Deep Hole Drilling for Aerospace: Titanium, Inconel, and Tight Tolerances

Aerospace deep hole drilling is the most demanding work I’ve done. The materials are expensive, the tolerances are tight, and the cost of a scrapped part can be five figures. Every hole needs to be right the first time.

The main applications I’ve seen are landing gear components, engine shafts, and hydraulic actuators. They all have the same challenge: deep holes in tough materials with no margin for error.

The Material Challenge

Aerospace parts are typically titanium (Ti-6Al-4V), Inconel (718 or 625), or high-strength steel like 300M. All of them are harder to drill than standard engineering materials.

Titanium is the one I’ve worked with most. It has low thermal conductivity, which means the heat stays at the cutting edge instead of going into the chip. That cooks the drill if you’re not careful with coolant pressure.

Inconel work-hardens as you cut it. If the drill dwells at the entry or during a peck retract, it work-hardens the surface and the next pass has to cut through a harder layer. I keep the tool moving at all times — no dwelling, no hesitation.

For titanium, I use these parameters:

The low cutting speed feels wrong if you’re used to steel. But titanium won’t let you run faster. Push it past 50 m/min and the edge doesn’t last a full hole.

For Inconel 718, the parameters shift downward:

The cutting speed feels absurdly low until you watch the results. At 30 m/min, I’ve seen the edge fail before it reaches 100mm depth in Inconel. At 20 m/min, the same tool runs through 300mm without a problem. The material wins if you push it — you have to accept the slow pace and plan around it.

Landing Gear Components

Landing gear parts are massive. I’ve worked on struts that were 2 meters long with a 40mm drilled hole through the center. The material is usually 300M steel, hardened to around 50 HRC.

The process is BTA drilling, not gun drilling, at those diameters. I start with a BTA head at around 70 m/min with a feed of 0.10-0.15 mm/rev. The coolant pressure needs to be high enough to clear the chips — 300-400 psi at the tool, which means 500+ psi at the pump after accounting for line losses.

Straightness is the critical spec on landing gear. A hole that drifts by even 0.5mm over 2 meters can affect the wall thickness. I check alignment before every job and verify straightness with a test bar at least once per shift.

Engine Shaft Drilling

Turbine engine shafts rank among the most demanding deep hole jobs I’ve touched. The material is Inconel 718 or sometimes Waspaloy, with a bore of 20-60mm through 500-1500mm of length. The concentricity between bore and OD is typically 0.05mm or tighter.

My standard approach is gun drilling from both ends. Unlike automotive crankshaft work, where the aspect ratio is lower and the material is steel, a single aerospace pass at 75:1 L/D pushes straightness too far. By coming from both ends and meeting in the middle, each pass stays under 40:1 and alignment is far easier to control.

I leave a 1mm witness at the meeting point and check alignment with a borescope before the cleanup pass. If the two holes meet within 0.5mm, a final reaming pass brings them to spec. If they’re off by more than that, I check wall thickness — a thin wall on one side means the part is scrap.

Hydraulic Actuators

Aerospace hydraulic actuators are similar to industrial hydraulic cylinders but with tighter tolerances and more exotic materials. (I covered industrial hydraulic cylinder drilling here — the aerospace version is the same principle, just harder.) A typical job might be an actuator barrel in 15-5PH stainless steel, with a 25mm bore through 400mm of material.

The challenge with stainless is chip control. 15-5PH produces tough, stringy chips that want to wrap around the tool. I run a higher feed than I would in carbon steel — around 0.06-0.08 mm/rev for a 25mm BTA head — to get the chips to break.

The surface finish spec on aerospace actuators is usually Ra 0.8μm or better. I’ve found that running a wiper insert on the BTA head gives a noticeably better finish than a standard geometry. The wiper flat burnishes the surface as the tool rotates, which can save a honing operation.

Inspection Requirements

Every aerospace deep hole gets inspected. The standard methods I’ve used are:

I’ve learned to inspect early and often. On a titanium landing gear strut, I check the first 50mm of the hole before drilling the full depth. If the entry is off, I can correct the alignment and save the part. If I drill the full 2 meters before checking, the part is scrap.

What Aerospace Taught Me

Aerospace work has a different mindset. In job shop work, you can scrap a part and make another. In aerospace, the material lead time alone might be six weeks, and the blank cost more than a small car.

I’ve become more conservative with parameters on aerospace work — not because the tools can’t take more, but because the cost of a mistake is too high. I change tools earlier, check alignment more often, and run coolant pressure at the high end of the range. It’s not the fastest way to drill a hole, but it’s the most reliable way.

Key Takeaways

• Titanium needs low cutting speed (30-50 m/min) and coolant pressure above 1500 psi to keep the heat from cooking the cutting edge.
• Inconel requires even slower speeds (15-25 m/min) and a continuous cut — any dwell work-hardens the surface and shortens tool life.
• Landing gear straightness is the critical spec; verify alignment with a test bar at least once per shift.
• On expensive aerospace parts, inspect the first 50mm before drilling full depth. A misaligned entry is fixable at that point; a full-depth drift is scrap.
• Run conservative parameters when the blank costs more than a small car and material lead time runs six weeks. Reliability beats cycle time.