Carbon fiber and glass laminates fail differently from metals. They don’t cut — they delaminate, fuzz, and burn. The hole exit is where composite drilling goes wrong: without backup support and a sharp diamond tool, up to 60% of aerospace parts can be rejected for delamination. This guide covers tooling, parameters, and backup strategy for CFRP, glass, aramid, and metal–composite stacks.
| Difference vs Metal | What Happens | Consequence |
|---|---|---|
| Heterogeneous structure | Hard abrasive fibers + soft resin matrix alternate in the cut | Rapid flank wear, edge dulling, fiber pull-out |
| Anisotropy | Each ply has its own fiber orientation crossing the cutting edge | Fuzzing, uncut fibers, variable cutting force |
| Low thermal conductivity | Friction heat stays in the layer ahead of the drill, worst at exit | Matrix melting, burns, glass fibers char |
| Weak interlaminar bond | Thrust force exceeds the critical interlaminar strength | Delamination, microcracks between plies |
| No plastic chip | Fibers shear, matrix crumbles into dust | Dust hazard, abrasive chip slurry in coolant |
| Material | Fiber Hardness | Dominant Damage | Cooling / Dust | Notes |
|---|---|---|---|---|
| Carbon (CFRP) | Very hard, abrasive | Delamination, uncut fibers, edge chipping | Dry or MQL; carbon dust conductive | Worn tools produce the greatest damage |
| Glass (GFRP) | Abrasive, fibrous | Fraying, fuzzing, splintering, matrix melt | Dry + dust collection | Glass drill temps roughly double basalt; diamond grit best |
| Aramid (Kevlar®) | Tough, resists shearing | Fiber pull-out, fuzzing, uncut fiber ends | Low RPM, sharp edge | Tough fibers must be sheared, not torn |
| Thermoplastic matrix (PEEK/PEKK) | Matrix softens with heat | Melting, burrs, dimensional drift | Cryogenic / MQL | Optimum Vc ~130 m/min found for PEKK/CFRP |
Delamination is the interlaminar fracture where the drill pushes the last uncut plies apart instead of cutting them. It comes in two forms:
| Type | Location | Cause | Severity |
|---|---|---|---|
| Peel-up delamination | Hole entrance (top plies) | Helix angle pulls plies upward away from the tool | Less critical |
| Push-out delamination | Hole exit (bottom plies) | Thrust force exceeds interlaminar strength of the last ply | Most critical — undermines structural health |
| Tool Type | Best For | Limitations |
|---|---|---|
| CVD diamond-coated carbide | Complex geometries, small drills <4 mm (Seco Feedmax C1/C2, Sandvik R854, OSG) | Coating dulls the edge; generally not resharpened |
| PCD-tipped (brazed on carbide) | Larger drills >4 mm; higher speeds; longest life (Seco CX1/CX2, CoreHog, ISCAR) | Flat wafers limit edge geometry; braze joint near the hot edge |
| Uncoated solid carbide | Laminate stacks where no single tool satisfies every material | Shortest life; quality preserved at cost of tool life (AMAMCO) |
| Diamond-grit edge drills | Fiberglass; ~3× faster than carbide | Mandatory dust collection; cannot run in bushings |
There is no universal CFRP parameter set. Optima depend on fiber architecture (woven vs unidirectional), matrix (thermoset vs thermoplastic), tool geometry, and diameter. These are defensible starting points from published studies.
| Material | Vc (m/min) | Feed (mm/rev) | Drill | Source / Notes |
|---|---|---|---|---|
| Woven CFRP (0/90°) | 50–90 | 0.2–0.4 | Ø8 mm carbide | Isik et al., Processes 2024, SA/GA-optimized |
| Thermoset CFRP | 15–45 | 0.05–0.2 | Ø3–5 mm twist | Bolat et al., Micromachines 2023; optimum 45 m/min, 0.05 mm/rev |
| CFRP, delamination-min | ~30 | 0.06 | 118° point carbide | Karabük University Taguchi study |
| PEKK/CFRP (thermoplastic) | ~130 | Low | PCD / coated carbide | Optimum Vc for min delamination, 2025 study |
| Glass fiber (GFRP) | Low RPM 5,000–15,000 spindle | Moderate steady pressure | Diamond-grit or carbide 55° point | Gill Corporation machining guidance |
| CFRP/Ti stack | Split per layer | Low, variable at interface | Double-margin one-shot PCD | Metal layer dominates wear & heat |
Composites don’t make metal chips — they make abrasive dust and broken fiber fragments. In deep holes this dust packs the flutes, burns the matrix, and gauges the bore. Dust extraction is mandatory, and carbon dust is conductive and a fire/electrical risk.
Vacuum at the drill point or machine enclosure. Carbon dust is conductive — keep it out of electrical cabinets and bearings.
Lift the drill periodically to clear dust and chips. This prevents galling, binding, and heat buildup (Gill Corporation guidance).
In metal–composite stacks, a flat-point / chipbreaker geometry breaks the long metal chip before it whips and damages the composite wall.
Glass and carbon dust is highly abrasive; fine filtration extends tool and seal life in any flooded application.
| Challenge | Consequence | Countermeasure |
|---|---|---|
| Chip evacuation over length | Dust packs flutes, friction burns the matrix | Peck cycles, internal coolant, diamond-edge tools |
| Entry drift / off-square start | Deviation grows with depth — bell-mouth entry | Spot-face or pilot hole; drill templates with hardened bushings |
| Exit unsupported over long span | Exit deflection releases stored energy → delamination | Backup plate or support under the final plies |
| Wear over the hole length | Force climbs as edge dulls | Monitor torque/thrust; regrind on schedule |
| Multiple-diameter bores | Step transitions catch uncut fibers | Step drills with controlled point transitions |
| Defect | Detection | Acceptance Typical |
|---|---|---|
| Delamination factor (Fd) | Ultrasonic C-scan, edge microscopy | Depends on ply count; exit damage most critical |
| Uncut fibers at exit | Visual / optical inspection of last ply circumference | None for structural holes |
| Surface roughness (Ra) | Profilometer; waterjet reference ~6–7 μm Ra | Drilled CFRP typically 1–4 μm Ra |
| Diameter / tolerance | Air gauge, CMM | H8 typical in aerospace stack drilling (SAE) |
| Sub-surface damage | X-ray CT, high-resolution ultrasonic | 3-D damage-zone extraction used for structural assessment |
Riveted and bolted aircraft joints are drilled as stacks — CFRP/Al, CFRP/Ti, or three-layer CFRP/Al/Ti — in one shot so holes stay aligned. The metal layer dominates tool wear and heat, and the composite layer dominates quality risk. This is where the process earns its reputation.
| Layer Transition | Problem | Countermeasure |
|---|---|---|
| CFRP → Al | Metal burr whips and scratches the composite wall | Flat-point / chipbreaker one-shot tool; PCD edge |
| CFRP → Ti | Titanium chips are hot, long, and hard to break; heat burns the CFRP | Low speed, split parameters, chilled air or MQL |
| Ti → CFRP interface | Performance mutation at the interface; oversize holes | Double-margin tool design; variable feed at interface |
| Wear accumulation | Standard twist drills wear rapidly in single-shot work | PCD or coated carbide; diamond/ta-C coatings |
When delamination cannot be tolerated, or the stack includes materials that ruin a drill, non-mechanical methods step in. They each trade one quality for another.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Exit delamination | Feed too high at break-through; no backup | Reduce feed at exit; add cork/foam backup; drill pilot |
| Fuzzing / uncut fibers | Dull edge, low relief angle, wrong point angle | Regrind to sharp edge ≤10 μm; relief >10°; 118° point |
| Burns / matrix melt at exit | Heat concentration in the last plies | Lower RPM, increase peck, MQL/chilled air |
| Oversize hole / drift | Off-square entry, worn tool, thin stack flex | Spot-face entry, drill template with bushings, clamp stack |
| Rapid tool failure | Wrong tool for stack; CVD coat on >4 mm drill | Switch to PCD; match geometry to the metal layer |
| Burr on metal ply in stack | Metal chip whipping in the composite bore | Flat-point chipbreaker, lower feed, one-shot PCD |