Landing gear components are the most structurally critical deep hole drilling jobs I have taken on. Every strut, axle, and pin carries the full weight of an aircraft during takeoff, landing, and taxi. A hidden drilling defect — a chip notch, a deviated bore, a surface tear — can become a crack initiation site that grounds a fleet. The materials are ultra-high-strength steels hardened past 50 HRC, the bores run deep, and the inspection standard is zero defects.
I have been drilling landing gear components for over a decade across three different shops serving aerospace primes and tier-one suppliers. This article covers what I have learned about the components, materials, parameters, and quality requirements that define this work.
I already covered the broader aerospace deep hole drilling landscape in another article. Landing gear deserves its own treatment because the materials and quality demands are distinct from engine shafts or hydraulic actuators.
Typical Landing Gear Components That Need Deep Hole Drilling
Landing gear assemblies are not single parts. They are systems of nested components, each with deep hole drilling operations. These are the three categories I have worked on most.
Main and Nose Landing Gear Struts — These are the large vertical members that absorb the impact load on touchdown. A typical main landing gear strut on a commercial airliner is 1.5 to 3 meters long with a through-bore of 30 to 80 mm. The bore serves as the oil reservoir for the shock absorber. Surface finish inside the bore matters because the seal rides directly on that surface. I have seen jobs with an Ra 0.4 micron spec on the bore ID, which requires either precision BTA drilling with wiper inserts or a follow-up skiving and roller burnishing operation.
Axles — The axle is the horizontal member that connects the wheels to the landing gear structure. Axles are shorter than struts, typically 300 to 800 mm long, but the aspect ratio is still high — a 25 mm bore through 600 mm of material is common. The critical spec on axles is concentricity between the bore and the outer diameter. If the bore is off-center, the wall thickness varies and the axle loses fatigue life. The concentricity tolerance I typically see is 0.1 mm TIR or tighter.
Pins and Trunnions — These are the pivot points where the landing gear rotates during retraction and extension. They are shorter components, 100 to 300 mm long, with a bore diameter of 12 to 40 mm. The bores are often cross-holes or intersecting features. The challenge is maintaining straightness when the drill enters at an angle or when the hole breaks into an existing cavity mid-length. Every intersection point is a stress concentration riser that gets scrutinized on the NDT inspection.
Materials Used in Landing Gear
The material selection for landing gear is driven by strength-to-weight ratio and fracture toughness. These are not materials that drill easily.
300M Steel — This is the most common landing gear material I have worked with. It is a low-alloy, ultra-high-strength steel with a nominal tensile strength of 280 ksi (1930 MPa). In the heat-treated condition, hardness runs 50 to 55 HRC. 300M is similar to AISI 4340 but with higher silicon content (1.6% versus 0.25%) for improved toughness. The high silicon makes 300M more abrasive at the cutting edge than 4340. Tool wear is faster, and the surface finish is harder to control.
AISI 4340 — Used in older or lower-stressed landing gear designs. Hardness after heat treatment is typically 38 to 45 HRC. 4340 drills more easily than 300M, but it is less common in modern landing gear because the strength-to-weight ratio is lower. I still see it in legacy military aircraft and some business jet components. The main challenge with 4340 is maintaining chip control at the higher feeds needed for production rates.
15-5 PH Stainless Steel — This precipitation-hardening stainless steel shows up in corrosion-resistant landing gear components, especially on naval aircraft that operate from carriers. Hardness ranges from 33 to 44 HRC depending on the heat treatment condition (H900, H1025, or H1150). The H900 condition is the hardest and most abrasive. The challenge with 15-5 PH is the stringy chip formation and the tendency to work-harden if the feed drops too low. I covered aerospace stainless applications in more detail here.
Drilling Parameters for High-Strength Steels
Parameters that work on standard alloy steel will destroy tooling on landing gear materials. The cutting speed, feed, and coolant pressure all shift downward or upward depending on the alloy condition. Here is the parameter table I reference when quoting landing gear work.
| Material | Hardness (HRC) | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (psi) | Recommended Tool Coating |
|---|---|---|---|---|---|
| 300M | 50-55 | 40-60 | 0.04-0.08 | 800-1200 | TiAlN or AlCrN |
| AISI 4340 (HT) | 38-45 | 55-80 | 0.06-0.12 | 600-1000 | TiAlN |
| 15-5 PH (H900) | 40-44 | 45-65 | 0.05-0.10 | 700-1100 | TiCN or TiAlN |
| 15-5 PH (H1025) | 33-38 | 55-75 | 0.06-0.12 | 600-900 | TiAlN |
A few notes on these parameters from my experience. The speed range for 300M looks low compared to what you might run on 4340 at the same hardness, but the high silicon content creates more friction at the cutting interface. I have tested 300M at 70 m/min on a 25 mm BTA head and watched the tool edge break down before reaching 200 mm depth. Dropping to 50 m/min let the same tool finish a full 1.5 meter bore with acceptable wear patterns.
For feed rate on 300M, I stay above 0.04 mm/rev even on small diameters. Below that, the cutting edge rubs instead of shearing, which generates excess heat and surface hardening. The chips also get thinner and harder to evacuate. If the machine cannot hold steady feed at that rate due to servo limitations, I back the speed down further rather than dropping the feed.
Coolant pressure on 300M needs to be aggressive enough to clear chips from the cutting zone. At 50 to 55 HRC, the chips are short and segmented, which helps evacuation. The real risk is not chip packing — it is localized heating from inadequate coolant volume. I set coolant flow to maintain a minimum of 60 L/min through a BTA drill head regardless of pressure reading.
Tool coating matters significantly on landing gear materials. For 300M above 50 HRC, I use AlCrN-coated tools. The aluminum chromium nitride coating provides thermal barrier properties that protect the carbide substrate from the heat generated at low cutting speeds. On 4340 and 15-5 PH at lower hardness ranges, TiAlN is sufficient and more economical. I avoid uncoated carbide entirely on landing gear components — the tool life is too short to justify the lower tool cost.
For gun drilling smaller diameters (under 20 mm), I typically drop the feed to 0.02-0.04 mm/rev and increase coolant pressure to 1200-1500 psi. Small-diameter gun drills in high-strength steel are fragile, and chip evacuation is the limiting factor. I covered the gun drilling coolant pressure fundamentals in this deep dive.
Quality and Inspection Requirements
Landing gear inspection is more rigorous than any other application I have worked on. The governing standards are AS9100 and customer-specific landing gear specifications. Here are the inspection methods and acceptance criteria I use.
| Inspection Method | What It Measures | Typical Acceptance Criteria | Frequency |
|---|---|---|---|
| Air gauging | Bore diameter | ±0.025 mm on diameter | 100% of parts |
| CMM | Position, true position | ±0.05 mm positional tolerance | First article + 1 per 10 parts |
| Dial bore gauge / ball bar | Straightness | 0.05 mm per 300 mm of length | 100% of parts |
| Profilometer | Surface finish (Ra) | Ra 0.8 μm maximum, Ra 0.4 μm for seal surfaces | 100% of parts |
| Ultrasonic testing | Wall thickness, subsurface defects | ±0.05 mm wall thickness variation, no indications > 0.5 mm | 100% of parts |
| Magnetic particle inspection (MPI) | Surface cracks | No linear indications | 100% of parts |
| Dye penetrant inspection | Surface flaws | No indications | First article + spot check |
The inspection sequence matters. On a typical landing gear strut, I check bore diameter and straightness immediately after drilling, before the part is removed from the machine. If the bore is within spec, the part moves to NDT. If the bore is out of spec, the part may be salvageable through reaming or honing — but only if there is enough stock remaining.
Wall thickness is the inspection that catches most landing gear defects. The structural analysis assumes a minimum wall thickness, usually 3 to 6 mm depending on the component. If the bore drifts off-center by even 0.3 mm over 2 meters, the thin side of the wall falls below the minimum. Every landing gear bore I have ever scrapped was due to wall thickness violation, not diameter or finish.
Surface finish is the second most common reason for rejection. The seal surfaces inside struts require Ra 0.4 μm. Standard BTA drilling with a wiper insert produces around Ra 0.6 to 0.8 μm, which means a burnishing or honing pass is usually required. I plan for a skiving and roller burnishing operation on any strut bore that serves as a seal surface. The burnishing pass also compresses the surface layer, which improves fatigue life.
Common Challenges
I have dealt with all of these in production. Some are material-specific, others are geometry-specific.
Material Hardness — The biggest challenge across all landing gear drilling is that the material is already heat treated before drilling. Unlike many automotive deep hole drilling applications, where parts are drilled in the annealed or normalized condition and hardened afterward, landing gear components are drilled in the hardened state. There is no post-drilling heat treatment to relieve residual stresses or correct distortion. Every degree of hardness above 40 HRC increases cutting forces by roughly 10%. At 55 HRC, the cutting forces are roughly double what they are at 35 HRC. The machine stiffness, tool holder, and fixturing all need to be designed for those forces.
Deep Bores and Straightness — Landing gear struts push the limits of BTA drilling depth capabilities. A 2.5 meter bore at 50 mm diameter gives a length-to-diameter ratio of 50:1. At that ratio, the drill head has a natural tendency to drift due to the asymmetric cutting forces inherent in single-lip BTA tools. I counter this with closely fitted guide pads and a stiff machine spindle. If straightness becomes a problem, I switch to counter-rotating BTA drilling where the tool rotates opposite to the workpiece rotation, which cancels some of the drift forces. The straightness requirement of 0.05 mm per 300 mm is tight enough that I verify the machine alignment with a laser every month.
Surface Finish on Seal Surfaces — Seal surfaces inside strut bores are unforgiving. A scratch, tear, or mark that is barely visible to the naked eye can leak oil during service. The finish is usually specified as Ra 0.4 μm with no individual scratch deeper than 0.5 μm. Standard BTA drilling produces Ra 0.6 to 1.2 μm depending on feed rate and tool condition. I have found that the fastest path to a reliable seal surface is BTA drilling followed by skiving with a single-point tool and then roller burnishing. The skiving pass removes the BTA feed marks and brings the finish to around Ra 0.6 μm, and the burnishing pass compresses the surface to Ra 0.2 to 0.4 μm. This combination is more reliable than trying to hold the finish directly from the BTA operation.
Chip Control at High Hardness — There is a common misconception that harder materials always produce better chip breakage. In landing gear steels, the chips are short and segmented, but they are also abrasive and hot. The challenge is not breaking the chips — it is evacuating them without scoring the finished bore surface. A chip that rubs against the bore wall on its way out leaves a scratch that fails the surface finish inspection. I use a high-pressure coolant system with a minimum of 800 psi at the tool to push chips out of the bore fast enough that they do not contact the wall.
Key Takeaways
- Landing gear components are drilled in the hardened state (50-55 HRC), not annealed. Cutting forces are roughly double what they are at 35 HRC — machine stiffness must match the material.
- 300M steel is the most common landing gear material and the most abrasive. Drop cutting speed to 40-60 m/min and use AlCrN-coated tools.
- Wall thickness variation is the leading cause of scrapped landing gear components. Check the bore position early and verify with ultrasonic inspection on every part.
- Seal surfaces inside struts typically require Ra 0.4 μm finish. Plan for a skiving and roller burnishing post-drilling operation — BTA drilling alone will not consistently hold that spec.
- Straightness of 0.05 mm per 300 mm length is achievable with BTA drilling using close-fitted guide pads and counter-rotation if needed.
- Inspect diameter and straightness before removing the part from the machine. Early detection of drift allows reaming or honing correction before the part is scrapped.
- Coolant pressure of 800-1200 psi is required for 300M. For gun drilling under 20 mm diameter, increase to 1200-1500 psi and reduce feed to 0.02-0.04 mm/rev.
- For a deeper breakdown of hardened steel drilling parameters and tool life, see this separate article.