Deep Hole Drilling for Aircraft Structural Components

When Aircraft Structures Need Deep Holes

Aircraft structural components like wing spars, bulkheads, and floor beams sometimes need through-holes for wiring harnesses, fluid lines, or fastener access. I have drilled these on CNC gun drilling machines with standard parameters. The holes are usually not structural themselves — they are pathways for systems that need to pass through the structure. But the parts they go through are highly stressed, so the drilling process cannot introduce stress risers or heat-affected zones.

The material is usually aluminum or titanium. Aluminum drills easily at 150-200 m/min with feeds of 0.08-0.12 mm/rev. Titanium needs slower speeds around 30-45 m/min with coolant pressure at 1500-2000 psi. I have drilled 7075-T6 aluminum wing spars with L/D ratios up to 30:1 and gotten consistent results. Titanium bulkheads are more demanding — the material work-hardens if the feed rate drops too low, and the heat generated at the cutting edge needs the high coolant pressure to carry it away.

I have worked with both Airbus and Boeing specifications for these types of holes. The requirements are similar between the two — position tolerance, surface finish, and burr limits are well-defined. The main difference I see is in the documentation requirements. Airbus typically requires more paperwork per hole, with separate sign-offs for each inspection step. Both require full traceability of the drill serial number and the inspection records for each hole.

Parameters for Aluminum and Titanium Structural Holes

I maintain a parameter card for each material and hole size combination. Here is what I have settled on after several years of aerospace work:

The titanium tool life numbers are low compared to aluminum, and that is just the nature of the material. I have tested coated and uncoated drills in titanium and found that AlTiN-coated carbide gives the best results. The coating handles the high cutting zone temperatures without breaking down. I have also tested TiAlN and AlCrN coatings in titanium, and AlTiN consistently gives 20-30% more tool life than the others.

Chip Control Challenges in Aerospace Materials

The main challenge with aluminum is chip control. Aluminum produces long stringy chips that can pack up in the bore if the feed rate is not high enough. I run 0.08-0.12 mm/rev to keep the chips short and broken. Below 0.08 mm/rev, the chips come out as long ribbons that wrap around the drill shank and jam in the bushing. Above 0.12 mm/rev on small diameters, the cutting forces get high enough to deflect the drill and cause position error.

For titanium, the challenge is different. Titanium produces short, segmented chips naturally, so chip control is less of an issue. The real problem is heat. Titanium has low thermal conductivity, so most of the heat stays in the cutting zone. Without high coolant pressure, the edge overheats and the titanium welds to the carbide. I have seen this happen at 1200 psi coolant pressure. At 1800 psi, the same job runs clean all day. The difference is the coolant velocity — at 1800 psi the coolant jet penetrates the cutting zone and breaks the vapor barrier that forms at lower pressures.

I have also dealt with chip packing in deep titanium holes. Even though the chips are short, they can still pack if the coolant flow is insufficient to push them out of the bore. I check coolant flow rate before every titanium job — minimum 20 liters per minute for an 8mm diameter hole. If the flow is below that, I stop and clean the coolant holes in the drill before proceeding. I have found that titanium chips can bridge across the bore and create a plug that blocks coolant return flow.

Position Tolerance and Inspection

Position tolerance is typically within 0.1mm of the design coordinate. This requires good machine alignment and a rigid setup. I check the first hole position with a CMM before drilling the rest. On one job, I found that the machine’s Z-axis had 0.03mm of play in the thrust bearing, which caused the drill to walk on entry. Replacing the bearing fixed the position drift. That experience taught me to check spindle thrust bearing condition as part of the regular machine maintenance schedule.

I use a guided bushing for every aerospace hole I drill. The bushing supports the drill within 1-2mm of the workpiece surface. Without the bushing, the drill walks on entry and the position error can exceed 0.2mm. I have tested unguided entry on aluminum and the position error was 0.15mm on average. With a bushing, the same job held 0.05mm consistently. The bushing wear is also important — I replace bushings when the ID wears more than 0.01mm oversize.

Here is the inspection sequence I follow for structural holes:

I have found that checking position on the first hole catches most setup errors before they turn into scrap. If the first hole is good, the rest of the batch usually follows. I also check the drill bushing alignment with a test indicator before every setup. A misaligned bushing by even 0.02mm will produce a hole that is within position tolerance but angled, and the angle can cause the hole to break through the opposite side in the wrong location on deep holes.

Burr Control for Aerospace Holes

Aerospace specifications are strict about burrs. I have rejected holes that had a burr smaller than 0.05mm because the inspection standard allowed zero burr on that particular surface. Burr control starts at the drill exit. I use a backup support on the exit side to minimize exit burr. The support can be a sacrificial aluminum plate or a dedicated backup bushing. Without the backup support, the exit burr on aluminum can exceed 0.15mm, which requires manual deburring that adds cost and can damage the surface finish.

On titanium, the exit burr is harder to control because the material is more ductile at the cutting temperature. I have found that reducing the feed rate by 50% for the last 2mm of the hole reduces exit burr significantly. I program this feed reduction into the CNC cycle so the operator does not have to remember to do it manually. I have measured exit burr heights of 0.08-0.12mm with the feed reduction versus 0.20-0.30mm without it.

For aluminum, the burr is softer and easier to remove, but it is also easier to create. A sharp drill with proper geometry produces minimal burr. A dull drill tears the material on exit and leaves a large burr that requires manual deburring. I change drills at 80% of expected life on aerospace jobs to avoid the dull-drill burr problem. I also use a deburring tool in a secondary operation for holes that require zero burr — a spring-loaded carbide blade that chamfers both edges in one pass.

I have also developed a carbide grade selection guide for aerospace deep hole drilling based on my experience. For aluminum in aerospace grades, micrograin carbide with a DLC coating gives the best tool life. For titanium, submicrograin carbide with AlTiN coating outlasts every other combination I have tested. I keep dedicated drills for aluminum and titanium jobs and never mix them — a drill used on titanium should never touch aluminum because the titanium can leave embedded particles in the edge that cause built-up edge in aluminum.

Documentation and Traceability Requirements

Aerospace drilling requires complete documentation. I maintain a drill log for every production run that records the drill serial number, the number of holes drilled, the parameter settings, and the inspection results for each hole. The customer can audit this log at any time, and I have had customers request copies of the log for specific serial-numbered parts.

I also tag each drill with a unique serial number and track its usage history. When a drill is used on a critical aerospace part, I can trace which holes it drilled and what the inspection results were for those holes. This traceability is required by AS9100 and most aerospace prime contractor requirements. I have a database that links drill serial numbers to part serial numbers, and I can generate a traceability report for any part within minutes.

Key Takeaways

• Aluminum structural holes need feed rates of 0.08-0.12 mm/rev to control chip formation. Too slow creates long ribbons that pack.
• Titanium deep holes need coolant pressure above 1500 psi to manage heat. Below that, edge welding is likely.
• Always use a guided bushing for aerospace structural holes. Unguided entry increases position error by 3-4x.
• Check first-hole position with CMM before running a batch. It catches machine alignment issues early.
• Reduce feed by 50% for the last 2mm of each hole to minimize exit burr on titanium.
• Change drills proactively at 80% of expected life. A dull drill creates burrs that require manual rework.
• Document everything. Aerospace customers audit process records, and missing paperwork can hold up an entire shipment.
• Check spindle thrust bearing condition regularly — 0.03mm of play causes detectable position drift.
• Use a deburring tool for critical holes that require zero exit burr on both entry and exit edges.
• Maintain a drill serial number database with full traceability to part serial numbers for AS9100 compliance.
• Use dedicated drills for aluminum and titanium — never cross-contaminate between the two materials.
• Position tolerance of 0.1mm requires guided bushings, CMM first-article inspection, and regular spindle bearing checks.
• AluminumTiN coating outperforms TiAlN and AlCrN in titanium by 20-30% based on side-by-side testing.