter | Value |
|---|---|
| Diameter | 6 — 30 mm |
| Depth | 50 — 500 mm |
| L/D ratio | 5:1 — 25:1 |
| Tolerance | IT8 — IT9 |
| Material | Al-Li alloys (2198, 2050), 7075-T6, 2024-T3 |
Al-Li alloy drilling challenges: Aluminum-lithium alloys present specific difficulties: built-up edge formation, swarf welding in flutes, and rapid tool wear. Conventional carbide drills achieve as few as 10 holes per insert.
Solution: Sumitomo WDX drills with Aurora DLC (diamond-like carbon) coating achieved 200 holes per insert — a 20× improvement — by suppressing BUE through low friction (coefficient 0.05 — 0.2) and high hardness (Machinery Magazine, 2024).
Typical parameters for Al-Li alloys:
| Hole Diameter | Speed | Feed |
|---|---|---|
| 14 mm | 13,500 rpm | 0.075 mm/rev |
| 20 mm | 10,000 rpm | 0.15 mm/rev |
| 26 mm | 7,700 rpm | 0.20 mm/rev |
Materials Comparison
| Material | Tensile Strength | Hardness | Thermal Cond. (W/m·K) | Machinability Rating |
|---|---|---|---|---|
| 300M steel | 1,900 — 2,100 MPa | 52 — 56 HRC | 36 | Difficult |
| Ti-6Al-4V | 900 — 1,100 MPa | 33 — 38 HRC | 6.7 | Very difficult |
| Inconel 718 | 1,310 — 1,510 MPa | 33 — 46 HRC | 6.5 | Extremely difficult |
| 17-4 PH stainless | 1,000 — 1,300 MPa | 33 — 44 HRC | 16 | Moderate |
| 7075-T6 aluminum | 540 — 570 MPa | 150 BHN | 130 | Easy |
| Al-Li 2198 | 480 — 520 MPa | 140 BHN | 120 | Moderate (BUE issues) |
Aerospace Quality Requirements
Deep hole drilling for aerospace is governed by standards that go beyond dimensional tolerance.
Surface Integrity
The surface condition of a deep hole bore directly affects component fatigue life. For aerospace, three factors are critical:
1. Surface roughness: Target Ra ≤ 0.8 µm for fatigue-critical components. Higher roughness creates stress concentration sites.
2. Residual stress: BTA drilling with properly designed guide pads produces compressive residual stress (-459 to -556 MPa) in the bore surface, which improves fatigue performance. Gun drilling produces less compressive stress but superior surface finish. The choice depends on which factor is more critical for the specific application.
3. White etching layers (WEL): These form at high cutting speeds and feeds when temperatures exceed the material’s austenitization point. WEL are brittle (up to 3× harder than the substrate) and reduce fatigue life. Aerospace applications require parameter selection that avoids WEL formation — verified through microstructural inspection.
Certification Requirements
- AS9100 / AS9102: First article inspection requirements for production parts
- NADCAP: Process certification for special processes including drilling
- Material traceability: Tooling and process parameters must be documented per component
- Non-destructive testing: Bores may require fluorescent penetrant inspection (FPI) or eddy current testing
See our surface finish guide for detailed information on Ra values, residual stress, and WEL in deep hole drilling.
Production Strategies
Single-Setup Machining
Aerospace landing gear components benefit from combination machines that can drill, bore, and contour in a single setup. UNISIG’s B-Series machines combine:
- BTA drilling for roughing
- Bottle boring for internal contouring
- Skiving and roller burnishing for final surface finish
- All in one machine, no part transfer
This eliminates the alignment errors and cycle time penalties of moving large, asymmetric landing gear components between machines.
Counter-Rotation
For extreme straightness requirements — typical in landing gear — counter-rotation (tool and workpiece rotating in opposite directions) cancels rotational errors and improves concentricity. This is a standard feature on aerospace-grade deep hole drilling machines but is not available on CNC conversions.
Automated Work Cells
UNISIG has documented cells producing over 100 aerospace parts per hour using multi-spindle machines with pick-and-place automation and conveyor loading.
Starting Points Summary
New aerospace deep hole drilling job — landing gear cylinder, 300M steel, 100 mm diameter, 30:1 L/D:
- Method: BTA drilling
- Speed: 35 m/min
- Feed: 0.12 mm/rev
- Coolant: 40 bar, oil-based, 200+ gpm
- Tool: Single-tooth BTA head, carbide inserts, TiAlN-coated
- Expected finish: Ra 0.8 — 1.6 µm, IT8 tolerance
Turbine shaft oil hole, Inconel 718, 6 mm diameter, 40:1 L/D:
- Method: Gun drilling
- Speed: 20 m/min
- Feed: 0.015 mm/rev
- Coolant: 80 bar, chlorine-free EP oil
- Tool: AlCrN-coated solid carbide gun drill
- Expected finish: Ra 0.4 — 0.8 µm, IT7 tolerance
References
- UNISIG. Aerospace Industry Applications — Landing Gear and Hydraulic Cylinders. 2024.
- UNISIG / Aero-Mag. Handling the Challenges of Landing Gear Drilling. Aerospace Manufacturing, 2024.
- UNISIG. Bottle Boring for Deep Internal Contouring. Technical Resource, 2024.
- Precihole Machine Tools. Deep Hole Drilling Solutions for the Aerospace Industry. 2024.
- Neway Aerotech. Superalloy Deep Hole Drilling for High-Performance Aerospace and Power Generation Parts. Case Study, 2024.
- CNC Machining Shops. Deep Hole Drilling — Process, Methods & Selection Guide. 2024.
- Tube Hollows. Gun Drilling for Aerospace Shafts and Precision Components. Technical Data, 2024.
- Goldcattle. CNC Deep Hole Drilling — Precision Long Depth Machining. 2024.
- Machinery Magazine. Drilling Issue Resolved — Al-Li Alloy Wing Spar Application. 2024.
- Kent CNC. Gun Drilling: Precision Holes at Extreme Depths. 2024.
- TAES. Deep Hole Drilling Gun Drill — Aerospace Applications. 2024.
- Aero-Mag / UNISIG. Alta Precision Landing Gear Case Study. Aerospace Manufacturing, 2024.
- ThomasNet. Drilling System Supports Large, Irregular Aerospace Components. 2024.
- Sumitomo Electric. WDX Drill with Aurora DLC Coating for Aluminum-Lithium Alloys. Technical Data, 2024.