Appearance
An aerospace manufacturer produces main landing gear shock strut cylinders for a regional jet. The outer cylinder is machined from 300M steel (AMS 6417, 52–54 HRC, 280 ksi tensile strength) with a 125 mm bore × 1,100 mm length. The bore is BTA drilled using coated carbide ISO K10 inserts at 400 RPM / 18 mm/min feed with 50 bar coolant pressure and high-viscosity cutting oil. After BTA drilling, the bore is semi-finish bored and honed to H7 tolerance (0/+0.040 mm) with Ra 0.4 µm finish for hydraulic seal performance. Axle bores (25 mm diameter × 300 mm) in the 4340 steel truck beam are gun drilled at 1,500 RPM / 12 mm/min feed with 80 bar coolant. All bores undergo nital etch inspection (AMS 2649) for grinding burn, magnetic particle inspection, and shot peening (AMS 2430) before hard chrome plating (AMS 2406, 50–100 µm). Post-plate hydrogen embrittlement relief baking is performed at 190°C for 23 hours.
Landing Gear Components Requiring Deep Hole Drilling
| Component | Material | Deep Hole Operation | Typical Bore Range | Length Range |
|---|---|---|---|---|
| Shock strut outer cylinder | 300M (52–54 HRC) | BTA drill + finish bore + hone | Ø80–250 mm | 500–2,000 mm |
| Shock strut inner piston | 300M / 4340 (48–52 HRC) | BTA drill or gun drill | Ø60–200 mm | 500–2,000 mm |
| Truck beam axle bore | 4340 (38–42 HRC) | Gun drill | Ø20–60 mm | 200–600 mm |
| Trunnion pin bore | 300M / 4340 | Gun drill or BTA drill | Ø30–80 mm | 200–500 mm |
| Torque link bore | 4340 / 7075 Al | Gun drill | Ø10–30 mm | 100–300 mm |
| Side strut clevis bore | 300M / 4340 | Gun drill | Ø15–50 mm | 150–400 mm |
| Actuator cylinder bore | 300M / 15-5PH | BTA drill + hone | Ø40–120 mm | 300–800 mm |
TIP
Landing gear components are classified as critical flight safety structures. All deep hole drilling operations must be performed with documented process control (per SAE AIR6813), and any subsequent machining or grinding of drilled bores must be verified by nital etch or Barkhausen noise inspection to confirm absence of grinding burns and surface damage.
Materials for Landing Gear Deep Hole Drilling
300M Steel (4340 Modified)
300M is the most widely used material for main landing gear structural components, modified from 4340 with higher silicon and vanadium addition:
| Property | Value |
|---|---|
| Tensile strength | 280–300 ksi (1,930–2,070 MPa) |
| Hardness | 52–54 HRC |
| Specification | AMS 6417 / AMS 6419 / MIL-S-8844 |
| Chemical addition | 1.45–1.80% Si, 0.05–0.10% V |
| Through-hardening | 52 HRC minimum in sections up to 89 mm |
| Cleanliness | AMS 2300 premium aircraft quality |
| Relative machinability | Very difficult (ISO material group S/H) |
4340 Steel
4340 (AISI 4340 / AMS 6414) is used for less critical structural components:
| Property | Value |
|---|---|
| Tensile strength | 180–220 ksi (1,240–1,520 MPa) |
| Hardness | 38–45 HRC typical for landing gear |
| Characteristics | Deep hardenability, good toughness |
| Work-hardening | Notable tendency — requires sharp tooling |
Other Landing Gear Materials
| Material | Typical Application | Machinability |
|---|---|---|
| 15-5PH (H900) | Actuator cylinders, fittings | Moderate |
| 7075-T6 / 7075-T73 | Torque links, support brackets | Good |
| Ti-6Al-4V | High-performance military gear | Very difficult |
| AerMet 100 | US Navy carrier gear (300 ksi+) | Very difficult |
BTA Drilling of Shock Strut Cylinders
BTA drilling is the primary rough-boring process for shock strut cylinders, creating the through-bore from solid forged bar stock or tube blanks. Landing grade shock struts require precise bore geometry for hydraulic seal performance and structural integrity.
BTA Drilling Parameters for 300M Steel
| Bore Diameter (mm) | Spindle Speed (RPM) | Feed Rate (mm/min) | Feed (mm/rev) | Coolant Pressure (bar) | Tool Material |
|---|---|---|---|---|---|
| 80 | 480 | 15 | 0.031 | 50 | Coated carbide K10 |
| 100 | 380 | 18 | 0.047 | 50 | Coated carbide K10 |
| 125 | 300 | 20 | 0.067 | 50 | Coated carbide K10 |
| 150 | 250 | 22 | 0.088 | 45 | Coated carbide K10 |
| 200 | 190 | 24 | 0.126 | 45 | Coated carbide K10 |
Recommended BTA Cutting Parameters
| Parameter | 300M (52–54 HRC) | 4340 (38–42 HRC) |
|---|---|---|
| Cutting speed (Vc) | 20–40 m/min (start at 25 m/min) | 40–70 m/min |
| Feed per revolution | 0.02–0.08 mm/rev | 0.05–0.15 mm/rev |
| Insert grade | Coated carbide K10–K20 | Coated carbide P20–P30 |
| Coating | TiAlN or AlTiN (CVD/PVD) | TiAlN recommended |
| Rake angle | 6°–10° positive | 8°–12° positive |
| Clearance angle | 6°–10° | 8°–10° |
BTA Drilling Quality for 300M
| Parameter | Achievable Value |
|---|---|
| Diameter tolerance | IT10–IT11 (0.1–0.2 mm for 125 mm bore) |
| Surface finish (as-drilled) | Ra 1.6–6.3 µm |
| Straightness | ≤0.1 mm per 1,000 mm |
| Concentricity | ≤0.1 mm TIR |
WARNING
300M steel at 52–54 HRC generates extreme cutting temperatures at the BTA tool edge. Coolant pressure of 45–50 bar minimum is essential for chip evacuation and thermal control. Loss of coolant flow during drilling will result in immediate tool failure, work hardening of the bore surface, and potential scrapping of the forging. Coolant flow monitoring with automatic machine stop is standard practice.
Gun Drilling of Landing Gear Components
Gun drilling is used for smaller diameter bores in landing gear components — axle bores, trunnion pin holes, clevis bores, and hydraulic passages.
Gun Drilling Parameters for Landing Gear Steels
| Component | Material (HRC) | Bore (mm) | Depth (mm) | Speed (RPM) | Feed (mm/min) | Coolant (bar) |
|---|---|---|---|---|---|---|
| Truck beam axle bore | 4340 (40 HRC) | 25 | 300 | 1,500 | 12 | 80 |
| Trunnion pin bore | 300M (52 HRC) | 35 | 200 | 800 | 8 | 100 |
| Torque link bore | 4340 (38 HRC) | 16 | 120 | 2,200 | 15 | 70 |
| Drag brace clevis | 300M (52 HRC) | 20 | 150 | 1,000 | 6 | 100 |
| Actuator port bore | 15-5PH (40 HRC) | 10 | 80 | 2,500 | 10 | 60 |
Gun Drill Tool Specifications
| Parameter | 300M | 4340 |
|---|---|---|
| Cutting speed | 15–30 m/min | 30–50 m/min |
| Tip material | Micrograin carbide K10 | Carbide K15–K20 |
| Point angle | 130°–140° | 120°–130° |
| Coolant type | High-viscosity oil, EP additives | High-viscosity oil |
Surface Treatment and Finishing
Hard Chrome Plating
Hard chrome plating (AMS 2406) is applied to landing gear cylinder bores and piston outer diameters for wear resistance and corrosion protection:
| Property | Requirement |
|---|---|
| Plating thickness | 50–100 µm functional hard chrome |
| Plating hardness | 800–1,200 HV (HRC 65–70) |
| Pre-plate shot peening | Required per QQ-C-320 Class 2E |
| Pre-plate surface finish | Ra 0.4–0.8 µm (16–31 RMS) |
| Post-plate finish (ground) | Ra 0.2–0.3 µm (8–12 µin) |
| Post-plate finish (honed) | Ra 0.1–0.2 µm (4–8 µin) |
| Hydrogen embrittlement relief | 190°C (375°F) for 23 hours minimum |
| Standard | AMS 2406 / MIL-STD-1501 |
HVOF Thermal Spray Coatings
HVOF (High Velocity Oxygen Fuel) tungsten carbide coatings (SAE ARP5935) are increasingly replacing hard chrome for external landing gear surfaces:
- Superior fatigue performance (no hydrogen embrittlement risk)
- As-coated surface finish Ra 0.4–0.6 µm
- Hardness 1,100–1,300 HV
- Corrosion resistance exceeds hard chrome
Shot Peening
Shot peening (AMS 2430) is required on landing gear components before chrome plating and as a surface enhancement for fatigue-critical areas:
- Intensity: Verified by Almen strip per SAE J443
- Coverage: 100% visual at 10× magnification
- Media: Steel shot, ceramic beads, or glass beads
- Pre-plating: Required for hard chrome to mitigate fatigue debit
- Post-machining: Applied to all machined surfaces in tension
TIP
For landing gear bores, shot peening is typically applied after final bore machining and before chrome plating. The peening must cover the full bore surface, verified by UV tracer dye and inspection under UV light. Peening intensity is selected to achieve at least 0.005A compensation on the Almen strip value for high-strength steels.
Quality Requirements and Standards
Bore Tolerances
| Component | Tolerance | Surface Finish (Ra) | Straightness | Standard |
|---|---|---|---|---|
| Shock strut cylinder bore | H7 (0/+0.040 mm for 125 mm) | 0.4 µm (honed) | 0.05 mm/m | SAE AIR6813 |
| Shock strut piston bore | H8 | 0.4 µm | 0.05 mm/m | SAE AIR6813 |
| Axle bore (truck beam) | H8 | 0.8 µm | 0.05 mm/m | Component drawing |
| Trunnion pin bore | H7 | 0.4 µm | 0.05 mm/m | Component drawing |
| Actuator cylinder bore | H7 | 0.2–0.4 µm | 0.05 mm/m | Component drawing |
Inspection and NDE Requirements
| Inspection Method | Standard | Application |
|---|---|---|
| Nital etch inspection | AMS 2649 | Verification of grinding burn on all machined bores |
| Magnetic particle inspection | ASTM E1444 | Surface crack detection on ferromagnetic components |
| Fluorescent penetrant inspection | ASTM E1417 | Surface crack detection on non-magnetic materials |
| Barkhausen noise analysis | SAE ARP4462 | Non-destructive grinding burn detection (alternative to nital etch) |
| Ultrasonic inspection | ASTM E213 / E428 | Subsurface defect detection in forgings |
| Dimensional inspection | CMM / air gauging | Bore diameter, roundness, straightness verification |
Surface Finish Measurement
Aerospace landing gear surface finish specifications commonly reference:
| Parameter | Seal Surface | Structural Bore | Chrome Plated Surface |
|---|---|---|---|
| Ra (µm) | ≤0.4 | ≤0.8 | ≤0.3 |
| Rz (µm) | ≤3.2 | ≤6.3 | ≤2.5 |
| Rmax (µm) | ≤4.0 | ≤8.0 | ≤3.0 |
Manufacturing Process Sequences
Shock Strut Outer Cylinder
1. Forged 300M bar stock (AMS 6417), UT inspected
2. Rough turning of external profile
3. BTA drill through-bore (full length)
4. Stress relief (if required per drawing)
5. Semi-finish boring (remove 0.5–1.0 mm from BTA bore)
6. Heat treatment (quench and temper to 52–54 HRC)
7. Hardness and tensile verification on witness coupon
8. Finish bore to H7 tolerance (0.2–0.4 mm stock remaining)
9. Nital etch inspection (AMS 2649) of bore surface
10. Honing to final H7 tolerance and Ra 0.4 µm
11. Magnetic particle inspection of bore
12. Mask non-plating surfaces
13. Shot peen bore surface (AMS 2430)
14. Hard chrome plate bore (AMS 2406, 50–100 µm)
15. Post-plate hydrogen embrittlement relief bake (190°C, 23 hr)
16. Final bore inspection (air gauge, profilometer)
17. Hydrostatic pressure testTruck Beam Axle Bore
1. Forged 4340 steel, quenched and tempered to 38–42 HRC
2. Rough machine external profile
3. Gun drill axle bore from both ends (meet at centre)
4. Semi-finish bore
5. Nital etch inspection
6. Finish bore to final size
7. Shot peen bore
8. Chrome plate or HVOF coat
9. Post-coat inspection
10. Final dimensional verificationCoolant and Tooling
| Parameter | BTA Drilling (300M) | Gun Drilling (300M) | BTA Drilling (4340) |
|---|---|---|---|
| Coolant type | High-viscosity oil (ISO VG 15–30) with EP | High-viscosity oil | High-viscosity oil |
| Pressure | 45–50 bar | 70–100 bar | 25–45 bar |
| Flow rate | 200–500 L/min (diameter dependent) | 30–80 L/min | 180–450 L/min |
| Filtration | ≤20 µm | ≤10 µm | ≤30 µm |
| Temperature control | 30–40°C | 30–40°C | 30–45°C |
Troubleshooting Common Issues
| Issue | Likely Cause | Solution |
|---|---|---|
| Rapid tool wear in 300M BTA drilling | Cutting speed too high or coolant inadequate | Reduce Vc to 20–25 m/min; verify coolant pressure ≥45 bar |
| Grinding burn on finish-bored 300M bore | Dull cutting edge or insufficient coolant | Replace insert earlier; increase coolant flow to cutting zone |
| Gun drill breakage in deep axle bore | Chip packing or feed too high | Increase coolant pressure; reduce feed; peck drill in 20 mm steps |
| Chrome plating pits in bore | Surface contamination or inadequate pre-plate cleaning | Improve degreasing and reverse etch cycle before plating |
| Nital etch indication after boring | Surface strain from worn insert | Verify insert condition; reduce feed; increase coolant |
| Bore surface cracking after heat treatment | Excessive cold work from BTA drilling | Reduce feed rate; verify BTA parameters pre-heat-treat |
| Straightness deviation >0.1 mm/1,000 mm | Misaligned guide bushing or worn guide pads | Check machine alignment; replace BTA guide pads |
| HVOF coating debonding in bore | Inadequate grit blast profile | Verify anchor profile Ra 3–5 µm; clean before coating |
FAQ
What material is most commonly used for landing gear shock strut cylinders?
300M steel (AMS 6417 / MIL-S-8844) is the most widely used material for main landing gear structural components. It is a modified 4340 steel with higher silicon (1.45–1.80%) and vanadium (0.05–0.10%), heat treated to 280–300 ksi tensile strength with 52–54 HRC hardness. It provides superior fatigue strength and fracture toughness compared to standard 4340.
What deep hole drilling process is used for shock strut cylinders?
BTA drilling is the standard rough-boring process for shock strut cylinders (80–200 mm bore range). Typical parameters for 300M steel: 250–480 RPM, 15–24 mm/min feed, 45–50 bar coolant pressure, coated carbide K10 inserts. The BTA-drilled bore is subsequently finish bored and honed to H7 tolerance with Ra 0.2–0.4 µm finish.
What is the nital etch inspection requirement for landing gear bores?
AMS 2649 nital etch inspection is mandatory for all machined and ground surfaces on landing gear components, including deep hole drilled bores. The process involves etching the bore surface with nitric acid in alcohol (nital), then inspecting for discolouration that indicates grinding burn or surface damage. Any burn indication requires engineering disposition; re-burnishing or re-machining beyond drawing limits results in component rejection.
What chrome plating specification is used for landing gear bores?
AMS 2406 (Electrodeposited Hard Chromium Plating) is the primary specification. Key requirements: plating thickness 50–100 µm, hardness 800–1,200 HV, pre-plate shot peening per AMS 2430, post-plate hydrogen embrittlement relief baking at 190°C (375°F) for 23 hours minimum. MIL-STD-1501 provides additional low-embrittlement process requirements for Air Force landing gear.
How are axle bores in landing gear truck beams manufactured?
Truck beam axle bores (20–60 mm diameter, up to 600 mm length) are gun drilled from 4340 steel (38–42 HRC). Typical parameters: 1,500 RPM, 12 mm/min feed, 80 bar coolant pressure. Because the bore length typically exceeds the gun drill's maximum single-pass capability, the bore is often gun drilled from both ends with a centre intersection. Straightness verification and nital etch inspection follow.
What surface finish is required for landing gear cylinder bores?
Honed seal surfaces require Ra ≤0.4 µm (16 µin) with H7 bore tolerance. Chrome-plated and honed surfaces can achieve Ra 0.1–0.2 µm (4–8 µin). Structural non-sealing bores typically require Ra ≤0.8 µm (32 µin). Surface finish is verified by profilometry at multiple positions along the bore length.
What is the role of shot peening in landing gear bore manufacturing?
Shot peening (AMS 2430) is applied to landing gear bores to introduce compressive residual stress, which improves fatigue life and mitigates the fatigue debit associated with hard chrome plating. It is mandatory before chrome plating per QQ-C-320 Class 2E. Coverage must be 100% verified at 10× magnification, with intensity measured by Almen strip per SAE J443.
What coolant is required for BTA drilling 300M steel?
High-viscosity deep hole cutting oil (ISO VG 15–30) with extreme-pressure additives is required for BTA drilling 300M steel. Coolant pressure must be maintained at 45–50 bar minimum with flow rate proportional to bore diameter (200–500 L/min). Filtration to ≤20 µm is essential. Coolant temperature should be controlled at 30–40°C to prevent thermal distortion of the workpiece.
What NDE methods are used to inspect deep hole drilled landing gear bores?
Multiple methods are used: nital etch (AMS 2649) for grinding burn detection, magnetic particle inspection (ASTM E1444) for surface cracks in ferromagnetic steels, Barkhausen noise analysis (SAE ARP4462) as a non-destructive alternative to nital etch, ultrasonic inspection for subsurface defects, and dimensional verification by air gauging or CMM for bore diameter, roundness, and straightness.
What is the manufacturing sequence for a landing gear shock strut outer cylinder?
The sequence is: forged 300M bar stock UT inspection → rough external turning → BTA drill through-bore → stress relief → semi-finish bore → heat treat to 52–54 HRC → finish bore → nital etch inspection → hone to H7 and Ra 0.4 µm → magnetic particle inspection → shot peen bore → hard chrome plate (AMS 2406, 50–100 µm) → hydrogen embrittlement relief bake (190°C, 23 hr) → final inspection.
Summary
Deep hole drilling for aerospace landing gear components demands rigorous process control, specialised tooling, and comprehensive quality verification:
- 300M steel (52–54 HRC, 280–300 ksi) is the primary material for shock strut cylinders, requiring BTA drilling at conservative parameters (Vc 20–40 m/min, feed 0.02–0.08 mm/rev) with coated carbide K10 inserts.
- BTA drilling produces the through-bore in shock strut cylinders (80–200 mm bore × up to 2,000 mm length), achieving IT10–IT11 as-drilled, subsequently finished to H7 by honing.
- Gun drilling is used for smaller bores in axles, trunnions, torque links, and hydraulic passages, operating at 15–50 m/min with 60–100 bar coolant pressure.
- Nital etch inspection (AMS 2649) is mandatory on all machined landing gear bores to verify absence of grinding burn and surface damage.
- Shot peening (AMS 2430) precedes chrome plating to introduce compressive residual stress and mitigate the fatigue debit of the plating process.
- Hard chrome plating (AMS 2406, 50–100 µm) with post-plate hydrogen embrittlement relief baking at 190°C provides wear resistance and corrosion protection.
- Quality verification includes bore air gauging, surface profilometry, nital etch, magnetic particle inspection, and hydrostatic pressure testing of the finished assembly.