Skip to content 
CNC Machining Aerospace Composites: Delamination, Tooling & Process Guide
Author: Marcus Chen, Quality Director, Rapid Precision
Marcus Chen has 16 years in aerospace and precision manufacturing quality, with direct experience qualifying composite machining processes for AS9100D-certified aerospace programmes including CFRP structural components and GFRP radome panels.
For aerospace structural engineers specifying CNC machining operations on carbon fibre reinforced polymer (CFRP) components, the failure mode that ends careers and programmes is delamination — the interlaminar separation of composite plies caused by cutting forces, heat, or vibration that splits the laminate at the resin-matrix interface. A single delamination event on a $5,000 CFRP blank scraps the part and delays the programme. At scale, delamination rates of 2–5% across a production run of complex composite aerostructures cost $50,000–$500,000 in scrap, rework, and programme schedule impact.
Composite machining is fundamentally different from metal machining in two critical ways: composites are anisotropic (properties vary with fibre direction), and composites cannot be plastically deformed — they fail by fracture and delamination rather than by chip formation. Every machining decision — tool geometry, cutting speed, feed rate, cutting direction relative to fibre orientation, coolant strategy, and fixturing — directly affects delamination risk. The process that works on 0°/90° cross-ply laminate may cause delamination on quasi-isotropic laminates with the same nominal dimensions.
This guide covers the delamination mechanism, tooling selection, cutting parameters by composite type, fibre orientation effects, NDT inspection requirements, and the AS9100D process framework that Rapid Precision uses for aerospace composite programmes.
Composite Types for Aerospace CNC Machining
| Composite | Fibre | Matrix | Machinability | Primary Aerospace Applications | Key Machining Risk |
|---|---|---|---|---|---|
| CFRP (standard) | Carbon fibre | Epoxy | Difficult — highly abrasive CF, brittle fracture | Primary structure: wing spars, fuselage frames, empennage | Delamination, fibre pullout, tool wear (carbide fails fast) |
| CFRP (woven) | Woven carbon fabric | Epoxy | Difficult — bidirectional cutting loads | Skins, fairings, access panels | Delamination at fabric interfaces, fraying at cut edges |
| GFRP (glass fibre RP) | E-glass / S-glass | Epoxy / polyester | Moderate — less abrasive than CF | Radomes, antenna fairings, non-structural panels | Fibre pullout, delamination at stack exit |
| CFRP/Ti hybrid (CFRP-titanium) | Carbon + titanium layers | Epoxy + Ti | Very difficult — dual material transition in single stack | Advanced wing structures, door surround structure | Galvanic corrosion on tool, delamination at CF-Ti interface |
| Thermoplastic CFRP (PEEK matrix) | Carbon fibre | PEEK | Challenging — high matrix toughness, gummy matrix | Next-gen aerostructures, brackets, clips | Matrix melting if heat not managed — gummy deposits on tool |
| Nomex/carbon honeycomb sandwich | Carbon face skins + Nomex core | Epoxy | Moderate — core crushing risk | Flight control panels, cabin floors, nacelle components | Core crushing under clamping force; delamination at face-core bond |
The Delamination Mechanism: Why It Happens and How to Prevent It
Delamination in composite machining occurs when cutting forces — principally the thrust force perpendicular to the laminate plane — exceed the interlaminar shear strength of the composite at a ply interface. In drilling, the critical zone is the exit surface, where the drill pushes the final plies forward before breaking through (‘push-out delamination’). In edge trimming and routing, delamination occurs at the cut surface when the tool deflects into the laminate rather than cutting clean fibres.
| Delamination Type | Machining Operation | Primary Cause | Prevention Strategy |
|---|---|---|---|
| Push-out (exit) | Drilling, countersinking | Axial thrust force at drill exit exceeds interlaminar shear strength | Reduce feed rate at exit, use backup plate, split-point drill geometry |
| Peel-up (entry) | Drilling (early zone) | Upward-peeling force from conventional helix drill | Use low-helix or brad-point drill; reduce feed on entry passes |
| Edge delamination | Routing, trimming | Radial cutting force peels surface plies away from edge | Climb milling direction; sharp PCD or diamond-coated router bits; reduce DOC |
| Thermal damage | Routing, drilling, grinding | Heat exceeds matrix glass transition temperature (Tg) — typically 120–180°C for epoxy | Dry machining (no wet coolant that can penetrate laminate); air blast; sharp tools to minimise friction heat |
| Core crushing | Drilling honeycomb sandwich | Insufficient support under honeycomb core during drilling | Support fixture underneath sandwich panel; drill skin and core separately where possible |
At Rapid Precision, all aerospace composite machining is qualified under our AS9100D quality system with delamination inspection per ASTM E2966 and NDT requirements defined in the customer’s CMM plan.
Tooling Selection for Aerospace Composites
| Tool Type | Application | Life vs. Carbide | Cost Index | Best For |
|---|---|---|---|---|
| Uncoated carbide | GFRP, thin CFRP laminates | Baseline | 1.0x | Low-volume GFRP, prototypes where tool cost is secondary |
| Diamond-coated carbide | CFRP all types | 3–5× carbide life | 2.5–4x | Standard CFRP production — best cost-per-part economics |
| PCD (polycrystalline diamond) — brazed | High-volume CFRP | 10–25× carbide life | 8–15x | High-volume production runs where tool change downtime is the constraint |
| CVD diamond (thick film) | Precision CFRP, woven composites | 15–30× carbide life | 12–20x | Highest precision requirements, cleanest edge quality on woven fabric |
| Brad-point drill | CFRP drilling, entry-exit delamination-sensitive | 2–4× standard twist drill | 1.5–2x | Parts where entrance and exit delamination are the primary quality risk |
| Step drill (pilot + reamer) | Precision bore holes in CFRP | 2–3× standard drill | 1.8–2.5x | Precision holes ±0.025 mm in structural CFRP requiring surface integrity |
Cutting Parameters: CFRP vs GFRP
| Parameter | CFRP (epoxy matrix) | GFRP (epoxy/polyester) | Notes |
|---|---|---|---|
| Cutting speed — routing | 200–800 m/min (PCD/CVD diamond) | 100–400 m/min (carbide) | Higher speed reduces interlaminar shear; below minimum speed increases delamination risk |
| Feed rate — routing | 0.1–0.3 mm/tooth | 0.15–0.4 mm/tooth | Excessive feed increases radial force → edge delamination |
| Drill feed rate | 0.025–0.075 mm/rev main body; reduce 50% at exit | 0.05–0.1 mm/rev | Critical exit zone requires feed rate reduction 2–3 mm before breakthrough |
| Climb vs conventional milling | Climb milling always preferred for edge quality | Climb preferred | Climb milling reduces tool engagement angle; less edge peel force |
| Coolant | Dry air blast — NO wet coolant on structural CFRP | Dry air blast or MQL | Wet coolant penetrates laminate via cut capillaries; compromises epoxy matrix |
| DOC (depth of cut) | ≤ 0.5–1.5 mm per pass (routing) | ≤ 0.5–2.0 mm per pass | Multiple light passes reduce thrust force and delamination risk |
NDT Inspection Requirements for Machined Composite Parts
Composite parts machined for aerospace programmes require non-destructive testing (NDT) inspection after machining to confirm no sub-surface delamination, porosity, or matrix cracking was induced. Standard NDT methods for machined composites:
- Ultrasonic inspection (UT): C-scan or through-transmission UT is the primary method for detecting interlaminar delamination. Detection threshold: 0.1 mm delamination area. Required on all structural CFRP machining per FAA AC 43.13 and EASA CS-25.
- Tap test: simple acoustic inspection — tapping with a coin or specialised electronic tap tester detects delamination by acoustic response change. Useful for field inspection; not as sensitive as UT for sub-surface defects.
- Thermography: flash thermography or lock-in thermography detects delamination by thermal response differential. Useful for large panel inspection and faster than full UT C-scan for first-line screening.
- Visual inspection: edge delamination visible at cut surfaces. Magnified inspection (10× loupe) on all cut edges. Fraying extending more than 0.5 mm from cut edge is typically a rejection criterion.
AS9100D Process Framework for Composite Machining at Rapid Precision
- Process validation (PV): all composite machining processes are IQ/OQ/PQ validated before production release. PQ data includes delamination rate, dimensional compliance, and tool life across a statistically significant sample (minimum 30 parts)
- Control plan: identifies delamination as a Special Characteristic (SC). SPC monitored on thrust force, cutting speed, and exit-zone feed rate. Process limits defined from PV data
- First Article Inspection (FAI): full dimensional inspection per AS9102 Form 3, plus mandatory UT C-scan on first article of each new part number
- ITAR compliance: all ITAR-controlled composite aerostructure work is registered and handled under Rapid Precision’s ITAR registration, with physical security and export compliance documentation
Frequently Asked Questions
What causes delamination in CFRP CNC machining?
Delamination in CFRP machining occurs when the cutting force perpendicular to the laminate plane (thrust force in drilling, radial force in routing) exceeds the interlaminar shear strength of the composite at a ply interface. The critical zones are: drill exit surface (push-out delamination from axial thrust), and cut edges in routing (peel-up from radial cutting force on surface plies). Prevention requires: reduced feed rate at drill exit, backup plates, climb milling direction for routing, sharp PCD or diamond-coated tooling, and depth-of-cut control.
Should CFRP be machined wet or dry?
CFRP should be machined dry with air blast cooling — not with wet coolant. Wet coolant (water-based or oil-based) penetrates the open composite structure through the cut surface via capillary action, softening the epoxy matrix and potentially causing delamination at subsequent ply interfaces. The correct thermal management approach for CFRP is: sharp tooling to minimise frictional heat, high cutting speed with PCD or diamond-coated tools (faster cutting reduces heat generation per unit volume removed), and continuous air blast to evacuate hot chips. PEEK-matrix CFRP is an exception — its higher matrix Tg (glass transition) allows light misting with compatible fluids.
What tools are required for machining CFRP in aerospace production?
Diamond-coated carbide tools are the production standard for CFRP aerospace machining — they provide 3–5× longer tool life than uncoated carbide at 2.5–4× tool cost, producing better per-part economics in production runs above 20 parts. PCD (polycrystalline diamond) tools provide 10–25× carbide life and are cost-effective in high-volume programmes where tool change downtime is the production constraint. CVD diamond (thick film) provides the highest edge quality and longest life, used for precision woven carbon fabric components where edge integrity is the primary quality requirement.
What NDT is required after machining CFRP aerospace components?
FAA AC 43.13 and EASA CS-25 requirements for structural CFRP aerostructures typically mandate ultrasonic C-scan inspection after machining. Detection threshold is typically 0.1–6.4 mm equivalent flat bottom hole depending on the part criticality category. Tap testing is accepted for secondary structure and field inspection. Thermography is increasingly accepted as a first-line screening method. All NDT requirements must be specified in the part’s CMM plan and process qualification documentation, not assumed from general practice.
Conclusion: Composite Machining Requires Process Discipline, Not Just Good Tools
- Delamination is prevented by process control — correct entry/exit feed strategy, climb milling, PCD tooling, dry air blast — not by choosing a better machine or tighter tolerances
- NDT inspection (UT C-scan for structural parts) is mandatory after machining, not optional — undetected sub-surface delamination is the failure mode that grounds aircraft
- AS9100D process validation with delamination as a Special Characteristic is the correct framework for aerospace composite machining — IQ/OQ/PQ plus SPC on thrust force and feed rate
Rapid Precision is AS9100D and ITAR registered for aerospace composite machining. Submit your composite part drawings for a DFM review at rapidcision.com.