Appearance
Landing gear components are subjected to the most extreme loads in aerospace — impact forces on touchdown, high-cycle fatigue from taxiing and takeoff, and corrosion from runway debris and de-icing chemicals. The deep holes drilled through struts, pins, and axles must maintain integrity under these conditions while meeting tolerances measured in microns. This article examines the specialized deep hole drilling processes used for landing gear manufacturing.
Landing Gear Components Requiring Deep Hole Drilling
Oleo Pneumatic Struts
Oleo (oil/gas) shock struts are the primary load-bearing components of aircraft landing gear. They contain pressurized oil and nitrogen that absorb landing impact. Deep hole drilling is required for:
- Inner cylinder bore: The精密 bore where the piston slides within the cylinder
- Oil and gas chambers: Precision bores that separate and seal the oil and gas volumes
- Metering pin bore: The central bore that controls the orifice through which oil flows during compression
| Parameter | Typical Requirement |
|---|---|
| Bore diameter | 50–300 mm (depending on aircraft size) |
| Bore length | 500–4,000 mm |
| Diameter tolerance | H8–H9 (IT8–IT9) |
| Surface finish | Ra 0.4–0.8 μm (seal surface) |
| Straightness | 0.05–0.15 mm per meter |
Torque Links
Torque links (scissors links) connect the upper and lower sections of the landing gear, preventing rotation of the inner cylinder relative to the outer cylinder. Deep hole drilling is used for:
- Pin bores: Precision holes for the pins that connect the links
- Oil passage holes: Drilled passages for lubrication
Axle Pins
Axle pins are the horizontal shafts that carry the wheel assembly. Deep hole drilling is used for:
- Center bore: Weight reduction and wiring passage
- Cross-holes: For retaining bolts and lubrication fittings
| Parameter | Typical Requirement |
|---|---|
| Pin diameter | 25–150 mm |
| Bore diameter | 10–50 mm (center bore) |
| Material | 300M, 4340 steel at 48–52 HRC |
| Concentricity | 0.03–0.08 mm (bore to OD) |
| Surface finish | Ra 0.8–1.6 μm |
Actuator Cylinders
Landing gear retraction/extension actuators are hydraulic cylinders that require deep hole drilling:
- Barrel bore: The main cylinder bore
- Fluid ports: Cross-drilled oil passages
Materials
Landing gear materials are selected for strength-to-weight ratio, fatigue resistance, and fracture toughness. They present significant deep hole drilling challenges.
| Material | Typical Application | Hardness | Machinability |
|---|---|---|---|
| 300M (4340M) | Main struts, axle pins | 48–52 HRC | Poor (very high strength) |
| 4340 steel | Actuators, fittings | 32–40 HRC | Fair |
| Ti-5553 | Modern landing gear | 38–42 HRC | Challenging (high strength, low conductivity) |
| Ti-6Al-4V | Lightweight components | 32–36 HRC | Fair to poor |
| 7075-T6 / 7075-T73 | Torque links, non-structural | — | Good |
| 17-4 PH stainless | Corrosion-resistant components | 38–44 HRC | Fair |
300M Steel
300M (AISI 4340 modified with silicon and vanadium) is the dominant material for main landing gear components. It offers:
- Ultimate tensile strength of 1,900–2,100 MPa
- Fracture toughness of 50–70 MPa√m
- Through-hardening capability in thick sections
Deep hole drilling challenges with 300M:
- High hardness (48–52 HRC) requires carbide tooling with AlTiN coatings
- High cutting forces cause tool deflection — counter-rotation is often required
- Low thermal conductivity concentrates heat at the cutting edge
- Chip control is difficult — tough, stringy chips
Ti-5553 (Ti-5Al-5Mo-5V-3Cr)
Ti-5553 is increasingly used in modern landing gear designs (Boeing 787, Airbus A350) for weight reduction. Compared to Ti-6Al-4V:
| Property | Ti-6Al-4V | Ti-5553 |
|---|---|---|
| Ultimate tensile strength | 950 MPa | 1,250 MPa |
| Maximum section thickness | 25 mm (for full properties) | 150 mm |
| Deep hole drilling difficulty | High | Very high |
Deep Hole Drilling Processes
Gun Drilling for Landing Gear
Gun drilling is used for smaller-diameter bores in landing gear components:
| Application | Typical Dia. | L/D Ratio |
|---|---|---|
| Axle pin center bore | 10–40 mm | 10:1–30:1 |
| Torque link pin bores | 6–20 mm | 3:1–8:1 |
| Oil passage holes | 3–12 mm | 20:1–80:1 |
For 300M steel at 48–52 HRC, typical gun drilling parameters:
| Parameter | Value |
|---|---|
| Cutting speed | 25–45 m/min |
| Feed rate | 0.010–0.025 mm/rev |
| Coolant pressure | 120–200 bar |
| Coolant type | Straight oil, EP additives |
| Tool material | Micro-grain carbide, AlTiN coated |
BTA Drilling for Landing Gear
BTA drilling is the standard process for main strut and actuator cylinder bores:
| Application | Typical Dia. | L/D Ratio |
|---|---|---|
| Oleo strut cylinder bore | 50–300 mm | 10:1–20:1 |
| Actuator barrel bore | 30–150 mm | 8:1–25:1 |
For BTA drilling of landing gear steels:
| Parameter | Value |
|---|---|
| Cutting speed | 40–70 m/min |
| Feed rate | 0.05–0.20 mm/rev |
| Coolant pressure | 20–60 bar |
| Coolant flow | 400–1,200 L/min |
| Tool type | Indexable carbide inserts |
Case Study: BTA Drilling of 4340 Landing Gear
A documented case study using Allied Machine BT-A tooling on 4340 steel landing gear components showed dramatic improvements:
| Metric | Conventional Tooling | BT-A (BTA) Tooling |
|---|---|---|
| RPM | 500 | 125 |
| Feed rate | 0.003 IPR | 0.006 IPR |
| Cycle time | 15 min | 1 min 20 sec |
| Holes per tool | 2 | 43 |
| Cost per hole reduction | — | 99% |
This case illustrates why BTA drilling has become the preferred process for landing gear components where diameter permits.
Bottle Boring
Bottle boring — creating an enlarged internal cavity at the mid-section while maintaining a smaller diameter at the ends — is extensively used in landing gear manufacturing for weight reduction.
Application to Oleo Struts
Oleo struts benefit from bottle boring because the internal pressure and stress distribution varies along the length. The bore can be larger in the mid-section where stresses are lower, while maintaining smaller diameters at the end fittings where stresses concentrate.
Machine Requirements
Bottle boring requires specialized machine tools with:
- CNC-controlled tool axes for internal contouring
- Real-time feed and position feedback for accurate cavity geometry
- High-pressure coolant delivery for chip evacuation from the enlarged cavity
The UNISIG B700 Drop Bed machine is designed for this application, offering:
- 63" (1,600 mm) swing over bed for large strut components
- Parts from 500–4,000 mm long
- Solid bores up to 200 mm diameter
- Counterbores to 300 mm diameter
- 126-hp tool spindle
- 250 GPM coolant flow
Surface Finish Requirements
Landing gear bore surface finish requirements are determined by function:
| Surface Type | Typical Ra | Why It Matters |
|---|---|---|
| Seal surface (oleo strut) | Ra 0.4–0.8 μm | Seal life, leak prevention |
| Bearing surface | Ra 0.8–1.6 μm | Bearing fit and wear |
| Non-sealing bore | Ra 1.6–3.2 μm | General structural |
| Threaded holes | Ra 3.2 μm | Thread integrity |
Achieving Surface Finish in High-Strength Materials
In 300M steel at 48–52 HRC:
| Process | Achievable Ra | Notes |
|---|---|---|
| Gun drilling | Ra 0.8–1.6 μm | Standard parameters |
| Precision gun drilling | Ra 0.4–0.8 μm | Optimized speeds, sharp tooling |
| BTA drilling | Ra 1.6–3.2 μm | Standard |
| Fine BTA | Ra 0.8–1.6 μm | Optimized |
| BTA + roller burnishing | Ra 0.2–0.4 μm | Often used for seal surfaces |
| BTA + honing | Ra 0.1–0.4 μm | Precision seal surfaces |
Straightness and Concentricity
Requirements
| Characteristic | Typical Tolerance |
|---|---|
| Bore straightness | 0.05–0.15 mm per meter |
| Concentricity (bore to OD) | 0.05–0.15 mm |
| Roundness | 0.01–0.05 mm |
Achieving Straightness
For long landing gear components, straightness is achieved through:
- Counter-rotation: Rotating the workpiece opposite to the tool cancels deflection
- Steady rests: Supporting long workpieces at intermediate points
- Guide bushing condition: A worn bushing at the entry point causes initial deviation that propagates
- Machine alignment: Regular verification of headstock-tailstock alignment
- Pilot hole accuracy: A straight pilot hole guides subsequent operations
TIP
In landing gear deep hole drilling, entry face preparation is often overlooked but is critical for straightness. The entry face must be square to the bore axis (within 0.05 mm over the face diameter) and the guide bushing bore must be concentric to the spindle axis. A 0.01 mm error at entry can become a 0.10 mm straightness deviation at 500 mm depth.
Inspection Methods
D Dimensional Inspection
| Characteristic | Method | Equipment |
|---|---|---|
| Bore diameter | Air gauging, bore micrometer | Multi-point air plug, internal micrometer |
| Straightness | Laser or mechanical | Laser straightness gauge, CMM |
| Concentricity | Rotary CMM | CMM with rotary table |
| Surface finish | Stylus profilometry | Profilometer |
| Roundness | CMM or roundness tester | Roundness measuring machine |
Non-Destructive Testing
| Method | Application |
|---|---|
| Fluorescent penetrant inspection (FPI) | Surface crack detection |
| Magnetic particle inspection (MPI) | Surface crack detection (ferromagnetic materials) |
| Ultrasonic testing | Subsurface defect detection |
| Eddy current | Surface crack detection in titanium |
| Borescope inspection | Visual bore surface inspection |
Typical Manufacturing Capabilities
Machine Tool Requirements
| Capability | Minimum | Recommended |
|---|---|---|
| Workpiece length | 3,000 mm | 4,000+ mm |
| Swing over bed | 500 mm | 800+ mm |
| Counter-rotation | Desirable | Required |
| Coolant pressure | 100 bar | 150–200 bar |
| Steady rests | 2 | 3–4 |
| CNC axes for bottle boring | — | Required for complex profiles |
Typical Process Flow for an Oleo Strut
| Step | Operation | Process |
|---|---|---|
| 1 | Rough turning | CNC turning of OD |
| 2 | Deep hole drilling | BTA drill main bore |
| 3 | Bottle boring | Internal profiling |
| 4 | Boring and finishing | Precision boring of seal surfaces |
| 5 | Roller burnishing | Seal surface finishing |
| 6 | Cross-hole drilling | Ports and oil passages |
| 7 | Heat treatment (if required) | Stress relief |
| 8 | Final inspection | Full dimensional + NDT |
FAQ
Q: What landing gear components require deep hole drilling? Oleo pneumatic struts (inner cylinder bores), axle pins (center bores), torque links (pin bores), actuator cylinders (barrel bores), and various fittings and structural components.
Q: What materials are used for landing gear deep hole drilling? 300M steel (dominant for main structures), 4340 steel, Ti-5553 (modern aircraft), Ti-6Al-4V, 7075 aluminum, and 17-4 PH stainless steel. These materials range from difficult to extremely difficult to machine.
Q: What is bottle boring and why is it used in landing gear? Bottle boring creates an enlarged internal cavity at the mid-section of a component while maintaining smaller diameters at the ends. It reduces weight without compromising strength at the critical end fittings, and is widely used in oleo strut manufacturing.
Q: What is the typical straightness requirement for landing gear bores? Typical straightness is 0.05–0.15 mm per meter of bore length, depending on the component and its function. Critical seal surfaces may require tighter tolerances.
Q: How is surface finish achieved in high-strength landing gear steels? Gun drilling and BTA drilling produce Ra 0.8–3.2 μm depending on parameters. For seal surfaces (Ra 0.2–0.4 μm), roller burnishing or honing is used as a secondary operation after drilling.
Q: What coolant pressure is needed for landing gear deep hole drilling? For gun drilling: 120–200 bar. For BTA drilling: 20–60 bar (but at much higher flow rates, up to 1,200 L/min).
Q: What is Ti-5553 and why is it used in landing gear? Ti-5553 (Ti-5Al-5Mo-5V-3Cr) is a high-strength beta titanium alloy with tensile strength of 1,250 MPa. It is used in Boeing 787 and Airbus A350 landing gear for weight reduction compared to 300M steel.
Q: What certifications are required for landing gear deep hole drilling? AS9100D (quality management system), Nadcap CMSP (conventional machining special process), and customer-specific requirements from landing gear OEMs (Safran, Collins, Heroux-Devtek, etc.).
Q: How are landing gear deep-drilled bores inspected? Using air gauges or bore micrometers for diameter, laser straightness gauges for straightness, CMM with rotary table for concentricity, and stylus profilometers for surface finish. NDT includes FPI, MPI, ultrasonic, and eddy current testing.
Q: What is the most common deep hole drilling challenge in landing gear? Tool deflection in high-strength materials (300M at 48–52 HRC, Ti-5553). The combination of high cutting forces and long tool extensions makes maintaining straightness and concentricity the primary challenge. Counter-rotation and intermediate steady rests are the most effective solutions.