Drilling Composite Materials: Carbon Fiber, Fiberglass, Plastics

Composites and engineering plastics present a different set of challenges than metals, and the geometry, speeds, and handling practices that work on steel will damage these materials. Delamination, fraying, melted matrix, and undersized holes all stem from treating composites like metal. The physics are different. The approach has to be.

Carbon Fiber Reinforced Polymer (CFRP)

CFRP is arguably the most demanding drilling material in common shop use. The challenges:

Delamination: Carbon fiber's layered structure means the drilling force can separate plies, especially at entry and exit. Delamination is often invisible on the surface and only detected under load testing or inspection. In aerospace, a delaminated hole is cause for rejection of the entire part.

The fiber orientation problem: In a woven or unidirectional layup, the cutting edge alternately cuts fibers perpendicular to the layup and at angle to it. The "worst angle" — where the fiber is oriented roughly 45° to the cutting direction — requires maximum force per unit material removed. Geometry matters enormously here.

Abrasiveness: Carbon fiber is ceramic-hard in the fiber phase (~1,000+ HV). The matrix (typically epoxy) is comparatively soft. This combination is aggressively abrasive. HSS drills used on CFRP are essentially single-use. CFRP is a carbide application.

CFRP Geometry Requirements

Point angle: 90° to 120° is standard for CFRP, versus 118–135° for metals. The lower point angle reduces thrust force and distributes the entry load over a larger area, reducing delamination tendency.

Helix angle: Very low or zero helix is preferred for thin laminates. High helix drills designed to evacuate chips aggressively will pull on the fibers in the laminate direction, promoting delamination. Brad-point or straight-flute (zero helix) designs are common in aerospace composite drilling.

Special geometries: Compression spiral drills (alternating up-cut and down-cut geometry) cut the top surface in compression (down-shear) and the bottom surface up-shear, holding the delamination-prone exit ply in place from both directions.

Edge sharpness: Sharp, sharp, sharp. Any dullness means higher cutting force, which means more delamination. CFRP tooling is often assessed on entry/exit quality rather than hole count. Many aerospace shops replace CFRP drills every 50–100 holes regardless of apparent condition.

CFRP Speed and Feed

Recommended ranges for carbide drilling CFRP:

• SFM: 200–400 (lower end for thick laminates, higher end for thin)
• Feed per rev: 0.002–0.006" (lower feeds reduce thrust, critical for delamination control)
• Coolant: Dry or compressed air blast — CFRP is not water-soluble, and liquid coolant creates disposal issues with carbon fiber dust

CFRP Dust Management

Carbon fiber dust is hazardous. Electrically conductive (it will short-circuit electronics — grounded machines are mandatory), potentially carcinogenic (fiber length below 3.5 microns is considered respirable), and it gets into everything.

Required: Dust extraction at the cutting zone, not just area collection. A vacuum nozzle directed at the drill entry point captures dust before it becomes airborne. Operators require respiratory protection (N95 minimum for incidental exposure, P100 for sustained work). Regular HEPA vacuuming of machines used for CFRP. Do not use compressed air to blow down CFRP dust — it aerosolizes it.

Fiberglass (GFRP)

Fiberglass composites share CFRP's abrasiveness but are less demanding in most other respects. Glass fiber hardness is lower (~700 HV vs 1,000+ HV for carbon fiber), and fiberglass matrix systems are often more forgiving.

Key difference from CFRP: Glass fibers are isotropic in behavior at the micro scale. You don't see the same directional delamination patterns, though exit delamination is still a concern in thin laminates.

Geometry for GFRP: Standard 118° or 135° carbide drills work adequately for general GFRP. For thin laminates, back up the exit face with a sacrificial backup plate (wood, MDF, or similar material) to prevent exit delamination.

Speed: HSS is essentially useless for production GFRP work — it dulls within a few holes. Carbide at 250–400 SFM with moderate feed.

Engineering Plastics

Plastics share one fundamental challenge: they're thermally insulating and have low melting points. Metal drilling generates heat; plastics can't dissipate it. The result is melted, deformed holes, gummed flutes, and dimensional problems.

Polycarbonate and Acrylic (PMMA): Acrylic melts at around 160°C (320°F). Normal metal-cutting feeds and speeds generate temperatures well above this. Modified geometry with zero to slightly negative rake makes thin chips rather than long curling chips and reduces heat generation. Moderate SFM (150–300 for carbide), high feed rate to minimize contact time per edge.

Nylon: Begins deforming at 80°C (176°F). Zero rake, HSS or carbide. Dry — liquid coolant contaminates some nylon grades which absorb moisture and swell. Compressed air cooling works.

UHMW-PE: Very high feed rate, sharp drill, no coolant. UHMW springs back elastically — drill 0.010–0.020" oversize to hit nominal diameter. After drilling, the material relaxes to near nominal.

PTFE (Teflon): Deforms under tool pressure. Back-supported drilling, very sharp edges, high feed. Peck drilling with frequent retract cycles to prevent chip packing in flutes.

Phenolic and FR4 (PCB Laminate)

Hard, abrasive matrix with glass or cotton fiber reinforcement. Treated like a light composite rather than a plastic. Same principles as GFRP — carbide tooling, low thrust, backup plate for exit. FR4 (fiberglass epoxy PCB laminate) is aggressively abrasive. For shops occasionally drilling FR4 for prototype boards or custom panels, sharp carbide with low feed and a sacrificial backup produces acceptable results.

Cross-Material Summary

• CFRP: Carbide compression/dagger drill, 90–118° point, dry/air blast, watch for delamination and dust
• GFRP: Carbide, 118° point, dry/air blast, backup plate for exit
• Acrylic/PC: Modified zero-rake geometry, 118° point, air or dry, melting is the enemy
• Nylon: Zero rake HSS, 118° point, dry only, watch for thermal deformation
• UHMW-PE: Sharp HSS, 118° point, drill oversize to account for elastic recovery
• PTFE: Sharp HSS, 90° point, dry, peck cycle to manage chip packing
• FR4: Carbide, 118° point, dry, backup plate required

The important takeaway: standard metal-cutting HSS drills are wrong for composites and many engineering plastics. Using them doesn't produce holes — it produces delaminated composites, melted bores, gummed tools, and scrapped parts. Maintain a small set of purpose-ground carbide drills for composites, and modified-geometry tooling for thermoplastics.