Appearance
A landing gear strut is the most structurally demanding hydraulic cylinder in existence. It supports the full weight of the aircraft on take-off, absorbs the impact of landing at 3 m/s vertical descent rate, and then cycles through compression and extension for taxi, take-off, and landing again — 100,000+ cycles over the life of the component. The bore of this cylinder is gun-drilled or BTA-drilled from a solid forging of 300M steel at 50 HRC, then skived and roller-burnished to a mirror finish of Ra 0.2 µm. The diameter tolerance is ±0.025 mm over a length of 2,000 mm. The straightness is 0.025 mm per 25 mm of length. A leak at any point means the component is scrapped — and the forging alone costs more than a luxury automobile.
Aerospace Deep Hole Drilling Applications
| Component | Typical Diameter | Length | Material | Key Requirement |
|---|---|---|---|---|
| Landing gear outer cylinder | 150–300 mm | 1,500–3,000 mm | 300M steel (50–54 HRC) | Leak-tight bore, fatigue resistance |
| Landing gear inner piston | 100–200 mm | 1,200–2,500 mm | 300M or 4340M | Wear-resistant surface, hard chrome or HVOF |
| Nose gear actuator | 50–120 mm | 500–1,200 mm | 15-5PH stainless | Corrosion resistance |
| Main gear actuator | 80–180 mm | 800–2,000 mm | Ti-6Al-4V or 300M | High strength-to-weight |
| Helicopter rotor shaft | 40–100 mm | 800–1,800 mm | 4340 or 300M | Concentricity, torsional fatigue |
| Wing flap actuator | 40–80 mm | 600–1,500 mm | Ti-6Al-4V | Straightness, surface finish |
| Thrust reverser actuator | 30–60 mm | 400–1,000 mm | Inconel 718 or 15-5PH | High-temperature capability |
Materials for Landing Gear Components
Primary Aerospace Steels
| Material | Tensile Strength | Typical Hardness | Machinability | Application |
|---|---|---|---|---|
| 300M (AMS 6257) | 1,900–2,100 MPa | 50–54 HRC | Very difficult — hardest common landing gear steel | Main landing gear, large actuators |
| 4340M (AMS 6419) | 1,600–1,900 MPa | 45–50 HRC | Difficult | Smaller actuators, secondary structure |
| 4330V (AMS 6427) | 1,500–1,700 MPa | 42–47 HRC | Moderate | Helicopter rotor components |
| 15-5PH (AMS 5659) | 1,100–1,300 MPa | 33–38 HRC | Fair | Corrosion-resistant actuators |
| Custom 465 (AMS 5936) | 1,500–1,700 MPa | 48–52 HRC | Very difficult | High-strength corrosion resistant |
Titanium and Superalloys
| Material | Tensile Strength | Hardness | Challenge | Application |
|---|---|---|---|---|
| Ti-6Al-4V (AMS 4928) | 900–1,100 MPa | 32–36 HRC | Low thermal conductivity, BUE | Actuator bodies, lightweight components |
| Ti-10V-2Fe-3Al | 1,200–1,400 MPa | 38–42 HRC | High cutting forces | High-strength landing gear components |
| Inconel 718 (AMS 5663) | 1,300–1,500 MPa | 35–45 HRC | Work-hardening, high temperature | Thrust reverser, hot section |
WARNING
300M steel at 50+ HRC is one of the most difficult materials to deep hole drill. The specific cutting force exceeds 5,000 N/mm², tool edge compressive stress approaches 3,000 MPa, and the material's high silicon content (1.6%) creates abrasive carbides that accelerate flank wear. Gun drilling 300M requires K30-grade carbide with AlCrN coating, T-land edge preparation of 0.05–0.08 mm, and cutting speeds below 25 m/min. Every other parameter must be subordinated to tool survival.
Drilling Process by Method
Gun Drilling (Small to Medium Diameters: 5–50 mm)
| Parameter | 300M Steel (50–54 HRC) | 4340M (45–50 HRC) | Ti-6Al-4V |
|---|---|---|---|
| Cutting speed | 15–25 m/min | 20–35 m/min | 25–40 m/min |
| Feed rate | 0.008–0.020 mm/rev | 0.010–0.025 mm/rev | 0.015–0.035 mm/rev |
| Coolant pressure | 150–200 bar | 120–180 bar | 100–150 bar |
| Coolant type | EP oil, high sulphur | EP oil | EP oil (chlorine-free) |
| Tool grade | K30–K35, AlCrN coated | K20–K30, TiAlN coated | K15–K20, TiAlN or DLC |
| Expected tool life | 5–20 holes per regrind | 20–50 holes per regrind | 15–40 holes per regrind |
| Surface finish (as-drilled) | Ra 0.4–0.8 µm | Ra 0.4–0.8 µm | Ra 0.6–1.2 µm |
BTA / STS Drilling (Large Diameters: 50–300 mm)
For landing gear outer cylinders, STS (Single Tube System) or BTA drilling is used:
| Parameter | 300M Steel (50–54 HRC) | 4340M | Ti-6Al-4V |
|---|---|---|---|
| Cutting speed | 15–25 m/min | 20–35 m/min | 20–35 m/min |
| Feed rate | 0.05–0.15 mm/rev | 0.08–0.20 mm/rev | 0.06–0.15 mm/rev |
| Coolant flow | 200–500 L/min | 200–500 L/min | 150–350 L/min |
| Coolant pressure | 40–80 bar | 30–60 bar | 30–50 bar |
| Insert grade | K30, AlCrN, wiper edge | K20, TiAlN | K15–K20, TiAlN |
| Number of inserts | 2–3 | 2–3 | 2 |
Bottle Boring
Aerospace landing gear cylinders often require internal profiling — a larger diameter at one end (for the piston seal gland) and smaller diameter along the main bore. This is achieved through bottle boring:
| Bottle Boring Feature | Capability |
|---|---|
| Bore diameter range | 50–400 mm |
| Internal step changes | Multiple diameters in one bore |
| Corner radii | R 1–10 mm (tool-dependent) |
| Diameter tolerance | ±0.025 mm |
| Positional accuracy of internal profile | ±0.1 mm from reference face |
| Surface finish in bottle-bored section | Ra 0.4–0.8 µm (as-bored) |
Bottle boring uses actuated tool heads that extend or retract cutting elements at programmed depths. The tool is positioned at the correct depth, the cutting elements expand to the programmed diameter, and the bore is cut on the retraction stroke.
Skiving and Roller Burnishing
For landing gear cylinders and actuators, skiving and roller burnishing (SRB) is the standard finishing process after deep hole drilling:
| Parameter | Typical Value |
|---|---|
| Skiving depth of cut (radial) | 0.2–0.5 mm per side |
| Skiving speed | 150–250 m/min |
| Skiving feed | 1–4 mm/rev (multi-blade tool) |
| Burnishing speed | 150–250 m/min |
| Burnishing feed | 1–4 mm/rev (same stroke) |
| Surface finish after SRB | Ra 0.1–0.4 µm |
| Diameter tolerance after SRB | H8 or better |
| Roundness | ≤ 0.02 mm |
| Cylindricity | ≤ 0.05 mm over 500 mm |
| Surface hardness increase | Up to 50% (cold working) |
SRB vs. Honing for Aerospace Cylinders
| Factor | Skiving + Roller Burnishing | Honing |
|---|---|---|
| Cycle time (2 m bore) | 15–30 minutes | 2–6 hours |
| Surface finish | Ra 0.1–0.4 µm | Ra 0.05–0.2 µm |
| Surface structure | Compressive residual stress, plateau finish | Sharp peaks, no compressive stress |
| Seal wear rate | Lower (plateau surface retains lubricant) | Higher (abrasive peaks wear seals) |
| Oil retention | Moderate (plateau finish) | Higher (more porous) |
| Stick-slip at low speed | Superior (low friction) | Inferior |
| Process cost | Lower (single pass, faster) | Higher (multiple passes, slower) |
TIP
For aerospace hydraulic actuators, skiving and roller burnishing has largely replaced honing as the finishing process of choice. The combination of compressive residual stress, plateau surface finish, and 8–20× faster cycle time makes SRB the preferred method. The seal manufacturers' surface finish specification of Ra 0.2–0.4 µm is reliably achieved in a single pass, and the resulting surface produces lower dynamic friction and longer seal life than honed surfaces.
Tolerances and Quality Requirements
Dimensional Tolerances
| Feature | Commercial Aircraft | Military / High-Performance |
|---|---|---|
| Bore diameter | H8 (±0.027 mm for 100 mm dia.) | H7 (±0.017 mm for 100 mm dia.) |
| Straightness | 0.025 mm per 25 mm length | 0.013 mm per 25 mm length |
| Concentricity (bore to OD) | ≤ 0.10 mm TIR | ≤ 0.05 mm TIR |
| Roundness | ≤ 0.03 mm | ≤ 0.015 mm |
| Cylindricity | ≤ 0.05 mm over 500 mm | ≤ 0.03 mm over 500 mm |
| Surface finish (final) | Ra 0.2–0.4 µm | Ra 0.1–0.2 µm |
Process Capability Requirements
| Metric | Requirement |
|---|---|
| Cpk (critical dimensions) | ≥ 1.33 (minimum), ≥ 1.67 (preferred) |
| Gauge R&R (measurement system) | ≤ 10% of tolerance |
| First article inspection (FAI) | AS9102 full dimensional report |
| Statistical process control (SPC) | Required for all critical features |
Quality Standards and Certification
| Standard | Scope | Key Requirements |
|---|---|---|
| AS9100D | Aerospace quality management system | Risk management, configuration control, traceability |
| AS9102 | First article inspection | Complete dimensional verification against engineering drawing |
| NADCAP (AC7109) | Aerospace drilling and machining | Process specification, operator certification, machine capability |
| AMS 2430 | Shot peening (if specified) | Almen intensity, coverage |
| AMS 2404 | Hard chrome plating (if specified) | Thickness, adhesion, hydrogen embrittlement relief |
| ASTM E1417 | Liquid penetrant inspection | Sensitivity level, developer type |
Required Documentation
For each landing gear or actuator component, the following records must be maintained:
| Document | Content | Retention |
|---|---|---|
| Process plan | All operations with parameters | Life of component |
| Machine qualification | Capability study results | Life of component |
| In-process inspection | Readings for all critical dimensions | Life of component |
| Nonconformance report | Any deviation with disposition | Life of component |
| Serial number traceability | Heat number, forging lot, serial | Life of component |
| NDT reports | UT, MPI, penetrant results | Life of component |
| Material certification | Chemical analysis, mechanical properties | Life of component |
Common Defects and Troubleshooting
| Defect | Cause | Corrective Action |
|---|---|---|
| Tool breakage at depth (300M) | Chip packing, insufficient coolant | Increase pressure, check chip form, reduce feed |
| Oversize bore | Tool deflection or BUE | Sharpen tool, check guide pad condition |
| Surface tear marks | Built-up edge in titanium | Increase speed, change coating to DLC |
| Bell-mouth at entry | Worn entry bush or excessive feed | Replace bush, reduce entry feed |
| Spiral marks on bore | Guide pad chatter | Adjust pad clearance, check coolant flow |
| Diameter taper (entry larger) | Drill vibration at entry | Use drill bushing, reduce speed at entry |
| Rough surface after burnishing | Insufficient stock for skiving | Increase skiving allowance to 0.3 mm minimum |
| Ovality | Inconsistent pad pressure | Check guide pad wear, re-calibrate tool |
FAQ
Q: What material is most commonly used for aerospace landing gear? 300M ultra-high-strength steel (AMS 6257) is the standard for main landing gear components on commercial aircraft. It offers 1,900–2,100 MPa tensile strength at 50–54 HRC. For corrosion-resistant applications, 15-5PH stainless or Custom 465 is specified.
Q: What diameter and depth tolerances are required for landing gear bores? Bore diameter tolerance is typically H8 (±0.027 mm for a 100 mm bore) or better. Straightness must be 0.025 mm per 25 mm of length. Surface finish after finishing (skiving + roller burnishing) is Ra 0.2–0.4 µm.
Q: What is bottle boring in landing gear manufacturing? Bottle boring uses an actuated tool head that expands cutting elements at programmed depths to create internal profiles — larger diameters at one end for seal glands and smaller diameters along the main bore — all in a single setup without repositioning the workpiece.
Q: What is the difference between gun drilling and BTA drilling for landing gear? Gun drilling is used for smaller diameters (5–50 mm) with external chip evacuation. BTA or STS drilling is used for larger diameters (50–300 mm) with internal chip evacuation through the drill tube. For landing gear outer cylinders (150–300 mm diameter), BTA/STS is the standard method.
Q: How is surface finish achieved in landing gear cylinder bores? After BTA drilling, the bore is skived (machined with multi-blade tool) and then roller-burnished (cold-worked with rolling elements) in a single pass. This achieves Ra 0.1–0.4 µm surface finish with compressive residual stress, which improves fatigue life and seal performance.
Q: What quality standards govern aerospace deep hole drilling? AS9100D (quality management system), NADCAP AC7109 (drilling and machining process certification), and AS9102 (first article inspection). These require documented procedures, certified operators, machine capability studies (Cpk ≥ 1.33), and full traceability.
Q: What coolant is used for deep hole drilling 300M steel? High-viscosity extreme-pressure oil with high sulphur content, delivered at 150–200 bar pressure. The EP additives are essential for preventing metal-to-metal contact at the high cutting pressures generated in 300M machining.
Q: How does deep hole drilling for aerospace differ from industrial applications? Aerospace requires tighter tolerances (H8 vs H11–H12 typical in industrial), harder materials (300M at 54 HRC vs P20 at 30 HRC), full material traceability, documented process control under AS9100, and 100% NDT inspection of every component.
Q: What is the typical tool life when gun drilling 300M steel? 5–20 holes per regrind for a 10 mm × 200 mm hole, depending on tool grade and parameters. This is 10–50× shorter than drilling annealed steel. K30–K35 carbide with AlCrN coating and proper T-land edge preparation is essential.
Q: Can landing gear cylinders be repaired if the bore is damaged? Minor surface damage can be removed by honing (up to 0.1 mm on diameter) if the remaining wall thickness meets design minimum. Chrome-plated or HVOF-coated surfaces may be stripped and re-coated. Damage beyond these limits typically requires scrapping the component.