Aerospace and EV structures join CFRP to aluminium and titanium with thousands of fastener holes — drilled in one pass through the whole stack. The catch: each layer wants different speed, feed and coolant. This page covers the dual-speed regime, stacking order, tooling and coolant that make single-shot stack drilling work.
Each material in a CFRP/Al/Ti stack has opposite machinability demands:
| Configuration | Effect on quality |
|---|---|
| Metal layer beneath CFRP | Acts as a backing plate — reduces push-out delamination in the composite |
| Drill Ti → CFRP → Al (Ti on top) | Improves hole quality: titanium swarf is evacuated without passing through the CFRP hole |
| Al on top (Al/CFRP/Ti) | Worse — aluminium burr and chips complicate evacuation |
| Parameter | Recommended range |
|---|---|
| Cutting speed — CFRP / Al layers | 40–120 m/min (high speed) |
| Cutting speed — Ti layers | 20–60 m/min (about half) |
| Feed rate | 0.03–0.15 mm/rev |
| Point angle | 130–140° |
| Helix angle | >24° |
Best reported tool life in stack drilling was 310 holes with an uncoated carbide drill at the lowest speed (20/40 m/min) and lowest feed (0.05 mm/rev) under flood coolant. Lower feed (0.03 mm/rev) at higher speed minimizes thrust and optimizes CFRP surface quality in some studies — the exact optimum depends on your stack order and tooling.
| Method | Result |
|---|---|
| High-pressure flood (~70 bar through-spindle) | Undersized holes (max errors ~14 μm Ti, 20 μm CFRP, 15 μm Al); better roundness |
| Spray mist | Oversized holes (up to 6/120/28 μm) — worse tolerance |
| MQL (through-tool) | Best overall: reduces friction and chip adhesion, improves tolerance and surface |
| Lubricated CO₂ cryogenic | Cuts tool-tip temperature, keeps holes within 0.5% of nominal, +90% fiber pull-out improvement, −33% burr height |
Helical (orbital) milling interpolates a milling cutter around the hole path and generally causes less damage than drilling — lower forces, less heat, easier chip evacuation and reduced delamination. The trade-off: it can leave oversized CFRP holes and undersized titanium holes because the two materials machine differently, and cycle time is longer than a single-shot drill. Choose helical milling for the highest hole-quality stacks; choose drilling for speed.