Deep Hole Drilling of Composites — CFRP GFRP Stack Drilling

An aerospace manufacturer drilling 6.35 mm diameter × 38 mm deep fastener holes through 19 mm CFRP/titanium stacks for a wing skin assembly was experiencing delamination in 12 % of holes at the CFRP exit ply, requiring costly rework and scrapping of $8,000 wing panels. The critical breakthrough phase — when the drill exits the CFRP and enters the titanium layer — generated thrust forces exceeding the critical threshold for the final CFRP plies. Switching to a diamond-coated step drill with a pilot point, combined with a 40 % feed reduction during the final 0.5 mm of CFRP penetration and ultrasonic vibration assistance, reduced the exit delamination rate from 12 % to 0.3 %, and extended tool life from 80 to over 400 holes per drill.

Composite Materials and Their Drilling Challenges ​

Fibre-reinforced polymer composites present fundamentally different drilling challenges from metals. Unlike homogeneous metallic materials, composites are heterogeneous, anisotropic, and abrasive, with material properties that vary by fibre orientation, ply stacking sequence, and matrix type.

Common composite materials in deep hole drilling:

Unique drilling challenges:

1. Delamination: The most critical defect. Separation of plies at the hole entry (peel-up delamination) or exit (push-out delamination). Caused by thrust forces exceeding the inter-laminar bond strength.
2. Fibre pull-out: Fibres are not cut cleanly but pulled from the matrix, leaving rough hole walls and reducing fatigue strength.
3. Thermal damage: The epoxy matrix (typically 180–220 °C glass transition temperature) can soften or degrade if cutting temperatures exceed 200 °C.
4. Tool wear: Carbon fibres are highly abrasive (carbon fibre hardness exceeds that of carbide). Diamond coating is needed for production volumes.
5. Uncut fibres: Remaining fibres at the hole exit that were not cleanly sheared.

Delamination Mechanisms and Control ​

Delamination is the primary quality concern in composite deep hole drilling. Two distinct mechanisms occur:

Peel-up delamination (entry side):

As the drill point contacts the top plies, the cutting edge exerts an upward peeling force that separates the top plies from the laminate. This mechanism is influenced by:

• Drill point angle: larger point angles (140°) increase peel-up delamination
• Feed rate: higher feed rates increase peel-up forces
• Chisel edge geometry: a large chisel edge increases axial force at entry

Push-out delamination (exit side):

As the drill approaches the exit side, the uncut plies beneath the drill point flex under thrust force. When thrust force exceeds the inter-laminar fracture toughness, the remaining plies separate — the most severe form of delamination. The critical thrust force at exit is governed by:

Critical thrust force ∝ √(G_IC × E × h³)

Where G_IC is the mode I inter-laminar fracture toughness, E is the flexural modulus, and h is the remaining uncut ply thickness. This means:

• Each ply thickness and fibre orientation has a critical thrust force threshold
• Thin remaining plies near exit are most vulnerable
• Higher fracture toughness materials (e.g., toughened epoxy systems) tolerate higher forces

Delamination factors used in industry:

Delamination control strategies:

1. Feed rate reduction at breakthrough: Reducing feed by 40–60 % in the final 0.5–1.0 mm before exit reduces push-out delamination by 50–80 %. This is the single most effective and easily implemented control.

2. Pilot hole pre-drilling: A small pilot hole (20–30 % of final diameter) reduces thrust force at the drill point by eliminating the chisel edge indentation zone.

3. Back-up support: A sacrificial backing plate or support block beneath the exit side provides mechanical support to the final plies, preventing flexural delamination.

4. Step drill geometry: A two-diameter step drill with a small pilot diameter followed by a reaming step reduces thrust force at breakthrough by spreading the cutting action over two stages.

5. Ultrasonic vibration assistance: A 2025 study found ultrasonic-assisted drilling reduced the damage coefficient by 15.7 % compared to conventional drilling.

Tool Geometry for Composite Drilling ​

Tool geometry is the most important factor in composite deep hole drilling quality. Unlike metal drilling, where chip evacuation dominates tool design, composite drilling prioritises clean fibre shearing and delamination prevention.

Drill geometries for composites:

Step drill geometry parameters:

For deep hole drilling of composites, the step drill configuration is most commonly used:

• Pilot diameter: 60–70 % of final diameter
• Pilot length: 2–4 mm (sufficient to clear the first material layer in stacks)
• Step angle: 90–120° (lower angle reduces thrust force)
• Point angle: 90–120° (lower angle for CFRP, higher for GFRP)
• Relief angle: 8–15°
• Helix angle: 20–30°

Tool materials for composite drilling:

Diamond-coated carbide is the standard for production composite drilling. The diamond layer (15–30 µm thick) provides the abrasion resistance needed against carbon fibres. Tool life of diamond-coated drills in CFRP is typically 300–800 holes, compared to 20–50 holes for uncoated carbide.

Stack Drilling — CFRP/Metal Combinations ​

Stack drilling — drilling through two or more dissimilar material layers in a single operation — is the most common deep hole drilling application in aerospace manufacturing. The most important stacks are:

CFRP/Titanium stacks (CFRP/Ti):

Used in airframe structural joints (wing skins, fuselage panels, empennage). The drilling challenge is managing the transition between materials with very different properties:

• CFRP layer: abrasive, temperature-sensitive matrix, delamination risk at exit
• Titanium layer: work-hardens, generates high heat, long chips
• Chip evacuation: titanium chips are abrasive to CFRP hole walls

Key findings from 2024–2025 research:

• A comprehensive 2025 review established aerospace industry tolerances: max delamination 1 mm, diameter deviation ±30 µm, burr height 150 µm, metal surface roughness Ra 1.6 µm, CFRP surface roughness Ra 3.2 µm
• Combined ultrasonic + low-frequency vibration drilling (CVAD, 2025) reduced CFRP entrance delamination by 72.6 % and exit delamination by 38.83 %, with Ti exit burr height reduced by 51.19 %
• DLC-Bn coated drills showed superior performance in CFRP/Ti stacks with reduced thrust forces and no edge chipping or fracture

CFRP/Aluminium stacks:

Used in wing and fuselage structure where aluminium is the primary structure and CFRP is a doubler or repair patch.

• Aluminium is easier to drill than titanium but produces long chips that can damage CFRP hole walls
• The aluminium layer provides excellent back-up support for CFRP exit — exit delamination is rare in CFRP/Al stacks
• Mean forces in aluminium are more than double those in CFRP
• Dry and flood cooling both produce delamination-free holes

CFRP/CFRP stacks (thick laminates):

For composite-only stacks thicker than 20 mm:

• Delamination risk exists at each ply interface, not just entry and exit
• Chip evacuation of abrasive carbon dust is critical — vacuum extraction recommended
• Diamond-coated step drills are preferred
• Orbital drilling (helical milling) produces the best hole quality for thick laminates

Process Parameters ​

Cutting parameters for composite drilling:

Parameter effects on hole quality:

• Spindle speed: Higher speeds increase frictional heating but can thermally soften the matrix, reducing thrust force. However, excessive speed (> 6,000 rpm in CFRP) causes matrix degradation and delamination from thermal stress.
• Feed rate: The most significant parameter for thrust force and delamination. Lower feed reduces thrust force proportionally. A 2024 study found that feed rate and tool type are the two most important features for delamination prediction.
• Point angle: Lower point angles (90–118°) distribute the axial force over a longer engagement, reducing peak thrust at breakthrough. Higher angles (140°) concentrate force and increase delamination risk.

Orbital Drilling (Helical Milling) ​

Orbital drilling — where the tool rotates on its own axis while simultaneously orbiting around the hole centre — offers distinct advantages for composite deep hole drilling:

Process characteristics:

• Tool diameter is smaller than hole diameter (typically 60–80 %)
• The tool follows a helical path, milling rather than pushing through the material
• Chips are small and easily evacuated (no chip packing in deep holes)
• Cutting is interrupted, allowing heat dissipation between passes

Benefits for composites:

• Delamination virtually eliminated (no axial thrust at breakthrough)
• Tool life extended by 2–5× compared to conventional drilling
• Burr-free holes in stack drilling
• Variable hole diameter with a single tool (within limits)
• Coolant not required (dry machining possible)

Limitations:

• Longer cycle time (2–5× slower than conventional drilling)
• Requires 5-axis CNC capability or special orbital drilling heads
• Limited to hole depths typically < 10× tool diameter
• Tool path programming is more complex

Orbital drilling parameters for CFRP:

• Spindle speed: 4,000–8,000 rpm
• Orbital speed: 200–500 rpm
• Axial feed per orbital revolution: 0.05–0.15 mm
• Radial depth of cut: 0.5–2.0 mm (depending on tool diameter)

Tool Wear in Composite Drilling ​

Tool wear in composite drilling is dominated by abrasion from carbon fibres. The wear mechanism is fundamentally different from metal drilling:

Wear mechanisms:

• Abrasive wear: Carbon fibres are harder than carbide (carbon fibre hardness: 2,000–3,000 HV, carbide: 1,500–2,000 HV). Each fibre acts as an abrasive particle, progressively eroding the cutting edge.
• Coating delamination: Diamond coatings fail by delamination from the carbide substrate, not by wear of the diamond itself. Coating adhesion quality is the primary determinant of tool life.
• Edge rounding: The cutting edge radius increases progressively, eventually preventing clean fibre cutting. Rounded edges pull fibres rather than shearing them, increasing delamination and surface roughness.

Tool life by coating type:

Monitoring tool wear in production:

• Thrust force increase of 20–30 % indicates significant tool wear
• Hole entry burr height or delamination diameter increase
• Surface roughness increase (Ra > 3.2 µm for CFRP)
• Acoustic emission monitoring (increased high-frequency noise from fibre rubbing)

Coolant and Chip Evacuation ​

Dry drilling: The preferred method for composite drilling, provided cutting temperatures remain below the matrix glass transition temperature (Tg). Dry drilling avoids moisture absorption into the composite edge, eliminates contamination of the composite by cutting fluids, and simplifies chip disposal (dry carbon dust).

Minimum quantity lubrication (MQL): Used when additional lubrication is needed for stack drilling (particularly the metal layer). MQL delivers 20–100 mL/h of biodegradable oil in a compressed air stream. A 2024 study found MQL cooling produced delamination within 24 % of hole diameter in CFRP/Al stacks.

Cryogenic cooling: LN2 cooling (−196 °C) is increasingly used for CFRP/Ti stack drilling. The low temperature embrittles the titanium chips and prevents matrix softening. The VibroCool EU project combines cryogenic cooling with ultrasonic vibration for FRP-metal stacks.

Chip evacuation: Carbon dust is hazardous — fine carbon fibres are conductive and can cause electrical shorts in machine tool electronics. Vacuum extraction systems with HEPA filtration are mandatory for production composite drilling. Chip evacuation is particularly important in deep holes (> 10×D) where carbon dust can pack and cause overheating.

Quality Assurance for Composite Holes ​

Aerospace industry acceptance criteria for composite drilled holes:

Troubleshooting Composite Deep Hole Drilling ​

Frequently Asked Questions ​

1. What is the most common defect in composite deep hole drilling? Exit delamination (push-out delamination) — the separation of the final composite plies as the drill exits the laminate. This is caused by thrust force exceeding the inter-laminar bond strength of the remaining plies.

2. Can standard twist drills be used for CFRP deep hole drilling? Not recommended for production. Standard twist drills are designed for metals and produce high thrust forces that cause delamination. Brad point, step, dagger, or diamond core drills are required for acceptable hole quality.

3. What is the best tool coating for drilling carbon fibre composites? Diamond coating (CVD diamond or PCD) provides the best wear resistance against abrasive carbon fibres. Diamond-coated carbide drills typically achieve 300–800 holes in CFRP, compared to 20–50 holes for uncoated carbide.

4. How does stack drilling differ from drilling a single composite laminate? Stack drilling must manage the transition between materials. In CFRP/Ti stacks, the CFRP generates abrasive dust while the titanium generates long, stringy chips and high heat. The drill geometry, feed rate, and coolant strategy must accommodate both materials in a single operation.

5. What feed rate should be used near the exit side of a CFRP laminate? Reduce feed by 40–60 % in the final 0.5–1.0 mm of CFRP penetration. This reduction lowers thrust force below the critical threshold for the remaining thin plies and is the single most effective delamination control strategy.

6. Is coolant required for drilling composites? Not for CFRP alone — dry drilling is preferred to avoid moisture absorption. For CFRP/metal stacks, dry drilling is still preferred for the CFRP layer, but coolant or MQL may be needed for the metal layer. Cryogenic cooling is an emerging option for high-performance stacks.

7. What is orbital drilling and why is it beneficial for composites? Orbital drilling (helical milling) uses a tool smaller than the hole diameter that follows a helical path. It virtually eliminates delamination because the tool cuts laterally rather than axially, producing no thrust force at breakthrough.

8. What is the acceptable delamination factor for aerospace composite holes? Typical acceptance limits are F_d < 1.10 (10 % oversize) for entry delamination and F_d < 1.15 for exit delamination. Critical structural applications may require F_d < 1.05.

9. How does ultrasonic vibration improve composite deep hole drilling? Ultrasonic vibration reduces thrust force by 27 %, improves chip evacuation, and reduces the damage coefficient by 15.7 %. Combined with low-frequency vibration (CVAD), delamination can be reduced by 72.6 % at the entrance and 38.8 % at the exit.

10. What are the health and safety considerations for composite drilling? Carbon fibre dust is conductive and can cause electrical shorts, skin irritation, and respiratory issues. Vacuum extraction with HEPA filtration is mandatory. Machining enclosures should be interlocked, and operators should use protective gloves and eye protection.

Summary ​

Deep hole drilling of composite materials is defined by the fundamental conflict between the abrasive, brittle nature of fibre-reinforced polymers and the requirement for clean, delamination-free holes. Success depends on four factors: tool geometry selection (step drill or diamond core preferred), feed rate control at breakthrough (the dominant process variable), tool coating (diamond coating essential for production volumes), and stack-specific parameter optimisation for CFRP/metal combinations. With the rapid growth of composite usage in aerospace (Boeing 787, Airbus A350, COMAC C919 are now >50 % composite by weight), deep hole drilling of composites and composite/metal stacks represents one of the most active areas of drilling research, with ultrasonic vibration assistance and orbital drilling emerging as the most promising process improvements.