Deep hole drilling was developed for metals, but modern manufacturing requires drilling holes through composites and non-metallic materials too. Carbon fiber reinforced polymers, fiberglass, technical ceramics, and engineered plastics all need precision bores for various applications.
I have drilled composite components for aerospace, automotive, medical, and industrial applications. The materials behave differently than metals, and the drilling parameters must be adjusted accordingly. For related work on difficult-to-machine materials, see BUE and aluminum gun drilling and micro-hole deep hole drilling.
Carbon Fiber Drilling
Carbon fiber reinforced polymer is the most common composite I have drilled. It is strong, lightweight, and used extensively in aerospace and automotive applications.
The challenge with drilling carbon fiber is the abrasive nature of the material. Carbon fibers are harder than carbide tooling, and they wear cutting edges rapidly. A standard carbide gun drill that lasts 50 meters in steel might last only 5 meters in carbon fiber.
For carbon fiber drilling:
| Parameter | Value |
|---|---|
| Cutting speed | 100-200 m/min |
| Feed rate | 0.03-0.08 mm/rev |
| Coolant | Compressed air or mist |
| Tool material | PCD or diamond-coated carbide |
| Expected tool life | 3-8 meters of drilling |
PCD (polycrystalline diamond) tooling is the preferred choice for carbon fiber. The diamond cutting edge resists the abrasive wear of the carbon fibers. A PCD-tipped drill can last 10-20 times longer than carbide in carbon fiber.
The feed rate in carbon fiber must be high enough to cut the fibers cleanly. If the feed is too low, the drill rubs rather than cuts, which generates heat and causes the resin matrix to soften. I have seen resin melting on the bore surface when the feed dropped below 0.02 mm/rev.
Delamination and Splintering
The most common defect in composite drilling is delamination, where the layers of the composite separate at the drill entry or exit. Delamination occurs when the drill pushes through the last few plies rather than cutting them.
I have found several methods to reduce delamination:
- Backing plate at the drill exit to support the last plies
- Peck drilling with small peck depths (1-2mm per peck)
- Reduced feed rate in the last 2mm before breakthrough
- Pilot hole to reduce the cutting forces at the exit
For aerospace-grade carbon fiber, I reduce the feed by 50% in the last 2mm of drilling. The reduced feed cuts through the exit plies cleanly without pushing them apart.
Fiberglass Drilling
Fiberglass reinforced plastic is used in marine, construction, and industrial applications. It drills differently than carbon fiber because the glass fibers are less abrasive but more likely to cause fuzzing at the bore surface.
For fiberglass drilling:
| Parameter | Value |
|---|---|
| Cutting speed | 80-150 m/min |
| Feed rate | 0.05-0.15 mm/rev |
| Coolant | Air mist or water |
| Tool material | Carbide (uncoated) |
The main issue with fiberglass is fuzzing. The glass fibers are not cut cleanly by the drill, and they leave a fuzzy surface inside the bore. The fuzz can cause problems in applications where the bore is a sealing surface or a bearing surface.
To minimize fuzzing, I use a sharp cutting edge and a higher feed rate. A higher feed rate cuts through the fibers before they can bend and fuzz. I have found that 0.12 mm/rev produces a cleaner surface than 0.05 mm/rev in fiberglass.
Ceramic Drilling
Technical ceramics like alumina, zirconia, and silicon carbide are used in applications requiring wear resistance and high-temperature capability. Drilling deep holes in ceramics is extremely difficult.
Ceramics are hard and brittle. They do not deform plastically like metals. The drilling process removes material by microfracture rather than shearing.
For ceramic drilling, I use:
- Diamond-plated core drills for large diameters
- Ultrasonic-assisted drilling with diamond abrasive
- Very low feed rates (0.001-0.005 mm/rev)
- Flood coolant for thermal control
The material removal rate in ceramics is dramatically lower than in metals. Drilling a 6mm hole through 50mm of alumina ceramic can take 30 minutes or more. The process is slow and methodical, with frequent tool inspection.
I have found that ultrasonic-assisted drilling is the most effective method for deep holes in ceramics. The ultrasonic vibration helps the diamond abrasive fracture the ceramic material more efficiently. Without ultrasonic assistance, the diamond tool loads up with ceramic dust and stops cutting.
Engineered Plastic Drilling
Engineered plastics like PEEK, PTFE, and polycarbonate are used in medical, food processing, and electrical applications. These materials are softer than metals but present their own challenges.
For drilling engineered plastics:
| Material | Cutting Speed | Feed Rate | Challenge |
|---|---|---|---|
| PEEK | 80-120 m/min | 0.05-0.12 mm/rev | Swarf melting |
| PTFE (Teflon) | 60-100 m/min | 0.04-0.10 mm/rev | Deformation |
| Polycarbonate | 100-200 m/min | 0.08-0.20 mm/rev | Cracking |
| Nylon | 100-150 m/min | 0.08-0.18 mm/rev | Swarf wrapping |
PEEK is the most common engineered plastic I have drilled for medical applications. It is strong and biocompatible but has a low melting point. If the cutting speed generates too much heat, the PEEK melts and re-solidifies on the bore surface.
For PEEK, I use compressed air cooling rather than liquid coolant. Liquid coolant can be absorbed by the plastic and cause dimensional changes. Air cooling removes the heat without affecting the material properties.
Key Takeaways
- Carbon fiber drilling requires PCD tooling for acceptable tool life; carbide wears out in 3-8 meters
- Feed rate reduction of 50% in the last 2mm prevents delamination at the drill exit in aerospace-grade carbon fiber
- Fiberglass drilling at 0.12 mm/rev feed produces cleaner bore surfaces than lower feeds
- Ceramic deep hole drilling is extremely slow (30+ minutes per hole) and benefits from ultrasonic assistance
- Engineered plastics like PEEK require air cooling to prevent melting at the bore surface